Temporal variations among planktonic diatom assemblages in a turbulent environment of the southern Strait of Georgia, British Columbia, Canada

Abstract

We examined the species composition and concentrat~on of planktonic algae as well as irradiance and near-surface temperature and sal~nity 3 times per week for 1 yr in a turbulent marine environment with con.tinuously high nutrient concentrations Seasonal variations in tempera.ture and sahn~ty were small, and diatoms composed the largest percentage of phytoplankton (25 pm) cell numbers throughout the year. We observed at least 96 diatom species, of which some were present throughout most of the year while others were observed for periods as short as 1 to 2 wk. Diatom species diversity reached minimum and maximum values during wlnter and summer months, respectively, and was positively correlated to temperature and daylength and negatively correlated to salinity. Winter and spring blooms occurred and were numerically dominated by small and large cells of Skeletonema costatum, respectively, and Pseudo-nitzschia delicatissirna, resulting in minimum seasonal values of species evenness, which peaked during summer months. The first 4 axes of a canonical correspondence analysis (CCA) explained 76 % of the variance in the diatom data; the measured environmental variables explained 90% of this variation, of which temperature had a dominant role. We discuss possible environmental optima of 44 diatom species based on results of CCA and make suggestions regarding altered measures of diversity.