Carol Robinson

A biogeochemical study of the coccolithophore, Emiliania huxleyi, in the North Atlantic

The biogeochemical properties of an extensive bloom (∼250,000 km2) of the coccolithophore, Emiliania huxleyi, in the north east Atlantic Ocean were investigated in June 1991. Satellite (NOAA-AVHRR) imagery showed that the bloom was centered initially at 60°–63°N by 13°–28°W and lasted approximately 3 weeks. Spatial variations in satellite-measured reflectance were well correlated with surface measurements of the beam attenuation coefficient, levels of particulate inorganic carbon, and coccolith density. Rates of both photosynthesis and calcification were typically relatively low within the coccolithophore-rich waters, suggesting the population was in a late stage of development at the time of the field observations. Levels of dimethyl sulphide (DMS) in surface waters were high compared to average ocean values, with the greatest concentrations in localized areas characterized by relatively high rates of photosynthesis, calcification, and grazing by microzooplankton. The estimated spatially averaged flux of DMS to the atmosphere was 1122 nmol m−2 h−1, somewhat greater than that determined for the same region in June-July 1987. Coccolith production (1 × 106 tonnes calcite-C) had a significant impact on the state of the CO2 system, causing relative increases of up to 50 μatm in surface pCO2 in association with alkalinity and water temperature changes. Gradients in pCO2 were as great as 100 μatm over horizontal distances of 20–40 km. The environmental implications of these findings are discussed in relation to the spatial and temporal distributions of E. huxleyi.

The Impact Of A Coccolithophore Bloom On Oceanic Carbon Uptake In The Northeast Atlantic During Summer 1991

Abstract Measurements of the carbonate system in the surface waters of the northeast Atlantic during summer 1991, following the main growth phase of a bloom of the coccolithophore Emiliania huxleyi are presented. We examine the perturbation of the carbonate system and assess the effect of calcification on the air-sea gradient of dissolved carbon dioxide in the surface ocean. An estimate of 1:1 organic to inorganic carbon uptake is calculated using the measurements of the surface carbonate parameters which is consistent with other estimates for E. huxleyi populations using radio-tracer methods. Using the changing ratio of dissolved carbon dioxide to nitrate concentration we demonstrate a relative increase in dissolved carbon dioxide due to calcification with evidence of this increase supported by estimates of the buffer factor and C:N assimilation ratios. Within the E. huxleyi bloom the effect of calcification on alkalinity appears to have reduced the air-sea gradient by ∼ 15 μ atms (corrected to a constant temperature) using measurements from a 440 km section along the 20°W meridian. This reduction could prove to be significant in terms of the overall drawdown of carbon during the spring-summer season in this area.

Plankton respiration in the Eastern Atlantic Ocean

Concurrent measurements of dark community respiration (DCR), gross production (GP), size fractionated primary production (14C PP), nitrogen uptake, nutrients, chlorophyll a concentration, and heterotrophic and autotrophic bacterial abundance were collected from the upper 200 m of a latitudinal (32°S–48°N) transect in the Eastern Atlantic Ocean during May/June 1998. The mean mixed layer respiration rate was 2.5±2.1 mmol O2 m−3 d−1 (n=119) for the whole transect, 2.2±1.1 mmol O2 m−3 d−1 (n=32) in areas where chlorophyll a was <0.5 mg m−3 and 1.5±0.7 mmol O2 m−3 d−1 (n=10) where chlorophyll a was <0.2 mg m−3. These values lie within the range of published data collected in comparable waters, they co-vary with indicators of heterotrophic and autotrophic biomass (heterotrophic bacterial abundance, chlorophyll a concentration, beam attenuation and particulate organic carbon concentration) and they can be reconciled with accepted estimates of total respiratory activity. The mean and median respiratory quotient (RQ), calculated as the ratio of dissolved inorganic carbon production to dissolved oxygen consumption, was 0.8 (n=11). At the time of the study, plankton community respiration exceeded GP in the picoautotroph dominated oligotrophic regions (Eastern Tropical Atlantic [15.5°S–14.2°N] and North Atlantic Subtropical Gyre [21.5–42.5°N]), which amounted to 50% of the stations sampled along the 12,100 km transect. These regions also exhibited high heterotrophic: autotrophic biomass ratios, higher turnover rates of phytoplankton than of bacteria and low f ratios. However, the carbon supply mechanisms required to sustain the rates of respiration higher than GP could not be fully quantified. Future research should aim to determine the temporal balance of respiration and GP together with substrate supply mechanisms in these ocean regions.

