ARTICLE IN PRESS
Deep-Sea Research II 53 (2006) 2597–2617 www.elsevier.com/locate/dsr2
Estimating potential diapause duration in Calanus finmarchicus Whitley J. Saumwebera,, Edward G. Durbinb a
2041 Lyttonsville Rd, Silver Spring, MD 20910, USA URI GSO, S. Ferry Rd., Narragansett, RI 02882, USA
b
Accepted 26 August 2006
Abstract Deep basins in the Gulf of Maine act as refuge for a large population of diapausing Calanus finmarchicus during the summer and fall. This population acts as the primary seed population for Georges Bank in the spring and is thought to be composed primarily of individuals that developed during the previous spring bloom. The factors affecting growth and mortality in the summer-fall population are not well understood, however, and loss terms from advection and starvation may be large. To assess the potential energetic limitation and loss of C. finmarchicus from the Gulf of Maine basins, a new nitrogen-specific respiration model has been developed for the resting stage of the species. Stage C5 C. finmarchicus were collected during July, September, and December 2003 from Wilkinson and Georges Basins. Animals were collected using both MOCNESS tows and zooplankton samplers on the Johnson Sea Link II submersible. Metabolic rates were measured using a Micro-Oxymax gas analyzer and Winkler incubation techniques both at sea and on animals kept in culture on shore. Respiration rates measured in the field were not significantly different from those measured on shore, with a mean of 130 mmol O2 gN1 h1 (14.4 mmol O2 gC1 h1) at 0 1C and a Q10 of 2.77 (2.58 for carbon-specific respiration). Using the nitrogen-specific rates in conjunction with visual estimates of nitrogen weight and lipid stores, we derived a discrete function for predicting potential diapause duration based on an animal’s length, oil sac volume, and the in situ temperature. The maximum potential diapause duration for a C5 C. finmarchicus is predicted to range from 280 days at 0 1C to approximately 90 days at 11 1C. The maximum potential diapause duration in the Gulf of Maine is predicted to be between 3.5 and 5.5 months. These results suggest that energetic limitation may play a role in controlling the population dynamics of diapausing C. finmarchicus in the Gulf of Maine. A reassessment of the importance of summer production to the Gulf of Maine C. finmarchicus population may be required to account for its year-round presence on the Northeast American Shelf. r 2006 Elsevier Ltd. All rights reserved.
1. Introduction The calanoid copepod Calanus finmarchicus is the dominant member of the mesozooplankton biomass during the spring and summer months across much Corresponding author.
E-mail addresses:
[email protected] (W.J. Saumweber),
[email protected] (E.G. Durbin). 0967-0645/$ - see front matter r 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.dsr2.2006.08.003
of the North Atlantic. While surface primary production remains high, C. finmarchicus typically remains within the epipelagic zone and may produce between one and three generations before the combination of grazing pressure and the onset of stratification reduces the phytoplankton biomass to levels that are food limiting (Conover, 1988; Mauchline, 1998; Durbin et al., 2000). C. finmarchicus, as is typical of its genus (Conover, 1988),
ARTICLE IN PRESS 2598
W.J. Saumweber, E.G. Durbin / Deep-Sea Research II 53 (2006) 2597–2617
responds to this period of limited resources by migrating out of the epipelagic zone and entering a period of reduced metabolism known as diapause. The migration to deeper water, typically undertaken at stage C5, is thought to both reduce the risk of predation and aid in the reduction of metabolism (Kaartvedt, 1996). Resting populations of C. finmarchicus are often found between 500 and 1000 m, though examples of both shallower and deeper populations do exist (Durbin et al., 1995; Hirche, 1991). Evidence of reduced enzymatic and digestive activity during diapause (Halberg and Hirche, 1980; Hirche, 1983, 1989) has led to the belief that C. finmarchicus does not feed during this period and instead relies entirely on the catabolism of stored lipid reserves, mostly in the form of wax ester. These wax ester reserves are stored in a large oil sac which nearly fills the body cavity of older copepodites (Lee et al., 1970; Kattner and Krause, 1987; Miller et al., 1998; Jonasdottir, 1999). Such reserves must sustain a copepod throughout the duration of diapause, which may last between 6 and 10 months (Hirche, 1996a). Most recent work on diapause in C. finmarchicus has focused on two broad issues: (1) how must an animal’s storage lipid be allocated in order for it to successfully complete diapause; and (2) what are the environmental and/or physiological cues that cause the initiation and terminate diapause? In answer to the first question, Hirche (1996b) suggests that the lipid reserves are likely to be more important for the final molt and gonad maturation than for maintenance metabolism while in diapause. This conclusion is common but there are some issues with its generality. At least two of the works cited by Hirche (1996b), Hirche and Kattner (1993) and Gatten et al. (1980) reported on experiments using copepods of the same genera but different species and extension of their conclusions to C. finmarchicus may be problematic. Others (Tande, 1982; Hopkins et al., 1984 as in Sargent and Falk-Petersen, 1988), based their conclusions on observations of resting populations in Fjords where, as Hirche (1996b) notes, stage distributions, and thus cohort separation, may differ from oceanic environments. Furthermore, Saumweber (2005a) reports on a series of observations in which lipid stores in resting animals do decrease appreciably over the course of a season. There are also conflicting reports regarding the amount of stored energy an animal may require upon arousal. Jonasdottir (1999) concluded that arousal and maturation would consume approxi-
mately 75% of a C. finmarchicus’ entire lipid reserve (based on estimates of maximal capacity found in this paper) while Rey-Rassat et al. (2002) concluded that only 32% was required. More work is clearly needed to clarify this issue. With regard to the question of dormancy cues, workers have tended to either favor an arousal mechanism linked to photic cues (Grigg and Bardwell, 1982; Hirche, 1989; Miller et al., 1991) or some sort of an internal timer based on reduced development time (Østvedts, 1955 as reviewed in Hirche, 1996a). Hind et al. (2000) tested the daylight cue hypothesis against the reduced development time hypothesis in a strategic model of C. finmarchicus life history. The authors found that the most persistent modeled populations were generated by assuming diapause was cued by food limitation and terminated based on the timing of constant, but reduced stage development throughout the resting phase. These results were found to be consistent with the prior work of Fiksen and Carlotti (1998), who noted in a detailed-individual-based model of C. finmarchicus life history that survival was most enhanced if diapause duration was based on the status of lipid reserves and was, as a result, highly sensitive to metabolic rate while in diapause. The findings of Hind et al. (2000) have recently been challenged, however, by Tittensor et al. (2003) and Speirs et al. (2005). Tittensor et al. (2003) modeled C. finmarchicus abundance in the Labrador Sea and found that predictions most closely matched observed abundances, and productivity was most successful, if diapause arousal varied with latitude. They did not link this variation to photic control but the results are consistent with that hypothesis. Speirs et al. (2005) argued that only changes in photoperiod coupled to some level of development could explain the degree of synchrony demonstrated by the C. finmarchicus population in the North Atlantic and Norwegian Sea. Of all these studies, however, only Fiksen and Carlotti (1998) considered the possibility that C. finmarchicus may become energetically limited during diapause. Very few direct measurements of C. finmarchicus diapause metabolism exist. Hirche (1983) and Ingvarsdottir et al. (1999) both found diapause respiration rates to be approximately 20% of active rates. Ingvarsdottir et al. (1999) constructed a simple carbon-specific respiration model to estimate potential diapause duration and found that at 0 1C, diapause could last nearly a year, but that survival
ARTICLE IN PRESS W.J. Saumweber, E.G. Durbin / Deep-Sea Research II 53 (2006) 2597–2617
2599
time dropped off considerably with increasing temperature. A number of issues prevent this model from being generally applicable however. Most notably, the authors did not generate a discrete, temperature specific, respiration function for diapausing animals. They did examine changes in diapause respiration with temperature, but only at two points. The aim of their paper was not to establish a statistical relationship between temperature and respiration, but the need for a more general function clearly exists. The authors also based their model on carbon-specific respiration rates. Carbonspecific rates are generally taken to be the standard for most metabolic studies and their use in Ingvarsdottir et al.’s (1999) work should be expected. Indeed, Ingvarsdottir et al. (1999) sites Ingvarsdottir (1998) in claiming to find no significant relationship between carbon weight and carbon-specific respiration. However, most of a stage C5 C. finmarchicus, carbon weight resides in the lipid within its oil sac and may not generate much in the way of metabolic demand. Literature regarding the morphology of calanoid oil sacs is scarce but there is some evidence that large copepod oil sacs may be analogous in structure to mammalian white adipocytes. Blades-Eckelbarger (1991) found that the oil sacs of female Eucheata marina were comprised of a single multinucleate cell wall surrounding a large intracellular lipid volume that was not transversed by any cellular structure. If C. finmarchicus has a similar structure for its oil sac, then it is likely that its respiratory demands would vary more consistently with structural, i.e. total nitrogen, rather than total carbon weight. A similar argument was first made by Harris (1983) and echoed by McLaren (1986) and McLaren and Leonard (1995). This paper attempts to respond to these issues in order to produce a more accurate and generally applicable model of diapause respiration for stage C5 C. finmarchicus. In doing so, we hypothesize that a C. finmarchicus in diapause may become energetically limited and that when this occurs, diapause must end. Our calculations are based on measurements of temperature and nitrogen-specific respiration rates and empirical estimates of lipid requirements.
where
2. Methods
2.1. Sampling for physiological measurements
In developing our diapause respiration model, we began with a simple exponential decay function
Sampling for individual physiological measurements took place during four research cruises
W t ¼ W 0 eNRt .
