CFU bacterial fraction in the estuarine upwelling ecosystem of Ria de Vigo, Spain: variability in abundance and their ecophysiological description

Between October 1990 and August 1991 colony-forming unit (CFU) bacteria at the centre of the Ria de Vigo, Spain, comprised on average 1% of total bact erioplankton, with a maximum of 6.8% in mid-April. In contrast to relativel y small fluctuations in total bacterioplankton, fluctuations in CFUs were c onsiderable, with the coefficient of variation ranging from 112 to 233%. Ov er 40% of the variance in CFU abundance in the surface layer during the who le period of observations (winter and summer) could be explained through an equation relating logCFU to incoming solar radiation, and during the summe r months through another equation relating logCFU to upwelling. Both physic al factors enhanced CFU concentrations. Biological variables, such as total bacterioplankton, chlorophyll and heterotrophic flagellates, had no furthe r effect on the explained variance. At the bottom of the photic zone, runof f and upwelling accounted for between 29 and 50% of the variance in CFU cou nts. In this layer, biological variables had a great influence on CFU abund ance, increasing the explained variance to 61% for the whole period studied . In this case, chlorophyll was negatively related to logCFU suggesting tha t the CFU bacterial fraction was more abundant in the water column after th e decline of the phytoplankton blooms and after the corresponding release o f bacteria attached to particles. At the bottom of the water column, seawat er temperature was the only important factor in explaining the variance in CFU counts (36%), with logCFU and temperature being positively related. We hypothesise that 3 factors are prominent in controlling the CFU standing st ock: solar radiation and upwelling stimulate the synthesis of particulate o rganic matter in surface waters, providing a food source for CFU bacteria, and terrestrial runoff which dilutes bacterial numbers. Based on cluster an alyses of physiological tests, on pure bacterial isolates from 2 contrastin g sampling dates, we show that prevailing hydro- and meteorological conditi ons select for different bacterial species in the CFU community. Lower CFU counts and diversity within the population, plus lower frequencies of posit ive responses in physiological tests arose through very high upwelling and NW winds which introduced CFU-poor water masses from the Atlantic Ocean int o the Ria de Vigo. Conversely, terrestrial runoff and NE winds enhanced div ersity within this population at the centre of the estuary, through the int roduction of terrestrial bacteria or those from shallow waters; these were characterised by higher numbers of positive responses in physiological test s.