tuar. cstl. Shelf Sci. 12:725-737. Hargrave, B. T., Prouse, N. J., Phillips, G. A., Neame, P. A. (1983). Primary production and respiration in pelagic and bentic com-.
Marine BiOlOgy
Marine Biology 99, 591-603 (1988)
9 Springer-Verlag 1988
The influence of macrofauna on estuarine benthic community metabolism: a microcosm study F . O . Andersen and E. Kristensen Institute of Biology, Odense University; Campusvej 55, DK-5230 Odense M, Denmark
Abstract
Effects of benthic macrofauna (Corophium volutator, Hydrobia sp., Nereis virens) on benthic community metabolism were studied over a 65-d period in microcosms kept in either light/dark cycle (L/D-system) or in continuous darkness (Dsystem). Sediment and animals were collected in January 1986 in the shallow mesohaline estuary, Norsminde Fjord, Denmark. The primary production in the L/D-system after 10 d acted as a stabilizing agent on the O z and CO z flux rates, whereas the D-system showed decreasing 0 2 and CO z flux throughout the period. Mean O z uptake over the experimental period ranged from 0.38 to 1.24 mmol m - 2 h - 1 and CO2 release varied from 0.80 to 1.63 mmol m -2 h - 1 in both systems. The presence of macro fauna stimulated community respiration rates measured in darkness 1.4 to 3.0 and 0.9 to 2.0 times for 0 2 and CO2, respectively. In contrast, macrofauna lowered primary production. Gross primary production varied from 1.06 to 2.26 mmol 02 m 2 h - 1 and from 1.26 to 2.62 mmol CO 2 m - 2 h-1. The community respiratory quotient (CRQ, CO2/O2) was generally higher in the beginning of the experiment ( 0 - 20 d, mean 1.89) than in the period from Days 20 to 65 (mean 1.38). The L/D-system exhibited lower CRQ (ca. I) than the D-system. The community photosynthetic quotient varied for both net and gross primary production from 0.64 to 1.03, mean 0.81. The heterotrophic D-system revealed a sharp decrease in the sediment content of chlorophyll a as compared to the initial content. In the autotrophic L/D-system, a significant increase in chlorophyll a concentration was observed in cores lacking animals and cores with C. volutator. (The latter species died during the experiment). Due to grazing and other macrofauna activities other cores of the L/D-system exhibited no significant change in chlorophyll a concentration. Community primary production was linearly correlated to the chlorophyll a content in the 0 to 0.5 cm layer. Fluxes of D I N (NH 4 + + NO z- + NO 3 -) did not reveal significant temporal changes during the experiment. Highest rates were found for the cores containing animals, m a i n l y
because of an increased NH4 + flux. The release of D I N decreased significantly due to uptake by benthic microalgae in the L/D-system. No effects of the added macrofauna were found on particulate organic carbon (POC), particulate organic nitrogen (PON), total carbon dioxide (TCO2) and NH4 § in the sediment. The ratio between POC and PON was nearly constant (9.69) in all sediment dephts. The relationship between TCO2 and NH4 + was more complex, with ratios below 2 cm depth similar to those for POC/PON, but with low ratios (3.46) at the sediment surface.
