Search

Zooplankton in the Barents Sea have been monitored annually with a standard procedure with determination of size-fractioned biomass since the mid-1980s. Biomass of copepods and cladocerans was estimated based on measured abundance and individual weights taken from literature. Calanus species were dominant, making up ~85% of the estimated biomass of copepods. The second most important taxon was Oithona spp. (~0.5 g dry weight (dw) m−2, ~10%), followed by Metridia spp. (~0.15 g dw m−2, 2–3%) and Pseudocalanus spp. (0.10–0.15 g dw m−2, 1–5%). Estimated biomass of cladoceran taxa (Evadne and Podon) was low (0.01 g dw m−2). Calanus spp. contributed most of the biomass of the medium size fraction (1–2 mm), whereas small copepod species (Oithona , Pseudocalanus and others) contributed to the small size fraction (<1 mm). Estimated biomass of Calanus spp. and of the sum of small copepod species were both positively correlated with measured total zooplankton biomass (R 2 = 0.72 and 0.34, respectively). The biomass ratio of small copepod species to Calanus was similar in Atlantic and Arctic water masses (~0.15–0.2) but tended to increase with decreasing total biomass. This suggests a shift to relatively larger roles of small copepods as Calanus and total biomass decrease. [ABSTRACT FROM AUTHOR]

In peninsular Florida, USA, rainfall is coupled with the Pacific Sea Surface Temperature Anomaly (SSTA), and rainfall affects mean depth and residence time of shallow lakes. We examined effects of two cycles of variation in rainfall using a 15-year data set from a shallow eutrophic lake dominated by small zooplankton, cyanobacteria and omnivorous fish. In high rainfall periods, the lake was deeper and cladoceran biomass was significantly higher than in dry periods. One factor was correlated with reduced biomass of cladocerans: a 3-fold higher biovolume of cyanobacteria. This led us to examine how variation in rainfall affects cyanobacteria. When cyanobacteria biovolume was high, the movement of water through the lake was low and invariant. Cyanobacteria grew unchecked. When cyanobacteria was reduced and cladocerans attained high biomass, there were intermittent flushing events that may have disrupted algal growth. Water color was elevated ~6-fold during the same time periods. Greater color may have made conditions less favorable for cyanobacteria by increasing light attenuation, and also more favorable for cladocerans, by reducing risk from fish. This study provides insight into how future variability in rainfall and drought, which may be exacerbated by global warming, could affect plankton in shallow subtropical lakes. [ABSTRACT FROM AUTHOR]

Production of diapausing eggs in many zooplankton species occurs after partial switching from parthenogenesis to sexual reproduction. Storage effect theory predicts the stable long-term coexistence of competitors investing in diapausing stages, but it does not address the effect of such investment on short-term coexistence. The freshwater rotifer Brachionus calyciflorus and the cladoceran Moina micrura were used in this study to test experimentally if diapause could promote coexistence of active individuals under inter-specific competition. Our results show that B. calyciflorus was more affected by inter-specific competition and exhibited a greater and earlier production of resting eggs compared with M. micrura. Costs of diapause investment in Moina were only observed in competition. This was translated into reduced probability of B. calyciflorus of being outcompeted by M. micrura. We show that investment in diapause might promote local and short-term coexistence of competing zooplankton species. Our study strongly suggests that the ecological effects of diapause encompass a wider range of temporal scales, from weekly to decadal, and levels of organization, from individuals to communities, than previously thought. [ABSTRACT FROM PUBLISHER]

The hypothesis that the joint effects of predation and cyanobacteria favor the performance of small-bodied cladocerans relative to large-bodied species was tested using three competition experiments in a two-factor full-factorial design. The large-bodied Daphnia pulex outcompeted the medium-bodied Simocephalus vetulus when fed a green alga Chlorella pyrenoidosa. However, the presence of the toxic cyanobacterium Microcystis aeruginosa and the coelenterate predator Hydra oligactis reversed the competitive outcome. Their joint effect was significant and contributed to the strongest dominance of S. vetulus. Similar dominance shifts were also observed in the competition between D. pulex and the small-bodied Bosmina longirostris, and in the competition between S. vetulus and B. longirostris. Life table and predation experiments showed that the order of sensitivity of three cladoceran species to Microcystis and to Hydra was the same as the order of competitive superiority in the absence of cyanobacteria. Therefore, cyanobacteria, predation and their joint effects are important drivers for competitive outcomes among cladocerans, potentially creating new opportunities for small-bodied species to dominate in a community. The present study underscores the importance of considering the combined effects of dietary palatability and predation on competitive dominance among herbivores, when predicting zooplankton responses to frequent cyanobacterial blooms. [ABSTRACT FROM PUBLISHER]

We investigated meso-zooplankton nutrient recycling as a driver of nitrogen deficiency for phytoplankton production in Arctic Alaskan lakes. Copepods and cladocerans differentially recycle inorganic nutrients based on nutrient demands for new tissue. Copepods typically sequester more nitrogen in the tissue, and can lead to nitrogen, relative to phosphorus, deficient conditions for primary production. Cladocerans sequester more phosphorus relative to nitrogen, potentially resulting in phosphorus deficiency for phytoplankton growth. We observed a positive relationship between meso-zooplankton communities with higher copepod relative to cladoceran biomass and levels of water column nitrogen deficiency. We also observed negative relationships between total precipitation over the 10 days prior to lake sampling and metrics of both nitrogen and phosphorus deficiency. These results support our predictions that relatively high watershed nutrient loading can overwhelm internal nutrient sources and sinks. However, when nutrient loading from the catchment is minimal meso-zooplankton community composition can affect nitrogen relative to phosphorus deficiency for primary production. Relationships between copepod biomass and nitrogen deficiency provide a potential mechanism for the high frequency of nitrogen limitation in Arctic Alaskan lakes. [ABSTRACT FROM AUTHOR]