FLOW CYTOMETRIC INVESTIGATION OF THE SIZE SPECTRUM OF NORTH SEA PHYTOPLANKTON COMMUNITIES
Marine biogeochemical processes are closely linked to phytoplankton community assemblages. Cell abundance and biomass are a measure of the successful conversion of inorganic to organic carbon. Carbon estimates are therefore often used to analyse metabolism and energy transfers within marine environments, and carbon is frequently the main parameter used in ecosystem models. Phytoplankton can be divided into functional types based on cell size: microplankton (<200 μm), nanoplankton (2-20 μm) and picoplankton (≤ 3 μm). Differences in cell volume govern variations in carbon content, nutrient uptake and influence cell fate. Reduced diameters equate to lower sedimentation rates and promote participation within the microbial loop and recycling of carbon within surface waters. Larger diameters can increase settling rates, resulting in the loss of carbon from surface waters. Current North Sea monitoring and research programmes typically only consider larger micro- and nanoplankton cells, or the bulk phytoplankton community as a whole: there is little separation by functional type. Inclusion of picoplankton and the delineation of biomass contribution by cell size are required for accurate depictions of phytoplankton productivity within this region, but this is not feasible with current water sampling protocols. Flow cytometry is a new multiparametric analysis technique offering high-speed enumeration and assessment of particles. Phytoplankton cells from 2-200 μm can be easily distinguished from debris and reproducible data on cell size and pigment content is supplied within minutes. This research uses flow cytometry to provide detailed assessments of phytoplankton community structure at a range of spatial and temporal scales. Distribution patterns are related to environmental parameters and observed patterns are used to test existing paradigm and advance current ecological theory.
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