Environmental variability in a transitional Mediterranean system (Oliveri–Tindari, Italy): Focusing on the response of microbial activities and prokaryotic abundance

The response of both microbial activities and prokaryotic abundances to environmental variability was studied in a transitional Mediterranean system (Oliveri–Tindari, Italy) during two yearly surveys (1997–'98 and 2005–'06). The total enzymatic (leucine aminopeptidase, β-glucosidase, alkaline phosphatase) and respiratory activity rates as well as of the abundances of total prokaryotes, culturable heterotrophic bacteria, faecal coliforms and enterococci were measured in surface waters of four brackish ponds, together with temperature, salinity, dissolved oxygen, pH, inorganic nutrients, chlorophyll-a and particulate organic carbon and particulate nitrogen determinations. The seasonal and interannual patterns of microbial parameters were investigated in relation to environmental variations.Both the microbial activities and the abundances of culturable heterotrophic bacteria, faecal coliforms and enterococci varied significantly among the ponds, reflecting their haline and trophic variability. Significant seasonal and interannual variations were found in the patterns of microbial activity, but not in total prokaryotic abundance, in response to temperature, dissolved oxygen and trophic changes. Microbial parameters showed a different response to environmental variability in the two examined periods: during 1997–'98, increases in the microbial activity rates and culturable bacterial counts were positively related with the trophic changes, while during 2005–'06 the patterns of leucine aminopeptidase, alkaline phosphatase and respiratory activities and of total prokaryotic and culturable bacteria abundances were uncoupled with those of temperature, salinity, dissolved oxygen, particulate organic carbon and chlorophyll-a. Changes in the relative importance of environmental and trophic variables between the two considered periods explain this discrepancy.