New production and photosynthetic rates within and outside a cyclonic mesoscale eddy in the North Pacific subtropical gyre

Abstract

Photosynthetic rates, autotrophic nitrogen assimilation, and the uptake and assimilation of nitrate were measured at three stations in the center of, near the edge of, and outside of a cyclonic mesoscale eddy near the island of Hawaii. Areal photosynthetic rates near the edge and at the center of the eddy were 31% and 66% higher, respectively, than outside the eddy. Nitrate uptake was equated to the drawdown of nitrate in incubated samples. Nitrate assimilation was calculated from the incorporation into particulate matter of 15N derived from K 15NO 3. Ambient nitrate concentrations were about 8 nM, and uptake of nitrate was undetectable above the nitracline. Addition of 55 nM K 15NO 3 spikes to samples taken above the nitracline stimulated an uptake and assimilation of nitrate which averaged 20–24 nM day −1. The stimulated nitrate uptake occurred at comparable rates during the day and night, but almost all stimulated assimilation occurred at night. Much of the stimulated nitrate uptake which occurred during the day was apparently excreted in a reduced form, most but not all of which was assimilated at night. In deeper samples that contained greater than 350 nM nitrate, almost all assimilation of nitrate occurred during the day. Nitrate uptake outside the eddy and the upward diffusion of nitrate outside the eddy were estimated to be 10–15 mg N m −2 day −1. The f-ratio was estimated to be 0.2 outside the eddy and 0.8 at the center of the eddy. Relative growth rates were about 70% outside the eddy and either nutrient-saturated or nearly so within the eddy. Areal chlorophyll a (chl a) concentrations were about 32% higher at the center of the eddy than outside. The differences in photosynthetic rates between the three stations could be largely explained by the differences in relative growth rates and chl a biomass. In samples taken from below the top of the nitracline, dark assimilation of 14C increased calculated photosynthetic rates by 25–35%, the percentage increase being positively correlated with nitrate concentration. No comparable dark assimilation was observed in samples taken from above the nitracline. This dark assimilation may reflect the release and subsequent uptake of carbon assimilated by phytoplankton exposed to recently upwelled water.