Spatial distribution of digestive enzyme activities of ...

Journal of Plankton Research Vol.11 no.3 pp.431-443, 1989

Spatial distribution of digestive enzyme activities of Calanus finmarchicus and C.hyperboreus in Fram Strait/Greenland Sea

Abstract. Digestive enzymes of copepodite V (CV) Calanus finmarchicus and C.hyperboreus from two different depths were compared during MIZEX 1984 (June/July) at stations in the ice and open water. CV of both species from 500-200 m showed reduced enzyme activity, indicating that they were in a resting stage. In moulting experiments at the end of June using CV from 100-0 m moulting was delayed and began only after 3 weeks in C.finmarchicus and after 3 months in C.hyperboreus. These results suggest that the deep CV populations are the seed of the new overwintering stock. In surface CV C.finmarchicus and C.hyperboreus enzyme activities were generally much higher than in deep CV. In neither species were enzyme activities correlated with chlorophyll concentrations. Activities in C.finmarchicus reflected overall phytoplankton distribution and were highest in the marginal ice zone, whereas they decreased under the ice except for polynya stations on the East Greenland Shelf. In surface C.hyperboreus digestive enzymes were not correlated with those of C.finmarchicus, pointing to different diets or regulatory mechanisms. Enzyme activity was lowest in the marginal ice zone and increased under the ice. High activities were found at polynya stations and other close pack ice. The utilization of ice algae by C.hyperboreus could explain these discrepancies in digestive enzyme activities of both species.

Introduction

The life cycle of herbivorous copepods in high latitudes is strongly influenced by the extreme seasonally of food availability. In the Greenland Sea there are also sharp spatial gradients in food supply resulting from the complex hydrographic regime and ice cover. Thus under the pack ice on the East Greenland Shelf, chlorophyll concentrations are very low during most of the year, while in polynyas phytoplankton blooms can develop much earlier. Highest chlorophyll concentrations were observed in the marginal ice zone and in the Polar Front region, where the Polar water meets Atlantic water (Smith et al., 1987; Spies, 1987). The two dominant copepod species in this region, Calanus finmarchicus and C.hyperboreus, have apparently chosen different life history strategies in coping with this variability: C.hyperboreus is at least biennial (Harding, 1966; Dawson, 1978), while C.finmarchicus has one generation per year in this area (Lie, 1965). Reproduction in C.finmarchicus is synchronized with the development of phytoplankton blooms by the dependency of egg production on food uptake (Marshall and Orr, 1955; Runge, 1984), while C.hyperboreus spawn in the absence of food, relying on fat reserves deposited in the previous year (Conover, 1967). Although C.finmarchicus is a boreal species inhabiting the North Atlantic (Marshall and Orr, 1955) and C.hyperboreus is an Arctic species (Grainger, 1963,1965), in the Fram Strait/Greenland Sea their distributions overlap (Smith, 1988). It is worthwhile to investigate the results of the two different life cycle strategies when they interact with the spatial and temporal pattern of food availability in the same habitat. © IRL Press

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Hans-Jurgen Hirche Alfred-Wegener-Institute for Polar and Marine Research, Columbusstrasse, D-2855 Bremerhaven, FRG

H.-J.Hlrche

Material and methods

Copepods were collected from the Fram Strait/Greenland Sea during the MIZEX and Post-MIZEX cruises of RV Polarstern between June 18 and August 4, 1984. Animals from the upper layers were collected with a bongo net (330 and 500 u,m mesh). For surface animals the net was lowered obliquely to 100 m depth and hauled to the surface at a steady speed while the ship was moving at 0.5 knots. Closed 2 1 jars served as cod ends to avoid damage to the material. Deep-living animals were taken from the 500-200 m fraction of vertical hauls using a multiple opening and closing net fitted with five 200 u.m mesh nets. Each catch was immediately diluted with prescreened surface water. Live animals were transferred into a sorting tray immediately after sampling and identified. For assays of digestive enzymes, 5-20 specimens were homogenized for 1 min in 3 ml iced distilled water using a Potter-Elvehjem homogenizer on crushed ice. The homogenates were deep frozen at — 20°C. In the laboratory ashore, following centrifugation at 5200 g for 10 min at 0°C, the supernatant liquid was used for determination of trypsin, amylase and proteins. Amylase was measured by the method of Street and Close (1956) at pH 6.8. Trypsin was determined according to Samain et al. (1977) at pH 8.3 and 39°C. Incubation time was 15 min in each case. Calibration was made with amylase (Merck 24507) and trypsin (Merck 8214) from bovine pancreas. Soluble protein was measured after Lowry et al. (1951) with bovine albumin as standard. For moulting experiments, ~50 C.finmarchicus CV and 25 C.hyperboreus CV from bongo net samples were sorted as described above and incubated in 1 1 (C.finmarchicus) or 3 1 bottles (C.hyperboreus) with filtered seawater at 1°C in 432


