Zooplankton Production Research Articles

Assessment of Missouri River floodplain invertebrates during historic inundation: implications for river restoration

Floodplain connectivity is important to aquatic organisms in large rivers. Anthropogenic alterations regulating the Missouri River have limited connectivity and negatively affected native fauna. Determining the biological response to rare inundation events may be important when considering potential restoration options on a regulated river; thus, we assessed benthic invertebrate and zooplankton communities at three floodplain sites during a historic Missouri River high-water event. Chironomid larvae dominated during most sampling trips and densities were often highest during initial sampling trips with lower densities as high water persisted. Similar trends were evident for rotifer, cladoceran, and copepod densities. Nonmetric multidimensional scaling also showed relatively high dissimilarity of densities between early and late sampling trips for benthic invertebrate and zooplankton communities. As such, short-term inundation may be more beneficial to Missouri River benthic invertebrate (mainly chironomid larvae) and zooplankton production than more prolonged inundation lasting a month or more. Furthermore, restoration projects may be designed at elevations allowing more short-term inundation, which would likely benefit native fishes with additional spawning, nursery, and foraging habitat. Levee setbacks may be an effective restoration option for increasing the amount of habitat available for short-term inundation while potentially providing socioeconomic, flood-risk reduction benefits by enhancing flow conveyance.

Spring and fall phytoplankton blooms in a productive subarctic ecosystem, the eastern Bering Sea, during 1995–2011

The timing and magnitude of phytoplankton blooms in subarctic ecosystems often strongly influence the amount of energy that is transferred through subsequent trophic pathways. In the eastern Bering Sea, spring bloom timing has been linked to ice retreat timing and production of zooplankton and fish. A large part of the eastern Bering Sea shelf (~500km wide) is ice-covered during winter and spring. Four oceanographic moorings have been deployed along the 70-m depth contour of the eastern Bering Sea shelf with the southern location occupied annually since 1995, the two northern locations since 2004 and the remaining location since 2001. Chlorophyll a fluorescence data from the four moorings provide 37 realizations of a spring bloom and 33 realizations of a fall bloom. We found that in the eastern Bering Sea: if ice was present after mid-March, spring bloom timing was related to ice retreat timing (p<0.001, df=1, 24); if ice was absent or retreated before mid-March, a spring bloom usually occurred in May or early June (average day 148, SE=3.5, n=11). A fall bloom also commonly occurred, usually in late September (average day 274, SE=4.2, n=33), and its timing was not significantly related to the timing of storms (p=0.88, df=1, 27) or fall water column overturn (p=0.49, df=1, 27). The magnitudes of the spring and fall blooms were correlated (p=0.011, df=28). The interval between the spring and fall blooms varied between four to six months depending on year and location. We present a hypothesis to explain how the large crustacean zooplankton taxa Calanus spp. likely respond to variation in the interval between blooms (spring to fall and fall to spring).

Gelatinous macroplankton in the Black Sea in the autumn of 2010

In October and November of 2010, the gelatinous macroplankton in the northwestern Black Sea and at the Crimean shelf was represented by the scyphozoan jellyfish Aurelia aurita, three species of ctenophores (Beroe ovata, Mnemiopsis leidyi, and Pleurobrachia pileus), and three species of hydromedusae. A. aurita was more common at the shelf, M. leidyi and P. pileus prevailed at the deeper sea stations, and B. ovata was almost ubiquitous with a biomass from below 1 to 49 g/m2A. aurita, which had an average biomass of 82–224 g/m2, was dominant at all the stations. M. leidyi, which had a biomass from below 1 to 115 g/m2, was recorded in October at only 14 of the 52 stations and in November at 20 of the 46 stations. The highest biomass of M. leidyi in October (105 to 116 g/m2) was recorded in the deep sea areas; in November, it was also the highest in these areas, but it reached at most 100 g/m2. The average daily ration of Aurelia ranged from 19.4 to 27.3 mg/m2 in October and from 7.0 to 17.2 mg/m2 in November; in both cases, it was insufficient to provide for the minimal physiological requirements. The average daily ration of the Mnemiopsis population (2.8–20.5 mg of zooplankton per m2) was invariably more than sufficient to provide for the minimal physiological requirements. Both predatory species together consumed less than 5% of the daily zooplankton production of the sea.

