Stable Carbon and Nitrogen Isotope Composition in Suspended Particulate Matter Reflects Seasonal Dynamics of Phytoplankton Assemblages in the Gulf of Riga, Baltic Sea

Abstract The ratio of stable carbon and nitrogen isotopes in the suspended particulate matter has been widely used to study processes occurring marine ecosystem. At same time, signals provided by isotope ratios coastal ecosystems can be difficult interpret, due several, often contradictory taking place simultaneously. In this study, we hypothesized that isotopic variation is predominantly affected seasonally phytoplankton species succession Gulf Riga, Baltic Sea. Cyclical seasonal patterns were observed for compositions both SPM data. Enrichment heavy Riga took during spring bloom (from on average between + 7.1 8.8 ‰, − 23.7 21.9 ‰ δ 15 N 13 C, respectively) pooled at significantly lower values 3.1 5.1 from 28.7 25.1 rest year. spatial gradient was sporadic inconclusive. results showed terrestrial anthropogenic input negligible autumn. Multivariate analysis revealed variability indeed driven composition. diatoms, dinoflagellates, ciliate Mesodinium rubrum facilitated enrichment C spring. contrast, atmospheric fixation cyanobacteria assimilation their released nutrients other organisms resulted summer. This requires careful considerations conducting food web studies temperate estuarine environments high biomass periods.