Krill aggregation in the St. Lawrence system, and supply of krill to the whale feeding grounds in the estuary from the gulf

Abstract

Persistent high-density krill aggregations make the St. Lawrence Estuary and the Gulf of St. Lawrence important feeding-grounds for large marine mammals. To estimate the effects of the circulation on the seasonal krill distribution, a krill biomass-concentration equation with active verti- cal migration was coupled to a 3D regional sea ice-ocean circulation model. The results show recur- rent spatial patterns of aggregation and advection controlled by the circulation and a high sensitivity to the parameters of the biological model. The time spent in the surface layer is crucial for the reten- tion of organisms in the estuary. The simulated krill aggregation areas are associated with 3 pro- cesses (tidal interactions with bathymetry, wind-driven upwelling and mean circulation). Zooplank- ton generally aggregate near the edges of the Laurentian Channel and other secondary channels, at locations that are consistent with the sparse synoptic information on the distributions of large marine mammals in the gulf. The simulations also indicate that changes in the seasonal circulation signifi- cantly affect the krill distribution within the gulf through gyre intensities, the seasonal thermocline and the strength of the estuarine circulation. Finally, the variability of zooplankton transport to the estuary from the gulf appears to be controlled by processes acting on the circulation mode at the mouth of the estuary and estuarine pumping of the krill layer towards the head of the Laurentian Channel. The simulated krill biomass imported into the estuary changed by a factor of 2 over the 3 simulated years.