Review of gross community production primary production net community production and dark community respiration in the Gulf of Lions

Abstract Thirty years of data were used to examine the influence of the Rhoˆne River discharge on the spatio-temporal distribution of primary production in the Gulf of Lions. These patterns are characterised for four hydrological provinces: (i) the Gulf of Marseilles, a coastal oligotrophic system exhibiting a typical diatom-based bloom succession; (ii) the mouth and plume of the Rhoˆne River, containing high levels of nutrients, constant levels of chlorophyll a , and a plankton assemblage dominated by nanoflagellates; (iii) the highly productive dilution area to the west of the Rhoˆne, with a diatom spring bloom and smaller winter diatom blooms; (iv) the southern area encompassing the oligotrophic Liguro-Provencal Current, a frontal zone of constantly high primary production and a divergence area exhibiting a typical spring bloom situation. No evidence of eutrophication due to river input was seen. In situ and in vitro net community production data suggest the Gulf of Lions as a whole is a balanced ecosystem on an annual time scale, with the mean primary production lying between 78 and 142 g C m −2 a −1 .

Water column and sea-ice primary production during Austral spring in the Bellingshausen Sea

Abstract The findings of a cruise to study the phytoplankton bloom dynamics associated with the marginal ice zone (MIZ) in the Bellingshausen Sea during Austral spring (November-December) 1992 are reported. Biomass and rate process measurements were carried out at stations located in the ice, ice edge and open water along the 85°W meridian in order to establish the productivity of the microalgae associated with sea-ice and in the water column. In addition, a series of transects along 85°W from sea-ice to open water conditions enabled an assessment of the development of phytoplankton populations. Low phytoplankton biomass and production were noted at ice-covered and ice-edge stations and in the open water close to the ice edge. Observations from the transects indicated no development of a classical ice edge bloom despite evidence that sea-ice had retreated more than 100 km during the study period. Survey data along the 85°W line revealed a region of high chlorophyll, centred on 67.5°S, which was initially observed during brash ice conditions. This feature, which remained geographically consistent, persisted for at least 25 days and was thought to be associated with a frontal region. Water column primary production ( 14 C) in this high chlorophyll region was ca 0.8 g C m −2 day − , more than 8 times higher than noted in the MIZ. Phytoplankton photosynthetic characteristics within this region indicated that cells were adapted to a low light regime. A critical depth of 80 m, estimated directly from oxygen flux measurements, was sufficient to permit the initiation and net growth of phytoplankton standing stocks in a mixed layer of ca 70 m. A modelling approach using 14 C observations suggested that phytoplankton growth was less than the sum of the algal loss terms within this feature. An advective supply of cells therefore would be required to sustain the observed high and constant algal biomass. In addition, although this high chlorophyll feature was initially observed during brash-ice conditions, the available data suggest that it was initiated under open water conditions.

Hydrothermal studies in the Aegean Sea

The aims of the Aegean Hydrothermal Fluxes and Biological Production project were to estimate the fluxes of fluids, chemicals, heat and bacteria from hydrothermal vents, establish the controls on venting dynamics, measure the productivity in the region of the vents and establish the effect of the vents on biodiversity of both prokaryotes and eukaryotes. This paper presents an initial synthesis of the project results. Research was done both by land-based SCUBA diving and from several vessels at a number of active sites in the near-shore coastal regions of Milos and Kos, with some additional studies at Methana, Lesbos and Santorini. Vent water composition showed very large variations. This was due to the mixing, of hydrothermal reservoir fluids, vapour condensate and seawater altered by interactions of fluid-sediment-bacteria in different proportions, in the gasohydrothermal vents. The composition ranged from nearly sea water with only slightly reduced pH, to higher or lower salinity fluids with a pH as low as 3 and with large enrichments in heavy and trace metals. Phase separation was a common feature at these shallow vents. The dry gas phase was mainly C02, but with significant amounts of H2S, CH4 and H2. These fluids commonly passed through soft sediments before venting from the seafloor and induced a convection cell of pore-water entrainment from deeper sediment layers into the water column with a consequent ‘re-charge’ down-flow of seawater into the sediment around the vent outlets. Such complex conditions may well explain the high biodiversity of Bacteria, Archaea and epifaunal species surrounding the vents. As many as 44 % of the archaeal lineages detected were found to represent novel phyla. Epifaunal diversity was particularly high with over 200 species recorded at the shallower Milos vents. These vents may form a ‘steppingstone’ for warmer water species to colonise the surrounding areas when water temperatures permit.

