Zooplankton Carcasses Research Articles

Hydrostatic pressure impedes the degradation of sinking copepod carcasses and fecal pellets.

Fast-sinking zooplankton carcasses and fecal pellets appear to contribute significantly to the vertical transport of particulate organic carbon (POC), partly because of low temperature that decreases microbial degradation during the descent into the deep ocean. Increasing hydrostatic pressure could further reduce the degradation efficiency of sinking POC, but this effect remains unexplored. Here, the degradation of carcasses and fecal pellets of the abundant marine copepod Calanus finmarchicus was experimentally studied as a function of pressure (0.1-100MPa). Samples were either exposed to elevated pressure in short 1-day incubations or a gradual pressure increase, simulating continuous particle sinking during a 20-day incubation. Both experiments revealed gradual inhibition of microbial respiration in the pressure range of 20-100MPa, corresponding to 2-10-km depth. This suggests that hydrostatic pressure impedes carbon mineralization of fast-sinking carcasses and fecal pellets and enhances the deep-sea deposition rate of zooplankton-derived organic material.

Composition of the sinking particle flux in a hot spot of dinitrogen fixation revealed through polyacrylamide gel traps

Diazotrophs regulate marine productivity in the oligotrophic ocean by alleviating nitrogen limitation, contributing to particulate organic carbon (POC) export to the deep ocean. Yet, the characterization of particles composing the sinking POC flux has never been explored in such ecosystems. Moreover, the contribution of the direct gravitational export of diazotrophs to the overall flux is seldom assessed. Here we explore the composition of the sinking POC flux in a hot spot of N2 fixation (the western sub-tropical South Pacific) using polyacrylamide gel-filled traps deployed at two stations (S05M and S10M) and three depths (170 m, 270 m, 1000 m) during the TONGA expedition (November-December 2019). Image analyses of particles collected in the gels was used to classify them into 5 categories (fecal aggregates, phytodetrital aggregates, mixed aggregates, cylindrical fecal pellets, and zooplankton carcasses). Fecal aggregates were the most abundant at both stations and all depths and dominated the flux (average of 56 ± 28% of the POC flux), followed by zooplankton carcasses (24 ± 19%), cylindrical fecal pellets (15 ± 14%) and mixed aggregates (5 ± 4%), whereas phytodetrital aggregates contributed less (<1%). Since N isotope budgets show that export is mainly supported by diazotrophy at these stations, these results suggest that the diazotroph-derived N has been efficiently transferred to the foodweb up to zooplankton and fecal pellets before being exported, pleading for an indirect export of diazotrophy. However, random confocal microscopy examination performed on sinking particles revealed that diazotrophs were present in several categories of exported particles, suggesting that diazotrophs are also directly exported, with a potential contribution to overall POC fluxes increasing with depth. Our results provide the first characterization of particle categories composing the sinking flux and their contribution to the overall flux in a hot spot of N2 fixation.

Major fine-scale spatial heterogeneity in accumulation of gelatinous carbon fluxes on the deep seabed

