Research Document - 2000/117

Oceanographic conditions in the Estuary and the Gulf of St. Lawrence in 1999: zooplankton.

By M. Harvey, J.A. Runge, J.F. St-Pierre, and P. Joly

Abstract

This report describes : 1) an overview of the spatial variability of the mesozooplankton and krill biomass in the lower St. Lawrence Estuary and the northwest Gulf of St. Lawrence (GSL) measured in September in each year between 1994-1999 and 2) results on the temporal variability of the zooplankton biomass, abundance, and species composition at two fixed stations of the Atlantic Zonal Monitoring Program (AZMP; Anticosti Gyre and Gaspé Current) in 1999.

Within the lower St. Lawrence Estuary and the northwest GSL, the biomass of mesozooplankton and krill was not evenly distributed and showed very high interannual variability. The estimated average wet biomass of mesozooplankton varied between 106.5 and 209.2 t · km^-2 from 1994 to 1999 with the highest and the lowest values found in 1994 and 1996 respectively. From 1994 to 1999, the overall average biomass of mesozooplankton for the whole lower St. Lawrence Estuary and northwest GSL was estimated at 1.6 ·10⁶ ± 0.3·10⁶ t. Likewise, the estimated average biomass of krill varied between 7.2 and 38.1 t·km^-2 from 1994 to 1999 with the highest and the lowest values found in 1994 and 1996 respectively. The overall average biomass of krill for the whole lower St. Lawrence Estuary and northwest GSL was estimated to be 1.7·10⁵ ± 0.9·10⁵ t.

The temporal variability of the zooplankton biomass, abundance, and species composition at the two AZMP fixed stations (Anticosti Gyre and Gaspé Current) showed that copepods were clearly dominant at all sampling dates in 1999, accounting for more than 80% of the zooplankton community for both stations. Small copepods (Oithona similis, Oncea borealis) dominated in abundance at all sampling dates at both stations except in the Gaspé Current in July and August, where larger species (Calanus finmarchicus, C. glacialis, Metridia longa) were more abundant. Total zooplankton biomass was much higher in the Anticosti Gyre than in the Gaspé Current for all seasons in 1999 except in May, when the biomass was equal at both stations. The integrated copepod abundance showed the same pattern of variation with season at both stations, with a maximum abundance of copepod eggs in spring and a maximum abundance of adult copepods and copepodite stages (CI-CV) in late fall. The total integrated abundance of adult copepods and copepodite stages (CI-CV) did not vary significantly between stations for all seasons. Finally, all proposed indices describing the state of the zooplankton community at each station in 1999 varied with season and their annual mean values varied between stations. However, it is too soon to say if they will be useful to describe the interannual variability of the zooplankton community or to detect any changes of the environmental conditions.

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