Meso-scale spatial variations in bacterial abundance and production associated with surface water convergence and divergence in the northeast equatorial Pacific

Abstract

To elucidate the effect of the water column structure associated with surface water convergence and divergence on bacterial parameters, we investigated the meso-scale spatial variations in bacterial abundance and production in the northeast equatorial Pacific from 5 to 12°N (longitude 131°30'W). The area is characterized by 3 major surface currents: the South Equatorial Current (SEC), North Equatorial Counter Current (NECC), and North Equatorial Current (NEC). Surface water convergence occurred at the boundary of the westward-flowing SEC and eastward-flowing NECC, and divergence occurred at the boundary of the NECC and westward NEC. A vertically well-mixed water column observed in the SEC/NECC area appeared to be high in nutrients and chlorophyll a (chl a), whereas the strongly stratified NEC area had a relatively low trophic status, with a surface layer depleted of nitrates and chl a. Accordingly, bacterial abundance, production, and mean biomass turnover rates, as well as grazing by heterotrophic nanoflagellates (HNF) in the well-mixed SEC/NECC area were consistently higher than in the highly stratified NEC area. The greater bacterial production and HNF grazing in the SEC/NECC region imply that heterotrophic bacteria are a significant trophic link between dissolved organic carbon and higher trophic levels in the microbial loop of the SEC/NECC area. Our results indicate that changes in the water column structure in relation to convergence and divergence play a key role in enhancing the amount of nutrients and chl a, thereby stimulating bacterial abundance and production and intensifying the trophic link between heterotrophic bacteria and heterotrophic protists in the microbial food web in the study area.