Pelagic bacteria and phytoplankton in oceanic waters near the Canary Islands in summer

Abstract

Bacterial abundance, biomass and heterotrophic activity along with hydrographic and phytoplankton variables were measured in waters surrounding the Canary Islands during Cruise BIOCAN-98 in September 1998. Three distinct oceanographic zones, defined using temperature and phytoplankton biomass concentrations, were studied. Zone A, located NW of La Palma Island, was characterized by the presence of alternating cold and warm eddies, and generally low phytoplankton biomass. Zone B was located between La Palma and Tenerife islands and showed a predominance of cold-core eddies and higher phytoplankton concentrations than Zone A. All stations occupied in the eastern part of the Canary Islands were grouped in Zone C and were characterized by relatively low surface temperature (<22°C) and high phytoplankton biomass. The vertical structure of the upper water column in these zones was indicative of relatively low mixing in stations of Zones A and B, while stations of Zone C displayed characteristics of frequent mixing events. Phytoplankton biomass showed deep relative maxima between 40 and 100 m at all stations. These maxima were shallower and more marked at stations with colder waters near the surface. Coccoid cyanobacteria were significantly correlated with chlorophyll a and reached abundances of up to 104 cells ml-1 in subsurface maxima. Heterotrophic bacteria were generally more abundant near the surface, and varied between 1.1 and 5.9 × 105 cells ml-1 in the upper 100 m. Bacterial abundance between 100 and 300 m was approximately constant. Bacterial production, estimated by leucine incorporation, was not correlated with bacterial abundance or chlorophyll a, and displayed relative maxima between 80 and 90 m depth but also near the surface. There were no significant differences in mean values Of abundance of cyanobacteria and heterotrophic bacteria between the oceanographic zones considered, but bacterial production and growth rates were significantly higher in Zone B. Using empirical conversion factors determined during the study, we estimated bacterial production values integrated in the upper 100 m that varied between 15 and 289 mg C m-2 d-1, with population generation times between 2 and 25 d. Taking into account phytoplankton biomass and primary production values, our results suggest that pelagic bacteria are able to consume dissolved organic carbon in excess of the amount produced in situ by phytoplankton in the upper surface waters of this region.