Introduction
Benthic communities are of major importance for the control of 02 concentration in the bottom layers of estuarine and coastal waters. Nutrients required for benthic and planktonic primary production are to a large extent supplied by benthic regeneration (e.g. Nixon 1981). Knowledge on O2-consuming and nutrient producing decomposition processes in sediments has become increasingly important due to the widespread eutrophication and oxygen depletion occurring in coastal waters (van Bennekom etal. 1975, Jorgensen 1980, Seliger et al. 1985, von Westernhagen et al. 1986). Most studies on benthic community metabolism in coastal areas have used sediment systems kept in darkness for long periods. However, the surface of coastal sediments is usually subject to solar radiation causing a benthic primary production. This may in turn affect gaseous and nutrient exchange across the sediment-water interface (Henriksen et al. 1980, 1983). The phytobenthos is a potential food source for benthic herbivores. Benthic fauna may therefore influence the sediment-community metabolism via the phytobenthos. Grazing on the benthic microalgae affects algal growth either positively or negatively, depending on the
592
F.O. Andersen and E. Kristensen: Estuarine benthic community metabolism
grazing pressure (Fenchel and Kofoed 1976, Pace et al. 1979, Darley et al. 1981, Connor et al. 1982, Davis and Lee 1983). Sediment mixing and ventilation activities of the infauna are known to redistribute organic matter in sediments (Aller 1982, Kristensen 1988). These activities stimulate aerobic and anaerobic microbial decomposition processes and solute fluxes in the bioturbated zone of sediments and thereby the overall community metabolism (Kristensen 1985, Aller 1988, Andersen and Helder 1987, Kristensen and Blackburn 1987). However, knowledge about the exact decay pattern of fresh organic matter mixed into the anaerobic zone of sediments is still limited. Benthic community metabolism is usually measured as O 2 consumption and CO2 production by the sediment system (e.g. Hargrave and Phillips 1981, Andersen and Hargrave 1984, Kepkay and Andersen 1985, Oviatt et al. 1986). These measurements provide an opportunity to estimate a community respiratory quotient (CRQ) for the sediment system (CO2/O2 flux ratio). The CRQ value is useful in stoechiometric calculations related to decomposition processes and flux measurements. Theoretically, CRQ values for aerobic systems are in the range of 0.7 to 1.0, depending on the type of organic matter oxidized (Lusk 1931). However, in natural systems in which a narrow oxic zone overlies deeper anoxic sediment, wide-ranging CRQ values have been determined; from I (Teal and Kanwisher 1961, Raine and Patching 1980, Anderson et al. 1986) to 4 (Hargrave and Phillips 1981, Andersen and Hargrave 1984, Kepkay and Andersen 1985). Many processes in a benthic community may modify the flux of 0 2 and CO2; for example when the reduced sulfur (S- -) created by anaerobic sulfate reduction is precipitated as FeS or FeS2 instead of being reoxidized in the oxic surface layer, the measured O 2 uptake underestimates the true community metabolism (Jorgensen 1977). The CO2 production, on the other hand, may be affected by chemolithoautotrophic fixation (Kepkay and Novitsky 1980) or by pH-dependent dissolution-precipitation processes (Anderson et al. 1986). In the present work we studied the effects of benthic macrofauna on community metabolism in experimental sediment microcosms kept in either light/dark cycle (autotrophic system) or in continuous darkness (heterotrophic system). The purpose was to evaluate the effects of benthic fauna and light on fluxes of O2, CO2, and dissolved inorganic nitrogen (DIN) by following the interactions between three infaunal species and microalgal biomass. The decomposition of organic matter in the sediment was examined from vertical profiles of inorganic and organic C and N. For the experiment we selected benthic animals with different feeding and burrowing behaviour: The amphipod Corophium volutator (Pallas), which lives in 3 to 5 cm deep Ushaped burrows, is a selective surface deposit feeder (Nielsen and Kofoed 1982, Henriksen et al. 1983). Epifaunal gastropods of the genus Hydrobia are selective surface deposit feeders (Jensen and Siegismund 1980). The polychaete Nereis virens (Sars), which lives in 10 to 40 cm deep burrows, is a non-selective omnivore (Goerke 1971).
