CONTRAST IN BIOLOGICAL RESPONSES TO TIDALLY-INDUCED VERTICAL MIXING FOR 2 MACROTIDAL ECOSYSTEMS OF WESTERN-EUROPE

The dynamics of phytoplankton blooms were studied during spring 1992 i n two typical coastal ecosystems of Western Europe, which differ in th e 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 riv ers. The Western English Channel is an open ocean situation and the st udied area was remote from any significant riverine influence during s pring. Both areas are macrotidal environments and are usually consider ed 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 whe re the bloom starts by early April. Water column stability induced by freshwater runoff, and local topography are responsible for the earlie r 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 Chann el where light is not a limiting factor, increased mixing during sprin g tides enables nutrient replenishment from the water-sediment interfa ce and phytoplankton responds immediately after nutrients have been re newed in the water column. In the more turbid waters of the Bay of Bre st, relaxation of vertical mixing during neap tides is required before phytoplankton is able to utilize nutrients originating from freshwate r inputs or in situ regeneration later in the season. (C) 1996 Academi c Press Limited