(1)
In this equation, W0 and Wt are the weight, in carbon, of an animal’s energy reserves at time 0 and time t, respectively. N is the animal’s weight in nitrogen, and R is a nitrogen and temperaturespecific carbon utilization rate (respiration). Two key assumptions are made in the derivation and application of this model: (1) stage C5 C. finmarchicus do not feed while in diapause; rather they survive entirely on energy derived from the catabolism of stored lipid reserves in the form of wax esters (WE); and (2) the respiration rate of animals in diapause remains relatively constant over the course of the diapause period. The proposed model also assumes the existence of an internal timer, as postulated by Hind et al. (2000), to control diapause cues. Unlike those authors, however, we link the timer to energy limitation and we do not necessarily consider it to be the sole arbiter of arousal. We postulate only that it is possible for a diapausing animal to become energetically limited, and that if this occurs, it must leave diapause. Under this scenario, arousal must be initiated when the amount of potential energy reserves reaches some critical fraction, F, of the original store. Restated in terms of Eq. (1), potential diapause duration, tD, may be found when Wt/W0 equals F as lnðF Þ . (2) NR All that that is required to solve this equation is for us to define the terms F and R. Two issues complicate this apparently simple task, however: (1) F may not be a universal constant for all individual stage C5 C. finmarchicus; and (2) N also may be a function of time (t). Defining these terms in a manner that is broadly applicable is the subject of the work described below. The critical lipid fraction (F) and the temperature-dependent respiration function (R) were defined using two sets of observations consisting of: (1) in situ measurements of changes in the physiology of individual diapausing C. finmarchicus; and (2) incubation experiments carried out with resting animals immediately postcapture and after some period of acclimation. tD ¼
ARTICLE IN PRESS 2600
W.J. Saumweber, E.G. Durbin / Deep-Sea Research II 53 (2006) 2597–2617
carried out in cooperation with the National Oceanic and Atmospheric Administration’s (NOAA) Ecosystem Monitoring Program (ECOMON). These cruises were conducted during June, August, and November 2001 and January 2002 on board the RVs Albatross IV and Delaware II (see Table 1). Five or six standard stations were sampled during each cruise, in and around the Gulf of Maine, including one each in the Slope Water (SW), Northeast Channel (NEC), Georges Basin (GB), Jordan Basin (JB), Wilkinson Basin (WB), and on the Scotian Shelf (SS) between Browns Bank and Cape Sable (see Fig. 1 and Table 1). The SW station was not sampled during the June and November cruises due to weather. A 1-m2 Multiple Opening Closing Net Environmental Sensing System (MOCNESS, Weibe et al., 1985) equipped with 150-mm mesh nets was used to sample the zooplankton from three depth strata at each station: 0–100 m (Net 3), 100–150 m (Net 2), and 150 m to the bottom (Net 1). Bongo Nets rather than the MOCNESS were used to sample the SS station because it was less than 100 m deep and consistently well mixed. It should also be noted that all surface tows (Net 3) during June 2001 only extended to 60 m rather than 100 m. One hundred and twenty individual stage C5 C. finmarchicus from each Net 1 and 3 were sorted and filmed at 120–160 magnification. Images were recorded on standard VHS tape to be used for body size and storage lipid analysis. The filmed animals were subsampled into groups of 30 for measurement of nucleic acid content and carbon and nitrogen weights. Animals sampled for nucleic acid analysis were frozen in liquid nitrogen on board ship until they could be moved to long-term storage at 80 1C
on shore. The carbon-nitrogen samples were placed individually in pre-weighed 6 4 mm tin weigh boats and stored in desiccation chambers until analysis. 2.2. Image analysis and lipid, C, and N content All images recorded during sampling were analyzed using the software packages NIH Image v. 1.60 and Image J v. 1.3 (http://rsb.info.nih.gov/ij/). For the purposes of this model, and all data reported herein, total body length was considered to be the prosome length (PL). PL was measured from the tip of the cephalasome to the end the last metasomal segment (Fig. 2). Oil Sac Area (OSA) was measured by outlining the apparent sac volume, and Oil Sac Length (OSL) was taken as the greatest dimension of the outlined region. Oil Sac Volume (OSV) was estimated as a cylindrical volume using the methods of Miller et al. (1998). All OSV calculations were made based on estimates of OSA taken from lateral views of the animals. In the few instances that only a dorsal view was available, the calculated OSV was converted to a lateral estimate based on a polynomial regression (R2 ¼ 0:88) between dorsal and lateral OSV estimates where OSVDorsal ¼ 8:0138ðOSVLateral Þ3 5:012ðOSVLateral Þ2 þ 1:4585ðOSVLateral Þ.
ð3Þ
Individual Wax Ester (WE) content was calculated from OSV using the relationship described by Miller et al. (1998) and individual Wax Ester Carbon content (WEC) was found by assuming that carbon comprised 74% of total WE weight (Katner and Krause, 1989).
Table 1 List of cruise dates and sampling locations for all animals sampled during 2001–2002 Abr.
No.
Date
Latitude (1N)
Longitude (1W)
Cruise June August November January
C1 C2 C3 C4
DE0105 AL0109 AL0111 AL0202
Jun 1–5, 2001 Aug 23–29, 2001 November 10–15, 2001 January 24–30, 2002
Na Na Na Na
Na Na Na Na
Station Slope Water Northeast Channel Scotian Shelf Georges Basin Jordan Basin Wilkinson Basin
SW NEC SS GB JB WB
S1 S2 S3 S4 S5 S6
Na Na Na Na Na Na
411440 421320 431040 421220 431460 421250
651290 661250 661020 671150 671410 691300
ARTICLE IN PRESS W.J. Saumweber, E.G. Durbin / Deep-Sea Research II 53 (2006) 2597–2617
2601
Cruise Stations, June 2001 –January 2002 45°
44°
Latitude (N)
JB
43°
SS WB GB
NEC
42°
SW
41°
40° 71°
70°
69°
68°
67°
66°
65°
Longitude (W) Fig. 1. Standard stations Gulf of Maine stations sampled during 2001–2002. For key see Table 1.
Fig. 2. Photo (160 ) of a stage C5 Calanus finmarchicus with prosome length, oil sac length (SL), and oil sac area (A) all marked.
OSV was also normalized to body length by expressing all OSV values as a percentage of the Apparent Maximum (%Amax) OSV for an
animal of a given length. We defined this Apparent Maximum as the upper bound of the OSV-Length data set, as determined by the regression quantiles method described in Cade et al. (1999). Cade et al. (1999) define a quantile, t, as that region above t% and below 1t% of the data. The authors suggested that the statistical bounds of a data set may be found by finding the greatest, or smallest, quantile through which a least absolute deviation polynomial regression may be a significant fit. We used a standard linear model stepwise regression of all OSV-Length measurements to determine the appropriate degree of polynomial and then found the data set’s upper bound by iteratively increasing t until the coefficients of the quantile polynomial regression were no longer significantly different from zero. The same technique was used to define a lower bound, or Apparent Minimum (Amin), for the OSVLength data set. All quantile regressions were performed using the Blossom v.W2001.08d software
ARTICLE IN PRESS W.J. Saumweber, E.G. Durbin / Deep-Sea Research II 53 (2006) 2597–2617
2602
package (http://www.mesc.usgs.gov/products/ software/blossom.shtml). The Apparent Maximum line is meant to represent the maximum potential storage volume (OSVmax) for any stage C5 C. finmarchicus of a given length. It was therefore defined using all of the animals measured during the 2001–2002 cruises (n ¼ 4080). Similarly, the Apparent Minimum line is meant to represent the minimum potential oil sac volume (OSVmin) for animals in a state of diapause. It was therefore defined using only data from animals collected below 150 m (n ¼ 1937), making the implicit assumption that these animals were in a resting state. The Amin represents our empirical attempt to answer the question of how much storage lipid is required for the final molt and maturation at the end of diapause. Thus the critical fraction, F, from Eq. (2) may now be defined for an animal of a given length as F¼
WECmin , WECmax
(4)
where WECmin and WECmax are equivalent to the wax ester carbon content of OSVmin and OSVmax, respectively. This model makes no assumptions about the fate of animals who reach the critical fraction, F, only that they leave the diapause population. This may happen either through mortality, emergence from diapause, or both. A biometric relationship with length also was developed for the estimation of individual weight. In this case, both length (mm) and OSV (mL) were used as predictors of individual C and N content (mg) in two separate multiple linear regressions of the form: Weight ðmgÞ ¼ mðL3 Þ þ oðOSVÞ þ n.
