Limiting nutrients of oyster pond seawaters in the Marennes-Oleron region for Haslea ostrearia: applications to the mass production of the diatom in mesocosm experiments

Bioassays were carried out with the 'blue diatom' Haslea ostrearia Simonsen , which is responsible for oyster greening during the fattening period of C rassostrea gigas Thunberg in oyster ponds. Samples of seawater were taken f rom two oyster ponds: one without oysters and the other with 20 oysters per m(2), maximal density allowed by the French AFNOR norm for 'refinement'. T he aims were to clarify the nutrient requirements of this diatom, also to e lucidate the eventual influence of C, gigas at this density on the seawater fertility and to envisage the mass production of this diatom by pond ferti lization. Examination of cell numeric densities at the end of bioassays all ows us to conclude that silicate was the first limiting nutrient, closely f ollowed by phosphate. Chlorophyll a concentrations led to different conclus ions: phosphate was the first limiting factor, but after the seawater stora ge period in ponds, seawater quality evolved to a deficiency of nitrogen. S ilicate addition increased cell division rate, and silicate depletion incre ased chi a synthesis for this species. Examination of nutrient assimilation ratios confirms that H. ostrearia requires a large amount of silicon. From these results, it was possible to prepare a N + P + Si simplified medium w hich has been tested in laboratory and field mesocosm conditions. In both c onditions, similar results were observed: a significant increase in H. ostr earia cell concentrations and consequently an evolution up to the greening stage. Applications of this work are numerous; the principal permits us to envisage the production of this species in 25-m(3) ponds, with the aim of a llowing constant production of the greening phenomenon. (C) 1999 Ifremer/Cn rs/Inra/Ird/Cemagref/Edition scientifiques et medicales Elsevier SAS.