Dominance shift of zooplankton species composition in the central Strait of Georgia, British Columbia during 1997

Resumo

A multivariate analysis of the zooplankton was completed during the 1997 annual cycle in the central Strait of Georgia, Canada. Sampling was conducted monthly in a single location (49° 15 0 N, 123° 44 9 W). The study used the species as environment descriptors, and examined possible patterns of species associations. Principal components analysis showed two groups of species coinciding with changes in vertical salinity structure arising from two phases of the Fraser River runoff. Group I was dominant during fall-winter to early spring related to low-runoff season. Group II was dominant during late spring to summer, and related to high-runoff season. Notable results were the scarcity of Neocalanus plumchrus and Euphausia pacifica, typically dominant species of zooplankton, coincident with river runoff levels lower than previous years and with an early phytoplankton bloom. We speculate that these results are a consequence of El Niño 1997 event, combined with the climate change expressed as a decreased freshwater runoff. Low abundance of Neocalanus plumchrus and Euphausia pacifica affected the zooplankton biomass. Consequently, substitute dominant P. pacifica and Cyphocaris challengeri probable play an important role in trophic dynamics while N. plumchrus and E. pacifica are scarce. Both amphipods feed on small zooplankters that feed on nanoplankton, concentrating food energy and biomass from small zooplankters. This suggests that P. pacifica and C. challengeri are an important link in the trophic ecology of the Strait of Georgia. In conclusion, shifts in zooplankton species dominance highly likely have an effect on juvenile zooplanktivorous salmon specie.