Research Document - 2004/080

Oceanographic conditions in the Estuary and the Gulf of St.Lawrence during 2003: zooplankton

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

Abstract

Zooplankton biomass, abundance, and species composition in the Québec Region (Lower St. Lawrence Estuary and Gulf of St. Lawrence) during 2003 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 and the northwest Gulf of St. Lawrence as measured in September of each year between 1994 and 2003.

The zooplankton biomass and abundance at the Québec fixed stations in 2003 were slightly higher than observations made in 2000, 2001, and 2002 at both stations. The relative proportion of copepods vs. the other groups of mesozooplankton increased from 70 to 95% between 1999 and 2003 in the Gaspé Current (GC) and from 70 to 85% between 1999 and 2001 in the Anticosti Gyre (AG). No larvacea were found at both fixed stations in 2003. The depth-integrated abundance of the Calanus finmarchicus stage composition showed that there were two periods of reproduction for this species at both stations in 2003. Reproduction occurred in summer (June-July) and fall (September-October) and was synchronized at both stations. The same situation was observed in 2000, 2001, and 2002. In 2003, the abundance of Calanus finmarchicus reached record high level since 1999 at both fixed stations (GC and AG). The zooplankton biomasses and abundances observed in early spring, late spring, and fall 2003 along the six AZMP sections were comparable to observations made in 2002, 2001, and 2000.

The mesozooplankton biomass in the Lower Estuary and the northwest Gulf of St. Lawrence in 2003 was slightly higher than in 2002 while the macrozooplankton biomass was comparable to the 2002 level. The year 2003 was characterized by a significant increase in the mean abundance of the pelagic amphipod Themisto libellula and the mysid Boreomysis arctica and a significant decrease in the mean abundance of the chaetognath Sagitta elegans and the gelatinous zooplankton Aglantha digitale, Obelia spp., and Boreo spp. The significant relationships between the mean annual abundance of the hyperiid amphipod T. libellula and the percentage of Labrador Shelf water supported the hypothesis that the significant increase in the mean abundance of this species observed in 2001, 2002, and 2003 in the Lower Estuary and the Gulf of St. Lawrence is associated with the intrusion of the cold Labrador Shelf water into the Gulf of St. Lawrence via the Strait of Belle Isle (Harvey et al 2003).

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