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

Results of parallel photosynthesis and respiration measurements in stratified, frontal and mixed shelf waters using I4CO, and oxygen methods are described. Particulate I4C fixation appeared to give an underestimate of gross photosynthesis. Estimates of net photosynthetic rates are compared with observed microheterotroph and calculated (from appropriate carbon biomass data) mesozooplankton respiration rates. In each of the 3 systems microheterotrophs were the major energy consumers. In surface stratified waters, turnover of plankton carbon was rapid compared to that in the light-limited, mixed water system and, under certain conditions, the phytoplankton may obtain > 50 % of their nitrogen requirement from ammonium 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.