Research Document - 2002/046

Oceanographic conditions in the Estuary and the Gulf of St. Lawrence during 2001 : zooplankton

By M. Harvey, J.F. St-Pierre, Pierre Joly and G. Morrier

Abstract

This report: 1) describes the results on the temporal variability of the zooplankton biomass, abundance, and species composition at two fixed stations and six transects of the Atlantic Zonal Monitoring Program (AZMP; Anticosti Gyre and Gaspé Current) in 2001 and 2) gives an overview of the interannual variability of the macrozooplankton species composition, abundance, and biomass in the Lower St. Lawrence Estuary and the northwest Gulf of St. Lawrence (GSL) as measured in September in each year between 1994 and 2001. We focus on the conditions during 2001 but compare those observations with previous information from 1999 and 2000 for the AZMP results and from 1994 to 2001 for the macrozooplankton results.

AZMP fixed stations and transects

• The annual minimum and maximum zooplankton biomasses occur in April in the Anticosti Gyre and the Gaspé Current respectively. This difference in the timing of the maximum and the minimum observed biomasses at the two stations seems to be "typical" since the same situation was observed in 1999 and 2000.
• Both the mean integrated zooplankton biomass and abundance observed in 2001 in the Anticosti Gyre and the Gaspé Current were on par with what we observed in 1999 and 2000.
• The total abundance of zooplankton in 2001 varied between 22,000 and 317,000 individuals · m-2 in the Gaspé Current and between 28,500 and 213,000 individuals · m-2 in the Anticosti Gyre. At both stations, the total abundance of zooplankton observed in 2001 was on par with observations made in 2000.
• Copepod eggs, juveniles, and adults were clearly dominant, accounting for more than 80% of the zooplankton community for all sampling dates in the Anticosti Gyre and the Gaspé Current except in May and July in the Gaspé Current.
• In the Anticosti Gyre, the minimum and the maximum copepod abundances occurred in May and November respectively and were synchronized with the minimum and the maximum values observed in 2000; the minimum and the maximum copepod abundances were observed in June and September in the Gaspé Current, ca. 1.5 months earlier than in 2000.
• The total zooplankton biomass varied between 3 and 208 g ww · m-2 along the six transects sampled in June and December 2001 in the Lower Estuary and the Gulf of St. Lawrence. The highest biomasses were found along the transects located over the Laurentian Channel (St. Lawrence Estuary, Sept-Îles, Anticosti, and Cabot Strait) and the lowest were in the northern (Bonne Bay) and the southern (Magdalen Island) regions.
• The zooplankton biomass observed in 2001 along all transects for both seasons (spring and fall) was on par with observations made in 2000 except along the Magdalen Island transect, where the zooplankton biomass was three and two times higher in spring and fall 2001 than in spring and fall 2000, and along the Cabot Strait transect, where the biomass was two times lower in fall 2001 than in fall 2000.
• The overall abundance of zooplankton was generally lower in 2001 than in 2000 for all regions and for both seasons except for fall in the southern Gulf (Magdalen Island transect), where the inverse was true.
• Globally, in both the Lower Estuary and the Gulf of St. Lawrence, the overall abundance of zooplankton was 64% and 41% lower in spring and fall 2001 than in 2000. This difference in abundance between the two years was due to the lower abundance of both copepod and invertebrate eggs in 2001.

Macrozooplankton species composition, abundance, and biomass for 1994-2001

• There were no significant changes in the macrozooplankton and the mesozooplankton biomasses in 2001 compared to 2000.
• There were significant changes in the mean abundance in some macrozooplankton species: 1) decreases in the euphausiid Thysanoessa raschii, 2) increases in the euphausiid Meganyctiphanes norvegica, the chaetognath Sagitta elegans, the gelatinous zooplankton Aglantha digitale, Obelia sp., and Boreo sp., and the pelagic amphipod Themisto libellula.
• The mean abundance of the latter species (T. libellula) increased from 0.17 ind · m-2 in 2000 to 9.2 ind · m-2 in 2001.
• There was a significant negative correlation between the annual CIL core temperature index (Gilbert, 2002) and the abundance of T. libellula in the Lower St. Lawrence Estuary and the northwest Gulf from 1994 to 2001.
• Based on this relationship, this species could be considered as an index of the intrusion of cold Labrador Current water into the Gulf of St. Lawrence. If this hypothesis is true, 2001, 1998, and 1995 would be years when there where important intrusions into the Gulf of St. Lawrence.

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