Spatial and temporal trends of diatom flux in British Columbian fjords

The monthly flux of diatom taxa at four sites in two fjordsof British Columbia, Canada, was determined over a 3-year periodby particle interceptor traps. Saanich Inlet is a small basinwith little horizontal density gradient while Jervis Inlet experiencesseasonal runoff and strong vertical stratification. The basicpattern of seasonal succession is the same in both fjords, butthe timing and volume of production differ. Production is spreadover a longer part of the year in Saanich Inlet, resulting ina more diverse assemblage, while in Jervis Inlet there is asingle production maximum in May, dominated by Skeletonema costatum.Most taxa occur simultaneously at all sites, but many show apreference for one fjord, or one area within a fjord. Severalspecies of Chaetoceros recur consistently in inner Saanich Inleteach fall, suggesting re-seeding by benthic spores. In contrast,Rhizosolenia setigera is more common in the outer reaches ofboth fjords, and is probably introduced from outside. In SaanichInlet interannual variability is relatively low, except forthe fall season, which may differ from year to year in bothvolume and taxonomic composition. Jervis Inlet is both spatiallyand temporally more variable: spring production may differ fromyear to year by an order of magnitude. In this fjord the degreeof stratification, controlled by freshwater influx, appearsto determine the timing and composition of the phytoplanktonassemblage. Large volumes of runoff may suppress the springbloom, and prolonged stratification through the summer producesan unusual assemblage which may be light limited. El Niñoof 1986–1987 had an opposite effect in the two fjords.In Saanich Inlet the unusual degree of sunshine during fallresulted in the largest bloom of the entire interval. Unusuallywarm air temperatures prevented the build-up of winter snowaround Jervis Inlet. As a result, the normal late-spring stabilizationof the water column from snowmelt did not occur, and the springbloom was eliminated.