Contrasting patterns of phytoplankton viability in the subtropical NE Atlantic Ocean

Abstract

The proportion of living phytoplankton cells (% LC) was quantified in natural com- munities along 2 transects of the subtropical NE Atlantic. The region is characterized by the domi- nance of nano- and micro-phytoplankton in the coastal NW African upwelling zone and pico- phytoplankton in the oligotrophic open ocean. Among the pico-cyanobacteria, Synechococcus sp. % LC (40 to 100% LC) was generally higher than that observed for Prochlorococcus sp. (20 to 90% LC). The % LC values of both Prochlorococcus sp. and Synechococcus sp. were low at the oli- gotrophic open ocean stations and decreased towards the coastal upwelling area. Both Prochlorococ- cus sp. and Synechococcus sp. % LC increased in waters in which temperatures reached >21°C. The vertical variability of Prochlorococcus sp. % LC was related to the underwater visible and ultraviolet radiation, showing lower viability in the surface waters. The % LC of nano- and micro-phytoplankton varied with the trophic conditions of the waters, showing lowest values (18.6 and 11.8% LC, respec- tively) in the oligotrophic open ocean and highest values (98.9 and 80.0% LC, respectively) in the coastal upwelling area. However, both nano- and micro-phytoplankton viability decreased as the water temperature increased. This pattern contrasted strongly with that found for pico-cyanobac- teria, indicating that the factors stressing the pico-sized and larger phytoplankton cells in the region were different. As a result of the % LC distribution of the different phytoplankton communities, the highest level of phytoplankton cell death was found in the oligotrophic area, while larger proportions of healthy, living cells were generally found in communities in the upwelling zone. The results identified cell death as an important factor structuring phytoplankton communities in the ocean.