Abyssal plain communities rely on the overlying water column for a settling flux of organic matter. The origin and rate of this flux as well as the controls on its fine-scale spatial distribution following seafloor settlement are largely unquantified. This is particularly true across regions where anthropogenically-induced seafloor disturbance has occurred. Here, we observed, quantified and mapped a mass deposition event of gelatinous zooplankton carcasses (pyrosomes) in July-September 2015 across one such physically disturbed region in the Peru Basin polymetallic nodule province (4150 m). Seafloor in this area was disturbed with a plough harrow in 1989 (as part of the DISCOL experiment) causing troughs in the sediment. Other parts were disturbed with an epibenthic sled (EBS) during a cruise in 2015 resulting in steep-walled, U-shaped troughs. We investigated two hypotheses: a) gelatinous food falls contribute significantly to the abyssal plain carbon pump and b) physical seafloor disturbance influences abyssal distribution of organic matter. We combined optical and bathymetric seafloor observations, to analyze pyrosome distribution on seabeds with different levels of disturbance. 2954 pyrosome colonies and associated taxa were detected in > 14,000 seafloor images. The mean regional carbon (C) deposition associated with pyrosome carcasses was significant compared to the flux of particulate organic C (182 to 1543%), and the total respired benthic C flux in the DISCOL Experimental Area (39 to 184%). EBS-disturbed seafloor tracks contained 72 times more pyrosome-associated C than an undisturbed reference site, and up to 4 times more than an area disturbed in 1989. Deposited pyrosomes collected had a higher proportion of labile fatty acids compared to the sediment. We document the temporal and spatial extent of an abyssal food fall event with unprecedented detail and show that physical seafloor disturbance results in the accumulation of detrital material. Such accumulation may reduce oxygen availability and alter benthic community structure. Understanding both the relevance of large food falls and the fine scale topography of the seafloor, is necessary for impact assessment of technologies altering seafloor integrity (e.g. as a result of bottom-trawling or deep seabed mining) and may improve their management on a global scale.

Degradation of dead cladoceran zooplankton and their contribution to organic carbon cycling in stratified lakes: field observation and model prediction

Abstract The contribution of dead zooplankton biomass to carbon cycle in aquatic ecosystems is practically unknown. Using abundance data of zooplankton in water column and dead zooplankton in sediment traps in Lake Stechlin, an ecological-mathematical model was developed to simulate the abundance and sinking of zooplankton carcasses and predict the related release of labile organic matter (LOM) into the water column. We found species-specific differences in mortality rate of the dominant zooplankton: Daphnia cucullata, Bosmina coregoni and Diaphanosoma brachyurum (0.008, 0.129 and 0.020 day−1, respectively) and differences in their carcass sinking velocities in metalimnion (and hypolimnion): 2.1 (7.64), 14.0 (19.5) and 1.1 (5.9) m day−1, respectively. Our model simulating formation and degradation processes of dead zooplankton predicted a bimodal distribution of the released LOM: epilimnic and metalimnic peaks of comparable intensity, ca. 1 mg DW m−3 day−1. Maximum degradation of carcasses up to ca. 1.7 mg DW m−3 day−1 occurred in the density gradient zone of metalimnion. LOM released from zooplankton carcasses into the surrounding water may stimulate microbial activity and facilitate microbial degradation of more refractory organic matter; therefore, dead zooplankton are expected to be an integral part of water column carbon source/sink dynamics in stratified lakes.

Vertical occurrence of copepod carcasses in the Cosmonaut Sea during austral summer

Zooplankton carcasses and their contribution to carbon export are increasingly garnering attention in food web studies of polar oceans. We investigated the occurrence of carcasses of dominant copepod species at four depths (100–200 m, 200–500 m, 500–1000 m, 1000–1500 m) and their potential contribution to particulate organic carbon (POC) concentrations in the Cosmonaut Sea, Antarctica, during summer, 2019/2020. The abundance of small copepods (< 2 mm), either live or dead individuals, was 1-3 orders of magnitude higher than the four dominant large copepod species Calanoides acutus, Calanus propinquus, Metridia gerlachei and Rhincalanus gigas at each depth. Most carcasses were observed at the surface (100–200 m), while the proportions of dead individuals increased with depth, from 22% in the 100–200 m layer to 70% of the 500–1500 m layer. The total number of passive sinkers (carcasses and moults) was dominated by carcasses of small copepods in each depth layer, while the carbon biomass of passive sinkers was dominated by carcasses of large copepods. Both abundance and carbon biomass of passive sinkers decreased with depth. The average contribution of passive sinkers to the POC concentration decreased from 0.27% in the 100–200 m depth stratum to 0.10% in the 500–1500 m layer. This study provides the first report of occurrence and contribution of carcasses from the surface to the mesopelagic layer of the Southern Ocean. This information can improve understanding of the role zooplankton play in downward carbon flux within the ocean’s twilight zone.

