Zooplankton biomass, advection and production on the northern Gulf of Alaska shelf from simulations and field observations

Abstract

Interpretation of zooplankton field observations on the Gulf of Alaska (GOA) shelf is complicated by the complex physical environment of currents, eddies and meanders which mix oceanic and coastal water masses to varying degrees. We therefore developed a lower trophic level model embedded in an ocean circulation model to examine the effects of wind, runoff and temperature on transport, biomass and production of microzooplankton and copepods on the northern GOA shelf and adjacent ocean. Mean microzooplankton biomass varied from 10–90 and 10–50mgCm−3 in the upper 50m for field measurements and simulations respectively. Mean simulated and measured copepod biomass in the upper 100m was about 1–40gCm−3. Field data indicated that Neocalanus biomass was negatively correlated with salinity in May. Simulations indicated that the negative correlation was due to elevated primary and secondary production on the shelf relative to the adjacent ocean. Floats programmed to simulate the ontogenetic vertical migrations of Neocalanus indicated that ~10 to 50% of Neocalanus originating near the shelf break spend at least some time on the shelf, depending on location. Residence time of floats on the shelf was affected primarily by wind, secondarily by runoff. Primary and secondary production in the simulations were highly correlated (r>0.9). Simulated secondary production was 10–90gCm−2y−1, with highest values in Lower Cook Inlet, around Kodiak and west of Kodiak.