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Research Document - 2005/043

Oceanographic conditions in the Estuary and the Gulf of St. Lawrence during 2004: zooplankton

By Harvey, M., J.-F. St.-Pierre, L. Devine, A. Gagné, Y. Gagnon, M.-F. Beaulieu


Zooplankton biomass, abundance, and species composition in the Québec Region (Lower St. Lawrence Estuary and Gulf of St. Lawrence) during 2004 are reviewed and related to conditions during the preceding years and over the longer term where applicable. In addition, this report gives an overview of the interannual variability of the mesozooplankton biomass and the macrozooplankton species composition, abundance, and biomass in the Lower Estuary (LE) and the northwest Gulf of St. Lawrence (NWGSL) as measured in fall of each year between 1994 and 2004.

Even with only four zooplankton samples collected in February, March, June and November 2004, at both Québec fixed stations in 2004, we can see that the zooplankton biomass follow the same temporal variations pattern then during the 5 previous years (1999-2003). Furthermore, the zooplankton biomass observed in February, March, June and November were comparable to the one observed previously (1999 – 2003) at the same period of the year except in June and November in the Anticosti gyre were the zooplankton biomass was ca 2 times lower than normal. Hierarchical community analysis revealed that copepods continue to numerically dominate the zooplankton year-round at both Québec fixed station in 2004. The depth-integrated abundance of the different Calanus finmarchicus stages reveled that the reproductive success of C. finmarchicus seems to have been lower in 2004 than during the previous years at both fixed stations. The overall abundance and biomass of zooplankton observed in 2004 along all sections in spring and fall were comparable to observations made since 2000.

The mean mesozooplankton biomass observed in November in the LE and the NWGSL in 2004 was 1.3 times lower than in 2003 and corresponded to the second lowest value observed since the last 10 years in the study area. On the other hand, the mean macrozooplankton biomass observed in 2004 was 1.2 times higher than in 2003. The most notable feature observed in the LE and the NW GSL was that 2004 had the lowest mean biomass of euphausiids in the last decade and that for the first time the mean biomass of the hyperiid amphipod T. libellula was higher than the mean biomass of euphausiids. Finally, the mean abundance of both the chaetognaths and the jellyfish observed in 2004 corresponded to the highest value of the time series and were up 8.5 and 10.5 times higher than the average value of the last ten years respectively.

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