The role of eutrophication reduction of two small man-made Mediterranean lagoons in the context of a broader remediation system: Effects on water quality and plankton contribution

In order to meet the requirements of the European Union Water Framework Directive for the Albufera de València (AV) Natural Park, in 2009, several areas of free water surface constructed wetlands (FWSCWs) planted with emergent vegetation and two small shallow lagoons planted with submerged macrophytes were created over a 40-ha area formerly occupied by rice fields. This area is currently a reserve known as Tancat de la Pipa. The dual goal of this programme was to improve the quality of the hyper-eutrophicated waters of the AV lagoon, the largest littoral lagoon in the Iberian Peninsula, and to restore former lost habitats to increase the biodiversity of the area. The lagoons were mainly fed with the outflows of the FWSCWs. The submerged macrophytes covered both lagoons and sustained a diverse waterfowl community. In this study, the nutrient reduction between the inflows and the outflows of the lagoons was evaluated, along with the change in the composition and biomass of plankton (i.e., phytoplankton and zooplankton), over the first four years of operation of the two lagoons. The nutrient reduction contribution of the lagoons to the overall Tancat de la Pipa system was also quantified. The results indicated that 42% of the total suspended solids, 30% of the total nitrogen and 49% of the total phosphorus were retained in the lagoons. An increase in the zooplankton and phytoplankton richness was observed in the outflows of both lagoons. These aquatic systems served as a site for zooplankton production and large cladocerans played a significant role in water clearing by consuming microalgae. Submerged vegetation was also critical for reducing nutrient loads, although macrophytes disappeared after the second year. Despite the macrophyte loss, the lagoons continued to act as nutrient sinks. Both lagoons facilitated over 30% mass removal of TP, TSS and Chl a and over 20% mass removal of DIN and TN for the overall Tancat de la Pipa system. Therefore, these lagoons represent a new sustainable aquatic ecosystem for improving water quality and promoting biological development. We recommend the configuration of this ecosystem as a complement to FWSCWs for managing eutrophicated waters in protected sites.

Stratification-mixing cycles and plankton dynamics in a shallow estuary (Limfjord, Denmark)

The biomass, production and consumption of phytoplankton, bacteria and zooplankton in a shallow Danish estuary (Limfjord) were analysed during a 9-day period. The water column changed between stratified and mixed conditions which influenced the dominant processes in the pelagic system. During strong stratification, phytoplankton was mainly controlled by microzooplankton grazing. A mixing event, which homogenized the water column, possibly provided food to a mussel-dominated benthic community. Concomitantly, zooplankton feeding and reproduction decreased. However, the nutrient input to the upper part of the water column during mixing and the subsequent stabilization provided the ideal conditions for the recovery of phytoplankton from the loss processes from previous days. Microzooplankton, which was also a significant consumer of bacteria throughout the sampling period, was not the only consumer controlling phytoplankton. The microbial food web was an important route for total plankton carbon during the study. However, the shorter food web increased in importance during mixing, possibly due to stronger benthic–pelagic coupling than during stratified periods. Stratification-mixing cycles, occurring during short-time periods, should be a key mechanism maintaining the benthic and pelagic communities in this shallow water system.

Phytoplankton Biomass and Composition in a River-Dominated Estuary During Two Summers of Contrasting River Discharge

Estuaries located in the northern Gulf of Mexico are expected to experience reduced river discharge due to increasing demand for freshwater and predicted periods of declining precipitation. Changes in freshwater and nutrient input might impact estuarine higher trophic level productivity through changes in phytoplankton quantity and quality. Phytoplankton biomass and composition were examined in Apalachicola Bay, Florida during two summers of contrasting river discharge. The 20 μm diatoms were an increasingly (20 to 70 %) larger component of phytoplankton biomass. Lower summer river discharges that lead to an areal contraction of lower (5–25 psu) salinity waters composed of higher phytoplankton biomass dominated by small ( 25 psu) salinity waters composed of relatively lower phytoplankton biomass and a higher percent contribution by >20 μm diatoms. A reduction in summer river discharge that leads to such a change in quantity and quality of estuarine phytoplankton available will result in a reduction in estuarine zooplankton productivity and possibly the productivity of higher trophic levels.