Distinguishing zooplankton fecal pellets as a component of the biological pump using compound‐specific isotope analysis of amino acids

AbstractZooplankton contribute a major component of the vertical flux of particulate organic matter to the ocean interior by packaging consumed food and waste into large, dense fecal pellets that sink quickly. Existing methods for quantifying the contribution of fecal pellets to particulate organic matter use either visual identification or lipid biomarkers, but these methods may exclude fecal material that is not morphologically distinct, or may include zooplankton carcasses in addition to fecal pellets. Based on results from seven pairs of wild‐caught zooplankton and their fecal pellets, we assess the ability of compound‐specific isotope analysis of amino acids (CSIA‐AA) to chemically distinguish fecal pellets as an end‐member material within particulate organic matter. Nitrogen CSIA‐AA is an improvement on previous uses of bulk stable isotope ratios, which cannot distinguish between differences in baseline isotope ratios and fractionation due to metabolic processing. We suggest that the relative trophic position of zooplankton and their fecal pellets, as calculated using CSIA‐AA, can provide a metric for estimating the dietary absorption efficiency of zooplankton. Using this metric, the zooplankton examined here had widely ranging dietary absorption efficiencies, where lower dietary absorption may equate to higher proportions of fecal packaging of undigested material. The nitrogen isotope ratios of threonine and alanine statistically distinguished the zooplankton fecal pellets from literature‐derived examples of phytoplankton, zooplankton biomass, and microbially degraded organic matter. We suggest that δ15N values of threonine and alanine could be used in mixing models to quantify the contribution of fecal pellets to particulate organic matter.

Zooplankton carcasses stimulate microbial turnover of allochthonous particulate organic matter

Carbon turnover in aquatic environments is dependent on biochemical properties of organic matter (OM) and its degradability by the surrounding microbial community. Non-additive interactive effects represent a mechanism where the degradation of biochemically persistent OM is stimulated by the provision of bioavailable OM to the degrading microbial community. Whilst this is well established in terrestrial systems, whether it occurs in aquatic ecosystems remains subject to debate. We hypothesised that OM from zooplankton carcasses can stimulate the degradation of biochemically persistent leaf material, and that this effect is influenced by the daphnia:leaf OM ratio and the complexity of the degrading microbial community. Fresh Daphnia magna carcasses and 13C-labelled maize leaves (Zea mays) were incubated at different ratios (1:1, 1:3 and 1:5) alongside either a complex microbial community (<50 µm) or solely bacteria (<0.8 µm). 13C stable-isotope measurements of CO2 analyses were combined with phospholipid fatty acids (PLFA) analysis and DNA sequencing to link metabolic activities, biomass and taxonomic composition of the microbial community. Our experiments indicated a significantly higher respiration of leaf-derived C when daphnia-derived OM was most abundant (i.e. daphnia:leaf OM ratio of 1:1). This process was stronger in a complex microbial community, including eukaryotic microorganisms, than a solely bacterial community. We concluded that non-additive interactive effects were a function of increased C–N chemodiversity and microbial complexity, with the highest net respiration to be expected when chemodiversity is high and the degrading community complex. This study indicates that identifying the interactions and processes of OM degradation is one important key for a deeper understanding of aquatic and thus global carbon cycle.

Fluxes of Sedimentary Material in the Lofoten Basin of the Norwegian Sea: Seasonal Dynamics and the Role of Zooplankton