During MIZEX 84 digestive enzyme activities of the copepodite V stage (CV) of both species collected at different depths and at various stations were studied to compare their feeding and physiological condition. Digestive enzyme activities were suitable for this purpose as they exhibit a positive relationship to various indices of potential food (Boucher and Samain, 1974; Mayzaud and Conover, 1976, 1984; Hirche, 1981; Harris et al., 1986) and enable distinction between the active feeding and resting (= overwintering) conditions (Hallberg and Hirche, 1982; Hirche, 1983). For additional information on the physiological condition, moulting experiments were conducted with surface dwelling CV of both species. These experiments should help to determine whether surface living CV had emerged from the previous overwintering stock or had developed from eggs spawned in the same spring. In the first case CV at the surface would just have ascended from depth and a certain percentage would always moult. This was observed during spring by Grigg and Bardwell (1982) and Hirche (1983) in C.finmarchicus and C.helgolandicus. In the second case moulting to adults would be delayed towards the entry of the resting period until, in the resting condition, development is blocked for several months in the field (Grigg and Bardwell, 1982; Hirche, 1983).

Digestive enzyme activities in Calanus

Results

Digestive enzymes The CV C.finmarchicus from both 100-0 and 500-200 m (Figure la) form two clusters with very little overlapping. The deep-living CV have low activities of both enzymes, while the surface-dwelling CV have generally high activities. In the latter trypsin and amylase are significantly correlated (P < 0.01, Figure 1). The activities of surface CV C.finmarchicus fell within the range of the deepliving population at five stations only. Unfortunately, for these stations no data are available for 500-200 m. When data are compared from stations where both depth strata were sampled, the means for corresponding data pairs are 3.24 ± 1.11 (100-0 m)/1.24 ± 0.37 (500-200 m) for amylase and 3.03 ± 1.05 (100-0 m)/0.64 ± 0.47 (500-200 m) for trypsin, respectively, and are thus significantly different (P < 0.001, paired r-test). In C.hyperboreus, as in C.finmarchicus, enzyme activities followed two different regressions (Figure 1), with the deep-living animals also characterized • O

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Fig. 1. Digestive enzyme activities (amylase = 10"2 U mg protein"1, trypsin = 1 0 " ' U mg protein"1) of C.finmarchicus and C.hyperboreus from two different depths. Roman letters indicate activity classes for Figures 2 and 3.

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the dark. The bottles were checked for moults at regular intervals by gently gathering the animals on a sieve mounted on a Petri dish to prevent the animals from drying. Adults and exuviae were removed and animals returned to the bottles. This procedure usually did not last longer than 2 min. Experiments were terminated, when >50% of the animals had moulted. A total of 81 samples of C.finmarchicus were analysed for digestive enzyme activities from 47 stations with CV from the upper layers and 22 stations with CV from 500-200 m tows. The samples of C.hyperboreus included 30 stations of CV from the upper layers and 12 from 500-200 m.

H.-J.Hirche

Fig. 2. Spatial distribution of digestive enzyme activities of CV C.finmarchicus. Activity classes from Figure 1. Ice edge position is given when samples were collected in its vicinity. Station numbers are indicated when mentioned in text. 434


