Abstract
Carbon and nitrogen stable isotope ratios are increasingly used to study long-term change in food web structure and nutrient cycling. We retrospectively analyse elemental composition (C, N and P) and stable isotopes (δ13C, δ15N) in archived monitoring samples of two important taxa from the bottom of the food web; the filamentous ephemeral macroalgae Cladophora spp. and the blue mussel Mytilus edulis trossulus from three contrasting regions in the Baltic Sea (coastal Bothnian Sea and Baltic Proper, open sea central Baltic). The aim is to statistically link the observed spatial and interannual (8–24 years’ time-series) variability in elemental and isotope baselines with their biomass trends and to the oceanographic monitoring data reflecting the ongoing environmental changes (i.e., eutrophication and climate) in this system. We find clear differences in isotope baselines between the two major Baltic Sea basins. However, the temporal variation in Mytilus δ13C was similar among regions and, at the open sea station, mussels and algae δ13C also correlated over time, likely reflecting a global (Suess) effect, whereas δ15N of both taxa varied with local and regional dissolved nitrogen concentrations in water. δ15N in source amino acids allowed detection of diazotrophic N in Mytilus, which was masked in bulk δ15N. 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.
Highlights
Introduction2007; Bouwman et al 2013; Carstensen et al 2020)
Coastal seas are highly involved in fundamental biogeochemical processes controlling nutrient and organic matter cycling (Middelburg and Herman2007; Bouwman et al 2013; Carstensen et al 2020)
Cladophora d13C was lower in the coastal Bothnian Sea station (Hoga Kusten) and Mytilus d13C was higher in the open Baltic Proper station (Gotland) compared to the other regions (Figs. 2, S2, Table S5)
Summary
2007; Bouwman et al 2013; Carstensen et al 2020) They receive nutrients and organic matter from both marine (e.g., primary production) and terrestrial (e.g., riverine inputs of terrestrial material, anthropogenic outfalls) origins and act as a filter between both realms (Asmala et al 2017). Organisms at the base of the food web such as filterfeeding bivalves or grazing snails, with low motility and long-life span, are commonly used as proxies for isotope baselines (i.e., the ultimate C and N sources) since they integrate intra-annual variability of nutrients (e.g., Vander Zanden and Rasmussen 1999; Post 2002) over longer time spans, several months or even year(s) (e.g., Gorman et al 2017), compared to organisms with shorter life spans such as zooplankton or phytoplankton. Perennial macrophytes typically reflect nutrient sources in the water column during the growth period (e.g., Savage and Elmgren 2004), the relatively fast turnover rates of their tissue should reflect a seasonal or shorter baseline
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