Phytoplanktonic biomass and environmental parameters were investigated in order to characterize the estuaries of the Maranhense Gulf. The samples had been collected bimonthly at four fixed points, in the period of May/2002 to March/2003 in the surface, in ebb and spring tides. The annual pluviometry during the period was of 1.898,5 mm; the salinity varied from 6 to 35; transparency from 18 to 61 cm; dissolved oxygen from 2,1 to 6,3 ml.L-1; oxygen saturation from 45 to 126%; ammonia from 0,02 to 26,45 μM; nitrite from 0,01 to 0,25 μM; nitrate from 0,24 to 3,53 μM and phosphate of 0,13 to 0,58 μM. The total chlorophyll a from 1,92 to 72,23 mg.m-3 and the fraction of nano and picophytoplankton from 0,86 (15%) to 36,80 mg.m-3 (97%), characterizing this area as quite productive. The principal component analysis (PCA) demonstrated that the chlorophyll a had a direct relationship with the salinity, transparency of the water, speed of the winds, phosphate and nitrate and inverse correlation with the rain, dissolved oxygen, saturation of the oxygen, temperature and ammonia. In the Maranhense Gulf wind, macrotides and the high turbidity are the main causes in the reduction of the euphotic zone and consequently of the chlorophyll a.
phytoplankton; chlorophyll a; estuary; environmental variables
