Numerous studies have reported sensitivities of diatom species to selected environmental variables in the field, but ignored complex interactions between these variables as well as the role of community structure. The aim of the present study was, therefore, to investigate and verify the joint effects of ecological and ecotoxicological variables on benthic diatom communities from two lowland river subsystems, differing in the degree of Zn and Cd contamination mainly. Seasonal trends and resilience of communities were studied by translocating intact communities. Bray-Curtis similarity values were used to discriminate diatom communities responses to metals and other factors, showing that succession of diatom consortia in the field was indeed steered by metal contamination. Intraspecific differences in metal adaptation due to chronic exposure in the field were detected in strains of the diatom species Gomphonema parvulum, isolated from the reference and the polluted stream. However, it seemed that metal tolerance was not the only selective factor, and other ecological variables may also have influenced the composition of the microphytobenthic communities. Interaction between structuring factors such as community development, trophic state and metal contamination were further analyzed in the laboratory. Community changes in the laboratory were evaluated using trophic indices for assessment of water quality based on field derived sensitivities.
The successional trends in experiments matched in general the observed difference in microphyte communities at the reference and metal polluted river stations. However, a simple interaction scheme of the three main factors (Zn, Cd and P) appeared to be insufficient to explain the species' role in detail. The developmental stage of biofilms combined with the different life-traits(i.e. growth form, position) of the species are suggested to be the major cause of complex, non-linear responses of multispecies biofilms even to simple factor combinations.

e-mail:castella@science.uva.nl
online Ph.D. dissertation and list of publications:
http://www.bio.uva.nl/onderzoek/aot/