Early spring bloom phytoplankton-nutrient dynamics at the Celtic Sea ShelfEdge

Phytoplankton production was measured at the shelf edge region of the Celti c Sea in April/May 1994 at the beginning of the spring bloom. Size fraction ated C-14 uptake experiments showed that phytoplankton >2 mu m dominated th e bloom although, in the period immediately before the increase in phytopla nkton biomass, picophytoplankton(< 2 mu m) was responsible for up to 42% of the production; in these late winter conditions, chlorophyll concentration s were generally < 0.7 mu g l(-1) and primary production was ca. 70 mmol C m(-2) d(-1). As the spring bloom developed, phytoplankton production rates of 120 mmol C m(-2) d(-1) were measured. Chlorophyll concentration increase d to >2 mu g l(-1) as a result of growth of larger phytoplankton, including diatoms, with large numbers of Nitzschia, Thalassionema and Chaetoceros do minating the assemblage. Picophytoplankton production declined as the sprin g bloom progressed. Nutrient concentrations were not depleted during the sa mpling period, and NO3- concentrations were > 6 mu mol l(-1). Nutrient assi milation rates were measured at the same time as primary production was est imated. Before the development of any substantial phytoplankton biomass, th e uptake rates for ammonium and nitrate were very similar, with S-ratios ra nging from 0.5 to 0.6. Assimilation of ammonium remained relatively constan t after the onset of stratification and bloom development, but nitrate upta ke increased by a factor of 2 or more, resulting in f-ratios >0.8. There wa s significant phosphate uptake in the dark, which was generally ca. 50% of the rate in the light. The C:N:P assimilation ratios changed as the bloom d eveloped; in the pre-bloom situation, when small phytoplankton cells domina ted the assemblage, the C:N assimilation ratio was variable, with some stat ions having ratios less than (ca 2.5), and some higher than (ca. 9), the Re dfield ratio. The most actively growing assemblages had N:P ratios close to the Redfield ratio, but the C:N ratios were consistently lower. New produc tion was found to be closely correlated with the size of the species making up the phytoplankton assemblage, and high f ratios were measured when larg er phytoplankton dominated the assemblage. (C) 1999 Elsevier Science Ltd. A ll rights reserved.