Biogeochemical fluxes through microzooplankton

[1] Microzooplankton ingest a significant fraction of primary production in the ocean and thus remineralize nutrients and stimulate regenerated primary production. We synthesized observations on microzooplankton carbon-specific grazing rate, partitioning of grazed material, respiration rate, microzooplankton biomass, microzooplankton-mediated phytoplankton mortality rate, and phytoplankton growth rate. We used these observations to parameterize and evaluate the microzooplankton compartment in a global biogeochemical model that represents five plankton functional types. Microzooplankton biomasses predicted in this simulation are closer to the independently derived evaluation data than in the previous model version. Most rates, including primary production, microzooplankton grazing, and export of sinking detritus are within observational constraints. However, the model underestimates microzooplankton and mesozooplankton biomasses and chlorophyll concentrations. Thus, we propose that sufficient carbon enters the model ecosystem, but insufficient carbon is retained. For microzooplankton, the low retention of carbon could be improved by parameterizing the model with ciliate gross growth efficiency only, indicating that ciliates may contribute more to microzooplankton activity than their biomass contribution suggests. By taking into account the model underestimation of biomass, we estimate that the ocean inventory of microzooplankton biomass is 0.24 Pg C (a range of 0.14-0.33 Pg C), which is similar to the biomass of mesozooplankton

Lake water level increase during spring affects the breeding success of bream Abramis brama (L.)

Probst WN, Stoll S, Peters L, Fischer P, Eckmann R. Lake water level increase during spring affects the breeding success of bream Abramis brama (L.). HYDROBIOLOGIA. 2009;632(1):211-224.In Lake Constance Eurasian bream Abramis brama (L.) spawn in very shallow littoral areas by the beginning of May. They attach their adhesive eggs to pebble and cobble substratum at less than 40 cm depth. Increasing water levels before spawning inundate bare substratum to which bream eggs may attach better than to deeper substratum covered by epilithon. Consequently, the water level increase prior to spawning should determine the amount of pristine spawning substratum available to bream and thus influence their breeding success. To test this hypothesis, the influence of hydrology and climate on the abundance of age-0 bream was combined with results from field investigations on the egg survival and abundance of age-0 bream. A strong positive correlation between the mean water level increase during the spawning season of bream (April-May) and the abundance of juvenile bream was found. By contrast, the absolute water level during spawning and during the nursery stage in summer, the cumulative temperature during the egg, larval and juvenile stages and two North Atlantic Oscillation (NAO) indices did not affect the abundance of juvenile bream. The field investigations confirmed that bream eggs attach better to and have higher survival rates on bare substratum than on substratum with epilithon cover. Accordingly, eggs within a spawning habitat of bream were most abundant between 10 - 20 cm depth, where the epilithon cover was lower than at depths exceeding 30 cm. The results of this study confirm an adverse influence of epilithon cover on the attachment and subsequent survival of bream eggs and emphasize the importance of spring inundations for the successful breeding of bream

Spring phenological responses of marine and freshwater plankton to changing temperature and light conditions

Shifts in the timing and magnitude of the spring plankton bloom in response to climate change have been observed across a wide range of aquatic systems. We used meta-analysis to investigate phenological responses of marine and freshwater plankton communities in mesocosms subjected to experimental manipulations of temperature and light intensity. Systems differed with respect to the dominant mesozooplankton (copepods in seawater and daphnids in freshwater). Higher water temperatures advanced the bloom timing of most functional plankton groups in both marine and freshwater systems. In contrast to timing, responses of bloom magnitudes were more variable among taxa and systems and were influenced by light intensity and trophic interactions. Increased light levels increased the magnitude of the spring peaks of most phytoplankton taxa and of total phytoplankton biomass. Intensified size-selective grazing of copepods in warming scenarios affected phytoplankton size structure and lowered intermediate (20–200 lm)-sized phytoplankton in marine systems. In contrast, plankton peak magnitudes in freshwater systems were unaffected by temperature, but decreased at lower light intensities, suggesting that filter feeding daphnids are sensitive to changes in algal carrying capacity as mediated by light supply. Our analysis confirms the general shift toward earlier blooms at increased temperature in both marine and freshwater systems and supports predictions that effects of climate change on plankton production will vary among sites, depending on resource limitation and species composition

Future Climate Scenarios for a Coastal Productive Planktonic Food Web Resulting in Microplankton Phenology Changes and Decreased Trophic Transfer Efficiency

We studied the effects of future climate change scenarios on plankton communities of a Norwegian fjord using a mesocosm approach. After the spring bloom, natural plankton were enclosed and treated in duplicates with inorganic nutrients elevated to pre-bloom conditions (N, P, Si; eutrophication), lowering of 0.4 pH units (acidification), and rising 3 degrees C temperature (warming). All nutrient-amended treatments resulted in phytoplankton blooms dominated by chain-forming diatoms, and reached 13-16 mu g chlorophyll (chl) a l(-1). In the control mesocosms, chl a remained below 1 mu g l(-1). Acidification and warming had contrasting effects on the phenology and bloom-dynamics of autotrophic and heterotrophic microplankton. Bacillariophyceae, prymnesiophyceae, cryptophyta, and Protoperidinium spp. peaked earlier at higher temperature and lower pH. Chlorophyta showed lower peak abundances with acidification, but higher peak abundances with increased temperature. The peak magnitude of autotrophic dinophyceae and ciliates was, on the other hand, lowered with combined warming and acidification. Over time, the plankton communities shifted from autotrophic phytoplankton blooms to a more heterotrophic system in all mesocosms, especially in the control unaltered mesocosms. The development of mass balance and proportion of heterotrophic/autotrophic biomass predict a shift towards a more autotrophic community and less-efficient food web transfer when temperature, nutrients and acidification are combined in a future climate-change scenario. We suggest that this result may be related to a lower food quality for microzooplankton under acidification and warming scenarios and to an increase of catabolic processes compared to anabolic ones at higher temperatures