Résumé
A multidisciplinary international oceanographic expedition was conducted in the Northeast Water Polynya (NEW, northeast of Greenland) from May to August 1993, to understand the formation of the polynya and its influence on the pelagic ecosystem. The residual circulation in the polynya is characterized by an anticyclonic gyre, which follows a system of troughs. The numerous data acquired during the expedition and the resulting publications provide the background for an ecological modelling study. Since the sampling scheme during this expedition was not appropriate for implementing ecological models, the first objective of the present study was to build up a multivariate time series. Stations in the time series were selected by taking into account the residual circulation in the anticyclonic gyre. Model outputs were compared to data along the time series. From the time series alone, it was not possible to fully understand the development of a second phytoplankton bloom, so that we formulated three hypotheses on the functioning of the ecosystem during that period: (H1) horizontal supply of nitrate from waters located to the north; (H2) vertical diffusion of nitrate; (H3) local remineralization processes in the surface layer. H2 and H3 are evaluated using two ecological models, in which the same components are simulated, but the first considers only the nitrogen cycle and the second simulates both the carbon and nitrogen cycles. Comparing the chemical and biological variables simulated by the models to the observed time series data by reference to the three hypotheses lead to the conclusion that H2 is the most likely hypothesis. This also means that the summer pelagic ecosystem in the anticyclonic gyre was perhaps dominated by a short food chain that mostly comprised large phytoplankton, copepods and appendicularians. However, because the parameterization of vertical mixing used to test hypothesis H2 may be an oversimplification of field conditions, the hypothesis cannot be fully tested.