Patterns and drivers of nearshore particulate organic matter composition

Suspended particulate organic matter (POM) is a primary food source for benthic and pelagic suspension feeders in aquatic ecosystems. POM is potentially composed of many sources, including phytoplankton, bacteria, zooplankton, and macrophyte (seaweed and seagrass) and terrestrial detritus. The relative importance of these sources to POM consumers is debated, in large part due to differing interpretations of stable isotope and fatty acid biomarkers. We investigated POM composition in the San Juan Archipelago using multiple methods including visual quantification of living and detrital components, multiple stable isotope (MSI) and fatty acid (FA) analyses. Sampling was conducted at multiple temporal and spatial scales to 1) determine the range of variation in POM biomarkers, 2) quantitatively compare δ13C, δ15N, δ34S and FA biomarkers with putative sources and 3) identify drivers of variation in POM composition. Using multivariate multiple regressions, MSI and FA explained 59.6% and 89.7% of the variation in POM composition, respectively. Variation of total FA concentration and proportions, and of δ13C and δ34S were strongly correlated to phytoplankton abundance. When phytoplankton were rare, bacterial FA were proportionally more abundant and corresponded to depletion of δ13C and δ34S and enrichment of δ15N. We observed a similar range of variation in phytoplankton biomass and biomarker composition at tidal (hourly) and seasonal time scales. Underlying the tidal and seasonal were strong correlations of POM composition and total FA concentration to salinity and temperature; decreased salinity was associated with an increase in the contribution marine of marine phytoplankton. These results suggest that temperature and salinity may be useful indicators for assessing marine primary production in the Salish Sea. This multi-scale study showed that POM composition is highly dynamic and largely driven by environmental controls on phytoplankton abundance