Exploring mixed layer dynamics and light as limiting factors in the diatom community in the SJI

Abstract

Stratification in the water column and its interplay with light has been historically observed to impact phytoplankton communities, most notably in Sverdrup’s critical depth hypothesis. This study aims to examine stratification, light, and its relationship with marine diatoms within the San Juan Islands and contribute to the 20-year PEF dataset, and overall understanding of the oceanographic processes within the San Juan Islands. Phytoplankton net tows and CTD casts were conducted at two stations through the fall seasons to pinpoint the depth of the mixed layer and euphotic zone and characterize the marine diatoms present. Cells in the phytoplankton samples were counted and identified down to the genus level, and the diversity of the communities was calculated using the Shannon-Wiener diversity index. The mixed layer depth was found to be controlled by tidal action and much more variable at Station North (between 8-71 m) and winds at Station South (between 19-41 m). The euphotic zone had no correlation with these variables. Phytoplankton abundance and diversity were overall higher at Station South (max 3778 cells/L, 1.79 SWDI) than Station North (max 2855 cells/L, 1.17 SWDI). Chaetoceros was the most abundant genus at each station. After statistical analysis using 1-way ANOVA tests and pairwise correlation, no relationship was found between the mixed layer depth, the depth of the euphotic zone, and phytoplankton abundance and diversity. Instead, the abundance of phytoplankton was found to be primarily associated with PAR. Future studies could further look into the role of salinity and temperature impacting the phytoplankton, why Chaetoceros is overwhelmingly prolific, and studying drivers of phytoplankton diversity in the SJI.