GLOBEC 2000: Responses of the Neocalanus spp. - Microplankton Community to Physical forcing in the Coastal Gulf of Alaska

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Ocean and atmospheric conditions in the coastal Gulf of Alaska vary widely over daily, seasonal, and interannual time scales. The abundance of dominant upper trophic level species, including salmon, has been shown to vary in concert with this environmental change, most notably on decadal time scales. The mechanisms linking these large-scale population shifts with climate are not clear, but may involve lower trophic level responses (i.e. bottom-up effects). Although lower trophic level species are less well studied in the CGOA, preliminary data indicates that the abundance and activity of microplankton populations also respond strongly at least to short-term changes in CGOA physical regimes. Additionally, microzooplankton are likely the dominant consumers of phytoplankton on the shelf, thus consituting a key link between physical forcing of primary production and higher trophic levels. The PIs will examine the processes structuring microplankton communities and link them with populations of Neocalanus spp., the dominant particle-grazing copepods in the coastal Gulf of Alaska. Collectively, 3 species of Neocalanus (N. flemingeri, N. plumchrus, N. cristatus) contribute substantially to total spring and summer mesozooplankton biomass in the CGOA. Neocalanus are capable of consuming both phytoplankton and microzooplankton, though the factors dictating this diet partitioning are not well understood. Furthermore, these copepods have been shown to alter individual body size, population biomass, and life cycle timing in response to variations in ocean conditions. Finally, the size and abundance of Neocalanus make them an important potential prey for pink salmon fry and other coastal fish species. Thus the microplankton - Neocalanus food web is a potential locus for the translation of environmental variation into higher trophic level responses. Specific measurements to be made are: 1) Microplankton abundance and composition (phytoplankton size structure, microzooplankton species and size composition); 2) Rates of microzooplankton herbivory; and 3) Rates of Neocalanus spp. grazing on microzooplankton and phytoplankton. A novel element of the work is the use of the FlowCAM (Flow Cytometer and Microscope) to characterize microplankton abundance and community composition during feeding experiments. This new imaging-in-flow technology should allow us to conduct many more copepod grazing experiments than would be possible with complete reliance on conventional microscopy. By sampling across a range of coastal physical regimes and seasons, this work will test the hypothesis that variation in the physical environment dictates production levels and food web structure in the CGOA, altering the timing, amount and quality of resources available to Neocalanus and ultimately to other higher trophic level species.