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 (1 59 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 compo sition), were distinguished during spring 1992, each corresponding, re spectively, to a bloom of Thalassiosira sp. and Skeletonema costatum d uring early spring (April), Rhizosolenia sp. during mid-spring (May) a nd Chaetoceros sociale during late spring (June). During each period t he production of biogenic silica (the mean rate of spring biogenic sil ica production was 13 mmol Si m-2 d-1), derived from C-14 primary prod uction 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 fro m the sediments. From this comparison, it is concluded that (1) the ea rly spring diatom bloom was mainly sustained by silicic acid from the watershed, (2) recycling of silicic acid within the water column playe d 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) bal ance the Si burial in sediments (net sink), and the contribution of th e sediment to the silica production equals that of the watershed. The factors that govern the quantitative and qualitative variations of phy toplankton blooms during these periods are discussed. In these nitrate -rich coastal waters, support is given to the hypothesis of Si-limitat ion of the diatom growth, at least during the early spring period when inputs of silicic acid from the watershed represented the major contr ibution to the silica production.