Secondary production at the Polar Front, Barents Sea, August 2007

To investigate spatial patterns of secondary production we sampled four core hydrographical regions of the Polar Front in the Barents Sea (Arctic Water, ArW; Polar Front Water, PFW; Atlantic Water, AtW; and Melt Water, MW) by towing an undulating instrument platform along a transect crossing the front from August 8–9, 2007. Sensors mounted on the platform provided data on the hydrography (CTD), fluorescence (Fluorometer, F) and zooplankton abundance in the size range between 0.1 and 30mm (Laser Optical Plankton Counter, LOPC). These continuous, biophysical data with high-spatial resolution were supplemented by discrete water and zooplankton net samples at stations for sensor calibrations. After in depth quality assessments of the biophysical data, estimates were made of the vital rates based on biovolume spectrum theory. Five size groups were distinguished from the LOPC data: small (S), mainly Oithona spp. and the appendicularian Fritillaria sp.; medium (M), mainly Pseudocalanus spp. and Calanus spp. CI–CIII; large (L), mainly Calanus spp. CIV–CV; and extra large (XL and 2XL), juvenile and adult euphausids. Size groups were further divided based on transparency of organisms. Vital rates based on the biophysical in situ data in combination with biovolume spectrum theories agreed generally well with data from empirical and numerical models in the literature. ArW was characterised by subsurface maxima of chlorophyll a (chl a), and an estimated population growth of ca. 13mgCm−3d−1 for CI–CIII Calanus spp. and some older Pseudocalanus within the chl a maxima. Frontal waters were characterised by low chl a concentrations, but high abundances and production (around 1gCm−3d−1) of small copepods (Oithona spp.) and appendicularians (Fritillaria sp.). The estimated production of small-size zooplankton was an order of magnitude higher than the production of all other size groups combined, including large copepods. The high loss rates (−166 to −271mgCm−3d−1) of small zooplankton may contribute a substantial amount of carbon to the benthos and to pelagic predators such as young capelin. AtW was the most productive water mass, with surface chl a maxima and an estimated population growth of 134mgCm−3d−1 for small zooplankton, 3.6mgCm−3d−1 for medium-sized copepods and 0.9mgCm−3d−1 for CIV–CVI Calanus. For those Calanus spp. in the surface layer, the estimated specific mortality rates were up to −0.35d−1, partly due to high predation pressure by hydrozoans and chaetognaths.

Forage fish quality: seasonal lipid dynamics of herring (Clupea harengus L.) and sprat (Sprattus sprattus L.) in the Baltic Sea

Abstract Røjbek, M. C., Tomkiewicz, J., Jacobsen, C., and Støttrup, J. G. Forage fish quality: seasonal lipid dynamics of herring (Clupea harengus L.) and sprat (Sprattus sprattus L.) in the Baltic Sea. – ICES Journal of Marine Science, 71: . This study investigates lipid content and fatty acid composition of two important forage fish, sprat (Sprattus sprattus) and herring (Clupea harengus) in the Baltic Sea ecosystem. Seasonal variation in lipids was studied during three periods following the annual reproductive cycle considering potential differences relating to fish size, sex, and reproductive status. The isopod Saduria entomon, being at times an important prey for predatory fish, was included for comparison. In both sprat and herring, lipid content and absolute contents of essential polyunsaturated fatty acids (PUFAs) varied seasonally with high levels towards the end of the annual zooplankton production cycle, succeeded by a decline. Lipid content and fatty acid composition differed significantly between sprat and herring. Sprat lipid content was higher than herring, increasing with fish size and characterized by large proportions of monounsaturated fatty acids. Herring lipid content was related to the reproductive cycle and proportions of PUFAs were high compared with sprat. Levels of essential PUFAs were high in S. entomon compared with clupeids rendering it a valuable alternative prey species in the Baltic Sea ecosystem. The lipid dynamics of forage fish and benthos, combined with changes in availability and abundance, will affect growth and reproduction of their predators.

