Stable isotope approaches for assessing spatial and seasonal patterns of organic pollution in a high-latitude coastal marine ecosystem

Abstract

Human population growth has exerted long-term pressures on marine environments, particularly in coastal systems affected by anthropogenic nutrient inputs. Sewage-derived organic enrichment represents a major source of nutrient loading and can significantly alter biogeochemical processes and food-web structure. Stable isotope analysis (SIA) of nitrogen (δ15N) and carbon (δ13C) has proven to be an effective tool for tracing nutrient sources, transport pathways, and their spatial and temporal variability in aquatic ecosystems. The aim of this study was to evaluate spatial and seasonal patterns of organic pollution using an isotopic approach in a high-latitude coastalmarine system, the Beagle Channel, subject to long-term anthropogenic impacts. Phytoplankton, zooplankton, and macroalgae were sampled across contrasting seasons, together with measurements of relevant environmental parameters (e.g., nutrient concentrations and temperature). Isotopic signatures revealed clear spatial and seasonal variability associated with organic enrichment. Zooplankton δ15N values during the cold season best reflected the isotopic enrichment in 15N linked to sewage inputs, while δ13C values in zooplankton and macroalgaehelped distinguish areas subjected to higher anthropogenic influence Overall, our results reinforce previous evidence indicating that proximity to contamination sources and seasonal dynamics are key factors governing the detectability of isotopic signals of organic pollution in marine environments. The pronounced seasonality to which the biota is exposed, combined with local hydrodynamic and topographic features, can modulate isotopic responses to organic pollution. This study contributes to the development of isotopic baselines that are valuable for long-term monitoring and management in coastal marine ecosystems of the Beagle Channel.