Contrasting patterns of phytoplankton viability in the subtropical NE Atlantic ocean

The proportion of living phytoplankton cells (% LC) was quantified in natural communities along 2 transects of the subtropical NE Atlantic. The region is characterized by the dominance 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 oligotrophic open ocean stations and decreased towards the coastal upwelling area. Both Prochlorococcus 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, respectively) 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-cyanobacteria, 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. © Inter-Research 2006.Peer Reviewe

Synechococcus and Prochlorococcus cell death induced by UV radiation and the penetration of lethal UVR in the Mediterranean Sea

11 páginas, 6 figuras, 5 tablas. -- Artículo Open Access.Irradiation experiments performed on natural communities of picocyanobacteria from the southwest Mediterranean Sea indicated that natural levels of solar ultraviolet radiation (UVR: 280 to 400 nm) induced important cell death in Prochlorococcus sp., although Synechococcus sp. appeared to be highly resistant. In the treatments where UVB radiation (UVBR, 280 to 315 nm) was excluded, Prochlorococcus also experienced high cell death with short half-life times of 3.01 ± 0.1 SE (h), showing the contribution of UVA radiation (UVAR, 315 to 400 nm) and photosynthetically active radiation (PAR, 400 to 700 nm) to Prochlorococcus cell death. Underwater radiometric measurements conducted during the cruise indicated that penetration of UVR was significant in the Mediterranean waters studied, with minimum diffuse attenuation coefficients of 0.165 and 0.071 m–1 for 313 and 380 nm, respectively. The lethal UV doses required to decrease the picocyanobacteria populations by half, UVLRD50, calculated experimentally, were related to underwater UVR penetration in the Mediterranean Sea measured during the cruise. By calculating (from incident irradiances and UVR penetration) the daily UV doses at different depths in the water column, we found that, for sunny days, experimental Prochlorococcus LRD50 (187 kJ m–2) could reach from 10 to 26 m depth. For Synechococcus, however, the depth receiving daily UVLRD50 (1375 kJ m–2) was always shallower, above 5 m depth. The differential sensitivity of the 2 genera and the UV transparency of the Mediterranean Sea suggest that solar radiation could be an important factor influencing the dynamics and distribution of cyanobacterial populations in the surface waters of this oligotrophic sea.Este estudio fue financiado por la concesión REN-2002-10606E/MAR para Susana Agustí, financiada por el Ministerio de España de Educación y Cienca, y por la Netherlands Science Foundation,Earth and Life Sciences (NWO-ALW, proyecto n. #812.03.001 para Gerhard J. Herndl), por el proyecto EUROTROPH (Nutrients Cycling and the Trophic Status of Coastal Ecosystems) financiado por el Quinto Programa Marco de la Unión Europea y por el proyecto SESAME financiado por la Unión Europea también. Moira Llabrés fue financiada por el proyecto REN2001-5060E/MAR.Peer reviewe

Seasonality of pico-phytoplankton abundance and cell death in a Mediterranean Bay (Bay of Palma, Majorca Island)

A pico-phytoplankton community, dominated by Synechoccoccus sp., was present through the year in the Bay of Palma, showing high dynamics, with changes across the Bay observed at a scale of a week. The populations of pico-phytoplankton showed high seasonality. Synechococcus sp. abundance was maximal during summer, with an important bloom observed at mid August. Prochlorococcus sp. showed three moderate blooms at spring, summer, and a larger one in winter. Pico-eukaryotes abundance varied seasonally, showing the highest cell concentration by mid April. Despite these differences in cell abundance, the three pico-phytoplanktonic groups showed similar patterns in the percentage of living cells, with peaks in late winter and in early summer, corresponding to the major peaks in nutrient concentration in the Bay. Similarly, the lowest % living cells was observed at late spring, indicative of high pico-phytoplankton cell death when nutrient concentration decreased markedly. The high pico-phytoplankton cell viability of late winter, was followed by moderate increase in cell abundances indicating that factors other than nutrients were controlling pico-phytoplankton populations. Despite the reduced pico-phytoplankton cell death at early summer, only Synechococcus sp. developed a sizeable bloom, indicating that summer conditions were more favourable for Synechococcus sp. as compared to other groups. Cell viability helped to elucidate the role of losses vs. stresses in controlling the population dynamics of pico-phytoplankton communities in the Bay of Palma.Peer Reviewe

Aerosol inputs enhance new production in the subtropical northeast Atlantic

8 pages, 3 figures, 1 table.Atmospheric deposition is an important source of limiting nutrients to the ocean, potentially stimulating oceanic biota. Atmospheric inputs can also deliver important amounts of organic matter, which may fuel heterotrophic activity in the ocean. The effect of atmospheric dry aerosol deposition on the metabolic balance and net production of planktonic communities remains unresolved. Here we report high inputs of aerosol-bound N, Si, P, Fe and organic C to the subtropical NE Atlantic and experimentally demonstrate these inputs to stimulate autotrophic abundance and metabolism far beyond the modest stimulation of heterotrophic processes, thereby enhancing new production. Aerosol dry deposition was threefold to tenfold higher in the coastal ocean than in the oyen ocean, and supplied high average (±SE) inputs of organic C (980 ± 220 μmol C m-2 d-1), total N (280 ± 70 μmol N m-2 d-1), Si (211 + 39 μmol Si m-2 d-1), and labile Fe (1.01 ± 0.19 μmol Fe m-2 d-1), but low amounts of total P (8 ± 1.6 μmol P m-2 d-1) to the region during the study. Experimental aerosol inputs to oceanic planktonic communities from the studied area resulted, at the highest doses applied, in a sharp increase in phytoplankton biomass (sevenfold) and production (tenfold) within 4 days, with the community shifting from a dominance of picocyanobacteria to one of diatoms. In contrast, bacterial abundance and production showed little response. Primary production showed a much greater increase in response to aerosol inputs than community respiration did, so that the P/R ratio increased from around 0.95 in the ambient waters, where communities were close to metabolic balance, to 3.3 at the highest nutrient inputs, indicative of a high excess production and a potential for substantial net CO2 removal by the community in response to aerosol inputs. These results showed that aerosol inputs are major vectors of nutrient and carbon inputs, which can, during high depositional events, enhance new production in the NE subtropical Atlantic Ocean.This research is part of the COCA (REN2000-1471-C02) and RODA (CTM-2004-06842-C03-O2) projects, funded by the Spanish "Plan Nacional de I+D". We thank NOAA Air Resources for access to the HYSPLIT model to generate the air mass back trajectories. We thank the crew of the R/V Hespérides and the technical UTM personnel involved for professional assistance. E A. and A. T. were supported by a fellowship from the Spanish Ministry of Education and Science, and C. M. D. and S. A. were supported by a sabbatical fellowship from the Spanish Ministry of Education and Science.Peer reviewe