PHYTOPLANKTON DYNAMICS IN RELATION TO THE BIOGEOCHEMICAL CYCLE OF SILICON IN A COASTAL ECOSYSTEM OF WESTERN-EUROPE

The Bay of Brest, France, a typical semi-enclosed coastal ecosystem (159 km2) of western Europe, was studied during spring 1992 with respect to the biogeochemical cycle of silicon. Three periods of nutrient and phytoplankton dynamics (chlorophyll a, biogenic silica, species composition), were distinguished during spring 1992, each corresponding, respectively, to a bloom of Thalassiosira sp. and Skeletonema costatum during early spring (April), Rhizosolenia sp. during mid-spring (May) and Chaetoceros sociale during late spring (June). During each period the production of biogenic silica (the mean rate of spring biogenic silica production was 13 mmol Si m-2 d-1), derived from C-14 primary production measurements, size fractionation experiments and appropriate Si:C ratios, has been compared with the sum of the silica acid inputs to the bay originating from rivers, from the adjacent Iroise Sea and from the sediments. From this comparison, it is concluded that (1) the early spring diatom bloom was mainly sustained by silicic acid from the watershed, (2) recycling of silicic acid within the water column played a major role during mid-spring to sustain the bloom of Rhizosolenia sp. and (3) silicic acid recycling at the sediment-water interface was the main contributor to the silica production during the late spring bloom. On a seasonal basis, the riverine inputs of Si (net source) balance the Si burial in sediments (net sink), and the contribution of the sediment to the silica production equals that of the watershed. The factors that govern the quantitative and qualitative variations of phytoplankton blooms during these periods are discussed. In these nitrate-rich coastal waters, support is given to the hypothesis of Si-limitation of the diatom growth, at least during the early spring period when inputs of silicic acid from the watershed represented the major contribution to the silica production.