Tidal effects on nutrients and phytoplankton distribution in Bertioga Channel, São Paulo, Brazil

Abstract

Abstract Dissolved nutrients and phytoplankton distribution in the Bertioga Channel, São Paulo State, were studied over a neap and a spring tide during the austral winter of 1991. Tidal influence and freshwater flow were the main forcing agents on the water column structure, nutrient availability and phytoplankton distribution as pointed out by the principal component analysis. The channel was vertically stratified during neap tide, but almost fully homogeneous during spring tide, especially in the flood phase. The inner area of the channel had high nutrient concentrations (up to 25 μM ammonium) and low dissolved oxygen saturation (minimum 20%). Phytoplankton biomass, measured as chlorophyll-a concentration, was low (maximum, 4.5 mg m−3) considering the high nutrient availability. The highest chlorophyll-a levels were associated with waters of coastal origin and flood periods. The phytoplankton community was dominated by phytoflagellates but the contribution of diatoms became very significant during spring tide. The major microphytoplankton forms were Skeletonema costatum and Pseudonitzschia species. The noticeable presence of freshwater species (Pinnularia, Synedra and Scenedesmus species), indicated the important role of freshwater inflow in the composition of local phytoplankton community. Data suggest that the high flushing rates and hydrodynamic instability at Bertioga Channel accounted for the low phytoplankton biomass observed in the environment.