We studied sedimentation processes in the Lofoten Basin of the Norwegian Sea based on data of the Automatic Deep-Sea Sedimentation Observatory (ADSO) deployed for a year (August 2017–July 2018). The article presents estimates of the vertical fluxes of total sedimentary material, organic carbon (Corg), calcium carbonate (CaCO3), biogenic silica (SiO2bio), aluminum, and the particles of planktonic origin (pteropods, foraminifera, fecal pellets, necrozooplankton). The seasonal pattern of total vertical flux is characterized by a unimodal distribution with a maximum in September 2017 (191 mg dry weight m–2 day–1) in the 0–500 m surface water layer. The total flux decreases markedly from November and reaches the minimum (2.6 mg m–2 day–1) in March–April 2018. The maximum Corg flux values (5.2 mg C m–2 day–1), CaCO3 (178 mg CaCO3 m–2 day–1), and SiO2bio (9.1 mg SiO2bio m–2 day–1) are observed in autumn. The value of fluxes and the portion of lithogenic material in it are higher at a depth of 2950 m due to redeposition from the nepheloid layer. The amplitude of flux variations during the year is noticeably smaller at this depth. The highest fluxes are related to the maass sedimentation of juvenile pteropods Limacina retroversa. The contribution of pteropods to vertical fluxes in autumn is 60–100% for CaCO3 and 56–73% for Corg. The contribution of foraminifera to the total CaCO3 flux does not exceed 6% and only reaches 60% at a depth of 550 m in October. The flux of fecal pellets, where ostracod pellets dominate, varies from <0.01 to 2.1 mgC m–2 day–1, making up from 0.1 to 90% of the Corg flux. The flux of zooplankton carcasses (ZCs) in the 0–550 m water layer varies from 0 to 0.45 mgC m–2 day–1, reaching the maximum value in February. The share of ZCs flux was 0–33% in the summer–autumn season, 12–52% in winter, and 23–41% in spring relative to the total (taking into account the ZCs) Corg flux. The ZCs flux varied from 0.04 to 0.22 mgC m–2 day–1 in the near-bottom water layer and amounted to 0.7–2% of the total Corg flux. The total contribution of all studied planktonic particles to the annual CaCO3 flux was 16–47%, and their contribution to the annual Corg flux was 37–45%. Comparative analysis of our results and published data obtained in the 1990s demonstrates that the observed climatic changes (increasing temperature and salinity, as well as an increase in the thickness of the Atlantic water layer) in the Lofoten Basin did not result in significant changes in the quantitative characteristics of sedimentation process.

Death and Decomposition in Aquatic Ecosystems

Resource subsidies affect nutrient cycling, species interactions and food webs in ways that influence ecosystem structure and function, but their effects depend on the history, magnitude, and recurrence frequency of the subsidies. In aquatic ecosystems, plant detritus has been considered the predominant form of such subsidies; however, while considered less abundant in many ecosystems, carrion represents subsidies with relatively rapid turnover and highly concentrated nutrient and energy release that can have strong and lasting effects on ecosystems. Carrion subsidies can be both autochthonous or allochthonous, and come in the form of natural senescence or disease related non-consumptive mortality, phenology-based programmed death (e.g., salmon spawning and death) or stochastic and episodic events (e.g., mass fish die-offs). All aquatic ecosystems have some level of non-consumptive mortality that provides a background level of carcasses to aquatic ecosystems, while others have a natural history of carrion resource subsidies (e.g., natural salmon-bearing streams), and some have only recently been exposed to phenology-based carrion subsidies (e.g., anthropogenic salmon introductions around the world). Many aquatic ecosystems experience episodic subsidies in the form of unexpected mass mortalities (e.g., eutrophication-, disease- or climate-related mass die-offs) or have seasonally dependent pulses, like that of marine or lake snow in the form of zooplankton carcasses. The responses of ecosystems to these different histories and frequencies of carrion subsidies have often been independently investigated, with little effort to compare and bridge research boundaries in the broader context of resource subsidies. In this review, we provide a synthesis of how pulsed carrion nutrient and energy subsidies have widespread and lasting impacts on many aquatic ecosystems. We do this with a synthesis of literature from freshwater and marine ecosystems along three themes of how carrion is produced and decomposes: autochthonous and allochthonous necromass; phenology-based mortality; and stochastic and episodic mass mortality subsidies. Studies of charismatic megafauna carrion describe significant impacts in oceans, but much less is understood for other animals. Quantifying the energy, nutrient and foodweb effects of carrion is needed for more species and among habitats to more fully understand how ubiquitous forms of necromass contribute to aquatic ecosystem structure and function.

