Macroalgal δ¹⁵N and N content as indicators of long-term and spatial variability in Northeast Atlantic nitrogen...

Coastal eutrophication driven by anthropogenic nitrogen (N) is a major pressure on estuarine ecosystems. Here, we analyzed stable N isotopes (δ¹⁵N) and tissue N content (%N) in the brown algae Fucus spp., collected in 171 sampling stations along the NW Spanish coastline (ca. 1500 km) between 1990 and 2023, to investigate long-term and spatial N enrichment patterns. We further related these indicators to seawater nutrients, land cover, and anthropogenic pressure proxies. Macroalgal δ¹⁵N ranged from 2.98 to 16.8‰ (median = 7.6‰) and was often above regional baseline thresholds for unimpacted systems (≤6‰), indicating widespread anthropogenic influence. Strong spatial gradients occurred, with higher δ¹⁵N in inner estuarine areas (Fucus ceranoides) and lower values at exposed sites (F. vesiculosus - F. spiralis). δ¹⁵N declined significantly since 1990, consistent with wastewater treatment improvements. However, the following stabilization in recent years, indicate continuing N pressures. Seawater nitrate and phosphate, agriculture land cover and, to a lesser extent, artificial surfaces were significantly associated with macroalgal N indicators, although effects were moderate, highlighting multiple interacting environmental drivers. Highest δ¹⁵N values in inner areas were consistent with sewage effluent and manure fertilization, whereas lower values at exposed areas likely reflected upwelling. Overall, macroalgal δ¹⁵N was a robust indicator of N enrichment across coastal gradients. However, source attribution remains limited by overlapping isotopic signatures and the limited spatial resolution of available datasets. These results support macroalgae as cost-effective biomonitors for long-term monitoring and underscore the need for integrated approaches to improve N source discrimination and management.