Previous studies conducted on the continental shelf in the Southeast Bay of Biscay influenced by Gironde waters (one of the two largest rivers on the French Atlantic coast) showed the occurrence of late winter phytoplankton blooms and phosphorus limitation of algal growth thereafter. In this context, the importance of dissolved organic phosphorus (DOP) for both algae and bacteria was investigated in 1998 and 1999 in terms of stocks and fluxes. Within the mixed layer, although phosphate decreased until exhaustion from winter to spring, DOP remained high and phosphate monoesters made up between 11 to 65% of this pool. Total alkaline phosphatase activity (APA, Vmax) rose gradually from winter (2-8 nM h−1) to late spring (100-400 nM h−1), which was mainly due to an increase in specific phytoplankton (from 0.02 to 3.0 nmol µgC−1 h−1) and bacterial APA (from 0.04 to 4.0 nmol µgC−1 h−1), a strategy to compensate for the lack of phosphate. At each season, both communities had equal competitive abilities to exploit DOP but, taking into account biomass, the phytoplankton community activity always dominated (57-63% of total APA) that of bacterial community (9-11%). The dissolved APA represented a significant contribution. In situ regulation of phytoplanktonic APA by phosphate (induction or inversely repression of enzyme synthesis) was confirmed by simultaneously conducted phosphate-enrichment bioassays. Such changes recorded at a time scale of a few days could partly explain the seasonal response of phytoplankton communities to phosphate depletion.

In biological modelling of the coastal phytoplankton dynamics, the light attenuation coefficient is often expressed as a function of the concentrations of chlorophyll and mineral suspended particulate matter (SPM). In order to estimate the relationship between these parameters over the continental shelf of the northern Bay of Biscay, a set of in situ data has been gathered for the period 1998-2003 when SeaWiFS imagery is available. These data comprise surface measurements of the concentrations of total SPM, chlorophyll, and irradiance profiles from which is derived the attenuation coefficient of the photosynthetically available radiation, K PAR. The performance of the IFREMER look-up table used to retrieve the chlorophyll concentration from the SeaWiFS radiance is evaluated on this new set of data. The quality of the estimated chlorophyll concentration is assessed from a comparison of the variograms of the in situ and satellite-derived chlorophyll concentrations. Once the chlorophyll concentration is determined, the non living SPM, which is defined as the SPM not related to the dead or alive endogenous phytoplankton, is estimated from the radiance at 555 nm by inverting a semi-analytic model. This method provides realistic estimations of concentrations of chlorophyll and SPM over the continental shelf all over the year. Finally, a relationship, based on non living SPM and chlorophyll, is proposed to estimate K PAR on the continental shelf of the Bay of Biscay. The same formula is applied to non living SPM and chlorophyll concentrations, observed in situ or derived from SeaWiFS radiance.

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From 22 April to 19 June 1990, 7 stations located off La Rochelle, France, from near-shore ('bassin Marennes-Oleron') to offshore waters (50 m depth contour) were vlslted 8 times, with the aim of determining whether the toxic dinoflagellates Dinophysls spp. ~n c r e a s e In cell denslty In nutr~ent-poor offshore water, in nutrient-rich Inshore waters, or both, and to determine t h e~r vertical d~stributlon In relation to stratification Temperature stratification developed offshore, but not inshore In April, few Dinophysis spp. were present offshore and none ~n s h o r e .In late May, up to 15 X 103 cells 1-' were recorded in the 10 to 15 m layer offshore, whereas inshore waters contained only a few cells I-' In late May, a short penod of wind partly modlfied the vertical structure, and Dinophysis spp. concentration fell, Increasing agaln when marked stratiflcatlon was agaln established Through the strait, flood-tide currents moved Dlnophysls spp. cells to the Inshore area, and ebb-tlde currents brought some of them back out. No relat~onshlp was found between Dinophysis spp. growth and availability of dissolved nutnents. We concluded that: ( l ) stratification of sufflclent magnitude ( A t > 5 "C) and duration ( > 2 wk) are the factors necessary for D~nophysls spp cell increase, and (2) inorganic nutnent Input of terrestrial ongin does not dlrectly promote their growth.

