Contrast in Biological Responses to Tidally-induced Vertical Mixing for Two Macrotidal Ecosystems of Western Europe

The dynamics of phytoplankton blooms were studied during spring 1992 in two typical coastal ecosystems of Western Europe, which differ in the influence of river discharges on the vertical stratification of the water column. The Bay of Brest is a semi-enclosed ecosystem, which is connected with the adjacent ocean and entered by two nutrient-rich rivers. The Western English Channel is an open ocean situation and the studied area was remote from any significant riverine influence during spring. Both areas are macrotidal environments and are usually considered as well-mixed. The beginning of the annual diatom bloom is delayed until late May in the Channel, in comparison with the Bay of Brest where the bloom starts by early April. Water column stability induced by freshwater runoff, and local topography are responsible for the earlier start of the phytoplankton bloom in the Bay of Brest. In both areas, the spring period is marked by a succession of diatom blooms which is strongly dependent upon the spring–neap tidal cycle. However, blooms develop under opposite mixing regimes: they occur during neap tides in the Bay of Brest and during spring tides in the Channel. In the Channel where light is not a limiting factor, increased mixing during spring tides enables nutrient replenishment from the water–sediment interface and phytoplankton responds immediately after nutrients have been renewed in the water column. In the more turbid waters of the Bay of Brest, relaxation of vertical mixing during neap tides is required before phytoplankton is able to utilize nutrients originating from freshwater inputs orin situregeneration later in the season.