Water column dynamics of Vibrio in relation to phytoplankton community composition and environmental conditions in a tropical coastal area

Vibrio abundance generally displays seasonal patterns. In temperate coastal areas, temperature and salinity influence Vibrio growth, whereas in tropical areas this pattern is not obvious. The present study assessed the dynamics of Vibrio in the Arabian Sea, 1-2 km off Mangalore on the south-west coast of India, during temporally separated periods. The two sampling periods were signified by oligotrophic conditions, and stable temperatures and salinity. Vibrio abundance was estimated by culture-independent techniques in relation to phytoplankton community composition and environmental variables. The results showed that the Vibrio density during December 2007 was 10- to 100-fold higher compared with the February-March 2008 period. High Vibrio abundance in December coincided with a diatom-dominated phytoplankton assemblage. A partial least squares (PLS) regression model indicated that diatom biomass was the primary predictor variable. Low nutrient levels suggested high water column turnover rate, which bacteria compensated for by using organic molecules leaking from phytoplankton. The abundance of potential Vibrio predators was low during both sampling periods; therefore it is suggested that resource supply from primary producers is more important than top-down control by predators.