RELATIONSHIPS BETWEEN THE ABUNDANCE OF BACTERIA AND OTHER BIOTA AND THE HYDROGRAPHIC VARIABILITY IN THE RIA DE VIGO, SPAIN

Hydrographic variability driven by changes in wind speed and direction and by meteorological parameters, such as incoming solar radiation, f luvial discharges, and temperature, affects biological standing stocks (e.g. of bacteria) in the marine environment. We studied the effects of such parameters in the Ria de Vigo (Galicia, Spain), between Octobe r 1990 and August 1991, where downwelling in winter results from predo minantly poleward winds, whilst in summer upwelling is driven by winds travelling towards the equator. Total bacterial abundance was strongl y affected by physical factors in the Ria de Vigo. Almost 60% of the v ariance in bacterial abundance in the surface layer could be explained by an equation relating logTC [log(total count of heterotrophic bacte rioplankton)] to incoming solar radiation, water temperature, run-off and coastal upwelling, where solar radiation, water temperature and mo derate upwelling are all positive terms, while run-off and strong upwe lling act negatively. Consideration of biological variables such chlor ophyll, ciliates and heterotrophic flagellates did not further explain the variance. Physical factors, however, only accounted for 40% of th e total variance in bacterial abundance at the bottom of the photic la yer. The inclusion of biological variables in the regression equations increased the explained variance to 55%; chlorophyll was the most imp ortant of these variables and was positively related to logTC. This in dicates that bacterial populations in the surface layer are mainly con trolled by physical factors with a weak phytoplankton influence; howev er, a stronger coupling between primary production and bacterial growt h must exist at the bottom of the photic layer. Physical variables als o explained 60 to 65% of the variance in cultivable spore-forming sapr ophytic bacteria, but, in contrast to effects on total bacterial numbe rs, run-off had a positive effect while moderate upwelling had a negat ive influence on spore-forming numbers. This suggests that terrestrial run-off has some ecological importance through the enrichment of near shore bacterial populations with cells of terrestrial origin.