Phytoplankton spring bloom in the Tagus coastal waters: hydrological and chemical conditions

Abstract

Spatial variability of phytoplankton as well as hydrological and chemical conditions in the Tagus coastal waters were studied during the spring of 1994. The highly patchy distribution of phytoplankton and the community structure were related to the specific abiotic conditions prevailing in the area. Two main water masses were distinguished: a tidally mixed one alongshore Lisbon-cape Espichel and a stratified region strongly related to the Tagus plume river. The latter seems to control, to some extent, the spatial phytoplankton variability and the timing and size of the spring bloom. The highest surficial levels of nutrients, specifically PO43−=1.00 μM, NO3−=16.0 μM and Si(OH)4=14.6 μM, were clearly associated with Tagus river discharges, while the lowest levels were attained offshore in the zone of maximum chlorophyll a concentrations (30 mg m−3) reflecting the occurrence of high nutrient consumption. Furthermore, the near depletion of PO43− indicates P limitation for phytoplankton growth. The diatom Detonula pumila (0.1×106 cells l−1) was the dominant species of phytoplankton communities in the mixed zone, alongshore Lisbon-cape Espichel, whereas, the small diatom Thalassiosira sp. (3×106 cells l−1) was blooming offshore, associated with the stratified region.