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Anthropogenic atmospheric loading of CO2 raises concerns about combined effects of increasing ocean temperature and acidification, on biological processes. In particular, the response of appendicularian zooplankton to climate change may have significant ecosystem implications as they can alter biogeochemical cycling compared to classical copepod dominated food webs. However, the response of appendicularians to multiple climate drivers and effect on carbon cycling are still not well understood. Here, we investigated how gelatinous zooplankton (appendicularians) affect carbon cycling of marine food webs under conditions predicted by future climate scenarios. Appendicularians performed well in warmer conditions and benefited from low pH levels, which in turn altered the direction of carbon flow. Increased appendicularians removed particles from the water column that might otherwise nourish copepods by increasing carbon transport to depth from continuous discarding of filtration houses and fecal pellets. This helps to remove CO2 from the atmosphere, and may also have fisheries implications. [ABSTRACT FROM AUTHOR]

Quantification of grazing losses of marine heterotrophic bacteria is critical for understanding nutrient and carbon pathways in aquatic systems. The dilution method is a commonly used experimental approach for quantifying bacterivory. However, valid estimates of grazing rates obtained using this method depend on several methodological assumptions including that the method does not influence specific growth rates of bacteria. Here, we hypothesize that filtration during the set-up of a dilution experiment has the potential to release allelochemicals from phytoplankton cells and thereby stimulate or inhibit bacterial growth with the consequence of biased grazing estimates. We tested this hypothesis during a natural Phaeocystis pouchetii bloom at two different locations within an Arctic fjord. Results from the dilution experiments suggest higher gross growth rate and grazing impact for bacteria in the outer fjord compared with the inner fjord. However, specific growth rates estimated by bacterial production cell−1 were significantly elevated in dilutions of water from the outer fjord but not the inner fjord. The analysis of dissolved metabolites in the seawater from both experiments prior and after filtration revealed altered metabolic profiles after filtration at both stations. As unaffected specific growth of prey on dilution is one of three fundamental assumptions of the dilution method, we conclude that it is important that empirically estimated bacterial specific growth rates be routinely included when using the dilution method to quantify bacterivory. [ABSTRACT FROM AUTHOR]

Knowledge of the complex life cycle of Phaeocystis is a key to understanding its role in marine ecosystems and global biogeochemistry. An existing life cycle model was modified and used to integrate understanding of the Phaeocystis life cycle. In model-driven research, models expose gaps in our understanding, empirical studies ensue, and feedback improves understanding. Following this scheme, three facets of the life cycle model were examined here. With four exceptions, the empirical studies described have been presented in other literature citations. The first facet involved testing for the existence of a process or producing its description. These studies included: demonstration of in vitro colony division in Phaeocystis pouchetii, description of in vitro change in colony shape for P. pouchetii associated with senescence, determining which P. pouchetii life stage is vulnerable to viral infection and lysis, and an experiment designed to determine whether the sediment could be a source of new Phaeocystis colonies to overlying waters; results suggested that more-detailed investigation of benthic particles as a physical substrate for colony formation is warranted. The second facet involved investigation of process rate quantification or process control parameters. Process rate quantification included measurements of colony division rate and growth rate using mesocosm-derived colonies. Process control experiments included testing diatom frustule enhancement of P. pouchetii colony formation from solitary cells, and investigation of mesozooplanktonic suppression and microzooplanktonic enhancement of Phaeocystis globosa colony formation by planktonic grazer infochemicals. The third facet pertained to the molecular identification of genetic differences between single cells and colonies of P. globosa. These studies were designed to provide insight to the question of control factors involved in the transition between single cell and colonial life stages. The life cycle model provided a ready place to incorporate new insights and understanding from empirical studies into an existing model, and can be used to improve simulation models of the direct and indirect effects of Phaeocystis on global biogeochemistry. [ABSTRACT FROM AUTHOR]