Some environmental factors influencing phytoplankton in the Southern Ocean around South Georgia

Data on phytoplankton and zooplankton biomass, and physical and chemical variables, are combined with a published multivariate description of diatom species composition to interpret variation within an area around South Georgia surveyed during an austral summer. Large-scale species distributions could be equated to the different water masses which reflected the interaction of the Antarctic Circumpolar Current with the island and the Scotia Ridge. Small-scale factors were found to act at an interstation scale and imposed local variation on the biogeographic pattern. Nutrient depletion could be related to phytoplankton biomass but no single inorganic nutrient of those measured (NO 3 −N, PO 4 −P and silica) could be identified as important. The ratio Si:P appeared to be more important as an ecological factor. The impact of grazing by krill and other zooplankton could only be resolved as differences in phytoplankton biomass and phaeopigment content. Diatom species composition showed a relation to local krill abundance very different from that suggested by published studies, but could be explained as the effect of earlier grazing outside the study area. The effects of vertical mixing could not account for interstation differences as pycnocline depth was uniformly greater than euphotic depth, and vertical stability very low. Some comparison was made with data collected in 1926–31 by the Discovery Investigations. Significant differences in the distribution of certain taxa such as Chaetoceros criophilum and C. socialis were traced to major differences in hydrology.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/46983/1/300_2004_Article_BF00443379.pd

Elements majeurs des particules en suspension de la Mediterranee occidentale

Chemical analyses of major elements (C, N, P, Si, Al, Fe, Ca, Mg) were performed on more than 500 samples of suspended matter from the western Mediterranean Sea. Four samples collected with a free-floating sediment trap were also analysed. The vertical distribution of particulate organic carbon, POC, reported herein is characteristic of oligotrophic mid-latitude regions. In open waters, the concentration of particulate aluminium is nearly constant. A fraction of particulate silicon Si sub(p) is of terrigenous origin, the rest is biogenic. The biogenic fraction of particulate iron and magnesium is noticeable only in the surface layer, where Fe sub(bio)/POC and Mg sub(bio)/POC ratios are about 10 super(-3). It is not possible to estimate the terrigenous and biogenic fractions of particulate calcium

Consumption and production on scales of a few days of inorganic carbon, nitrate and oxygen by the planktonic community: results of continuous measurements at the Dyfamed Station in the northwestern Mediterranean Sea (May 1995)

Continuous measurements between 0 and 200 m depth were performed every 2 h over two separate periods of four days at a station in the open northwestern Mediterranean Sea (Dyfamed Station) during the Dynaproc cruise in May 1995, Estimates of the daily variations in profiles of temperature, partial pressure of CO,, oxygen, chlorophyll a and nutrients were obtained. The distributions of the various physical and chemical properties were clearly different during the two time series, which were separated by a period of 11 days during which a wind event occurred. The mean daily utilization or production due to biological processes of dissolved inorganic carbon (DIC), nitrate + nitrite and oxygen were calculated along isopycnals using a vertical diffusion model. Between the surface and about 20 m depth, DIC was consumed and 0, released during the two time series while the nitrate + nitrite concentrations as well as supplies were zero, After the wind event, the O-2:C:N ratios of consumption (or production) were, on average, near the Redfield ratios, but during the first time series, the C :N utilization ratio between 20 and 35 m was two to three times that of Redfield stoichiometry and the oxygen release was low. The integrated net community production (NCP) in terms of carbon was equivalent during the two time series, whereas the chlorophyll a biomass was twice as high, on average, during the first time series but did decrease, These results imply that the production systems were different during the two periods. The first time series corresponds toa period at the end of production, due to the nutrient depletion in the euphotic layer. The formation of degradation products of the living material in dissolved organic form is probably important as indicated by the high C:N utilization ratios. The second time series corresponds to a reactivation of the primary production due to the upward shift of nutrients after the wind event. (C) 2000 Elsevier Science Ltd. All rights reserved

Distributions of carbonate properties and oxygen along the water column (0-2000m) in the central part of the NW Mediterranean Sea (Dyfamed site): influence of winter vertical mixing on air-sea CO2 and O-2 exchanges

Monthly measurements of pH, alkalinity and oxygen over two years (February 1998-February 2000) at the Dyfamed site in the central zone of the Ligurian-Provencal Basin of the Mediterranean made it possible to assess the vertical distributions (5-2000 m) and the seasonal variations of these properties. Alkalinity varies linearly with salinity between surface water and the Levantine Intermediate Water (marked by a maximum of temperature and salinity). In deep water, total alkalinity is also correlated linearly to salinity, but the slope of the regression line is 15% less. In surface water, the pH at 25degreesC varies between 7.91 and 8.06 on the total proton scale depending upon the season. The lowest values are observed in winter, the highest in spring and in summer. These variations are primarily due to biological production. The pH goes through a minimum around 150-200m and a small maximum below the intermediate water. The total dissolved inorganic carbon content (deduced from pH and alkalinity) is variable in surface water (2205-2310 mumol kg(-1)) and has a maximum in intermediate water, which is related to the salinity maximum. Normalized total inorganic carbon at a constant salinity is strongly negatively correlated with pH at 25degreesC. The fugacity of CO2, (fCO(2)) varies between 320 and 430 muatm in surface water, according to the season. Below the seasonal thermocline, the maximum fCO(2) (about 410 muatm) is located around 150-200 m. The presence of a minimum of oxygen in the intermediate water of this area has been observed for several years, but our measurements made it possible to specify the relationship between oxygen and salinity in deep water. Data from the intense vertical mixing during the winters of 1999 and 2000 were used to calculate the oxygen quantity exchanged with the atmosphere during these periods. The estimated quantity of oxygen entering the Mediterranean Sea exceeds that deduced from exchange coefficients calculated with the formula of Wanninkhof and McGillis. During the vertical mixing in the 1999 winter, fCO(2) in surface water was on average below equilibrium with atmospheric fCO(2), thus implying that CO2 was entering the sea. However, on this time scale, even with high exchange coefficients, the estimated CO2 uptake had no significant influence on the inorganic carbon content in the water column. (C) 2002 Elsevier Science Ltd. All rights reserved

