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Selph, KE, Landry MR, Laws EA.  2003.  Heterotrophic nanoflagellate enhancement of bacterial growth through nutrient remineralization in chemostat culture. Aquatic Microbial Ecology. 32:23-37.   10.3354/ame032023   AbstractWebsite

Heterotrophic nanoflagellates are the principal consumers of picoplankton in the ocean. Their role as nutrient remineralizers is also well established. However, the coupled interactions between grazer consumption and prey growth are less well understood. In this work, we demonstrate a tight coupling among flagellate grazing, nitrogen remineralization, and prey growth, resulting in bacterial growth rates averaging 2- to 14-fold higher in the presence of flagellate grazers. These results were obtained using 2-stage, nitrogen-limited chemostats containing a mixed culture of heterotrophic bacteria enriched from seawater and Paraphysomonas bandaiensis, a chrysomonad flagellate. Abundance and biovolume of the flagellates were monitored on a daily basis, as was bacterial abundance. Grazing rates were measured using short-term tracer uptake experiments (fluorescently-labeled bacteria and beads), and these data were used to calculate gross bacterial growth rates in the presence of grazers. A mass balance approach was used to estimate reduced nitrogen regenerated by the protist and nitrogen demand of the heterotrophic bacteria. These independent methods of assessing grazer growth and feeding, coupled with estimates of flagellate gross growth efficiency, provided strong, internally consistent constraints on the estimates of bacterial growth rates in the presence of grazers. Under these culture conditions, P. bandaiensis had a carbon-based gross growth efficiency averaging 28%. This work shows that independently measured grazing rates are essential in protist culture work if system dynamics are to be understood. It also underscores the necessity of including protist remineralization pathways in models if realistic simulations are to be obtained.

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Decima, M, Landry MR, Stukel MR, Lopez-Lopez L, Krause JW.  2016.  Mesozooplankton biomass and grazing in the Costa Rica Dome: amplifying variability through the plankton food web. Journal of Plankton Research. 38:317-330.   10.1093/plankt/fbv091   AbstractWebsite

We investigated standing stocks and grazing rates of mesozooplankton assemblages in the Costa Rica Dome (CRD), an open-ocean upwelling ecosystem in the eastern tropical Pacific. While phytoplankton biomass in the CRD is dominated by picophytoplankton (<2-mu m cells) with especially high concentrations of Synechococcus spp., we found high mesozooplankton biomass (similar to 5 g dry weight m(-2)) and grazing impact (12-50% integrated water column chlorophyll a), indicative of efficient food web transfer from primary producers to higher levels. In contrast to the relative uniformity in water-column chlorophyll a and mesozooplankton biomass, variability in herbivory was substantial, with lower rates in the central dome region and higher rates in areas offset from the dome center. While grazing rates were unrelated to total phytoplankton, correlations with cyanobacteria (negative) and biogenic SiO2 production (positive) suggest that partitioning of primary production among phytoplankton sizes contributes to the variability observed in mesozooplankton metrics. We propose that advection of upwelled waters away from the dome center is accompanied by changes in mesozooplankton composition and grazing rates, reflecting small changes within the primary producers. Small changes within the phytoplankton community resulting in large changes in the mesozooplankton suggest that the variability in lower trophic level dynamics was effectively amplified through the food web.

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Steinberg, DK, Landry MR.  2017.  Zooplankton and the ocean carbon cycle. Annual Review of Marine Sciences, Vol 9. 9:413-444., Palo Alto: Annual Reviews   10.1146/annurev-marine-010814-015924   Abstract

Marine zooplankton comprise a phylogenetically and functionally diverse assemblage of protistan and metazoan consumers that occupy multiple trophic levels in pelagic food webs. Within this complex network, carbon flows via alternative zooplankton pathways drive temporal and spatial variability in production-grazing coupling, nutrient cycling, export, and transfer efficiency to higher trophic levels. We explore current knowledge of the processing of zooplankton food ingestion by absorption, egestion, respiration, excretion, and growth (production) processes. On a global scale, carbon fluxes are reasonably constrained by the grazing impact of microzooplankton and the respiratory requirements of mesozooplankton but are sensitive to uncertainties in trophic structure. The relative importance, combined magnitude, and efficiency of export mechanisms (mucous feeding webs, fecal pellets, molts, carcasses, and vertical migrations) likewise reflect regional variability in community structure. Climate change is expected to broadly alter carbon cycling by zooplankton and to have direct impacts on key species.