Northeast Water Polynya 1993: construction and modelling of a time series representative of the summer anticyclonic gyre pelagic ecosystem

A multidisciplinary international oceanographic expedition was conducted in the Northeast Water Polynya (NEW, northeast of Greenland) from May to Augu st 1993, to understand the formation of the polynya and its influence on th e pelagic ecosystem. The residual circulation in the polynya is characteriz ed 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 schem e during this expedition was not appropriate for implementing ecological mo dels, the first objective of the present study was to build up a multivaria te time series. Stations in the time series were selected by taking into ac count 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 ec osystem during that period: (H1) horizontal supply of nitrate from waters l ocated to the north; (H2) vertical diffusion of nitrate; (H3) local reminer alization processes in the surface layer. H2 and H3 are evaluated using two ecological models, in which the same components are simulated, but the fir st considers only the nitrogen cycle and the second simulates both the carb on and nitrogen cycles. Comparing the chemical and biological variables sim ulated by the models to the observed time series data by reference to the t hree hypotheses lead to the conclusion that H2 is the most likely hypothesi s. This also means that the summer pelagic ecosystem in the anticyclonic gy re was perhaps dominated by a short food chain that mostly comprised large phytoplankton, copepods and appendicularians. However, because the paramete rization of vertical mixing used to test hypothesis H2 may be an oversimpli fication of field conditions, the hypothesis cannot be fully tested. (C) 20 00 Elsevier Science B.V. All rights reserved.