(5)
Animals used in these regressions included all stage C5 C. finmarchicus sampled for C and N content during the 2001–2002 cruises (n ¼ 851). Individual C and N content was determined using a Carlo Erba Strumentazione Carbon–Nitrogen Analyzer 1500. Animals were heated at 55 1C for 24 h prior to analysis in order to drive off any excess moisture. 2.3. Relative RNA:DNA ratios RNA:DNA indices are well established as providing good estimates of growth and nutritional condition for larval fish (Buckley, 1984; Buckley et al., 1999) and the same approach has been extended to copepods (Wagner et al., 1998, 2001).
Wagner et al. (1998) established that a relative RNA:DNA ratio (RRD) is a good index of relative growth in C. finmarchicus. We thus used mean RRD values as a qualitative method of comparing the metabolic state of animals collected during the 2001–2002 cruises with those used in the incubation experiments described below. Thirty stage C5 C. finmarchicus were picked for nucleic acid analysis from Nets 1 and 3 at each station during the 2001–2002 cruises and also from the sample population prior to each incubation experiment. Total RNA and DNA content was found using the flourometric technique described in Wagner et al. (1998) and the RRD index is expressed as a percentage of the maximal temperature specific RNA:DNA ratio described therein. 2.4. Sampling for shipboard experiments Eight incubation experiments were carried out over the course of two cruises during July and September 2003 on board the RVs Endeavor (EN383) and Seward Johnson (SJ0308), respectively (Table 2). Animals sampled during EN383 were collected using a MOCNESS with 150-mm mesh nets from four locations in the western Gulf of Maine and lower Bay of Fundy. All animals were collected at depths greater than 100 m. Specific sample depths on this cruise were selected to correspond to regions of peak particle density in the size range of C5 C. finmarchicus as determined by vertical casts with an Optical Plankton Counter (OPC) (Table 2). Animals sampled during SJ0308 were collected from three locations in Georges Basin using the suction samplers of the Johnson Sea Link II submersible (Table 2). The suction samplers use a diaphragm pump to draw water into 4-L chambers capped by 200-mm mesh. Sample depths were always below 100 m and chosen based on visual estimates from the submersible of peak C. finmarchicus abundance (see Table 2). Depths given in Table 2 represent the nominal center of a 10 m depth interval over which the suction sampler was run. On both cruises, animals were collected from cod ends or sample chambers prior to net rinsing and diluted in filtered sea water before sorting for incubation. 2.5. Shipboard respiration experiments All shipboard incubations were conducted based on methods described in Ikeda et al. (2000) for the
ARTICLE IN PRESS All sample dates were in 2003. ‘Sample Source’ refers to the cruise number referenced in the text and ‘Exp. Type’ refers to either Winkler or Oxymax techniques used at sea or on-shore.
30.39 38.78 5.58 24.83 43.47 21.10 57.51 41.28 74.02 34.88 20.00 71.22 99.73 118.28 31.32 213.30 256.57 214.37 277.24 336.92 265.34 316.91 297.77 317.67 268.64 169.74 251.52 306.12 271.99 129.53 3.10 4.06 0.57 2.57 4.69 2.16 6.45 4.46 6.45 4.21 2.35 6.64 10.89 11.86 3.50 22.76 27.04 20.59 29.66 36.01 28.19 34.62 32.33 34.28 29.06 18.92 24.00 35.52 23.96 14.29 5.4 5.0 5.4 7.1 7.9 5.9 7.9 7.9 9.7 7.2 3.6 7.3 7.5 7.3 7.5 661270 661280 671290 671290 671300 691420 671310 671310 691550 691550 691550 691390 Na Na 691550 EN383 EN383 EN383 SJ0308 SJ0308 SJ0308 SJ0308 SJ0308 GC GC GC EN383 EN385 EN385 GC J-W2 J-W3 J-W4 S-W2 S-W4 S-W6 S-W8 S-W9 D-W2 D-W3 D-W4 J-O1 A-O1 A-O2 D-O1
July 3 July 4 July 6 Sep. 11 Sep. 12 Sep. 16 Sep. 21 Sep. 21 Dec. 13 Dec. 13 Dec. 13 July 7 Aug 17 Aug 17 Dec. 13
120–180 98–149 150–220 197 207 165 197 197 125–158 125–158 125–158 199–266 150–220 150–220 125–158
441440 441390 431420 421180 421180 421280 421180 421180 431000 431000 431000 421390 Na Na 431000
W-Sea W-Sea W-Sea W-Sea W-Sea W-Sea W-Sea W-Sea W-Shore W-Shore W-Shore Oxymax Oxymax Oxymax Oxymax
Mean R (mmol O2 gC1 h1) Exp. T (1C) Exp. type Lon. (1W) Lat. (1N) Sample Z (m) Sample date Sample source Exp. ID
Table 2 List of respiration experiments, the source of animals for each experiment, and the results from each experiment
Standard deviation
Mean R (mmol O2 gN1 h1)
Standard deviation
W.J. Saumweber, E.G. Durbin / Deep-Sea Research II 53 (2006) 2597–2617
2603
determination of oxygen consumption via Winkler titration. Each experiment used seven 300-ml Biological Oxygen Demand (BOD) bottles as experimental test chambers, five sample and two control. Each sample bottle was loaded with 30 stage C5 C. finmarchicus from the dilute net sample no more than an hour after capture. All experimental bottles were filled with surface water filtered through a 0.2-mm mesh. Incubation temperature was nominally set to the in situ mean temperature over the depth range at which the test animals were captured and was monitored continuously with data loggers. Incubation durations ranged between 24 and 48 h, at the end of which, water was sampled for oxygen analysis and sample animals were sorted for analysis of carbon and nitrogen content as described above. The presence of any dead or damaged animals in a bottle at the end of an experiment caused it to be excluded from the final calculation of mean respiration. An experiment was required to have at least three usable sample bottles to be included in developing the respiration function described below. 2.6. Sampling for experiments on shore Previous workers (Ikeda, 1977; Skjodal et al., 1984) have suggested that zooplankton respiration experiments carried out immediately post-capture may be biased upwards due to the effects of capture stress, and that, any respiration experiment is likely to include some bias due to handling stress accrued during the sorting process. Others (e.g., Ingvarsdottir et al., 1999) have seen slight, but insignificant, increases in respiration over time post-capture. In an attempt to at least partially account for these possibilities, we performed four sets of respiration experiments on shore in addition to those carried out at sea. The shore-based experiments used stage C5 C. finmarchicus that had been captured at sea, but which were then allowed to acclimate for a period of time prior to the start of an incubation. These experiments took place during July, August, and December 2003 and used animals collected from at or below 150 m in Wilkinson Basin in the Gulf of Maine. The July and August 2003 animals were collected using a 1-m2 MOCNESS during the last 48 h of EN383 and EN385. The December 2003 animals were collected during a day trip aboard the RV Gulf Challenger (labeled as GC in Table 2) using a 0.25-m2 MOCNESS. In each case the animals were brought back to shore in filtered sea water, at
ARTICLE IN PRESS 2604
W.J. Saumweber, E.G. Durbin / Deep-Sea Research II 53 (2006) 2597–2617
relatively high concentration, and kept in the dark at in situ temperatures. After reaching shore, the animals were placed in 120-L tanks and diluted with filtered sea water to approximately 12 individuals L1. The animals were then allowed to acclimate nominally at their in situ temperature for approximately 48 h in the dark. Gentle aeration was provided with bubblers to prevent hypoxia. 2.7. Respiration experiments on shore Because it is not clear what effect the process of being placed in culture has on animals in a state of diapause, multiple incubations were conducted over the course of 1–2 weeks during each set of on-shore experiments to monitor the change in respiration rate and activity level over time. Incubations were only included in developing the respiration function described below if the RRD level of the animals just prior to the start of an experiment did not exceed levels measured in animals captured from below 150 m during the 2001–2002 field season. This limitation, and the stricture on using only experiments with completely healthy animals present at the end, resulted in only seven of 18 on-shore experiments being included in the respiration function and reported on here. Three of these experiments used the Methods of Ikeda et al. (2000), as described above, but in the remaining four experiments, oxygen consumption was measured with a Columbus Instruments Micro-Oxymax closed circuit respirometer. At the start of every experiment, the Micro-Oxymax requires a period of time to let the headspace of each experimental chamber reach gaseous equilibrium. This leads to a period of approximately 4 h between when animals are first loaded into a test chamber and the start of an experiment. This lag differs from the Winkler titration method in which the experiment effectively begins as soon as a sample bottle is capped. We postulated that any bias due to handling stress in the Winkler method would be illuminated by an apparent reduction in respiration rates as measured by the Micro-Oxymax. Differences in light exposure between the two experimental methods were not measured, but every effort was made to minimize light exposure to the extent possible in both sets of experiments. A number of authors have reported that field caught C. finmarchicus will apparently terminate diapause and complete their final molt in culture but only after prolonged exposure to light and a lag time of
between 1 and 3 weeks (Grigg and Bardwell, 1982; Hirche, 1989; Miller and Grigg, 1991; Hirche, 1996a). The Micro-Oxymax experimental configuration used ten 100-ml test chambers, six for samples and four as control. Each sample chamber was loaded as in the shipboard experiments but with only 10 stage C5 C. finmarchicus in 100 ml of filtered sea water. All the Micro-Oxymax incubation experiments were nominally run at the in situ temperature of capture, though some variation was experienced.
