Ecology of Benthic Epifauna of the Estuary and Gulf of St. Lawrence: Factors Influencing Their Distribution and Abundance on Buoys

Abstract

The invertebrate fauna on 200 buoys moored in the Estuary and Gulf of St. Lawrence was sampled from May through December from 1975 to 1976. A total of 45 species (13 sessile, 32 motile) was collected. The distribution of the most abundant sessile species (Obelia longissima, Mytilus edulis, Hiatella arctica, Balanus crenatus, and Balanus balanoides) was studied in detail. Numbers of organisms and biomass decreased markedly on passing from the Gulf of St. Lawrence to the Lower Estuary and from the Lower Estuary to the Upper Estuary. Numbers of species showed stepwise diminution along with a stepwise increase in turbidity, while community densities decreased progressively with salinities. No sessile species and four motile species were observed in the riverine portion of the Estuary. In the upper portion of the Estuary, freshwater species were found along the south shore whereas euryhaline species were observed on the north shore. Water outflow from the Saguenay Fjord coincided with the limit of penetration or with discontinuities in the distribution of all sessile species within the Estuary. In the Upper Estuary, patches of M. edulis, B. crenatus, and the hydroids O. longissima and Clytia sp. were observed in regions of upwelling. Similarly, in the Gulf, O. longissima was very abundant on buoys near Mingan Islands where there is also an upwelling. The relatively high abundance of hydroids where conditions were generally unfavorable for settlement of sessile organisms was due to their capacity to colonize substrata from the budding of a few individuals. Mytilus edulis was the most abundant species elsewhere in the Gulf, except along the Lower North Shore. Low water temperature influencing larval development of M. edulis in this region best explains low community densities. Variations of growth indices (maximum lengths and mean weights per individual) for M. edulis were related to water temperature and to relative abundance of suspended organic matter in seawater.Key words: benthic epifauna, St. Lawrence Estuary, Gulf of St. Lawrence, ecological distribution, biomass, population density, Mytilus edulis, Balanus crenatus, B. balanoides, Hiatella arctica, Obelia longissima, buoys