Processes controlling annual variations in the partial pressure of CO2 in surface waters of the central northwestern Mediterranean Sea (Dyfamed site)

Measurements of the partial pressure of CO, (pCO(2)) in surface waters, and other water properties were performed monthly during a 2-year period from February 1998 through February 2000, at a station in the open northwestern Mediterranean Sea (Dyfamed Station). On seasonal timescale, the pCO(2) minimum of 300 muatm in winter was followed by an increase of 120 muatm (pCO(2) reaching 420 muatm) related to warming of surface waters in summer. Estimates of the underlying processes (mixing, biological activity and air-sea gas exchange) governing the monthly variations of the upper layer PC02 were obtained from observed variations in total inorganic carbon content (TCO2) in the surface, and from the vertical distribution of physical parameters and TCO2, Monthly variations in TCO2 due to gas exchange were determined from wind speed and from the air-sea PC02 gradient. The impact of biological activity was estimated from the difference between the observed variations in TCO2 and the evaluations of air-sea exchange and carbon supply by physical processes. Mixing at the base of the mixed layer counteracts the late winter to summer TCO2 drawdown (about 80 mmol m(-3)) due to a net organic production of about 100 mmol m(-3). The carbon consumption continues until early summer despite the absence of nutrients in the upper layer from April or May. The net carbon production in the mixed layer during the warming period exceeds by a factor of 1.6 the carbon production deduced from nitrate fluxes and using the usual Redfield C:N ratio of 6.6:1. The TCO2 increase during the autumn is primarily associated with convective vertical mixing induced by upper layer cooling and deepening. On the other hand, the contribution of air-sea gas exchange to TCO2 variations remains relatively small aside from summer months, when the CO2 oversaturation is high and the mixed layer is only 15-20 m depth. (C) 2002 Elsevier Science Ltd. All rights reserved

Distribution of sterol and fatty alcohol biomarkers in particulate matter from the frontal structure of the Alboran Sea (S.W Mediterranean Sea)

Sterol and fatty alcohol biomarkers were analyzed in suspended and sinking particles from the water column (20-300 m) of the Almeria-Oran frontal zone to characterize the biogenic sources and biogeochemical processes. Diatom- and haptophyte-related sterols were predominant at all sites and vertical distributions of sterol, and fatty alcohol biomarkers in sinking particles were markedly different from suspended particles. In contrast to the relatively fresh sinking particles with elevated concentrations of phyto- and zooplanktonic sterols, suspended particles were extensively degraded with increasing depth and exhibited a more terrestrial and zooplanktonic signature with depth. Sterol and alcohol biomarkers distributions and delta(13)C values from the jet core and the associated gyre of Atlantic waters showed a decoupling between the sinking particles of 100- and 300-m depth, demonstrating the influence of lateral advection in the frontal zone. In contrast, vertical transport of the particulate organic matter in Mediterranean waters was interpreted from the similar isotopic and molecular composition at both depths. The high abundance of phytosterols and phytol below the euphotic zone at 100 m signified that downwelling of biomass occurred on the downstream side of the gyre. The high concentrations of phytosterols and POC, in combination with the high phytosterols/phytol ratio, indicated the accumulation of detrital plant material in the oligotrophic Mediterranean waters near the frontal zone. A higher contribution of phytol in the sinking particles collected during the night at the surface of the jet and at the upstream side of the gyre provided evidence of diel vertical zooplankton migration and important grazing by herbivorous zooplankton. Carbon isotope ratios of sterols confirmed that the 24-ethylcholest-5-en-3beta-ol, commonly associated with terrestrial sources, was a substantial constituent of the phytoplankton in this area. However, the more delta(13)C depleted values obtained for this compound in suspended particles suggested that there was some terrestrial contribution that only becomes evident after degradation of the more labile marine organic matter. (C) 2003 Elsevier Science B.V. All rights reserved

Regional variation in the particulate organic carbon to nitrogen ratio in the surface ocean

The concept of constant elemental ratios in plankton communities—the Redfield ratio—is of central importance to ocean biogeochemistry. Recently, several studies have demonstrated regional differences in the plankton C:P and N:P ratio. However, less is known about potential systematic variations in the C:N ratio. Here we present an analysis of the particulate organic carbon to nitrogen ratio of 40,482 globally distributed samples from the upper 200 m of the ocean water column. Particulate organic carbon and nitrogen concentrations are highly correlated (R 2 = 0.86) with a median value of 6.5. Using an artificial neural network analysis, we find regional variations in the C:N ratio linked to differences in environmental conditions. The ratio is lower in upper latitude cold water as well as upwelling regions in comparison to the warm oligotrophic gyres. We find substantial differences between ocean gyres that might be associated with differences in the nutrient supply ratio. Using cell sorting, we also quantified the C:N ratio of Prochlorococcus,Synechococcus, and picoeukaryotic field populations. The analysis demonstrates that picophytoplankton lineages exhibit a significantly higher ratio than the bulk particulate material but are only marginally significantly different from each other. Thus, the dominance of picophytoplankton in ocean gyres may contribute to the elevated ratios observed in these regions. Overall, the median C:N ratio derived from 40,482 samples is close to the canonical Redfield ratio, but significant regional deviations from this value are observed. These differences could be important for marine biogeochemistry and the regional coupling between the ocean's carbon and nitrogen cycles