3. Results 3.1. Apparent minimum and maximum wax ester content The ordinary least squares stepwise regression of the complete 2001–2002 biometric data set, demonstrated that there was an overall quadratic shape to the relationship between OSV and length. The 99th quantile was found to be the upper bound of the complete data set (OSVmax, Fig. 3A) and is defined as OSVmax ¼ 0:274052ðL2 Þ 0:5987ðLÞ þ 0:254568. (6) The lower bound of the data set comprised of animals captured below 150 m (OSVmin, Fig. 3B) was found to be the fifth quantile and is defined as OSVmin ¼ 0:279779ðL2 Þ 1:06674ðLÞ þ 1:060073. (7) When expressed as a percentage of OSVmax, OSVmin is greatest for the smallest animals measured (62%), then decreases with increasing animal length to a minimum (24%) for animals 2.15 mm in length, before increasing again with increasing animal length (40% for animals 3 mm in length). The pattern is similar when considering the relationship in units of WEC rather than OSV, except that the apparent minimum WEC (WECmin) is slightly higher at 27% OSVmax. The corresponding apparent minimum weights of WEC at lengths 1.8, 2.15, and 3.0 mm, are 27, 33, and 180 mg, respectively (Fig. 4). The mean length of all animals captured below 150 m during 2001–2002 is 2.38 mm. At this length, an animal has an apparent minimum OSV of 27% OSVmax and an apparent minimum WEC of 29% WECmax or 55 mg C.
ARTICLE IN PRESS W.J. Saumweber, E.G. Durbin / Deep-Sea Research II 53 (2006) 2597–2617
0.8
(A)
2605
All Data
0.7 0.6 0.5 0.4 0.3 0.2
Oil Sac Volume (µL)
0.1 0 1.80
0.8
1.90
(B)
2.00
2.10
2.20
2.30
2.40
2.50
2.60
2.70
2.80
2.90
3.00
Deep Animals
0.7 0.6 0.5 0.4 0.3 0.2 0.1 OSVmax 0 1.80
1.90
2.00
2.10
2.20
2.30
2.40
2.50
2.60
2.70
OSVmin 2.80
2.90
3.00
Length (mm) Fig. 3. Relationship between C. finmarchicus length at stage C5 and oil sac volume (OSV). (A) Shows the complete data set (n ¼ 4080) and (B) shows only data from animals collected below 150 (n ¼ 1937). OSVmax was calculated based on the data shown in (A). OSVmin was calculated based on the data shown in (B).
Both C and N weight show a strong predictive relationship with OSV (mL) and length (mm). Examination of the relationship between OSV and C weight suggested an exponential shape to the data (Fig. 5). Inclusion of this exponential term in the multiple linear regression produced a fit with R2 ¼ 0:92 where C ðmgÞ ¼ 7:641879ðL3 Þ þ 0:723668½374:99ðOSV0:536 Þ 54:7409.
ð8Þ
There did not appear to be any exponential relationship between OSV and N weight and a simple multiple linear regression, with no exponential term, produced a fit with R2 ¼ 0:90 where N ðmgÞ ¼ 1:16387ðL3 Þ þ 0:723668ðOSVÞ 0:94817. (9) Using the OSV term together with the length term in the nitrogen equation only slightly increased the
ARTICLE IN PRESS W.J. Saumweber, E.G. Durbin / Deep-Sea Research II 53 (2006) 2597–2617
2606
500 450
Wax Ester Carbon (µg)
400 350 300 250 200 150 100 50 OSVmin
OSVmax 0 1.80
1.90
2.00
2.10
2.20
2.30
2.40
2.50
2.60
2.70
2.80
2.90
3.00
Length (mm) Fig. 4. OSVmin and OSVmax expressed in units of wax ester carbon as a function of length.
400 350
Carbon Weight (µg)
300 250 200 150
y = 374.99x0.536 R2 = 0.8548
100 50 0 0.00
0.10
0.20
0.30
0.40
0.50
0.60
0.70
Oil Sac Volume (mm3) Fig. 5. Relationship between OSV and carbon weight.
quality of the fit over a regression with length alone, but the change was statistically significant. 3.2. Weight specific respiration vs. temperature Weight specific respiration rates (r), as measured using the Winkler incubation technique, ranged
from 191.64 mmol O2 gN1 h1 (21.41 mmol O2 gC1 h1) at 3.6 1C to 317 mmol O2 gN1 h1 (34.28 mmol O2 gC1 h1) at 9.7 1C (Table 2). The relationship between temperature and respiration rate was linearized by plotting log10 (r) against temperature and simple linear regressions were used to generate the equations shown in Fig. 6. The slopes of these
ARTICLE IN PRESS W.J. Saumweber, E.G. Durbin / Deep-Sea Research II 53 (2006) 2597–2617
2607
Log Respiration Rate (µmol O2 gN-1hr-1)
(A) 3.0 2.9
y = 0.0442x + 2.117 R2 = 0.7538
2.8
y = 0.0442x + 2.1154 R2 = 0.6966
2.7 2.6 2.5 2.4 2.3 2.2 2.1
Dec-O
2.0 3.0
4.0
5.0
6.0
7.0
8.0
9.0
10.0
9.0
10.0
Log Respiration Rate (µmol O2 gC-1hr-1)
(B) 1.8 1.7 1.6
y = 0.0441x + 1.1528 R2 = 0.762
y = 0.0412x + 1.1584 R2 = 0.5294
1.5 1.4 1.3 1.2 1.1 1.0 Dec-O
0.9 0.8 3.0
4.0
6.0
5.0
7.0
8.0
Temperature (°C) J-W
S-W
Winkler Regression
D-W
Oxymax
All Data Regression
Fig. 6. Log nitrogen (A) and carbon (B) specific respiration as functions of experimental temperature. Different symbols represent different experimental groups (see Table 2). The Winkler regressions do not include any Micro-Oxymax data while the all data regressions do. The Dec-O data points are treated as outliers and not included in any regression. Error bars are 95% confidence intervals.
equations correspond to Q10 values of 2.77 for nitrogen-specific respiration and 2.76 for carbonspecific respiration. To facilitate further statistical comparison of the respiration data, all sample nitrogen specific respiration values were adjusted to a theoretical rate at 7 1C using Q10 ¼ 2:77. The means of all adjusted data, including the Micro-Oxymax experiments, were compared using a one-way analysis of variance (ANOVA) and a
subsequent Tukey–Kramer multiple comparison test (a ¼ 0:05). Only one experiment, the December 2003 Micro-Oxymax incubation (Dec-O), was found to have produced respiration estimates significantly different from the others. The Dec-O results showed significantly lower respiration rates than 13 of the other 14 experiments. It is not readily apparent why the December Micro-Oxymax results should be so low. Winkler incubation data from
ARTICLE IN PRESS W.J. Saumweber, E.G. Durbin / Deep-Sea Research II 53 (2006) 2597–2617
2608
December show slightly, but not significantly, reduced respiration rates as compared with other experiments. Comparison of the three simultaneous Winkler and Micro-Oxymax incubations found that the difference in respiration measured by the two methods was only significant when the results from Dec-O were included in the comparison. For these reasons, the mean Dec-O respiration was treated as an outlier and excluded from further analysis while acknowledging the lack of an acceptable explanation. Oxygen saturation was found to be above 80% at the end of all experiments save one, and was generally well above 85%. In the one experiment below 80%, oxygen concentrations reached 71% of saturation but the adjusted mean respiration rate from this experiment was not found to be statistically different from the others. Because the adjusted Micro-Oxymax respiration values were not found to be significantly different than the Winkler values (with the exception of the Dec-O), they were included in the final respiration function used in the derivation of the diapause duration model described below (Fig. 6A). Including these values in the respiration function did not change the Q10 of the nitrogen-specific function but the Q10 for the carbon-specific function did decrease
from 2.76 to 2.58. In neither case did the intercept change significantly. The greater change in the carbon-specific function may reflect a change in the carbon weight of animals between the time of capture and analysis on-shore which would support our reasoning in using only nitrogen specific rates for deriving the diapause duration model. 3.3. Relative RNA:DNA ratios (RRD) The lowest RRD levels found during each of the four 2001–2002 cruises ranged from 40% in June to 28% in August, and least at 17% in both November and January (Fig. 7). The decrease in RRD levels between June and November suggests that there was some change in the mean physiological state of the diapause population during this period. All of the populations used in the incubation experiments described above had RRD levels which were within the range of lowest values found during 2001–2002 and most were near the low end of that spectrum (Fig. 7). 3.4. Respiration model for diapause Defining the critical fraction F from Eq. (2) as the ratio of WECmin/WECmax, and both OSV and nitrogen weight in terms of length, allows Eq. (1)
100
Relative RNA:DNA Ratio (%)
90 80 70 60 50 40 30 20 10 Dec-O1
Aug-O2
Aug-O1
Jul-O1
Dec-I4
Dec-I3
Dec-I2
Sepl-I9
Sepl-I8
Sepl-I6
Sepl-I4
Sepl-I2
Jul-I4
Jul-I3
C4S1
Jul-I2
C3S6
C2S6
C1S6
0
Station/Experiment Fig. 7. Relative RNA:DNA ratios (RRD) for animals sampled during the 2001–2002 field season and from animals used in all respiration experiments. The first four values are the lowest mean RRD values found during each of the 2001–2002 cruises, the rest are mean values from experimental populations. All IDs correspond to those found in Tables 1 and 2. All experimental animals are shown to exhibit mean RRD values within the range of the values from 2001 to 2002 (horizontal line). Error bars are 95% confidence intervals.