Can Humic Water Discharge Counteract Eutrophication in Coastal Waters?

A common and established view is that increased inputs of nutrients to the sea, for example via river flooding, will cause eutrophication and phytoplankton blooms in coastal areas. We here show that this concept may be questioned in certain scenarios. Climate change has been predicted to cause increased inflow of freshwater to coastal areas in northern Europe. River waters in these areas are often brown from the presence of high concentrations of allochthonous dissolved organic carbon (humic carbon), in addition to nitrogen and phosphorus. In this study we investigated whether increased inputs of humic carbon can change the structure and production of the pelagic food web in the recipient seawater. In a mesocosm experiment unfiltered seawater from the northern Baltic Sea was fertilized with inorganic nutrients and humic carbon (CNP), and only with inorganic nutrients (NP). The system responded differently to the humic carbon addition. In NP treatments bacterial, phytoplankton and zooplankton production increased and the systems turned net autotrophic, whereas the CNP-treatment only bacterial and zooplankton production increased driving the system to net heterotrophy. The size-structure of the food web showed large variations in the different treatments. In the enriched NP treatments the phytoplankton community was dominated by filamentous >20 µm algae, while in the CNP treatments the phytoplankton was dominated by picocyanobacteria <5 µm. Our results suggest that climate change scenarios, resulting in increased humic-rich river inflow, may counteract eutrophication in coastal waters, leading to a promotion of the microbial food web and other heterotrophic organisms, driving the recipient coastal waters to net-heterotrophy.

The Effects of Juvenile American Shad Planktivory on Zooplankton Production in Columbia River Food Webs

AbstractColumbia River reservoirs support a large population of nonnative American Shad Alosa sapidissima that consume the zooplankton that native fishes also rely on. We hypothesized that the unprecedented biomass of juvenile American Shad in John Day Reservoir is capable of altering the zooplankton community if these fish consume a large portion of the zooplankton production. We derived taxon‐specific estimates of zooplankton production using field data and a production model from the literature. Empirical daily ration was estimated for American Shad and expanded to population‐level consumption using abundance and biomass data from hydroacoustic surveys. Daphnia spp. production was high in early summer but declined to near zero by September as shad abundance increased. American Shad sequentially consumed Daphnia spp., copepods, and Bosmina spp., which tracked the production trends of these taxa. American Shad evacuation rates ranged from 0.09 to 0.24/h, and daily rations ranged from 0.008 to 0.045 g·g−1·d−1 (dry weight) over all years. We observed peak American Shad biomass (45.2 kg/ha) in 1994, and daily consumption (1.6 kg/ha) approached 30% (5.3 kg/ha) of zooplankton production. On average, American Shad consumed 23.6% of the available zooplankton production (range, &lt;1–83%). The changes in the zooplankton community are consistent with a top‐down effect of planktivory by American Shad associated with their unprecedented biomass and consumption, but the effects are likely constrained by temperature, nutrient flux, and the seasonal production patterns of zooplankton in John Day Reservoir. American Shad add to the planktivory exerted by other species like Neomysis mercedis to reduce the capacity of the reservoir to support other planktivorous fishes. The introduction of American Shad and other nonnative species will continue to alter the food web in John Day Reservoir, potentially affecting native fishes, including Pacific salmon Oncorhynchus spp.

