Phytoplankton community indicators of changes associated with dredging in the Tagus estuary (Portugal)

This work reports changes in suspended particulate matter, turbidity, dissolved Cr, Ni, Cu, Cd, Hg and Pb concentrations, and phytoplankton biomass and composition during a 5-month period dredging operation, in a trace element contaminated area of the Tagus estuary (Portugal). Phytoplankton biomass, diatom:other groups ratio, benthic:pelagic diatom ratio, Margalef's, Simpson's diversity, Shannon-Wiever's, and Warwick and Clarke's taxonomic diversity and distinctness indices, and individual taxa were investigated as indicators of dredging induced changes. Significant rise in sediment resuspension and trace element mobilisation caused by dredging influenced the community structure but not the overall biomass. Benthic diatom displacement into the water column maintained species diversity, and therefore, none of the indices highlighted community changes. Contrastingly, diatom:other groups ratio and benthic:pelagic diatom ratio were reliable indicators for the assessment of dredging induced changes. A shift in composition towards species less susceptible to trace elements was observed, disclosing some individual taxa as potential indicators.