Thermostratification and seasonal light increase are generally considered the first causes of phytoplankton spring blooms in temperate waters. The objective of this study is to confirm the existence of winter phytoplankton blooms, responsible for the early exhaustion of phosphate, within the Gironde plume waters (southeast Bay of Biscay), and to understand what may initiate them so early. Two cruises, BIOMET 2 and BIOMET 3, were carried out respectively in early (8 to 21 January) and late winter 1998 (25 February to 11 March). An increase of phytoplankton biomass (chl a) between the 2 cruises and non-conservative nutrients observed in late winter confirm an early phytoplankton growth both in the Gironde plume and adjacent oceanic waters. Among factors that possibly initiate these blooms, light availability seems to be the best candidate. First, the status of available light for phytoplankton changed dramatically between the 2 cruises when comparing the instantaneous depth-averaged irradiance (E m ) received by phytoplankton with the light saturation parameter E k determined from P versus E curves. Light was limiting for phytoplankton growth in early winter according to systematically lower values of E m than E k in the daytime. However, light was not limiting during a large part of the day in late winter since E m was above E k during 40% of the daylight.

Shrimp farming in New Caledonia typically uses a flow-through system with water exchange rates as a tool to maintain optimum hydrological and biological parameters for the crop. Moreover, the effluent shows hydrobiological characteristics (minerals, phytoplankton biomass and organic matter) significantly higher than that of the receiving environment. Separate surveys were carried out in a bay (CH Bay) with a medium-size intensive farm (30 ha) (PO) and in a mangrove-lined creek (TE Creek) near a larger semi-intensive farm (133 ha) (SO). Net loads of nitrogen exported from the semi-intensive farm and the intensive farm amounted to 0.68 and 1.36 kg ha(-1)day(-1), respectively. At CH Bay, discharge effects were spatially limited and clearly restricted to periods of effluent release. The high residence time at site TE favoured the installation of a feedback system in which organic matter was not exported. Mineralization of organic matter led to the release of nutrients, which in turn, caused in an increased eutrophication of this ecosystem. The study of the pico- and nanophytoplankton assemblages showed (i) a shift in composition from picophytoplankton to nanophytoplankton from offshore towards the coast and (ii) a shift within the picophytoplankton with the disappearance of Prochlorococcus and the increase of picoeucaryotes towards the shoreline. These community changes may partially be related to a nitrogen enrichment of the environment by shrimp farm discharges. Thus, in view of the recent addition of the New Caledonian lagoon to the UNESCO World Heritage list, the data presented here could be a first approach to quantify farm discharges and evaluate their impact on the lagoon.

A phytoplankton bloom was observed in late winter 2000, on the continental shelf offshore of southern Brittany, in northwestern Bay of Biscay. This bloom appeared initially along the 120-m isobath, in stratified and clear waters, at the interface between the oceanic water and the plumes of southern Brittany rivers (mainly the Loire and Vilaine). The development of the bloom was triggered by favourable meteorological conditions, characterised by solar irradiance reaching the maximum level expected for that period of the year. Outside of the bloom area, the phytoplankton photosynthesis was irradiance limited: inshore, because of the stronger attenuation of the light; offshore, because of the weak stratification. The hydrological conditions at the onset of the bloom were observed in the field, during the oceanographic cruise MODYCOT. However, without SeaWiFS, the only observations related to this major event in the primary production would have been those of the coastal phytoplankton network (REPHY (REseau PHYtoplankton)). Observed initially offshore by SeaWiFS, as early as March 5, 2001, the phytoplankton bloom extended onshore and was observed at REPHY stations by mid-March. On, March 15, fishermen reported that they were handicapped in trawling, the presence of mucilage in their nets carried clogging by very heavy slime. This mucilage has been shown to be produced by the diatom Coscinodiscus and, especially, C. wailesii. These observations were made as the main bloom was declining. A simple calculation, based upon the SeaWiFS chlorophyll concentration maps observed from March 5-16, shows that phosphate was probably totally depleted offshore, by March 16. The former bloom area appeared very poor in chlorophyll on the SeaWiFS image of April 7, which was the first image available after 3 weeks of an overcast sky. A second cruise undertaken at the end of April confirmed the phosphate depletion and the low chlorophyll concentration in that area.