Evaluation of chitobiase-based estimates of biomass and production rates for developing freshwater crustacean zooplankton communities

Increasing attention has been devoted to the development of alternative (often biochemical) methods for measuring crustacean zooplankton productivity because conventional methods are not globally applicable and rarely practical when community-level rates are required. Here we evaluate the chitobiase method as a rapid, routine and instantaneous method for measuring the productivity of freshwater crustacean zooplankton communities. Chitobiase, a moulting enzyme, is liberated into water following moulting and production rates are calculated by measuring its turnover rate in the water column. First, using literature-based instarand stage-specific individual body mass values, we found a common relationship between post-moult body size (and individual chitobiase activity) and the biomass produced between successive moults for common freshwater groups. Secondly, using a time-series of weekly measurements in a North-Temperate lake, we found a good correspondence between the standing activity of chitobiase in the water column (CBANAT) and the biomass sampled by a plankton net and laser optical plankton counter (LOPC). Overall, however, CBANAT-based biomass more closely corresponded to LOPC-based biomass estimates. Lastly, depth-specific biomass production rates and daily production to biomass estimates varied positively with temperature. Daily production to biomass ratios also varied closely with

Diel variation in the productivity of Acartia lilljeborgii and Acartia tonsa (Copepoda: Calanoida) in a tropical estuary (Taperaçu, Northern Brazil)

ABSTRACT Magalhaes, A., Nobre, D.S.B., Bessa, R.S.C., Pereira, L.C.C. and Costa, R.M. da, 2013. Diel variation in the biomass and productivity of Acartia tonsa (Copepoda: Calanoida) in a tropical estuary (Taperacu, northern Brazil). In: Conley, D.C., Masselink, G., Russell, P.E. and O'Hare, T.J. (eds.) Diel variation in the productivity of the copepods Acartia lilljeborgii and Acartia tonsa was investigated in the Taperacu estuary in July (rainy season) and October (dry season), 2004. The estimated productivity of A. lilljeborgii (min-max: 3.932,385.41 mg C m−3 d−1; mean: 246.60 mg C m−3 d−1) and A. tonsa (min-max: 0.32–160.21 mg C m−3 d−1; mean: 16.99 mg C m−3 d−1) highlight the importance of their contribution to zooplankton production in the Taperacu estuary. The absence of significant short-term variation in the productivity of A. lilljeborgii and A. tonsa in relation to the period of the day may be related to the combined effect of the shallow depth of the waters of this estuary and its strong tidal ...

An integrated fish–plankton aquaculture system in brackish water

Integrated Multi-Trophic Aquaculture takes advantage of the mutualism between some detritivorous fish and phytoplankton. The fish recycle nutrients by consuming live (and dead) algae and provide the inorganic carbon to fuel the growth of live algae. In the meanwhile, algae purify the water and generate the oxygen required by fishes. Such mechanism stabilizes the functioning of an artificially recycling ecosystem, as exemplified by combining the euryhaline tilapia Sarotherodon melanotheron heudelotii and the unicellular alga Chlorella sp. Feed addition in this ecosystem results in faster fish growth but also in an increase in phytoplankton biomass, which must be limited. In the prototype described here, the algal population control is exerted by herbivorous zooplankton growing in a separate pond connected in parallel to the fish–algae ecosystem. The zooplankton production is then consumed by tilapia, particularly by the fry and juveniles, when water is returned to the main circuit. Chlorella sp. and Brachionus plicatilis are two planktonic species that have spontaneously colonized the brackish water of the prototype, which was set-up in Senegal along the Atlantic Ocean shoreline. In our system, water was entirely recycled and only evaporation was compensated (1.5% volume/day). Sediment, which accumulated in the zooplankton pond, was the only trophic cul-de-sac. The system was temporarily destabilized following an accidental rotifer invasion in the main circuit. This caused Chlorella disappearance and replacement by opportunist algae, not consumed by Brachionus. Following the entire consumption of the Brachionus population by tilapias, Chlorella predominated again. Our artificial ecosystem combining S. m. heudelotii, Chlorella and B. plicatilis thus appeared to be resilient. This farming system was operated over one year with a fish productivity of 1.85 kg/m2 per year during the cold season (January to April).

Seasonal variations in the production and distribution of Zooplankton in Coringa back waters of Kakinada Bay.

Seasonal variations in the production and distribution of Zooplankton in Coringa back waters of Kakinada Bay.