by low activities. However, nine surface samples fell into the same regression but their enzyme activities were higher, especially amylase. The remainder of the surface CV followed another, steeper regression. In order to map the regional distribution, digestive enzyme activities from 100-0 m animals were grouped into four classes (Figure 1). The class with the lowest activity was defined by the values of deep-living CV, while the other classes were established by drawing zones of equal distance on the regression line. The resulting maps (Figures 2 and 3) reveal pronounced patterns in relation to the ice edge. In C.finmarchicus activities in the range of deep-living CV are restricted to the pack ice zone on the East Greenland Shelf, with one exception north of Spitsbergen. Areas of high activities are found in the open water both on a Fram Strait transect and in the vicinity of the ice edge. The position of the ice edge changed rapidly during the cruise due to changes in the winds and currents. In addition, various eddies shaped its appearance by their own drift and by advecting ice into the open water (Gascard et al., 1988). Therefore position of the ice edge is shown at the time of sampling in its vicinity. Examples of its variability are the ice edges from June 19 and 21. Also, in the 5 days between transect 174-182 and station grid 223-226 the ice edge receded rapidly towards the northwest. High enzyme activities were then found in the neighbourhood of former low activity stations (178 and 182).

Digestive enzyme activities in Calantu

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Fig. 3. Spatial distribution of digestive enzyme activities of CV C.hyperboreus. Activity classes from Figure 1. Ice edge position is given when samples were collected in its vicinity. Station numbers are indicated when mentioned in text. 435


In C.hyperboreus stations with high activities are all found in the East Greenland Current (Figure 3). These include the polynya stations 245 and 296 as well as locations under thick multiyear pack ice. In the open water and the marginal ice zone, and at some stations in the vicinity of the ice edge also under the ice, activities were very low. Differences in the spatial distribution of enzyme activities in the 0-100 m depth range of the two species are obvious when their enzyme activities are compared in relation to the position of the ice edge at the time of sampling (Figure 4). In C.finmarchicus the highest activities are found in the marginal ice zone, where variability is also high. Elevated activities are also found in the Westspitsbergen Current (stations 384 and 385) and at a station on the East Greenland Shelf (245). This station was located in a large polynya, that forms regularly in this region. Here the high enzyme activities may be correlated with a diatom dominated phytoplankton bloom (M.Baumann, personal communication). Activities decreased rapidly from the ice edge into the ice, especially trypsin. When trypsin activities within 150 km of the ice edge only are considered, this decrease follows a linear regression significantly (r = 0.71, P < 0.001). In C.hyperboreus there is a strong tendency for both enzyme activities to increase from the ice edge into the ice. This trend is significant within 150 km of

H.-J.Hircbe

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DISTANCE FROM ICE EDGE (KM) Fig. 4. Digestive enzyme activities (amylase = 10~2 U mg protein"1, trypsin •= 10"1 U mg protein"1) of C.futmarchicus and C.hyperboreus in relation to the ice edge position during collection. Regressions for C.finmarchicus (trypsin) and C.hyperboreus (amylase) were calculated using data within —150 to 100 km of the ice edge (which represents the transition from open water to the pack ice zone).

the ice edge (r = 0.73, P < 0.001). Lower activities are again found at some stations on the East Greenland Shelf. Thus there is an opposing tendency in the spatial distribution of the enzyme activities of the two species. A direct comparison of both enzymes, amylase and trypsin, at stations where samples for both species are available showed no correlation (r = 0.17 for amylase and r = 0.16 for trypsin, n = 22). The relationship of the digestive enzymes of both species to ambient food conditions was studied using the data set of chlorophyll values provided by J.Lenz et al. (personal communication). Correlations are not significant (C.finmarchicus: amylase r = 0.15/trypsin r = 0.58; C.hyperboreus: 0.31/0.47). As the two species usually inhabit different temperature regimes, the effect of temperature on digestive enzyme activities was studied using the 5 m sea water temperatures (CTD data from K.P.Koltermann, personal communication). In 436


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C.finmarchicus CV there is no temperature optimum for enzyme activities (Figure 5). However, most of the overwintering activity levels that were found in surface stages occurred where surface temperatures were below 0°C. In C.hyperboreus high activities are only found at stations with water temperatures below 0°C, but in this temperature range low activities were also frequently encountered (Figure 5). Moulting experiments Moulting experiments with CV C.finmarchicus collected at 17 stations in the upper 100 m were set up between June 18 and July 6. Within the first 2 weeks of incubation moulting was insignificant (Figure 6). After 3 weeks at a few stations a considerable part of the copepodites had moulted. The moulting time of 50% of the original number (MSQ) was calculated assuming a linear increase in moults between the controls. The average M50 is 30.5 ± 4.6 days. Herein experiments 437


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