NEW PRODUCTION AND PHOTOSYNTHETIC RATES WITHIN AND OUTSIDE A CYCLONICMESOSCALE EDDY IN THE NORTH PACIFIC SUBTROPICAL GYRE

Photosynthetic rates, autotrophic nitrogen assimilation, and the uptak e and assimilation of nitrate were measured at three stations in the c enter of, near the edge of, and outside of a cyclonic mesoscale eddy n ear 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 nitra te in incubated samples. Nitrate assimilation was calculated from the incorporation into particulate matter of N-15 derived from (KNO3)-N-15 . Ambient nitrate concentrations were about 8 nM, and uptake of nitrat e was undetectable above the nitracline. Addition of 55 nM (KNO3)-N-15 spikes to samples taken above the nitracline stimulated an uptake and assimilation of nitrate which averaged 20-24 nM day(-1). The stimulat ed nitrate uptake occurred at comparable rates during the day and nigh t, but almost all stimulated assimilation occurred at night. Much of t he stimulated nitrate uptake which occurred during the day was apparen tly excreted in a reduced form, most but not all of which was assimila ted at night. In deeper samples that contained greater than 350 nM nit rate, almost all assimilation of nitrate occurred during the day. Nitr ate uptake outside the eddy and the upward diffusion of nitrate outsid e 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 (chi a) concentrations were about 32% higher at the center of the eddy than outside. The differences in photosynthetic rates between the thr ee stations could be largely explained by the differences in relative growth rates and chi a biomass. In samples taken from below the top of the nitracline, dark assimilation of C-14 increased calculated photos ynthetic rates by 25-35%, the percentage increase being positively cor related with nitrate concentration. No comparable dark assimilation wa s observed in samples taken from above the nitracline. This dark assim ilation may reflect the release and subsequent uptake of carbon assimi lated by phytoplankton exposed to recently upwelled water. Copyright ( C) 1996 Elsevier Science Ltd.