Tidal changes in copepod abundance and maintenance of a summer Coscinodiscus bloom in the southern San Juan Channel, San Juan Islands, USA

Abstract

Tidal currents interacting with complex topographies are common features of coastal environments. These interactions are hypothesized to have significant effects on local plankton distribution and abundance, and therefore on food availability to planktivorous fishes. The purpose of this study was to test the hypothesis that tidal currents interacting with an island archipelago create significant and predictable increases in copepod availability to planktivores. Copepod densities during flood and ebb tides were sampled weekly during July-October of 1995-1997. Copepods of the genera Pseudocalanus, Paracalanus, and Corycaeus were the numerically dominant zooplankton except during a bloom of dinoflagellates (Noctiluca spp.) in July 1996. At sampling locations within the main tidal current, median copepod densities were 47 to 252 copepods m-3 greater during flood tides. In contrast, median densities outside of the main current were not significantly different between tides. An unexpected finding was the presence of an abundant, large centric diatom (Coscinodiscus wailesii), which showed a prolonged bloom from early July through early October in all years. Changes in copepod abundance and maintenance of the C. wailesii bloom were most likely caused by the advection of copepod aggregations and nutrients from near or below the pycnocline in the Strait of Juan de Fuca. Conversion of copepod numerical abundance to biomass estimates suggests that tidal differences in copepod abundance could affect fish growth. The predictability of changes in copepod abundance may explain why tidal rips and jets are important feeding areas for planktivorous fishes, as well as for fish predators such as marine mammals and seabirds.