The marine plankton diatom, Skeletonema costatum, was isolated from local waters and cultured in enriched sea water. The effects of light, temperature, salinity, and concentrations of phosphorus and nitrogen on its growth and photosynthesis were determined separately and simultaneously. The effects of exocrine production by Skeletonema and grazing by copepods also wer e tested . The experimental results were used in developing an index of the photosynthetic potential of the diatom for seasonal hydrographic conditions found in the continental shelf waters south of Long Island....