BIOLOGICAL EFFECTS OF GULF-STREAM MEANDERING

A modeling study was conducted to examine the effects of time-dependent mesoscale meandering of a jet on nutrient-phytoplankton-zooplankton (NPZ) dynamics. The jet was represented as a quasi-geostrophic flow using the method of contour dynamics. Two cases for biology were examined: 1) plankton in a mixed layer of fixed depth and 2) plankton at the base of a mixed layer (i.e., pycnocline) of variable depth. When the mixed layer depth is fixed, nutrient upwelling and dilution of the phytoplankton and zooplankton populations occur along the northward branch of the meander. The additional nutrients and reduced grazing pressure leads to significant enhancement (10-20%) of the phytoplankton production and biomass, while the zooplankton biomass decreases similarly. For plankton on a material surface of variable depth, phytoplankton growth in the pycnocline is increased by the higher light levels encountered during along-isopycnal upwelling. The nutrients decrease and the zooplankton mass in the pycnocline increases by a small amount downstream of the phytoplankton peak. Although the biological enhancements found are not large, the results suggest that vertical motions resulting from mesoscale oceanographic features such as jet meanders and mid-ocean eddies can be an important source of new nutrients for oceanic plankton production.