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

The ratio of stable carbon and nitrogen isotopes in the suspended particulate matter has been widely used to study processes occurring in the marine ecosystem. At the same time, the signals provided by isotope ratios in coastal ecosystems can be difficult to interpret, due to several, often contradictory processes taking place simultaneously. In this study, we hypothesized that the carbon and nitrogen isotopic variation is predominantly affected by seasonally occurring phytoplankton species succession in the Gulf of Riga, Baltic Sea. Cyclical seasonal patterns were observed for carbon and nitrogen isotopic compositions of both SPM and phytoplankton data. Enrichment of heavy isotopes in the Gulf of Riga took place during spring phytoplankton bloom (from on average between + 7.1 and + 8.8 ‰, and between − 23.7 and − 21.9 ‰ for δ¹⁵N and δ¹³C, respectively) and pooled at significantly lower values (from + 3.1 to + 5.1 ‰ and from − 28.7 to − 25.1 ‰ for δ¹⁵N and δ¹³C, respectively) for the rest of the year. At the same time, the spatial gradient of isotope ratios was sporadic and inconclusive. The results showed that terrestrial and anthropogenic input to particulate matter is negligible from spring to autumn. Multivariate analysis revealed that the observed seasonal variability was indeed driven by variation in phytoplankton species composition. The diatoms, dinoflagellates, and the ciliate Mesodinium rubrum facilitated enrichment of ¹⁵N and ¹³C in spring. In contrast, atmospheric nitrogen fixation by cyanobacteria and the assimilation of their released nutrients by other organisms resulted in lower δ¹⁵N values during summer. This variability requires careful considerations for conducting food web studies in temperate coastal and estuarine environments during high phytoplankton biomass periods.