Seasonal and short-time-scale dynamics of microplankton community production and respiration in an inshore upwelling system

16 pages, 7 figures, 3 tables.An intensive study of pelagic primary production and microplankton community respiration was carried out during an entire upwelling season in the Ria de Vigo (NW Spain). From April to November measurements of oxygen production and respiration using the light-dark bottle technique were made twice a week at the surface, 1% light depth (1% LD, 12 ± 4 m) and 40 m (8 m above sea floor) alongside routine physical, chemical and biological measurements. During the major part of the survey period intermittent intrusions of cold, nutrient-rich upwelled water were observed in the ria with a periodicity of about 2 wk. Rates of gross primary production (GPP) were high but variable averaging 37.3 ± 30.7 µM O2 d-1 and 3.6 ± 4.8 µM O2 d-1 at the surface and 1% LD respectively over the period of survey (n = 50). Rates of dark community respiration (DCR) were also high and variable with maximum values being observed in the surface layer where the seasonal average was 12.2 ± 9.8 µM O2 d-1. At the 1% LD and 40 m, DCR averaged 5.3 ± 4.4 and 2.8 ± 3.0 µM O2 d-1 respectively. Although seasonal average and maximal DCR (up to 46.5 µM O2 d-1) were among the highest reported for coastal areas, microplankton production over the period of survey was dominated by autotrophic processes. Respiration losses by the microplankton community in the euphotic zone represented on average 43% of estimated mean seasonal water column GPP (2.1 to 2.7 g C m-2 d-1). Net heterotrophy in the aphotic layer consumed the equivalent of a further 25% of estimated water column GPP. The degree of coupling between primary production and respiration was primarily controlled by upwelling. During upwelling events respiration was generally low in the water column but it increased as a linear function of chlorophyll a concentration (R2 = 0.55, n = 13) and GPP (R2 = 0.47, n = 13) in the surface layer. Under such condition phytoplankton appears as the dominant component of community respiration consuming 14% of GPP. During periods of upwelling relaxation respiration was high relative to GPP. High water column respiration rates extending occasionally down to 40 m took place at the expense of organic matter trapped inside the bay. The seasonal breakdown of thermal stratification in autumn presented a relationship between surface respiration and chlorophyll a or GPP similar to that observed during upwelling events. The large excess primary production during this period was not remineralised inside the ria, suggesting that a large fraction may be exported towards the shelf.The project was funded by the EU MAST Contract CT90-0017 'Control of Phytoplankton Dominance'. G.M. is grateful to the British Council for a 6 mo fellowship which enabled her to continue her study at the Queen's University of Belfast. X.A.A.-S. acknowledges support from a visiting fellowship of the Departamento de Postgrado y Especializacion (CSIC) during the preparation of the paper. The authors thank the staff of Queen's University Marine Laboratory in Portaferry and of the Grupo de Oceanoloxia at the Instituto de Investigacions Marbias in Vigo for their help and support during the project.Peer reviewe

Thermohaline, chemical and biological characterisation of the poleward flowing slope current off the NW Iberian Upwelling System

Poster.-- 32nd International Liege Colloquium on Ocean Hydrodynamics Exchange Processes at the Oceans Margins, Liège, Belgium, May 8-12 2000A high-salinity poleward-flowing compensation undercurrent, carrying subtropical Eastern North Atlantic Central Water (ENACW1), is recurrently observed off NW Spain during the "upwelling season" (April to September). The poleward flow surfaces during the transition from upwelling-favourable northeriy winds to downwelling-favourable southerly winds by September-October. The poleward transport of ENACW1 in the upper ocean produces unexpectedly warm and salty waters off NW Spain during the autwnn and winter compared to the surrounding subpolar waters of the NE Atlantic. NO3 -, HPO4- and Si(OH)4 levels in ENACWT -fipm south of 40°N. the latitude of Cape Mondego, Portugal- are much lower than in subpolar ENACW (ENACWr)-from north of 43°N, the latitude of Cape Finisterre. Spain. Contrasting nutrient status affect the microplankton community structure and the distributions of the reduced forms ofN-nutrients (NO2- and NH4-). Seasonal changes in the thermohaline, chemical and biological setting of the poleward flowing slope current are studied in response to the decreasing stratification from the middle summer to the middle winter. Data collected during cruises Be9815c (June 27- July 7, 1998; OMEX-II. MAST-III). Galicia XII (September 1991, MAST-I), MORENA II (December 1993. MORENA. MAST-II) and CD110b (10-16 January 1998, OMEX-11, MAST-III)N

A Lagrangian biogeochemical study of an eddy in the North East Atlantic

We report the results of an experiment in the Northeast Atlantic in which sulphur hexafluoride (SF 6) was released within an eddy and the behaviour of trace gases, nutrients and productivity followed within a Lagrangian framework over a period of 24 days. Measurements were also made in the air above the eddy in order to estimate air–sea exchange rates for some components. The physical, biological and biogeochemical properties of the eddy resemble those of other eddies studied in this area, suggesting that the results we report may be applicable beyond the specific eddy studied. During a period of low wind speed at the start of the experiment, we are able to quantitatively describe and balance the nutrient and carbon budgets for the eddy. We also report concentrations of various trace gases in the region which are similar to those observed in other studies and we estimate exchange rates for several trace gases. We show that the importance of gas exchange over other loss terms varies with time and also varies for the different gases. We show that the various trace gases considered (CO 2 , dimethyl sulphide (DMS), N 2 O, CH 4 , non-methane-hydrocarbons, methyl bromide, methyl iodide and volatile sele-nium species) are all influenced by physical and biological processes, but the overall distribution and temporal variability of individual gases are different to one another. A storm disrupted the stratification in the eddy during the experiment, resulting in enhanced nutrient supply to surface waters, enhanced gas exchange rates and a change in plankton community, which we quantify, although overall productivity was little changed. Emphasis is placed on the regularity of storms in the temperate ocean and the importance of these stochastic processes in such systems

The Portugal coastal counter current off NW Spain: new insights on its biogeochemical variability

Time series of wind-stress data, AVHRR and SeaWiFS satellite images, and in situ data from seven cruises are used to assemble a coherent picture of the hydrographic variability of the seas off the Northwest Iberian Peninsula from the onset (September-October) to the cessation (February-May) of the Portugal coastal counter current (PCCC). During this period the chemistry and the biology of the shelf, slope and ocean waters between 40degrees and 43degreesN have previously been undersampled. Novel information extracted from these observations relate to: 1. The most frequent modes of variability of the alongshore coastal winds, covering event, seasonal and long-term scales; 2. The conspicuous cycling between stratification and homogenisation observed in PCCC waters, which has key implications for the chemistry and biology of these waters; 3. The seasonal evolution of nitrite profiles in PCCC waters in relation to the stratification cycle; 4. The Redfield stoichiometry of the remineralisation of organic matter in Eastern North Atlantic Central Water (ENACW)-the water mass being transported by the PCCC; 5. The separation of coastal (mesotrophic) from PCCC (oligotrophic) planktonic populations by a downwelling front along the shelf, which oscillates to and fro across the shelf as a function of coastal wind intensity and continental runoff; and 6. The photosynthetic responses of the PCCC and coastal plankton populations to the changing stratification and light conditions from the onset to the cessation of the PCCC