Algal assay of relative abundance of phytoplankton nutrients in northeast United States coastal and shelf waters

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Regulation of phytoplankton growth by chemical water quality, in the coastal and shelf region between Cape Hatteras and the Gulf of Maine, was assessed by enrichment bioassay. Water sample collections, made in five surveys over an annual cycle, were assayed in the laboratory. The assay results, based on growth of a centric diatom, Thalassiosira pseudonana (clone 3H) Hasle and Heimdal, suggest that nitrogen, potentially, was the most important growth-limiting nutrient region-wide and year-round. The results also suggest that phosphorus had considerable importance and that silicon and vitamin B 12 occasionally were limiting. The potential regulatory importance of phosphorus apparently was less consistent, temporally and spatially, than that of nitrogen. Phosphorus limitation was detected primarily in just two of the collections, from March and July surveys. In the March collection it was evident predominantly in the inshore samples, but in the July collection its incidence was approximately the same among samples from inner, mid and outer shelf. Inhibitory water quality was prevalent in two of the collections, particularly in samples from inshore locales. The cause of this inhibition has not been identified but it was not associated with nutrient concentration. It is possible that adverse water quality was an important growth regulator in the region, at least for species having sensitivities analogous to that of the assay organism.