Materials and methods
Sampling Sediment and animals were collected in January 1986 in the shallow mesohaline estuary, Norsminde Fjord, Denmark (Jorgensen and Sorensen 1985). Sediment-cores were taken by hand at ~0.5 m water depth, using 25-cm long and 9-cm i.d. plexiglas corers. For the experiment 20 cores were obtained. Additional 10 cores (5 cm i.d.) were taken to examine sediment characteristics before the start of the experiment. Nereis virens (Polychaeta), Corophium volutator (Crustacea), Hydrobia sp. (Gastropoda) were collected by digging and gentle sieving. To establish the chosen species composition and density of infauna for the experiment, we removed the animals present in the cores after sampling. Immediately upon arrival at the laboratory, the experimental cores were purged with N 2 for 20 rain, sealed with rubber stoppers and left at 20 ~ for 24 h. The presence of free sulfide during anoxia in the overlying water is known either to kill the infauna or to induce the animals to ascend to the sediment surface in search of sulfide-free oxygenated water (Jorgensen 1980). The following day all visible individuals were removed before the anoxic water was replaced with fully oxidized water at 15 ~ Simultaneously, sediment depth was adjusted to 12 cm in all cores. Two days later, a known number of infaunal animals were added: to 4 cores no animals (0-cores); to 4 cores 30 individuals of Corophium volutator (C-cores, ~ 5964 m - 2); to 4 cores 30 individuals of Hydrobia spp. (H-cores, ~ 5964 m-2); to 4 cores 4 individuals of Nereis virens (N-cores, ~795 m-2); and to 4 cores 30, 30 and 4 individuals of Corophium volutator, Hydrobia spp. and Nereis virens, respectively (CHN-cores). All cores were kept at the experimental conditions 15 ~ and 17%o S, for 5 d before incubation.
Experiments Ten of the experimental cores, i.e., two of each type, were exposed to a 12 h light/12 h dark cycle (L/D-system) and 10 were kept in continuous darkness (D-system). The light intensity at the sediment surface of the L/D-system in light was 158 and I I I # E cm 2 S--1 between and during incubation, respectively. Horizontal light penetration, below the sediment surface, through the transparent corers was eliminated by placing an opaque supporting plate in the container at the level of the sediment surface. The overlying water (volume: ~ 500 hal) was continuously renewed at a rate of ca. 50 ml min -1 from a 1000-1itre thermostated reservoir. The excurrent overflow from the cores returned to the reservoir via two 50-liter containers (one for L/D-system and one for D-system), that simultaneously acted as core support and thermostat. To prevent the escape of hydrobiid snails, lids of 9 cm i.d. plastic petri dishes were fitted to the top of each corer. The excurrent water was allowed to pass through a series of 0.5 m m perforations bored in the lids.
F.O. Andersen and E. Kristensen: Estuarine benthic community metabolism Once each week for 9 wk ( ~ 6 5 d) all cores were incubated to determine solute exchange across the sedimentwater interface. The L/D-system was alternately incubated in light and dark, while the D-system was dark-incubated. Before incubation, the inside walls of the corers above the sediment surface were gently cleaned to remove microalgal and bacterial films, which otherwise might affect the results. During the ~4-h incubation period the corers were closed with magnetic stirrers fitted to the corers, allowing no headspace. Stirring was continuous and below the limit of resuspension. Before and after incubation, water samples were taken for analysis ofO2, TCO2 (CO2, HCO3 - and CO 3- -), DIN (NH4 +, NO2 - and NO 3 -) and DOC (dissolved organic carbon). Those for dissolved gases were analyzed immediately. Oxygen was determined in duplicate by standard Winkler technique, using an automated titrator (Radiometer Autoburette ABU 13). The TCO 2 was analyzed on a Beckman Model 865 Infrared Gas Analyzer according to the method of Salonen (1981). Samples for D I N and DOC were quickly frozen and analyzed in duplicates as soon as possible. The standard autoanalyzer methods of Solorzano (1969) for NH4 + and of Armstrong et al. (1967) for NO2and NO 3- were used. The DOC was determined as CO 2 after combustion at ~1000~ on a Beckman TOC Analyzer. Exchange rates were calculated from concentration changes during the incubation period, using the volume of water trapped above the sediment and given as mmol m - 2 h-1 for O z and TCO 2 and gmol m -z h -a for DOC and DIN.