ARTICLE IN PRESS W.J. Saumweber, E.G. Durbin / Deep-Sea Research II 53 (2006) 2597–2617
to be rewritten and solved (see Tables 3–5). Eq. (26) from Table 4 defines maximum potential diapause duration (t) as ðWECt þ ðd=cÞÞ 1 t ðdaysÞ ¼ ln c . ðWEC0 þ ðd=cÞÞ This derivation is incomplete, however. Wax esters, though dominant, are not the only storage lipids of consequence in C. finmarchicus. Triacylglycerides (TAGs) have also been found to be important and they are the main form of lipid found in C. finmarchicus eggs (Sargent and Falk-Petersen, 1988; Hirche, 1996b; Jonasdottir, 1999). In addition to being associated with reproduction, TAGs are thought to be the more mobile of the storage lipid types and have been associated with recent feeding. Miller et al. (1998) found that there was a near constant amount of TAG in stage C5 animals. They suggested that this level represented a threshold of TAG storage above which excess energy is stored as WE. This threshold corresponded to
2609
17 mg TAG or approximately 12 mg of TAG carbon (TAGC). Miller et al. (1998) found that this number remained constant for a variety of animals over a 3-month sampling period and thus may be considered relatively independent of size. These results are consistent with those of Jonasdottir (1999), who found a small volume of TAGs in diapausing animals at the beginning of the resting period but that this volume was rapidly used up over the course of the season and was absent in the spring. Taking these considerations into account, we made the assumption that an animal begins its rest period with a full complement of both WEC and TAGC. Thus, from Eq. (26), WEC0 is replaced with total storage lipid carbon (TLC) as TLC ¼ WECmax þ TAGC;
(29)
where TAGC ¼ 12 mgC. The maximum potential diapause duration for any C5 C. finmarchicus may now be stated in terms of its length (L) and in situ temperature (T) by simply rewriting Eq. (28) from
Table 3 List of basic equations used in deriving the diapause duration model Equations
Comments 2
OSVmax ðmLÞ ¼ eL þ fL þ g
(10)
OSVmin ðmLÞ ¼ jL2 þ kL þ l
(11)
900ðmgWE mL1 Þ OSVi þ 10:8 WECi ðmgÞ ¼ 0:74ðmgC mgWE1 Þ 1:44
(12)
OSVi ðmLÞ ¼ 0:002162ðWECi Þ 0:012
(13)
NðmgÞ ¼ oOSVi þ mL3 þ n
(14)
a ¼ o ð0:002162Þ
(15)
b ¼ mL3 þ n o 0:012
(16)
NðmgÞ ¼ aWECi þ b
(17)
rðmmolO2 gN1 h1 Þ ¼ 10ðpTþqÞ
(18)
RðmgC mgN1 day1 Þ ¼
1 24ðhÞ QðmmolCO2 mmolO1 2 Þ 12:011ðmgC mmolCO2 Þ r 1 10ðmgN gN Þ
(19)
Oil sac volume calc. based on Eqs. (6) and (7) in the text
Relationship of WE carbon to OSV after Miller et al. (1997)
Relationship of N with OSV and L (Eq. (9) in the text) and then rewritten as a linear function of WEC
Weight specific resp. as a function of T (from Fig. 6A) and rewritten in terms of carbon usage
ARTICLE IN PRESS W.J. Saumweber, E.G. Durbin / Deep-Sea Research II 53 (2006) 2597–2617
2610
Table 4 Derivation of the diapause duration model Equations
Steps
WECtþDt ¼ WECt DtRN
(20)
dWEC ¼ RaWECt Rb dt
(21)
c ¼ Ra
Recognizing Eq. (21) as linear
d ¼ Rb dWEC ¼ cWECt þ d dt
WECt ¼ WECt þ
dWEC dt dWEC dt dWEC dt dWEC dt
¼
Rewriting Eq. (1) using Eq. (4) and the relationships in Table 4
dWEC dt
ð22Þ
d c
¼ c WECt
Because c and d are constants, WEC* can be substituted for WEC and Eq. (22) can be simplified to Eq. (24) d c
þd
¼ cWECt d þd ¼ c WECt c
ð23Þ
dWEC ¼ c dt WECt
(24)
WECt ¼ WEC0 ect d d ct e WECt þ ¼ WEC0 c c
ð25Þ
t ¼ ln
ðWECt þ ðd=cÞÞ 1 c ðWEC0 þ ðd=cÞÞ
Integrating Eq. (24) allows Eq. (25) to be solved for t where t is the time until WECt lipid reserves remain
(26)
" # WECmin þ ððRðmL3 þ nÞÞ= RaÞ ðRaÞ1 t ¼ ln WECmax þ ððRðmL3 þ nÞÞ= RaÞ
(27)
" # 0:74 ð900 ðjL2 þ kL þ lÞÞ=1:44 þ ðRðmL3 þ nÞÞ= Ra ðRaÞ1 t ¼ ln 0:74 ð900 ðeL2 þ fL þ gÞÞ=1:44 þ ðRðmL3 þ nÞÞ= Ra
(28)
Using the definition of F from Eq. (4), t is redefined as the maximum potential diapause duration and can be described entirely in terms of animal length at a given T. To predict the time remaining in diapause for a given animal, WEC at time of capture replaces WECmax in Eq. (27). ‘t’ is in units of days
Table 4 to include TAGC as t ðdaysÞ ¼ ln
ð0:74 ðð900 ðjL2 þ kL þ lÞ þ 10:8Þ=1:44Þ þ ðRðmL3 þ nÞ= RaÞÞ ðRaÞ1 , ð0:74 ðð900 ðeL2 þ fL þ gÞ þ 10:8Þ=1:44Þ þ 12 þ ðRðmL3 þ nÞ= RaÞÞ
where R is defined as the daily nitrogen-specific respiration rate in units of carbon (mg C gN1 d1), L is length (mm), and all other terms are defined in
(30)
Table 5. The respiratory quotient of 0.74 mmol CO2 mmol O1 (defined in Table 5) assumes complete 2 lipid metabolism (Schmidt-Nielsen, 1994). Fig. 8
ARTICLE IN PRESS W.J. Saumweber, E.G. Durbin / Deep-Sea Research II 53 (2006) 2597–2617
2611
Table 5 List of terms, their values, and the equations where they are first defined or used Term
Value
Source Eq.
Units/description
e f g j k l m n o p q Q TAGC
0.274052 0.5987 0.254568 0.279779 1.06674 1.060073 1.163287 0.94817 18.80358 0.0442 2.1154 0.74 12
Eq. (6) Eq. (6) Eq. (6) Eq. (7) Eq. (7) Eq. (7) Eq. (9) Eq. (9) Eq. (9) Fig. 6A Fig. 6A Eq. (19) Eq. (29)
Coefficients and intercept for the relationship between OSVmax (mL) and length (mm) Coefficients and intercept for the relationship between OSVmin (mL) and length (mm) Coefficients and intercept for the relationship between nitrogen weight (mg), OSV (mL) and length (mm) Coefficient and intercept for the relationship between temperature (1C) and log nitrogen specific respiration rate (mmol O2 gN1 h1) The respiratory quotient (mmol CO2 mmol O1 2 ) The initial carbon weight (mg) of TAG storage lipid in diapausing animals
3 240 180
2.7
90
2.8
120
210
150
2.9
2.5
240
2.3
180
2.2
90
270
120
150
2.4
210
Length (mm)
2.6
2.1 2
0
0
15
21
180 150
1.9
120
1.8 0
1
2
120
90
90
3
4
5
60
6
7
8
9
10
11
12
Temperature (°C) Fig. 8. Maximum potential diapause duration (in days on the z axis) as a function of animal length and in situ temperature. This figure assumes that animals begin diapause with their maximum potential lipid stores as determined by OSVmax. The greatest potential diapause duration shown in this figure is 280 days at 0 1C for an animal 2.34 mm in length.
shows the parameter t as a two-dimensional function of both length and temperature. Maximum potential diapause duration is approximately 9 months at 0 1C, with the longest duration predicted to be 280 days for animals 2.34 mm in length. If one wishes to calculate the potential time remaining in diapause for an individual animal, the WECmin term from the numerator of Eq. (30) can simply be replaced with the observed WEC, as determined visually using the methods described above.