Effects of zooplankton carcasses degradation on freshwater bacterial community composition and implications for carbon cycling.

Non-predatory mortality of zooplankton provides an abundant, yet, little studied source of high quality labile organic matter (LOM) in aquatic ecosystems. Using laboratory microcosms, we followed the decomposition of organic carbon of fresh 13 C-labelled Daphnia carcasses by natural bacterioplankton. The experimental setup comprised blank microcosms, that is, artificial lake water without any organic matter additions (B), and microcosms either amended with natural humic matter (H), fresh Daphnia carcasses (D) or both, that is, humic matter and Daphnia carcasses (HD). Most of the carcass carbon was consumed and respired by the bacterial community within 15 days of incubation. A shift in the bacterial community composition shaped by labile carcass carbon and by humic matter was observed. Nevertheless, we did not observe a quantitative change in humic matter degradation by heterotrophic bacteria in the presence of LOM derived from carcasses. However, carcasses were the main factor driving the bacterial community composition suggesting that the presence of large quantities of dead zooplankton might affect the carbon cycling in aquatic ecosystems. Our results imply that organic matter derived from zooplankton carcasses is efficiently remineralized by a highly specific bacterial community, but does not interfere with the bacterial turnover of more refractory humic matter.

Freshwater copepod carcasses as pelagic microsites of dissimilatory nitrate reduction to ammonium.

ABSTRACTA considerable fraction of freshwater zooplankton was recently found to consist of dead specimens that sink to the lake bottom. Such carcasses host intense microbial activities that may promote oxygen depletion at the microscale. Therefore, we tested the hypothesis that sinking zooplankton carcasses are microsites of anaerobic nitrogen cycling that contribute to pelagic fixed-nitrogen loss even in the presence of ambient oxygen. Incubation experiments were performed with the ubiquitous copepods Eudiaptomus sp. and Megacyclops gigas at different ambient oxygen levels that sinking carcasses encounter during their descent in stratified lakes. 15N-stable-isotope incubations revealed intense carcass-associated anaerobic nitrogen cycling only at low ambient oxygen levels (<25% air saturation). Dissimilatory nitrate reduction to ammonium (DNRA) dominated over denitrification and thus the potential for fixed-nitrogen loss was low. Consistent with this partitioning of anaerobic nitrogen cycling, the relative abundance of the carcass-associated marker gene for DNRA (nrfA) was ∼20–400 times higher than that for denitrification (nirS). Additionally, the relative nrfA and nirS abundances were ∼90–180 times higher on copepod carcasses than in lake water. This functional distinctiveness of carcass-associated bacterial communities was further substantiated by 16S rDNA-based fingerprinting. We conclude that the unique bacterial communities and microenvironments provided by zooplankton carcasses influence pelagic nitrogen cycling in lakes, but mainly at seasonally low ambient O2 levels in the bottom water.

Role of Zooplankton in the Vertical Mass Flux in the Kara and Laptev Seas in Fall

The role of zooplankton in the vertical mass flux in the Kara and Laptev seas was studied during cruise 63 of the R/V Akademik Mstislav Keldysh in August–October 2015. Mass fluxes were estimated using sediment trap samples. The maximum values of the total vertical flux (19600 mg m−2 day−1) and particulate organic carbon (POC) flux (464 mg C m−2 day−1) were measured close to the Lena River Delta in the Laptev Sea. In the Kara Sea, the total flux (80–2700 mg m−2 day−1) and the POC flux (17–130 mg C m−2 day−1) were substantially higher than the estimates published earlier. The fecal pellet flux varied from 2 to 92 mg C m−2 day−1 and made up 4–190% of the total organic carbon flux. The mineral composition of fecal pellets largely mirrored that of suspended particulate matter. Clay minerals in the fecal pellets were more abundant than in particulate matter in the areas with noticeable freshwater impact. The flux of zooplankton carcasses varied from 0.1 to 66.4 mg C m−2 day−1 and made up 0.2–72% of total POC flux. The results are discussed in relation to the abundance and composition of zooplankton, the concentration and composition of suspended particulate matter, hydrophysical conditions, and methods of sample preparation for analysis.

