Decreasing δ13C and δ15N values in four coastal species at different trophic levels indicate a fundamental food-web shift in the southern North and Baltic Seas between 1988 and 2016

Abstract

Marine ecosystems are exposed to increasing human pressures and climatic change worldwide. It has therefore become essential to describe ecosystem statuses with respect to multinational protection schemes, often necessitating long-term monitoring programmes. Changes in the food-web structure, which can be monitored via stable isotope measurements, represent an important descriptor of the status of marine ecosystems. We investigated long-term changes (29 years) in isotopic values (δC and δN) in four indicative organisms at different trophic levels in the southern North and Baltic Seas: bladderwrack (Fucus vesiculosus), blue mussel (Mytilus ssp.), eelpout (Zoarces viviparus), and herring gull (Larus argentatus). Time series analyses using generalised additive models revealed largely consistent declines in δC and δN throughout all trophic levels of the coastal food web at all study sites, indicating a clear change in these coastal regions from 1988 to 2016. There were no clear long-term patterns in egg biometrics for herring gulls, except for a consistent increase in eggshell thickness. The declines in stable isotope values were in line with the results of previous long-term studies of single higher-trophic-level species, which suggested that the noted changes were mainly caused by altered foraging patterns of the studied species. The current results demonstrate that declines in δC and δN have occurred throughout the whole food web, not just in particular species. We discuss the possible reasons for the decrease in stable isotope values, including decreasing eutrophication and an increase in terrestrial carbon sources.

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

“…Archived biological samples from environmental monitoring programs can be retrospectively analysed for elemental composition (C, N, P) and stable isotope ratios of carbon and nitrogen (δ 13 C, δ 15 N) to study nutrient cycling and reconstruct food webs in relation to a changing environment. Carbon isotopes provide information about ultimate carbon sources for primary production (Fry and Sherr 1984), while nitrogen isotopes can be used to trace speci c nitrogen sources, such as anthropogenic (e.g.…”

“…During summer it reaches its full growth and bene ts from nutrient enrichment (Thybo-christesen et al 1993). Isotope composition in bivalves, including Mytilus, is commonly used to study pelagic organic matter origin (Magni et al 2013) and its variability over time and space (Briant et al 2018;Corman et al 2018). Bivalves are considered suitable baselines for food web studies (e.g.…”

“…Tracing C and N origin in the Baltic Sea is complex, due to multiple interacting sources, especially in the coastal area. For instance, in the Baltic Proper, eutrophication is associated with elevated δ 15 N values in sediment (Voss et al 2000) and sewage waters enriched in 15 N, which is traceable in macrophytes (Savage and Elmgren 2004). However, the depleted 15 N signal of synthetic N fertilizers used in agriculture (Bateman and Kelly 2007) can be confounded with the similar signal of diazotrophic cyanobacteria (Rolff 2000), the latter considered an indirect effect of eutrophication (Vahtera et al 2007).…”

“…However, the depleted 15 N signal of synthetic N fertilizers used in agriculture (Bateman and Kelly 2007) can be confounded with the similar signal of diazotrophic cyanobacteria (Rolff 2000), the latter considered an indirect effect of eutrophication (Vahtera et al 2007). In the Bothnian Sea, the naturally low δ 15 N-NO 3 from pristine rivers (Voss et al 2005) should equally be re ected in relatively depleted 15 N baselines. The typically low δ 13 C of terrestrial carbon from the extensive riverine input in the north (Rolff and Elmgren 2000) is similarly expected to be re ected in a low δ 13 C baseline in the Bothnian Sea.…”

Long‐term changes in trophic ecology of blue mussels in a rapidly changing ecosystem

“…Also, a general multidecadal decrease in Mytilus spp. elemental and/or isotope composition from the southern North Sea, Baltic Sea (Corman et al 2018), and other European seas (Briant et al 2018) has been linked to climate change. However, the mechanisms behind such decreases remain hypothetical and no study statistically link decadal environmental change(s) with isotope composition, including population data on trophic niche, or stoichiometric relationships in Mytilus , or any other species.…”