Environmental factors affecting bacterioplankton and phytoplankton dynamics in confined Mediterranean salt marshes (NE Spain)

Environmental factors accountable for bacterioplankton or phytoplankton biomass dominance were analysed in a confined Mediterranean salt marsh (Empordà Wetlands, NE Spain). Two basins located in the same salt marsh, and with differences in size and catchment's area were compared, during four characteristic situations of the hydroperiod. Since bacterio- or phytoplankton relationships may be affected by other factors such as diel variations or vertical differences in nutrient composition and distribution, high frequency fluctuations due to these factors were also taken into account. Differences in catchment area appeared to be the more plausible explanation of differences in nutrient and organic carbon accumulation among basins, since during confinement basins essentially accumulate the allochthonous nutrient and organic matter supplies that previously entered by runoff. DOC (Dissolved Organic Carbon) favoured the bacterioplankton biomass increase, but also was the main variable significantly affecting phytoplankton biomass. Basins showed marked differences in bacterio- and phytoplankton dominances. Relationships between phytoplankton and bacterioplankton were positive, negative or not significant, depending on the basin and on the period of the year. The phytoplankton mixotrophic capabilities, both phagotrophy and osmotrophy, and their production of UV-screening compounds, as sunscreen, may explain the significant correlation between DOC and phytoplankton biomass, and the significant effect of phytoplankton on bacterioplankton found in these ecosystems.