Topographical and hydrographical impacts on the structure of microphytoplankton assemblages on the Abrolhos Bank region, Brazil

This study was conducted at the Abrolhos Bank (15°60′-21°30′S; 37°00′-40°30′W), Brazil, in July and August 2007, to evaluate the topographic and hydrographic influences on microphytoplankton composition and relative abundance. Net phytoplankton was collected from the top 200 m of the water column to assess diversity proxies (species richness, Shannon index, dominance and equitability) and compared with thermohaline, nutrient and chlorophyll profiles. A total of 326 taxa occurred in the area. Patterns in spatial distribution of microphytoplankton assemblages were two-fold: a north-south gradient linked to variations in temperature and nitrite, and a coast-offshore gradient associated with the depth of the mixed layer and the Brunt-Väisälä maximum frequency. Microphytoplankton assemblages were typical of tropical oligotrophic environments. However, the inshore community found on the Abrolhos Bank was enriched by bottom dwelling, large-sized cells ressuspended from local sediments as a result of the highly dynamic coastal circulation. Species diversity was high in oceanic sites where water column stability as measured by the Brunt-Väisälä frequency achieved its maxima, but high values of ecological indexes were also found in the southern part of the study area influenced by bottom intrusions of nutrient-rich oceanic waters, giving support to the notion that phytoplankton diversity increases at intermediate levels of environmental disturbance.