Методология использования седиментационных ловушек для изучения вертикального потока и скорости осаждения взвешенных частиц большого размера: морского снега, фекальных пеллет и трупов зоопланктона (обзор литературы)

Методология использования седиментационных ловушек для изучения вертикального потока и скорости осаждения взвешенных частиц большого размера: морского снега, фекальных пеллет и трупов зоопланктона (обзор литературы)

Fixed-Nitrogen Loss Associated with Sinking Zooplankton Carcasses in a Coastal Oxygen Minimum Zone (Golfo Dulce, Costa Rica)

Oxygen minimum zones (OMZs) in the ocean are of key importance for pelagic fixed-nitrogen loss (N-loss) through microbial denitrification and anaerobic ammonium oxidation (anammox). Recent studies document that zooplankton is surprisingly abundant in and around OMZs and that the microbial community associated with carcasses of a large copepod species mediates denitrification. Here, we investigate the complex N-cycling associated with sinking zooplankton carcasses exposed to the steep O2 gradient in a coastal OMZ (Golfo Dulce, Costa Rica). 15N-stable-isotope enrichment experiments revealed that the carcasses of abundant copepods and ostracods provide anoxic microbial hotspots in the pelagic zone by hosting intense anaerobic N-cycle activities even in the presence of ambient O2. Carcass-associated anaerobic N-cycling was clearly dominated by dissimilatory nitrate reduction to ammonium (DNRA) at up to 30.8 nmol NH4+ individual-1 d-1, followed by denitrification (up to 10.8 nmol N2-N individual-1 d-1), anammox (up to 1.6 nmol N2-N individual-1 d-1), and N2O production (up to 1.2 nmol N2O-N individual-1 d-1). In contrast, anaerobic N-cycling mediated by free-living bacteria proceeded mainly through anammox and denitrification in the anoxic bottom water, which underpins the distinctive microbial metabolism associated with zooplankton carcasses. Pelagic N-loss is potentially enhanced by zooplankton carcasses both directly through N2 and N2O production, and indirectly through NH4+ production that may fuel free-living anammox bacteria. We estimate that in the hypoxic water layer of Golfo Dulce, carcass-associated N2 and N2O production enhance N-loss as much as 1.4-fold at a relative carcass abundance of 36%. In the anoxic bottom water, however, N-loss is likely enhanced only marginally due to high ambient rates and low zooplankton abundance. Thus, zooplankton carcasses may enhance N-loss mainly at the hypoxic boundaries of OMZs which are usually more extensive in open-ocean than in coastal settings. Notably, these contributions by zooplankton carcasses to pelagic N-loss remain undetected by conventional, incubation-based rate measurements.

Plankton dynamics and zooplankton carcasses in a mid-latitude estuary and their contributions to the local particulate organic carbon pool

Estuaries are among the most productive aquatic ecosystems in coastal areas. Their productivity is linked to the formation of fronts generating mixing and retention of nutrients that can be used by autotrophs. Estuaries exhibit strong thermoclines and haloclines that may significantly affect zooplankton survival, while producing carcasses that could act as an alternative pathway of particulate organic carbon recycling. We investigated the in situ abundance of dead mero- and holozooplankton along the Valdivia River estuary, south-central Chile, during contrasting fresh water discharge conditions (summer, winter and spring), including the potential contribution of zooplankton carcasses to the particulate organic carbon standing stock along the estuary. Zooplankton samples were collected at four stations along the estuary during high tide at the surface, in the pycnocline and below the pycnocline. During autumn and winter the zooplankton community was mostly dominated by copepods, while during summer barnacle nauplii outnumbered copepods fourfold on average. During this study 29.5%±1.8% S.E. of the net-captured zooplankton community, including Acartia tonsa, Paracalanus spp., Oikopleura spp., copepod nauplii, Podon spp. and barnacle nauplii, appeared to have been dead at collection. Highest overall mortality occurred in winter (44±3.1% S.E.), with lower mortality during spring (26±3.8% S.E.) and summer (19±2.7% S.E.). The instantaneous mortality of copepods (Paracalanus spp. and A. tonsa) and copepod naupliar stages was always greater at the surface, associated with brackish water, while dead barnacle nauplii were usually distributed homogenously in the water column. The zooplankton carcass standing stock averaged ~0.2 mg C m-3 (in spring and winter) contributing to 0.03–0.22% of the POC produced in the estuary, while in summer carcasses reached up to 2.99 mg C m−3 with a contribution up to 0.87% of the POC. The summer contribution is linked to the presence of large amounts of dead barnacle nauplii that are a pulsed source to the estuary of highly labile POC.

