A spatio-temporal survey of some echinoderm larvae was conducted along a transect of 28 nautical miles from the Bay of Villefranche, France, halfway to Corsica. Distribution patterns and mechanisms responsible for larval dispersal, viz. water currents, hydroclimate and vertical movements, were studied. The echinoderm larvae exhibited weak vertlcal migration (a few meters), remaining in the surface layer. They are therefore subjected to hydrodynamic constraints which either limit or promote dispersal away from the nursery beds. Larval concentration decreased with distance from the coast and was generally restricted to within 16 miles of the coast This distance coincided with the location of a coastal divergence zone where rising subsurface waters confined larvae to the Ligurian Current.

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We examined changes in carbon fluxes in the marine unicellular diazotrophic cyanobacterium (UCYN) Crocosphaera watsonii in response to irradiance. We estimated changes in the total organic carbon and nitrogen contents, incorporation of carbon into reserves, and exudation processes of extracellular exopolymeric substances (EPS), as well as in the formation of transparent exopolymeric particles (TEP). Variability of cellular carbon pools and extracellular EPS and TEP under conditions of different irradiance and the significance of carbon release in the environment is discussed. Our results show a relationship between growth rate, metabolic processes of carbon incorporation in cells, exudation and irradiance. When growth is light-limited, both carbon production and exudation increase with light. No inhibition occurs at growth-saturating irradiances and half of the total particulate organic carbon is then found as intracellular carbohydrates. While the overall growth becomes light saturated, carbon fixation is further stimulated by higher irradiances, leading to an increased EPS exudation. We found that EPS exudation in C. watsonii is not only a way for cells to release excess carbon, but also occurs when growth is light-limited, even at the lowest irradiance tested. The formation of TEP increases with irradiance, with TEPcarbon amounting to up to 30% of the total particulate organic carbon. Our data will help to quantify the role of unicellular diazotrophs in the export of both carbon and nitrogen through and below the euphotic zone in the marine environment.

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