Photosynthesis, respiration and nitrogen supply of plankton populations in stratified, frontal and tidally mixed shelf waters

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Abstract
Results of parallel photosynthesis and respiration measurements in stratified, frontal and mixed shelf waters using 14CO2 and O2 methods are described. Particulate 14C fixation appeared to give an underestimate of gross photosynthesis. Estimates of net photosynthetic rates are compared with observed microheterotroph and calculated (from appropriate C biomass data) mesozooplankton respiration rates. In each of the 3 systems microheterotrophs were the major energy consumers. In surface stratified waters, turnover of plankton C was rapid compared to that in the light-limited, mixed water system and, under certain conditions, the phytoplankton may obtain > 50% of thier N requirement from NH excretion by zooplankton. Development of dinoflagellate blooms in the frontal region could be explained in terms of high rates of nitrate assimilation at the base of the thermocline, upward movement of the cells and low grazing mortality.