Sediment treatment Porosity, density, particulate organic carbon (POC) and nitrogen (PON), chlorophyll a, and pore-water solute concentrations (TCO2, DOC and DIN) were determined before (on 5 cm i.d. cores) and after (on experimental cores) the 65-d experimental period. The cores were cut into 0 to 0.5, 0.5 to 1, 1 to 2, 2 to 4, 4 to 6, 6 to 8 and 8 to 10 cm sections. During this all animals were collected, counted and weighed. The sediment POC and PON was analyzed on subsamples by a Hewlett-Packard 185B CHN-analyzer acording to the method of Kristensen and Andersen (1987) and expressed as ktmol cm-3 sediment. Subsamples of surface sediment for chlorophyll a determinations were added 90% acetone followed by ultrasonic disintegration (150 W in 2 rain, amplitude 30 #m, MSE). The samples were extracted for 20 h at 4~ after which they were centrifuged at 3000 rpm for 10 rain. The absorbance of the supernatant was read at 664 and 750 nm (Spectronic 700) before and after addition of HC1. The chlorphytl a content was calculated according to Parsons et al. (1984) and expressed as mg m - z on the basis of dry weight (DW) and density of the sediment. Pore water was obtained by centrifugation of sediment samples for 10 min at 2000 rpm in double centrifuge tubes. The sediment was placed in the inner tube. During centrifugation, the supernatant was forced through perforations covered with a filter (Whatman GF/C) into the outer tube. The supernatant
593
was analyzed for Y C O 2 , DOC and D I N as described earlier, and expressed as ttmol cm-3 sediment.
Results
Animals The countings of the animals after the experimental period revealed that our method for removing macrofauna from the sediment prior to the experiment was not completely efficient (Table 1). A number of Peloscolex sp. (Oligochaeta) and Pygospio sp. (Polychaeta) together with some juvenile nereids and hydrobiids, apparently survived the defaunation. However, the biomass of remaining animals was lower than that of the animals added. The animals initially added to the cores exhibited varying degrees of survival during the experimental period (Table 1). Corophium volutator was not found in any of the cores at the end of experiment, and it is uncertain when the individuals died. During the first days a number ofHydrobia sp. escaped from the cores. These individuals were quickly collected and added to the H- and CHN-cores in equal numbers before the petri dishes were mounted on top of the cores to prevent further escape. The number of animals in the cores generally decreased during the experiment (Table 1). Only the Hydrobia sp. in the H-cores of the 1/D-system and Nereis virens in CHNcores of both systems maintained the original number. The number of animals in the other cores (except C-cores) constituted only 25 to 62% of those added initially. The cores of the L/D-system all had a higher density and animal biomass than D-system cores.
02 and CO2 flux rates The 0 2 and CO2 flux rates in the various cores during the experiment are shown in Fig. 1. The rates in cores of the D-system were generally highest at the beginning of the experiment and decreased gradually with time. Such a pattern was not observed in cores of the L/D-system during darkness; an initial decrease was in most cores of the L/Dsystem superseded by increasing or steady rates after 10 d. The L/D-system in light, on the other hand, showed increasing rates until ca. Day 30, followed by decreased or steady levels thereafter. Mean 0 2 and CO 2 fluxes for the 65-d incubation period appear from Table 2. The presence of macrofauna in cores stimulated community respiration rates measured in darkness 1.4 to 3.0 and 0.9 to 2.0 times for 0 2 and CO2, respectively (Table 3). The 02 uptake in the D-system ranged from 0.38 to 1.14 mmol m - 2 h - 1 and CO 2 production from 0.80 to 1.63 mmol m -2 h - 1. The L/D-system generally exhibited higher rates in darkness than the D-system, however, with a smaller range (0 2 uptake: 0.68 to 1.24; CO 2 production: 1.18 to 1.59 mmol m -2 h - t ) . The following sequence appeared for both systems: O < C < H < CHN < N (Table 2),
