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Research Document - 2009/083

Oceanographic conditions in the Estuary and the Gulf of St. Lawrence during 2008: zooplankton

By M. Harvey and L. Devine

Abstract

This document provides an overview of the temporal variability of zooplankton biomass, abundance, and species composition in 2008 at four fixed stations and seven sections of the AZMP as well as an overview of the interannual variability of the macrozooplankton species composition, abundance, and biomass in the Lower St. Lawrence Estuary (LSLE) and the northwest Gulf of St. Lawrence (NWGSL) from 1994 to 2008.

Compared to the 1999–2007 average, the state of the zooplankton at the four Québec fixed stations in 2008 was estimated as lower than normal (zooplankton biomass) and above normal (total non-copepod abundance, total copepod abundance, Calanus finmarchicus abundance, Pseudocalanus spp. abundance) at AG and GC; lower than normal (zooplankton biomass), normal (total non-copepod abundance, total copepod abundance, Pseudocalanus spp. abundance), and above normal (C. finmarchicus abundance) at RS; and normal (zooplankton biomass, C. finmarchicus abundance, Pseudocalanus spp. abundance) and above normal (total non-copepod abundance, total copepod abundance) at SV. In addition, some changes in the zooplankton community structure were observed over the time series, including some changes in the rank of the top ten taxa and the appearance of new taxa in the dominant species (top ten): Temora spp. and appendicularians at AG; Temora spp., cladocera, and bivalve larvae at GC; Eucheata norvegica and Calanus glacialis at SV; and polychaete larvae at RS.

The biomass indices of Calanus hyperboreus and the mesozooplankton along the seven Québec sections in spring and fall 2008 were estimated to be normal or below normal except in the LSLE (TESL), where the C. hyperboreus biomass was evaluated as above normal. For the nine zooplankton abundance indices, most were evaluated as normal or above normal in spring 2008 except that below normal abundances were found for copepod nauplii along the TESL and TIDM sections, for mesozooplankton (excluding copepods) in the centre and the northeast GSL (TCEN, TBB), and for krill larvae in Cabot Strait (TDC). The situation was very different for the fall period: some of the abundance indices were still evaluated as normal or above normal (total copepods, copepod nauplii, small copepods, carnivorous zooplankton, mesozooplankton excluding copepods, krill larvae) and some others as normal or below normal (large copepods, C. finmarchicus CIV–CV, meroplankton). In addition, some changes in zooplankton composition were observed over the time series in each region, including some changes in the rank order of the top ten taxa and the appearance of new taxa in the top ten species: Eurytemora spp. and C. glacialis in the LSLE (TESL); Temora spp. and copepod eggs in the northwest GSL (TSI); copepod eggs southwest of Anticosti Island (TASO); Euchaeta norvegica and C. glacialis in the centre GSL (TCEN); C. glacialis in the northeast and southern GSL (TBB, TIDM); and Ostracoda in Cabot Strait (TDC).

The mean mesozooplankton biomass observed in November 2008 in the LSLE and in the NWGSL was 1.8 and 1.4 times higher than in 2006 and 2007, respectively. This corresponds to the second highest value observed in the last 15 years in the study area. The mean macrozooplankton biomass decreased from 15.4 ww g/m² in 2005 to 5.9 (2006), 8.6 (2007), and 6.2 (2008). The macrozooplankton biomass values observed in 2006, 2007, and 2008 correspond to the lowest values observed over the last 15 years.  The most notable feature observed in the LSLE and NWGSL was the sharp decrease in the abundance of Thysanoessa raschii, which was 5.3 times less abundant in 2006, 2007, and 2008 compared to the previous 15 years. In addition, the mean abundance of the hyperiid amphipod Themisto. libellula estimated in both regions in 2006, 2007, and 2008 corresponds to the lowest value observed over the previous last 15 years (except 2000).

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