Changes in mesozooplankton size structure along a trophic gradient in the California Current Ecosystem and implications for small pelagic fish

Abstract

Stocks of sardine and anchovy inhabiting eastern boundary upwelling systems of the world's oceans have exhibited large fluctuations in population size, often with peaks in biomass of one taxon alternating with that of the other over multidecadal periods. One hypothesis offered to explain such variability attributes changes in population growth to distinctions in the optimal size of the fishes' planktonic prey. However, the factors affecting size structure in mesozooplankton assemblages are poorly understood. Here, plankter sizes and concentrations were measured for samples collected across a trophic gradient in the California Current Ecosystem with coincident measures of nutrient concentrations. There was a clear distinction between mesozooplankter sizes in samples from oligotrophic and eutrophic waters, with the relative abundances of large individuals being greater in areas where upwelling conditions enhanced nutrient availability and increased abundances of large phytoplankters. The relative contributions of small zooplankters were greater in oligotrophic waters. In light of the observed variability in the biomasses and size structures of phytoplankton and zooplankton assemblages, the potential growth rates of sardine and anchovy are estimated using previously established models of ingestion, absorption, excretion, and respiration. These bioenergetic models suggest that the potential for anchovy growth is limited to nearshore, eutrophic waters where large zooplankters are abundant. In contrast, growth of sardine is possible under more oligotrophic conditions and is influenced by oceanographic conditions in the offshore region of the ecosystem.