Development and assessment of an analytical system for the accurate and continual measurement of total dissolved inorganic carbon

Abstract An automated coulometric titration system based on that described by Johnson, Sieburth, Williams and Brandstrom (1987, Mar. Chem., 21 : 117–133) has been evolved for the accurate and continual measurement of total dissolved inorganic carbon (TCO 2 ). The instrument achieves an analytical precision (1 SD) of ±0.5–1.0 μmol kg −1 (0.025–0.05%). The accuracy of the system has been examined by a limited comparison with other coulometric-based titrators and with a manometric-based system; agreement was to 1 μmol kg −1 . The capability for automatic continual analysis allows surface mapping of TCO 2 ; a sample rate of 10 analyses h −1 gives a mapping resolution of 1–2 km. Provision for frequent standardization with a liquid substandard has been included in the development. The ability to achieve high-density analyses while maintaining interlaboratory consistency and standardization constitutes a vital contribution to surveys of ocean carbon chemistry (e.g. Joint Global Ocean Flux Study, World Ocean Circulation Experiment).

BIOGEOGRAPHIC DIFFERENCES IN THE NET ECOSYSTEM METABOLISM OF THE OPEN OCEAN

In vitro measurements, predictions, and geochemical estimates of the balance between gross primary production (GPP) and community respiration (CR) in the open ocean do not agree. This has generated an active debate about the carbon balance in unproductive pelagic marine ecosystems. The analysis of generalized GPP:CR relationships that sustains this debate assumes a continuous trophic gradient or the simple partition of the World Ocean into productive and unproductive regimes. We measured euphotic zone GPP and CR along a latitudinal (40° N–30° S) transect across the Atlantic Ocean, which included two open-ocean oligotrophic provinces: the eastern area of the North Atlantic Subtropical Gyre (NAST-E) and the center of the South Atlantic Gyre (SATL). Net heterotrophy prevailed in the euphotic zone of the NAST-E, while the plankton community in the central SATL was net autotrophic. A review of published studies of net plankton metabolism in the open ocean substantiates this geographic pattern, suggesting the e...

The temperature response of gross and net community production and respiration in time-varying assemblages of temperate marine micro-plankton

Abstract Micro-organism community respiration and net community production rates and their response to temperature were determined as oxygen flux rates in the Menai Strait during a 6-month period including the spring and summer of 1993. The rates for gross community production were calculated from polynomial fits of community respiration and net community production data. The cardinal temperatures of gross community production were estimated from these equations. The optimal temperature was positively correlated to the in situ temperature. The natural population gave no evidence of being shocked due to experimental temperature manipulation. Frequency histograms of the temperature coefficients of community respiration and gross community production were distinct in this environment. Q 10 values for respiration were greater than Q 10 values for photosynthesis, in contrast to published observations from the Southern Ocean where they overlapped. It was argued that this was a consequence of the short-term temperature variability of the environment.

Biological and physical forcing of carbonate chemistry in an upwelling filament off northwest Africa: Results from a Lagrangian study

The Mauritanian upwelling system is one of the most biologically productive regions of the worlds oceans. Coastal upwelling transfers nutrients to the sun-lit surface ocean, thereby stimulating phytoplankton growth. Upwelling of deep waters also supplies dissolved inorganic carbon (DIC), high levels of which lead to low calcium carbonate saturation states in surface waters, with potentially adverse effects on marine calcifiers. In this study an upwelled filament off the coast of northwest Africa was followed using drifting buoys and sulphur hexafluoride to determine how the carbonate chemistry changed over time as a result of biological, physical and chemical processes. The initial pHtot in the mixed layer of the upwelled plume was 7.94 and the saturation states of calcite and aragonite were 3.4 and 2.2, respectively. As the plume moved offshore over a period of 9 days, biological uptake of DIC (37 ?mol kg?1) reduced pCO2 concentrations from 540 to 410 ?atm, thereby increasing pHtot to 8.05 and calcite and aragonite saturation states to 4.0 and 2.7 respectively. The increase (25 ?mol kg?1) in total alkalinity over the 9 day study period can be accounted for solely by the combined effects of nitrate uptake and processes that alter salinity (i.e., evaporation and mixing with other water masses). We found no evidence of significant alkalinity accumulation as a result of exudation of organic bases by primary producers. The ongoing expansion of oxygen minimum zones through global warming will likely further reduce the CaCO3 saturation of upwelled waters, amplifying any adverse consequences of ocean acidification on the ecosystem of the Mauritanian upwelling system.