Phytoplankton and hydrography of the Kennebec estuary, Maine, USA

The biomass, abundance and species composition of phytoplankton in the Kenn ebec estuary, Maine, USA, were investigated in relation to hydrography and Light regime during 7 seasonal survey cruises. The salinity distribution ra nged from 32 at the mouth to between 0 and 5 at the head, depending on the magnitude of freshwater discharge at the time of each survey. Maximum Verti cal salinity and temperature gradients were observed at the mouth. while lo cal tidal mixing, combined with the freshwater flow, produced a well-mixed water column at the head of the estuary. The middle portion of the estuary was stratified on flooding and ebbing tides, but was vertically well mixed at high and low tides. Phytoplankton biomass was lowest in winter (chloroph yll a approximate to 1 mu g l(-1)) and highest in summer (up to 10 mu g l(- 1)) The phytoplankton species assemblages at the seaward and the riverine e nds of the estuary were made up of taxa with corresponding salinity prefere nces. Both cell numbers and biomass (chlorophyll a) exhibited a bimodal dis tribution along the length of the estuary in the warmer months, with the mi ddle portions of the estuary having depressed phytoplankton standing stocks compared with the seaward and landward ends. This bimodal distribution was related to Light limitation and nutrient regeneration in the middle portio n of the estuary and to the production of and advective contributions of ph ytoplankton from both the freshwater and seaward ends.