594
F.O. Andersen and E. Kristensen: Estuarine benthic community metabolism
Table 1. Biomass and density of fauna in experimental cores. Right column shows density of fauna initially added to cores. Cores were either kept in continuous darkness (D-system) or in a 12 h light/12 h dark cycle (L/D-system).O: with no animals; C: with Corophium volutator; H: with Hydrobia spp.; N: Nereis virens; CHN: with all three species Species
Corophium volutator density (m- 2)
D-System
L/D-System
O
C
H
N
0
0
0
0a
Hydrobia spp. density (Ill- 2) biomass (g drywt m-2)
0 0
0 0
Nereis virens density (m- 2) biomass (g drywt m-2)
0 0
100 0.1
3 678 b 9.8
398 1.0
0 0
199 c 5.8
CHN
0"
Added
O
C
H
N
0
0~
0
0
CHN
0~
5 964
2 982 b 7.0
597 1.9
299 0.4
6 362 b 21.2
199 0.5
3 579 b 9.4
5 964 -
795 c 14.8
100 0.5
398 0.8
299 2.9
497 c 23.8
895 c 26.2
795 -
Peloscolex sp. occurrence
XX
X
X
X
X
XX
XX
XX
X
X
0
Pygospio spp. occurrence
XX
X
X
X
X
XXX
XX
XX
X
X
0
Corophiurn volutator was added Nereis virens was added c Hydrobia spp. was added • =sparse, x x =common, x x x =abundant
Table 2. Mean 0 2 and CO 2 flux rates over 65-d experimental period for sediment cores with no animals (O), Corophium volutator (C), Hydrobia spp. (H) and Nereis virens (N), all three species (CHN). Cores were kept in a light/dark cycle (L/D-cores) or permanently in dark (D-cores). All values are in mmol m -2 h-1. Positive figures indicate flux into sediment. Negative figures indicate flux from the sediment. NPP: net primary production; GPP: gross primary production; Resp: respiration Incubation
Core type 0
C
H
N
CHN
L/D-system Light (NPP) 02 CO 2
-0.99 1.24
-1.20 1.34
-0.66 0.64
-0.53 0.74
0.13 -0.20
Dark (RESP) Oz CO 2 O2-animal a
0.68 -1.38 0.02
1.06 -1.18 0.03
0.97 -1.44 0.13
1.24 -1.59 0.18
1.18 -1.46 0.39
Light-Dark (GPP) 02 CO 2
- 1.67 2.62
-- 2.26 2.52
- 1.63 1.91
- 1.76 2.31
- 1.06 1.26
D-system Dark (RESP) 02 CO 2 O2-animal"
0.38 --0.80 0
0.54 --0.90 0
0.61 --1.02 0.06
1.14 --1.63 0.08
1.05 -1.57 0.26
uptake by Nereis virens and Hydrobia spp. is estimated on basis of Kristensen (1983) and Jensen and Siegismund (1980), respectively 0 2
except for CO z production in O- and C-cores o f the L / D system. Oxygen production and CO z uptake in light represents the net primary production (NPP) o f the community. M e a n N P P was in the range o f - 0 . 1 3 to 1.20 m m o l O z m - z h - t and - 0 . 2 0 to 1.34 m m o l CO 2 m - 2 h - 1 (Table 2). The N P P was slightly lower in the O-cores than in the C-cores, and ca. 50% lower in H- and N-cores. The C H N - c o r e s in light had on average a higher respiration than photosynthesis, resulting in a negative N P P (Table 2); however, a positive N P P was obtained during the last incubation on D ay 65 (Fig. 2). Gross primary production (GPP) of the co m m u n i t y may be estimated as: light minus dark fluxes (Table 2). The highest G P P was found in C- and O-cores (like NPP) and the lowest in CHN-cores. The G P P in O-, H- and N-cores was o f the same order, especially when G P P is expressed as O 2 production. The presence of macrofauna, expressed as animal O2 uptake (from Table 2), was negatively related to primary production ( r = - 0 . 9 2 7 for N P P and - 0 . 7 8 3 for GPP).
The co m m u n i t y respiratory (CRQ) and photosynthetic quotient (CPQ) The ratio between m e a n C O 2 and 02 different core types is shown in Fig. 2. darkness initially showed high values, crease approaching more constant levels
flux (CRQ) in the Since the fluxes in followed by a de(Fig. 1), we present
F.O. Andersen and E. Kristensen: Estuarine benthic community metabolism D-SYSTEM
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