Zooplankton carcasses and non-predatory mortality in freshwater and inland sea environments

Zooplankton carcasses are ubiquitous in marine and freshwater systems, implicating the importance of non-predatory mortality, but both are often overlooked in ecological studies compared with predatory mortality. The development of several microscopic methods allows the distinction between live and dead zooplankton in field samples, and the reported percentages of dead zooplankton average 11.6 (minimum) to 59.8 (maximum) in marine environments, and 7.4 (minimum) to 47.6 (maximum) in fresh and inland waters. Common causes of non-predatory mortality among zooplankton include senescence, temperature change, physical and chemical stresses, parasitism and food-related factors. Carcasses resulting from non-predatory mortality may undergo decomposition leading to an increase in microbial production and a shift in microbial composition in the water column. Alternatively, sinking carcasses may contribute significantly to vertical carbon flux especially outside the phytoplankton growth seasons, and become a food source for the benthos. Global climate change is already altering freshwater ecosystems on multiple levels, and likely will have significant positive or negative effects on zooplankton non-predatory mortality. Better spatial and temporal studies of zooplankton carcasses and non-predatory mortality rates will improve our understanding of this important but under-appreciated topic.

Modeling sinking rate of zooplankton carcasses: Effects of stratification and mixing

Using the carcass sinking rate and density determined in laboratory for several freshwater zooplankton species, we developed a model of zooplankton carcass sinking as affected by turbulence and stratification. The model was subsequently used to estimate the residence time of zooplankton carcasses in the water column of Lake Stechlin, a typical temperate dimictic lake in northeastern Germany. The residence time varied among the different species and was strongly affected by thermal stratification. At the peak of summer stratification, the carcasses stayed up to 5 d in the 70 m‐deep water column before reaching the lake bottom. Residence time was long enough that zooplankton carcasses could serve as an important matter and energy source for bacteria in the lake's pelagic zone and hence have the potential to significantly affect aquatic carbon and nutrient cycling. The proposed model of sinking rates, based on physically sound relationships, can be easily applied to other passively sinking particles, and be integrated into large ecosystem models.

Zooplankton and aggregates as refuge for aquatic bacteria: protection from UV, heat and ozone stresses used for water treatment

Aggregates and zooplankton may provide refuge for aquatic bacteria against external hazards. The ability of attached bacteria to survive and recover from stressors commonly used for water treatment was tested in the laboratory. Without zooplankton or aggregates, both UV and ozone significantly reduced abundance of free-living bacteria in both freshwater and marine medium. The presence of zooplankton carcasses and aggregates, however, allowed some of the attached bacteria to survive and recover quickly within 3 days. Heat exposure was the least effective as both free-living and attached bacteria were able to recover quickly. Selective survival of bacterial phylotypes led to large changes in bacterial community composition after stress exposures, and some of the bacteria that recovered belonged to groups with known pathogens. This study demonstrates that zooplankton and aggregates protected various aquatic bacteria from external stressors, and organic remains generated from zooplankton and aggregates after stress exposure even enabled the surviving bacteria to quickly regrow and subsequently be released into the surrounding water. Hence, water disinfection treatments that overlooked the potential persistence of bacteria associated with organisms and aggregates may not be effective in preventing the spread of undesirable bacteria.

