GLOBEC: Northeast Pacific Study: Mesoscale Zooplankton Distribution and Productivity

University Of Minnesota-Twin Cities, Minneapolis MN

Investigators

Abstract

This project addresses one of the three central hypotheses of the U.S. GLOBEC Northeast Pacific Study: "Spatial and temporal variability in mesoscale circulation constitutes the dominant physical forcing on zooplankton biomass, production, distribution, species interactions and retention and loss in coastal regions." The PIs respond to specific components of the GLOBEC announcement which call for (a) three dimensional mesoscale surveys aimed at determining the distribution and productivity of zooplankton in relation to their physical environment, and (b) process studies focused on understanding zooplankton in situ population dynamics processes and the interaction between physical and biological processes. It is not well understood how mesoscale features in the California Current System impact zooplankton biomass, production, distribution, or retention and loss from coastal regions. These interacting phenomena have been studied rarely off California, and even more rarely off Oregon. Mesoscale physical dynamics are relatively easy to measure, but require a high sampling frequency. Most standard methods of measuring zooplankton are simply not compatible with the sampling frequency required to resolve mesoscale features. This fundamental challenge must be met for GLOBEC to succeed. The group of investigators has invested a decade in the development and application of an integrated methodology for measuring zooplankton biomass, distribution and productivity at the high resolution required for mesoscale studies. They find significant mesoscale interactions between physical forcing and zooplankton productivity in the California Current System off northern California. In a 1993 pilot study, they found net decreasing productivity in the central jet of the California Current, and net increasing productivity in adjacent eddy systems. The mesoscale match of physical biological processes was striking - but underlying causes remain obscure. The GLOBEC NEP study offers an unparalleled opportunity to understand these dynamics upstream - off Oregon - and so to shed light on fundamental processes. The investigators will study both physical and biological processes, and their spatio-temporal coupling, by measuring and mathematically deducing individual processes of advection, vertical migration, and rates of productivity. They will use the integrated Sea-Soar-Optical Plankton Counter and ADCP in mesoscale surveys, coupled with critical net sampling, to resolve spatial and temporal distributions of size- and species-structured zooplankton at the mesoscale. Rates pertinent to population dynamics will be determined from the biomass spectral method (Zhou and Huntley, 1997) in conjunction with certain complementary and independent field measurements. The approach is aimed at producing estimations of biomass and productivity - resolved at the same scale as mesoscale physics - and will clarify factors controlling variations in zooplankton productivity. These results are not only critical to defining the "food field" of juvenile salmon (a key GLOBEC NEP target species), but can improve our general understanding of interactions between zooplankton populations and their dynamic physical environment, with is the central goal of GLOBEC.