Spatio-Temporal Variation in Stable Isotope and Elemental Composition of Key-Species Reflect Environmental changes in the Baltic Sea

Carbon and nitrogen stable isotope ratios are increasingly used to study long-term change in food web structure and nutrient cycling. Whether isotope composition in primary producers and consumers (so-called isotope baselines) reflect environmental changes in a similar manner is largely unknown. We take advantage of long-term oceanographic monitoring data and archived biological samples for the well-studied Baltic Sea to retrospectively analyse elemental composition (C, N and P) and stable isotopes (δ ¹³ C, δ ¹⁵ N) in the filamentous ephemeral macroalgae Cladophora spp. and in blue mussel Mytilus edulis trossulus from three contrasting regions (coastal Bothnian Sea and Baltic proper, open sea central Baltic) with the aim to statistically link the observed spatial and interannual (8–24 years’ time-series) variability in elemental and isotope baselines with environmental changes. We find clear differences in isotope baselines between the two major Baltic Sea basins. However, the temporal development in Mytilus δ ¹³ C was remarkably similar among regions and, at the open sea station, mussels and algae δ ¹³ C also correlated over time, likely reflecting a global Baltic Sea or Northern Hemisphere pattern. In contrast, δ ¹⁵ N of both taxa responded to regional and local drivers (water nutrient concentrations). δ ¹⁵ N in source amino acids allowed detection of diazotrophic N signal in Mytilus , which was masked in bulk δ ¹⁵ N. Finally, Cladophora N:P reflected regional nutrient levels in the water while P %, which differed for both taxa, was linked to food quality for Mytilus . This study highlights the potential of a multi-taxa and multi-stable isotope approach to understand nutrient dynamics and monitor long-term environmental changes.