Pelagic diatom assemblages are good indicators of mixed water intrusions into Saanich Inlet, a stratified fjord in Vancouver Island

Abstract

Abstract We assessed the potential importance of tidally-driven intrusions of autochthonous protistan plankton into Saanich Inlet, a fjord in Vancouver Island, as potential sources of fossils in the fine intra-annual sediment laminations of ODP cores. We determined the structure of protistan assemblages throughout the Inlet during July 1995 at times of successive neap and spring tides. Results were compared to a concurrent physical study of tidal flows and nutrient advection into and out of the Inlet, which allowed us to identify protistan species indicative of mixed Satellite Channel and stratified Saanich Inlet waters. These identifications were either confirmed or rejected by temporal analyses of protistan assemblages based on weekly sampling at two fixed positions, one in Saanich Inlet and one in Sidney Channel from May through October 1997. Results of both studies supported the hypothesis that the presence of a suite of diatom species, including most Chaetoceros spp., a large-celled variant of Skeletonema costatum and Thalassiosira rotula indicates the intrusion of water with elevated levels of dissolved inorganic nitrogen into Saanich Inlet from the Strait of Georgia. This indicator group of species contained additional species, but not all were observed in large numbers in 1997. Many of these species are important components of the laminated sediments of Saanich Inlet, whose presence is probably indicative of paleo-environmental conditions in the southern Strait of Georgia rather than Saanich Inlet. Effects of tidal cycles on dinoflagellate and other flagellated cells, exclusive of mastigophorans, were inconclusive and additional studies are required to elucidate details of their ecology in waters of the southern Strait of Georgia.