Distribution and Abundance of Zooplankton Along Tamil Nadu Coastal Waters, India

Zooplanktons have a limited ability to control their fine-scale distribution in the water column, but they are also at the mercy of oceanic currents and water movements. The distribution and abundance of zooplanktons are strongly influenced by many factors. The present study consists of 26 species at three different stations. In this study, maximum of these species are found in station-3, i.e., Pichavaram, than the other 2 stations. The species recorded in the present study belongs to families: Foraminifera (9), Calanoida (6), Cyclopoida (4), Appendicularia (4) Hydroida(1), Rotatoria(1) and Sagittoida(1). The zooplankton production gradually decreases with increasing depth, where the offshore stations yield less abundance than the coastal waters stations.

Spatio-temporal dynamics of growth and survival of Lesser Sandeel early life-stages in the North Sea: Predictions from a coupled individual-based and hydrodynamic–biogeochemical model

Accounting for the individual variability and regional variations are important when predicting recruitment in fish species. Spatially explicit descriptions for recruitment in sandeels are necessary and sandeel growth and survival depend locally on zooplankton prey. We investigate the responses of larval and early juvenile Lesser Sandeel (Ammodytes marinus) in the North Sea to local feeding conditions by an adapted version of a generic bioenergetic individual-based model for larval fish describing growth and survival. Prey encounter and physiological processes are described explicitly in the model, which allows analyzing the influence of prey on the growth and survival of sandeel. The model is coupled to a hydrodynamic–biogeochemical model with physical and prey fields and implemented in temporal and three-dimensional spatial settings. Zooplankton biomass simulated by the biogeochemical model is validated by Continuous Plankton Recorder survey time series data. Spatio-temporal dynamics of the sandeel cohorts are simulated by the integrated model framework for the period 2004–2006 and five major area divisions of suitable sandeel habitats in the North Sea. This allows obtaining insight into the influence of temperature variation and zooplankton availability on the growth and survival. To determine areas promising for recruitment, area divisions are compared and optimal time of hatching for higher survival to recruitment due to match–mismatch with prey is determined by comparing different hatching times. The effect of vertical diel migration behavior of sandeel on the model outcomes is also examined. Vertical migration of sandeel results in increased feeding ability and growth and decreased starvation mortality of individuals. Results show that areas of German Bight and Southern Bight with high zooplankton production support high growth of sandeel. Most sandeel survive to settling in the Dogger Bank area that has large retention on average and still productive in zooplankton. Hatching at the optimal time of March/February with matching zooplankton peak concentrations enhances the growth and survival. Growth correlates positively with the observed temperature trend. Survival correlates negatively with temperature and prominently, when the hatching is in winter.

Secondary production of crustacean zooplankton and biomass of major rotifer species in Lake Bosumtwi/Bosomtwe, Ghana, West Africa

AbstractStudies have shown a strong linkage between zooplankton and fisheries' potential in tropical lakes. High zooplankton production provides the basis for fish production, but knowledge of zooplankton production dynamics in African lakes is extremely limited. Crustacean zooplankton production and the biomass of dominant rotifers in Lake Bosumtwi were assessed over a 2‐year period. The crustaceans comprised an endemic and extremely abundant cyclopoid copepod, Mesocyclops bosumtwii and the cladoceran Moina micrura. Mean standing stock of the crustaceans was 429 mg dw m−3, whilst annual production averaged 2.1 g dw m−3 y−1. Production doubled from 1.4 g dw m−3 y−1 in 2005 to 2.8 g dw m−3 y−1 in 2006. Copepods accounted for 98.5% of crustacean production. The biomass of the dominant rotifers Brachionus calyciflorus and Hexarthra intermedia was less than 1% of total zooplankton biomass. Daily turnover rate and turnover time of the crustaceans was 0.19 day−1 and 6.2 days respectively. Crustacean production yielded no statistical relationship with phytoplankton biomass. Production was well within the range of tropical lakes. Peak crustacean production synchronized maximum rainfall, lake mixing and phytoplankton production. Most importantly, no one year's set of dynamics can be used to characterize zooplankton production in the lake.

