Recent intrusion of Labrador Shelf waters into the Gulf of St. Lawrence and its influence on the plankton community and higher trophic levels

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Wintertime hydrographic data obtained in the Gulf of St. Lawrence (GSL) since 1996 suggest large intrusions of dense and cold surface waters from the Labrador shelf in 2001. These intrusions contributed significantly to the summertime coldness of the intermediate layer in the GSL despite relatively mild winter conditions. Several changes in the plankton community were associated with this intrusion. A major change is the first occurrence of the diatom Neodenticula seminae covering most of the GSL in spring 2001. This is very unusual since this diatom is usually found in the North Pacific, not in the North Atlantic. Because of its parallel occurrence on the Labrador Shelf, it is inferred that this Pacific species was introduced into the GSL via natural advection processes across the Arctic and down the Labrador Current. Other phytoplankton observations of typical Labrador shelf waters in the GSL support this hypothesis. Another important change is a highly significant increase in the abundance of the Arctic hyperiid amphipod Themisto libellula. Numbers increased from 0.17 ind. m-2 in 2000 to 10 ind. m-2 in September 2001. This is the most important increase in abundance of this species in the last decade. These changes in ocean circulation processes probably have a significant impact on higher trophic level interactions as suggested by an increase of the occurrence of this zooplankton species in the cod diet.