Microbial decomposition of proteins and lipids in copepod versus rotifer carcasses

Zooplankton carcasses are common within aquatic systems and potentially serve as organic-rich substrates for bacteria. We compared the microbial decomposition of representative crustacean (copepod) and non-crustacean (rotifer) zooplankton carcasses and monitored changes in carcass protein and lipid contents. Our results showed that carcass decomposition was mainly driven by bacteria colonizing from the surrounding water. Carcass-associated bacteria displayed higher protease and lipase activities than free-living bacteria. Protein content of copepod carcasses decreased by 70% within the first 8 h and shifted from larger to smaller sized proteins, while protein loss in rotifer carcasses was insignificant. Carcass lipid content did not change significantly over 24 h in either zooplankton type, although polar branched fatty acids increased on copepod carcasses indicating an increase in viable microbial biomass. Our results suggest differential turnover of protein versus lipid within a zooplankton carcass and that carcasses from different zooplankton groups would affect water column microbial processes differently.

Microbial activities accompanying decomposition of cladoceran and copepod carcasses under different environmental conditions

AME Aquatic Microbial Ecology Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsSpecials AME 57:89-100 (2009) - DOI: https://doi.org/10.3354/ame01331 Microbial activities accompanying decomposition of cladoceran and copepod carcasses under different environmental conditions Kam W. Tang1,*, Samantha L. Bickel1, Claudia Dziallas2, Hans-Peter Grossart2 1Virginia Institute of Marine Science, 1208 Greate Road, Gloucester Point, Virginia 23062, USA 2Leibniz Institute of Freshwater Ecology and Inland Fisheries, Alte Fischerhuette 2, 16775 Stechlin, Germany *Email: kamtang@vims.edu ABSTRACT: Cladoceran and copepod carcasses in both marine and freshwater environments represent concentrated reservoirs of organic substrates for water column bacteria. We studied the microbial abundance, activities, and diversity associated with decomposing carcasses of different zooplankton species over short and long time scales, and in oligotrophic vs. eutrophic environments. Fresh carcasses of Daphnia cucullata, Diaphanosoma brachyurum, and Eudiaptomus gracilis were rapidly colonized by bacteria, which reached peak abundances within 1.5 d at 20°C and then decreased. Cell-specific exoenzymatic activity on protein and lipid analogs and production rate of bacteria associated with the carcasses were all higher than in the ambient water. ANOSIM analyses of DGGE banding patterns revealed that bacterial communities associated with both cladoceran and copepod carcasses rapidly diverged from the initial bacterial community in the ambient water. The high similarity of bacteria on both types of carcasses indicates that the carcasses were decomposed by similar bacterial groups. Estimated carcass decomposition rate was lower at 6°C, with an estimated Q10 of 2.4. Carcasses suspended in the eutrophic Lake Dagow had a higher average carbon loss rate than those suspended in the oligotrophic Lake Stechlin. Cladoceran carcasses were initially colonized by bacteria faster than copepod carcasses in both laboratory and field experiments, suggesting that cladoceran carcasses were more prone to exploitation by bacteria, yet copepod carcasses lost carbon at higher rates. Overall, our results suggest that pelagic zooplankton production can be directly converted to water column bacterial production via carcass decomposition, especially during the mid-summer zooplankton decline commonly observed in lakes. KEY WORDS: Decomposition · Exoenzymes · Bacterial production · Bacterial community composition · Zooplankton carcasses Full text in pdf format PreviousNextCite this article as: Tang KW, Bickel SL, Dziallas C, Grossart HP (2009) Microbial activities accompanying decomposition of cladoceran and copepod carcasses under different environmental conditions. Aquat Microb Ecol 57:89-100. https://doi.org/10.3354/ame01331 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in AME Vol. 57, No. 1. Online publication date: September 18, 2009 Print ISSN: 0948-3055; Online ISSN: 1616-1564 Copyright © 2009 Inter-Research.