Mécanismes et incidences écologiques de l'homogénéisation annuelle de densité dans un milieu eutrophe stratifié

Résumé La présente étude porte sur deux séries de données récoltées en baie de Biétri (lagune Ebrié, Côte d'Ivoire). La première concerne un programme d'échantillonnage à basse fréquence (octobre 1985–mars 1986), au cours duquel ont été mises en évidence les conditions hydroclimatiques nécessaires à la suppression dans ce milieu du gradient de densité (augmentation de la salinité de surface, refroidissement des eaux lagunaires consécutif à une remontée côtière). La seconde se rapporte à un échantillonnage à haute fréquence (janvier–février 1987), sur le même site, afin de définir les mécanismes et les conséquences écologiques de la déstratification. Lorsque les conditions hydroclimatiques sont réunies, la déstratification proprement dite se réalise à l'occasion d'une séquence de vive-eau, lors d'une basse basse-mer exceptionnelle, par l'accélération des déplacements d'eau de surface dans la baie, traduite par un nombre de Richardson minimal. Cette déstratification permet une homogénéisation des profils verticaux des concentrations en sels nutritifs, mais surtout, elle entraîne une disparition de 90% de la charge en N-NH₄ et de 60% de celle en P-PO₄. Une modification de l'environnement chimique des eaux de l'hypolimnion lors de la réoxygénation est suggérée pour expliquer cette brutale et transitoire disparition. A l'opposé, la répartition verticale des biomasses semble peu affectée par la suppression du gradient de densité, la topographie des fonds interdisant l'établissement d'importants mouvements convectifs. Ainsi, les conséquences biologiques de la déstratification dans ce milieu estuarien eutrophe sont peu marquées, et sans comparaison avec les modifications hydrodynamiques et hydrochimiques observées à cette occasion.

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Two sets of data were collected in the Biétri bay (Ebrié lagoon, Côte d'Ivoire). The first one concerns a program with a low frequency sampling (October 1985–March 1986), during which hydroclimatic conditions necessary to the suppression of the density gradient (increasing surface salinity, cooling of surface waters in relation with a coastal upwelling) were defined. The second one refers to a high frequency program (January–February 1987), in the same area, in order to describe the mechanisms and the ecological consequences of the destratification. When hydroclimatic conditions are favorable, destratification occurs during a spring tide period, corresponding to an exceptional low low waters, induced by a higher tidal current velocity in the bay, evidenced by a low Richardson number. That destratification allows an homogenization of vertical profiles of nutrient concentrations, but leads mainly to the disappearance of 90% of N-NH₄ and 60% of the P-PO₄ loads. A modification of the chemical environment in the hypolimnion during its re-oxygenation is proposed to explain this sudden and transitory disappearance. On the contrary, the vertical distribution of biomass seems to be little affected by the gradient suppression, the bottom topography preventing from important convective movements. Thus, the biological consequences of the destratification in this eutrophic estuarine area are not as important as the hydrodynamical and hydrochemical modifications observed during this phenomenon.

     

Kinetics of zooplankton in an aquatic continuum: from the Marne River and reservoir to the Seine estuary

A study was carried out within a 700-km river sector, including three types of ecosystems (a reservoir, a river and its estuary) to characterise the major features of zooplankton communities in the Seine Basin. In rivers, zooplankton biomass becomes significant only when the growth rate of the organisms is higher than the dilution rate (4-5th orders rivers, according to River Continuum Concept). Upstream, short residence times favour the development of small species (Rotifers) with low individual body weight and biomass. Conversely, larger species (microcrustaceans) develop more downstream, where increased residence time leads to autochthonous production (Riverine Productivity Model). Such a pattern is greatly modified by human impact. Zooplankton input from the Marne reservoir represents one type of disruption in the general upstream-downstream trend (according to the Serial Discontinuity Concept). This reservoir is a source of microcrustaceans; they rapidly disappear mainly through fish predation, and therefore have little impact on the river phytoplankton. Discontinuities, such as confluences, have a relatively small effect on the stock of zooplankton with regard to the water release from the reservoir, but they persist more downstream, because they have the same lotic origin. A few microhabitats with macrophytes play a small role for this canalised river, but they can modify locally the plankton community structure and composition. As a whole, the flux of zooplankton rises exponentially, whereas discharge increases linearly from upstream (4th order) to downstream (8th order). In the canalised sectors, Dreissena larvae build up an important biomass, adding to that of the zooplankton sensu stricto. Especially abundant in the downstream sector of the Marne and Seine Rivers, the larvae show a widespread colonisation of the benthic substrates by the adult Dreissena. One of the largest mussel colonies in the middle estuary can contribute to a rapid decrease of zooplankton. Estuary ecosystems form a transitional zone between freshwater and seawater, with zooplankton dynamics closely linked to the particular conditions on this part of the river system.     

Intertidal macrozoobenthos evolution of the "Baie des Veys' between 1985 and 2000

Intertidal macrozoobenthos evolution of the 'Baie des Veys' between 1985 and 2000. The distribution of intertidal macrozoobenthos in the 'Baie des Veys' (French coast of the eastern English Channel) has been studied in 2000. Results were compared with those of 1985, which are included in a long-term monitoring program established since 1973. The populations remained distributed along an estuarine-marine gradient, with a particularity on the eastern side, which is isolated from the rest of the bay by a river channel. The western and central tidal flats became more subjected by marine influence, which led to a homogenisation of the benthic communities. Oyster farming locally caused a fast decrease of the typical eastern community, which was characterized by Scoloplos armiger and Urothoe poseidonis.