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Research Document - 2008/037

Oceanographic conditions in the Estuary and the Gulf of St. Lawrence during 2007: zooplankton

By M. Harvey and L. Devine

Abstract

Zooplankton biomass, abundance, and species composition in the DFO Quebec Region (Lower St. Lawrence Estuary [LSLE] and the Gulf of St. Lawrence) during 2007 are reviewed and compared to the eight previous observations (1999–2006) at the Anticosti Gyre (AG), the Gaspe Current (GC), and the Shediac Valley (SV) stations and to the two previous years (2005–2006) at the Rimouski (RS) station. In addition, this report gives an overview of the interannual variability in the mesozooplankton biomass and macrozooplankton species composition, abundance, and biomass in the LSLE and northwest Gulf of St. Lawrence (NWGSL) as measured in the fall of each year between 1994 and 2007. The state of the zooplankton at the four Quebec’s fixed stations in 2007 was considered as normal (zooplankton total abundance and copepod total abundance) and lower than normal (zooplankton biomass and Calanus finmarchicus abundance) at the AG station; normal (zooplankton biomass, zooplankton total abundance, and copepod total abundance) and above normal (C. finmarchicus abundance) at the GC station; and normal (C. finmarchicus abundance), lower than normal (zooplankton biomass), and above normal (zooplankton and copepod abundance) at the SV station. The average abundance of C. finmarchicus at RS in 2007 was 2.6 times higher than during the two previous years. In addition, some changes in the zooplankton community structure were observed over the time series at AG, GC, SV and RS including some variations in the rank of the top ten taxa and the appearance for the first time of some new taxa in the dominant species (top ten): appendicularians and Oncaea spp. at AG, echinoderm and bivalve larvae at GC, euphausiids (eggs, nauplii, juveniles) and Calanus glacialis at the RS station, and echinoderm and polychaete larvae at SV. Zooplankton biomass along the seven Quebec sections in spring 2007 was estimated as higher than normal along the Lower St. Lawrence Estuary (TESL), Sept-Îles (TSI), Cabot Strait (TDC), and Îles-de-la-Madeleine (TIDM) sections; normal along the southwest Anticosti (TASO) section; and lower than normal along the Centre Gulf of St. Lawrence (TCEN) and the Bonne Bay (TBB) sections. Concerning the total zooplankton abundance, anomalies were above normal for all sections except for TSI, TDC, and TIDM, where the total abundance of zooplankton was evaluated as normal. On the other hand, during fall, both the total zooplankton biomass and total zooplankton abundance anomalies were normal for all sections except TESL, TSI, and TASO, where these were evaluated as above normal, and along TCEN, where the total biomass was evaluated as lower than normal. In addition, some changes in the 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: appendicularians, echinoderm larvae, and ostracods in LSLE and the NWGSL, C. glacialis in the NEGSL, pteropods and Oncaea spp. in the southern GSL, and pteropods and Metridia spp. in Cabot Strait. The mean mesozooplankton biomass observed in November 2007 in the LSLE and NWGSL was 1.4 times higher than in 2006 and corresponds to the fourth highest value observed over the last 14 years in the study area. The mean macrozooplankton biomass decreased from 15.4 in 2005 to 5.9 and 8.6 wet weight (g/m²) in 2006 and 2007, respectively. The macrozooplankton biomass values observed in 2006 and 2007 correspond to the lowest values observed over the last 14 years. The most notable feature observed in the LSLE and NWGSL was that 2006–2007 had the lowest mean biomass of euphausiids in our dataset due to a strong decrease in the abundance of Thysanoessa raschii, which was 5.3 times less abundant in 2006–2007 compared to the last 14 years. In addition, the mean abundance of the hyperiid amphipod Themisto libellula estimated in both regions in 2007 corresponds to the lowest value observed over the last 14 years.

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