4. Discussion 4.1. Storage lipid volume as a diapause cue A number of authors (Sargent and Falk-Petersen, 1988; Hirche, 1989, 1996a) have suggested that most of the energy stored as lipid reserves in diapausing C. finmarchicus is required for the final molt and gonad maturation. While we would argue that there is still some debate over the relative energetic costs
ARTICLE IN PRESS 2612
W.J. Saumweber, E.G. Durbin / Deep-Sea Research II 53 (2006) 2597–2617
of diapause metabolism and the final maturation, it does seem likely that the latter are not insignificant. Jonasdottir (1999) broke the likely energetic costs of final maturation into three categories: (1) the cost of migration from deep water, (2) the cost of molting, and (3) the cost of gonad development (Jonasdottir, 1999). In this paper, we have made an empirical attempt to define the sum of these costs, as a function of body size, using the term OSVmin (Fig. 3) and its corollary term WECmin. These represent our prediction of the minimum amount of lipid reserves required by a C5 C. finmarchicus to leave diapause and successfully enter the surface population as a mature adult. A number of previous authors have attempted to define these costs using both laboratory experiments and field observations, but in each case there were qualifications which make the estimates suspect or lacking in generality with respect to C. finmarchicus. Gatten et al. (1980) used observations of multiple cohorts in the wild to conclude that the related copepod, Calanus helgolandicus, requires approximately half of its storage lipid reserves in the transition between C5 and female. However, different species, even within the same genus, may have differing approaches to reproductive strategies and it is not clear that these results can be applied to C. finmarchicus. Jonasdottir (1999) estimated the cost of arousal and maturation in C. finmarchicus based on measurements of Wax Ester and Triacylglyceride content in two synchronous populations of surface females and deep C5s from the Faroe-Shetland Channel in the eastern North Atlantic. The author estimated the total cost of migration and maturation to be 150 mg storage lipid carbon. Approximately half of this estimate was allocated to respiration associated with the ascent process, 8 mg were associated with molting and 72 mg were associated with the gonad maturation process. Given our estimate of OSVmax, 150 mg represents approximately 77% of the maximum potential WEC available for an animal of average size (2.4 mm). This value contrasts with our estimate of OSVmin, which predicts that a diapausing animal, 2.4 mm in length, has a minimum WEC content of 57 mg, or only 29% of the maximum potential WEC (Fig. 4). This is the fraction our model predicts should be available for migration and maturation. In fact, of the nearly 2000 animals we sampled from below 150 m, approximately three-quarters (74%) had oil sac volumes equivalent to less than 77% of
their maximum potential WEC content. These differences cannot easily be explained and may be attributed to a number of factors. Jonasdottir (1999) based her estimates on animals sampled from a population diapausing at significantly greater depths than the one discussed in this paper, and it is likely that there were significantly greater energetic costs associated with the migration. However, it seems unlikely that C. finmarchicus populations from different regions incorporate differences in migration cost into the timing of their ascent. Alternatively, Jonasdottir’s (1999) values may overestimate the cost of ascent. The author’s calculations were based on an observed difference in storage lipid between two populations in different depth layers which made the implicit assumption that the two sampled populations represented a single cohort. This may not have been a valid argument given the findings of Heath and Jonasdottir (1999), who described the arousal of the Faroe-Shetland Channel C. finmarchicus population as an asynchronous event which may be spread out over more than 2 months. The deep population Jonasdottir (1999) assumed to be nearing arousal may actually have been at an earlier stage of diapause. Jonasdottir (1999) also recognized that her calculation did not allow for the role buoyancy changes may play in assisting C. finmarchicus migration from deep water. Visser and Jonasdottir (1999) and Campbell and Dower (2003) have suggested that the expansion of Wax Ester storage lipids with increasing temperature and decreasing pressure make it likely that a diapausing copepod’s ascent from depth is greatly aided by increasing buoyancy. It also may be true that the diapause population in the Faroe-Shetland channel is not energetically limited due to the extremely cold nature of the water in which it resides. If such is the case, it would not be appropriate to assume that all storage material left over at arousal must be used up in the subsequent processes of maturation and migration. Lastly, the difference in estimates of required lipid reserve may be linked to differences in body size. The Jonasdottir (1999) lipid requirement estimate of 150 mg storage lipid carbon is consistent with our predictions for an animal 2.9 mm in length (Fig. 4). Length at stage is negatively correlated with temperature (Campbell et al., 2001) and stage C5 C. finmarchicus greater than 3 mm in length are common in extremely cold regions such as the Gulf of St. Lawrence (J. Runge pers. comm.).
ARTICLE IN PRESS W.J. Saumweber, E.G. Durbin / Deep-Sea Research II 53 (2006) 2597–2617
Unfortunately, no length data were reported in Jonasdottir (1999). More recently, Rey-Rassat et al. (2002) used the same visual estimation techniques used in this paper (after Miller et al., 1998) to estimate the change in storage lipid volume between late stage C5s and recently molted female C. finmarchicus. Animals used in these experiments were raised in culture at 8 1C and subjected to two different feeding regimes upon reaching stage C5. The authors calculated that the maturation and molt process required between 63 and 69 mg of wax ester carbon and found no significant difference in lipid requirement between animals raised through stage C5 at high or low feeding regimes. They did not measure length, but the predicted length for a stage C5 animal raised under good food conditions at 8 1C is 2.4 mm according to the equations of Campbell et al. (2001). Thus, the estimates of Rey-Rassat et al. (2002) appear to agree with our predicted lipid requirement of 57 mg WEC, especially considering that their estimate of wax ester carbon content per unit lipid (0.78 mgC mgWE1) was slightly greater than the one used here (0.74 mgC mgWE1). It should be noted that, because their experiments were conducted on animals reared in culture, the lipid expenditures they measured naturally exclude any potential costs associated with vertical migration and arousal. Nevertheless, the agreement with our predictions is remarkable. 4.2. Respiration rates Although reports of respiration rates for C. finmarchicus while in diapause are scarce, the measurements presented in this paper are remarkably consistent with those that do exist. Hirche (1983) reported on respiration rates for two populations of apparently diapausing C. finmarchicus in the Gullmar and Kors fjords off the coasts of Sweden and Norway, respectively. He found respiration levels in Korsfjord to range between 22.8 and 33.4 mmol O2 gC1 h1 at 6 1C (reported as ml O2 g dry wt1 h1, converted assuming C wt/Dry wt ¼ 0.59, see Saumweber, 2005a). Using carbonspecific terms, we found a respiration rate of 25.4 mmol O2 gC1 h1 at 6 1C. Hirche (1983) reports even lower respiration values for the C. finmarchicus population in Gullmarsfjord. However, these estimates were based on experiments using the Electron Transport System (ETS) assay (Owens and King, 1975), and this method has been called into question
2613
for a variety of reasons explained in Henandez-Leon and Gomez (1996). The Korsfjord measurements were taken using a Winkler technique similar to the one described in this paper. Marshall and Orr (1958) report on seasonal changes in the respiration rate of C. finmarchicus and found the lower bound of the rates to be between 50 and 70 mmol O2 gC1 h1 at 101 (converted from rates per individual to gC1 assuming 150–200 mgC animal1). These values are slightly higher than our predicted rate of 37.2 mmol O2 gC1 h1 at 10 1C. Ingvarsdottir et al. (1999) measured respiration rates in a diapausing population of C. finmarchicus from the Faroe-Shetland Channel, northwest of Scotland, over the course of a season. The lowest rate they measured was 14.4 mmol O2 gC1 h1 at 0 1C. This value is identical to the 0 1C respiration rate predicted by the regression in Fig. 6B of this paper. Hirche (1983)’s lowest value predicts a similar 0 1C rate of 12.9 mmol O2 gC1 h1 using the carbon-specific Q10 determined in this paper. These similarities suggest that even though our respiration function is based on measurements taken during all phases of the GOM C. finmarchicus diapause season, it still predicts rates in keeping with the lowest values reported elsewhere. It therefore appears likely that our estimates of diapause duration may be conservative (in the sense that longer estimates are conservative) despite valid concerns over the assumption of constant respiration rates throughout the course of diapause. This conclusion is supported by the fact that activity levels of all experimental animals, as indicated by RRD values, were near the lowest levels observed in the field (Fig. 7). Both Ingvarsdottir et al. (1999) and Hirche (1983) reported that the respiration rates of the deep C5 population were approximately 15–20% of those found on the surface. We find a similar ratio when comparing the predicted diapause rates with other measurements of surface C5 populations (e.g., Mayzaud, 1976; Marshall and Orr, 1958). 4.3. Diapause duration predictions and implications Hirche (1996a) reports that C. finmarchicus diapause durations typically range between 6 and 10 months. Our model predicts that an animal must spend the bulk of its resting phase in waters below 5 1C to achieve such potential diapause duration. While this appears to be quite possible in oceanic
ARTICLE IN PRESS 2614
W.J. Saumweber, E.G. Durbin / Deep-Sea Research II 53 (2006) 2597–2617