Zooplankton production in the Bay of Quinte 1975–2008: relationships with primary production, habitat, planktivory, and aquatic invasive species (Dreissena spp. andCercopagis pengoi)

The Bay of Quinte, a long, shallow, high-phosphorus system in Lake Ontario, spans a trophic range from eutrophic (upper Bay) to meso-oligotrophic (mouth). Phosphorus control (PC) and the invasion of dreissenid mussels and Cercopagis pengoi have influenced its biology and environment. We elucidated the principal drivers behind zooplankton production in the Bay from pre-PC times to the present (1975–2008). Production of abundant species was calculated using the egg-ratio method. Production estimates were improved by determining system-specific production/biomass (P/B) relationships for rarer species. Planktivory governed zooplankton production initially, likely preventing a response to PC. With changes in the fish community, zooplankton production increased. Thereafter, the principal drivers were the structure and size of the fish community, shifts in the predatory arena associated with changing light penetration, temperature, and addition of C. pengoi — all within a general context of nutrient availability. Dreissenid impacts were indirect. Interactive variables combining measures of predation, nutrient level, light, and temperature were the best at accounting for variability in zooplankton production.

Hypoxia modifies planktivore–zooplankton interactions in Lake Erie

We evaluated vertical distributions of fish and zooplankton, planktivore consumption, and prey production in Lake Erie during 2005 to determine how hypolimnetic hypoxia alters fish (i.e., rainbow smelt (Osmerus mordax) and emerald shiners (Notropis atherinoides)) and invertebrate planktivore (i.e., Bythotrephes longimanus and Leptodora kindtii) relationships with their mesozooplankton prey. Hypoxia concentrated 45%–76% of fish into a narrow (&lt;2 m) metalimnetic layer, but only 3%–13% of zooplankton production was in this layer. The epilimnion may have served to some degree as a refuge for mesozooplankton because high temperatures may have excluded rainbow smelt. High concentrations of fish above the hypolimnion likely resulted in increased competition for large prey (i.e., predatory claodcerans). Although hypoxia did not result in overall high predation demands by planktivores relative to total zooplankton production, planktivore consumption rates within the metalimnion exceeded zooplankton production in that layer.

An isotopic (Δ&amp;lt;sup&amp;gt;14&amp;lt;/sup&amp;gt;C, δ&amp;lt;sup&amp;gt;13&amp;lt;/sup&amp;gt;C, and δ&amp;lt;sup&amp;gt;15&amp;lt;/sup&amp;gt;N) investigation of the composition of particulate organic matter and zooplankton food sources in Lake Superior and across a size-gradient of aquatic systems

Abstract. Food webs in aquatic systems can be supported both by carbon from recent local primary productivity and by carbon subsidies, such as material from terrestrial ecosystems, or past in situ primary productivity. The importance of these subsidies to respiration and biomass production remains a topic of debate. While some studies have reported that terrigenous organic carbon supports disproportionately high zooplankton production, others have suggested that phytoplankton preferentially support zooplankton production in aquatic ecosystems. Here we apply natural abundance radiocarbon (Δ14C) and stable isotope (δ13C, δ15N) analyses to show that zooplankton in Lake Superior selectively incorporate recently fixed, locally produced (autochthonous) organic carbon even though other carbon sources are readily available. Estimates from Bayesian isotopic modeling based on Δ14C and δ13C values show that the average lake-wide median contributions of recent in-lake primary production and terrestrial, sedimentary, and bacterial organic carbon to the bulk POM in Lake Superior were 58%, 5%, 33%, and 3%, respectively. However, isotopic modeling estimates also show that recent in situ production contributed a disproportionately large amount (median, 91%) of the carbon in mesozooplankton biomass in Lake Superior. Although terrigenous organic carbon and old organic carbon from resuspended sediments were significant portions (median, 38%) of the available basal food resources, these contributed only a small amount to mesozooplankton biomass. Comparison of zooplankton food sources based on their radiocarbon composition showed that terrigenous organic carbon was relatively more important in rivers and small lakes, and the proportion of terrestrially derived material used by zooplankton correlated with the hydrologic residence time and the ratio of basin area to water surface area.