waters at high latitudes, where temperatures can approach 0 1C at diapause depths (Ingvarsdottir et al., 1999), it may be problematic on the shelves or in slope water in the Western Atlantic. Average temperatures in the Gulf of Maine deep water range between 5 and 10 1C (Loder et al., 2001; Saumweber, 2005a). The diapause duration model shown in Fig. 8 predicts a maximum duration of 3.5–5.5 months at such temperatures. This period of time may be too short for animals to begin initiation of diapause in June as has previously been hypothesized, but it is consistent with observations made by Durbin et al. (1997). They calculated that the stage distribution of active populations seen early in the year implied an emergence from diapause in December. C. finmarchicus arousal patterns are thought to be generally associated with the timing of the regional spring bloom (Hirche, 1996a; Tittensor et al., 2003) but in the Gulf of Maine, the bloom does not typically occur until March or April (Durbin et al., 1995; O’Reilly and Zetlin, 1998). Not surprisingly, Durbin et al. (2003) found that C. finmarchicus populations in the surface waters of the Gulf of Maine early in the year may be severely food limited. This begs the rhetorical question for Gulf of Maine Calanus: why wake up early? Durbin et al. (2003) suggested that early arousal allows the Gulf of Maine population to take advantage of anomalous stratification events that may cause an early bloom, as was observed in 1999. An alternative answer, as suggested by this model, may be that C. finmarchicus in the Gulf of Maine has no choice but to wake up in December due to energetic limitation. Saumweber (2005b) proposes that the first generation (G0) of GOM C. finmarchicus in a new year is the result of the previous year’s summer production and composed mostly of animals that did not begin the migration to deep water until August or September. Miller et al. (1991) reported that the diapausing population of C. finmarchicus in the slope water south of Georges Bank was predominantly centered around 500 m. Temperatures at this depth are typically near 5 1C (Saumweber, 2005a). Diapause duration is predicted to be approximately five and a half months at this temperature, which should be sufficient for animals entering diapause in late summer or early fall as was proposed by Miller et al. (1991). Water at this depth is generally classified as Labrador Slope Water (Gatien, 1976), and its temperature is closely related to the flux of water leaving the Labrador Sea and turning south-
west around the Grand Banks. In years of strong flow, typically associated with a negative North Atlantic Oscillation index (Green and Pershing, 2000), potential diapause duration may be increased as animals developing in the slope water migrate into cooler water. The maximum potential diapause duration at all temperatures is found for animals 2.34 mm in length. This length appears to represent the best balance between the rate of increase in lipid storage capacity and the rate of increase in respiration with body size. It is therefore interesting to note that the mean length for all animals captured below 150 m during the course of this study was 2.38 mm. The similarity between the mean length of diapausing animals in the Gulf of Maine and the optimal length for diapause duration would seem to support the general shape of the diapause duration function, and at least suggests that C. finmarchicus in the Gulf of Maine have faced evolutionary pressure to maximize their diapause potential. Such pressure may be indicative of the fact that the Gulf of Maine C. finmarchicus population resides near the southern limit of the species’ range. It may be that other C. finmarchicus populations do not face the same degree of energetic limitation and potential diapause duration could exceed the time required. If such is the case, then some other physiological cue besides energetic limitation must be linked to arousal. We must therefore emphasize that this model aims only to predict the potential diapause duration and that this time frame will necessarily only coincide with actual diapause duration in populations that are energetically limited. In other populations, photic period or reduced stage development rates may be more appropriate arousal cues as has been suggested by a number of workers (Grigg and Bardwell, 1982; Hirche, 1989; Miller et al., 1991; Hind et al., 2000). 5. Conclusions This paper presents a new method for estimating the potential diapause duration of an individual stage C5 C. finmarchicus. The model makes a general prediction of maximum potential diapause duration based on an animal’s size and in situ temperature. It also can be used to make specific predictions of the potential time remaining in diapause for an animal based on observations of its size, available lipid reserves, and in situ temperature. Multiple such observations might be
ARTICLE IN PRESS W.J. Saumweber, E.G. Durbin / Deep-Sea Research II 53 (2006) 2597–2617
used to predict loss rates due to energetic limitation for local resting populations. Acknowledgements We thank: Bob Campbell, Maria Casas, Melissa Wagner and the rest of the planktoneers for assistance in the lab and at sea; Sharon McLean and Jerry Prezioso at NOAA NMFS, Erica Head at the Bedford Institute of Oceanography, Marsh Youngbluth at the Harbor Branch Oceanographic Institute, and Jan Whitting and Eric Zettler at the Sea Education Association for donating ship time; and Mary-Lynn Dickson for the use of the MicroOxymax. A special thanks goes to the Captains and crews of the RVs Delaware II, Albatross IV, CCGS Hudson, and SSV Corwith Cramer. We would also like to thank the two anonymous reviewers who contributed their time and advice. This research was supported by the National Oceanic and Atmospheric Administration and National Science Foundation Biological Oceanography program and is GLOBEC contribution #305.
References Blades-Eckelbarger, P.I., 1991. Comparative ultrastructure of lipid storage sites in female Euchaeta marina and Pleuromamma xiphias (Copepoda: Calanoida). Marine biology 108, 49–58. Buckley, L.J., 1984. RNA-DNA ratio: an index of larval fish growth in the sea. Marine Biology 80, 291–298. Buckley, L.J., Calderone, E., Ong, T.-L., 1999. RNA-DNA ratio and other nucleic acid based indicators for growth and conditions of marine fishes. Hydrobiologia 401, 265–277. Cade, B.S., Terrell, J.W., Schroeder, R.L., 1999. Estimating effects of limiting factors with regression quantiles. Ecology, 311–323. Campbell, R.W., Dower, J.F., 2003. Role of lipids in the maintenance of neutral buoyancy bby zooplankton. Marine Ecology Progress Series 263, 93–99. Campbell, R.G., Wagner, M.M., Teegarden, G.J., Boudreau, C.A., Durbin, E.G., 2001. Growth and development rates of the copepod Calanus finmarchicus reared in the laboratory. Marine Ecology Progress Series 221, 161–183. Conover, R.J., 1988. Comparative life histories in the genera Calanus and Neocalanus in high latitudes of the northern hemisphere. Hydrobiologia 167/168, 127–142. Durbin, E.G., Gilman, S.L., Campbell, R.G., Durbin, A.G., 1995. Abundance, biomass, vertical migration and estimated development rate of the copepod Calanus finmarchicus in the southern Gulf of Maine during late spring. Continental Shelf Research 15, 571–591. Durbin, E.G., Runge, J.A., Campbell, R.G., Garrahan, P.R., Casas, M.C., Plourde, S., 1997. Late fall-early winter
2615
recruitment of Calanus finmarchicus on Georges Bank. Marine Ecology Progress Series 151, 103–114. Durbin, E.G., Garrahan, P.R., Casas, M.C., 2000. Abundance and Distribution of Calanus finmarchicus on the Georges Bank during 1995 and 1996. ICES Journal of Marine Science 57 (6), 1664–1685. Durbin, E.G., Campbell, R.G., Casas, M.C., Ohman, M.D., Niehoff, B., Runge, J., Wagner, M., 2003. Interannual variation in phytoplankton blooms and zooplankton productivity and abundance in the Gulf of Maine during winter. Marine Ecology Progress Series 254, 81–100. Fiksen, O., Carlotti, F., 1998. A model of optimal life history and diel vertical migration in Calanus finmarchicus. Sarsia 83, 129–147. Gatien, M.G., 1976. A study in the slope water region south of Halifax. Jounal of the Fisheries Research Board of Canada 33 (10), 2213–2217. Gatten, R.R., Sargent, J.R., Forsberg, T.E.V., O’Hara, S.C.M., Corner, E.D.S., 1980. On the nutrition and metabolism of zooplankton. XIV. Utilization of lipid by Calanus helgolandicus during maturation and reproduction. Journal of the Marine Biological Association of UK 60, 391–399. Green, C.H., Pershing, A.J., 2000. The response of Calanus finmarchicus populations to climate variability in the Northwest Atlantic: basin-scale forcing associated with the North Atlantic Oscillation. ICES Journal of Marine Science 57, 1536–1544. Grigg, H., Bardwell, S.J., 1982. Seasonal observations on moulting and maturation in Stage V copepodites of Calanus finmarchicus from the Firth of Clyde. Journal of the Marine Biological Association of the UK 62, 315–327. Halberg, E., Hirche, H.-J., 1980. Differentiation of mid-gut in adults and overwintering copepodids of Calanus finmarchicus (Gunnerus) and C. helgolandicus (Claus). Journal of Experimental Marine Biology and Ecology 48, 283–295. Harris, J.R.W., 1983. The development and growth of Calanus copepodites. Limnology and Oceanography 28 (1), 142–147. Heath, M.R., Jonasdottir, S.H., 1999. Distribution and abundance of overwintering Calanus finmarchicus in the FaroeShetland Channel. Fisheries Oceanography 8 (Suppl. 1), 40–60. Henandez-Leon, S., Gomez, M., 1996. Factord affecting the respiration/ETS ratio in marine zooplankton. Jornal of Plankton Research 18 (2), 239–255. Hind, A., Gurney, W.S.C., Heath, M., Bryant, A.D., 2000. Overwintering Strategies in Calanus finmarchicus. Marine Ecology Progress Series 193, 95–107. Hirche, H.-J., 1983. Overwintering of Calanus finmarchicus (Gunnerus) and C. helgolandicus (Claus) in a Swedish fjord. Marine Ecology Progress Series 11, 281–290. Hirche, H.-J., 1989. Spatial distributionof digestive enzyme activities of Calanus finmarchicus and C. hyperboreas in Fram Straight/Greenland Sea. Journal of Plankton Research 11, 431–443. Hirche, H.-J., 1991. Distribution of dominant calanoid copepod species in the Greenland Sea during late fall. Polar Biology 11, 351–362. Hirche, H.-J., 1996a. Diapause in the marine copepod, Calanus finmarchicus—a review. Ophelia 44, 129–143.
ARTICLE IN PRESS 2616
W.J. Saumweber, E.G. Durbin / Deep-Sea Research II 53 (2006) 2597–2617
Hirche, H.-J., 1996b. The reproductive biology of the marine copepod, Calanus finmarchicus—a review. Ophelia 44, 111–128. Hirche, H.-J., Kattner, G., 1993. Egg production and lipid content of Calanus glacialis in spring: indication of a fooddependent and a food-independent reproductive mode. Marine Biology 117, 615–622. Hopkins, C.C., Tande, K.S., Gronvik, S., Sargent, J.R., 1984. Ecological investigations of the zooplankton community of Balsfjorden, northern Norway: an analysis of growth and overwintering tactics in relation to niche and environment in Metridia longa (Lubbock), Calanus finmarchicus (Gunnerus), Thysanoessa inermis (Kroyer), and T. rashi (M. Sars). Journal of Experimental Marine Biology and Ecology 82, 77–99. Ikeda, T., 1977. The effect of laboratory conditions on the extrpolation of experimental measurements to the ecology of marine zooplankton. IV. Changes in respiration and excretion rates of boreal zooplankton species maintained under fed and starved conditions. Marine Biology 41, 241–252. Ikeda, T., Torres, J.J., Hernandez-Leon, S., Geiger, S.P., 2000. Metabolism. In: Harris, R.P., Wiebe, P.H., Lenz, J., Skjoldal, H.R., Huntley, M. (Eds.), ICES Zooplankton Methodology Manual. Academic Press, San Diego, CA, pp. 455–532. Ingvarsdottir, A., 1998. Seasonal changes in physiology and chemical composition of Calanus finmarchicus late copepodite stages. Ph.D. Thesis. University of Aberdeen. Ingvarsdottir, A., Houlihan, D.F., Heath, M.R., Hay, S.J., 1999. Seasonal changes in respiration rates of copepodite stage V Calanus finmarchicus (Gunnerus). Fisheries Oceanography 8 (Suppl. 1), 73–83. Jonasdottir, S.H., 1999. Lipid content of Calanus finmarchicus during overwintering in the Faroe-Shetland Channel. Fisheries Oceanography 8 (Suppl. 1), 61–72. Kaartvedt, S., 1996. Habitat preference during overwintering and timing of seasonal vertical migration of Calanus finmarchicus. Ophelia 44, 145–156. Katner, G., Krause, M., 1989. Seasonal variations of lipids (Wax Esters, Fatty Acids, and Alcohols) in Calanoid copepods from the North Sea. Marine Chemistry 26, 261–275. Kattner, G., Krause, M., 1987. Changes in lipids during development of Calanus finmarchicus s.l. from copepodid I to adult. Marine Biology 96, 511–518. Lee, R.F., Nevenzel, J.C., Paffenhoffer, G.A., 1970. Wax esters in marine copepods. Science 167, 1510–1511. Loder, J.W., Shore, J.A., Hannah, C.G., Petrie, B.D., 2001. Decadal-scale hydrographic and circulation variability in the Scotia-Maine Region. Deep-Sea Research II 48 (1–3), 3–35. Marshall, S.M., Orr, A.P., 1958. On the biology of Calanus finmarchicus. X. Seasonal changes in oxygen consumption. Journal of the Marine Biological Assoiation UK 37, 459–472. Mauchline, J., 1998. The Biology of Calanoid Copepods: Advances in Marine Biology, vol. 33. Academic Press, San Diego, CA, pp. 349–400. Mayzaud, P., 1976. Respiration and nitrogen excretion of zooplankton. IV. The influence of starvation on the metabolism and the biochemical composition of some species. Marine Biology 37, 47–58. McLaren, I.A., 1986. Is ‘‘structural’’ growth of Calanus exponential? Limnology and Oceanography 31 (6), 1342–1346.
McLaren, I.A., Leonard, A., 1995. Assessing the equivalence of growth amd egg production in copepods. ICES Journal of Marine Science 52, 397–408. Miller, C.B., Grigg, H., 1991. An experimental study of the resting phase in Calanus finmarchicus (Gunnerus). Bulletin of the Plankton Society of Japan. (Spec. vol.: Procedings of the fourth International Conference on Copepoda): 479–493. Miller, C.B., Cowles, T.J., Wiebe, P.H., Copley, N.J., Grigg, H., 1991. Phenology in Calanus finmarchicus: hypotheses about control mechanisms. Marine Ecology Progress Series 72, 79–91. Miller, C.B., Morgan, C.A., Prahl, F.G., Sparrow, M.A., 1998. Storage lipids of the copepod Calanus finmarchicus from Georges Bank and the Gulf of Maine. Limnology and Oceanography 43 (3), 488–497. O’Reilly, J.E., Zetlin, C., 1998. Seasonal, horizontal, and vertical distribution of phytoplankton chlorophyll ‘a’ in the Northeast US Continental Shelf ecosystem. NOAA National Marine Fisheries Service Report # NOAA TR-NMFS-139. Østvedts, O.J., 1955. Zooplankton investigations from weathership ‘‘M’’ in the Norwegian Sea, 1948–1949. Hvalrad Skr 40, 1–93. Owens, T.G., King, F.D., 1975. The measurement of respiratory Electron Transport System activity in marine zooplankton. Marine Biology 30, 27–36. Rey-Rassat, C., Irigoien, X., Harris, R., Carlotti, F., 2002. Energetic cost of gonad development in Calanus finmarchicus and C. helgolandicus. Marine Ecology Progress Series 238, 301–306. Sargent, J.R., Falk-Petersen, S., 1988. The lipid biochemistry of calanoid copepods. Hydrobiologia 167/168, 101–114. Saumweber, W.J., 2005a. Energetic constraints on diapause in Calanus finmarchicus: implications for population dynamics in the Gulf of Maine. II. Changes in the physiology and abundance of Calanus finmarchicus in the Gulf of Maine between June 2001 and January 2002. Ph.D. Thesis, Graduate School of Oceanography, University of Rhode Island. Saumweber, W.J., 2005b. Energetic constraints on diapause in Calanus finmarchicus: implications for population dynamics in the Gulf of Maine. III. The implications of energetic limitation for diapausing Calanus finmarchicus: towards a Gulf of Maine Calanus budget. Ph.D. Thesis, Graduate School of Oceanography, University of Rhode Island. Schmidt-Nielsen, K., 1994. Animal Physiolgy: Adaptation and Environment. Cambridge University Press, New York, NY, pp. 5–62. Skjodal, H.R., Bamstedt, U., Klinken, J., 1984. Changes with time after capture in the metabolic activity of the carnivorous copepod Euchaeta norvegica. Journal of Experimental Marine Biolgy and Ecology 83, 195–210. Speirs, D.C., Gurney, W.S.C., Heath, M.R., Wood, S.N., 2005. Modeling the basin scale demography of Calanus finmarchicus in the north-east Atlantic. Fisheries Oceanography 14 (5), 333–358. Tande, K.S., 1982. Ecological investigations on the zooplankton community of Balsfjorden northern Norway: generation cycles, and variations in body weight and body cntent of carbon and nitrogen related to overwintering and reproduction in the copepod Calanus finmarchicus. Journal of Experiemental Marine Biology and Ecology 62, 129–142. Tittensor, D.P., Deyoung, B., Tang, C.L., 2003. Modeling the distribution, sustainability, and diapause emergence timing of
ARTICLE IN PRESS W.J. Saumweber, E.G. Durbin / Deep-Sea Research II 53 (2006) 2597–2617 the copepod Calanus finmarchicus in the Labrador Sea. Fisheries Oceanography 12 (4–5), 299–316. Visser, A.W., Jonasdottir, S.H., 1999. Lipids, buoyancy and the seasonal vertical migration of Calanus finmarchicus. Fisheries Oceanography 8 (Suppl. 1), 100–106. Wagner, M., Durbin, E.G., Buckley, L., 1998. Nucleic acids as indicators of condition in the copepod Calanus finmarchicus. Marine Ecology Progress Series 162, 173–181.
2617
Wagner, M.M., Campbell, R.G., Boudreau, C.A., Durbin, E.G., 2001. Nucleic acids and growth of Calanus finmarchicus in the laboratory under different food and temperature conditions. Marine Ecology Progress Series 221, 185–197. Weibe, P.H., Morton, A.W., Bradley, A.M., Backus, R.H., Craddock, J.E., Barber, V., Cowles, T.J., Flierl, G.R., 1985. New developments in the MOCNESS, an apparatus for sampling zooplankton and micronekton. Marine Biology 87, 313–323.
