Seaweed strandings as a nitrogen source for benthic algae: Evidence from in situ and in vitro experiments in the English Channel

Seaweed strandings are recurrent in the eastern Seine Bay (English Channel) and may strongly affect nutrient dynamics at the sediment–water interface. This study investigated whether nitrogen released from decaying stranded seaweeds contributes to nitrogen availability for benthic Ulva during summer, using a combined approach of 19 months of in situ monitoring and an in vitro experiment. Variation in δ 15 N in Ulva (δ 15 N Ulva ) was observed, with lower values (<7 ‰) in spring and higher values (>9 ‰) in summer. In spring, low δ 15 N Ulva values and the positive correlation between concentrations of dissolved inorganic nitrogen (DIN) in intertidal seawater and the River Orne’s flow indicate a terrigenous nitrogen origin, mainly from agricultural fertilizers. In summer, concentrations of dissolved organic nitrogen (DON) and ammonium increased in both intertidal and interstitial waters. Significant positive correlations were found between δ 15 N Ulva , the extent of algal wracks, and DON concentrations, the latter also increasing with the extent of wracks. These relationships suggest that, in summer, when riverine nitrogen inputs are minimal, decaying stranded seaweeds constitute an endogenous nitrogen source for benthic seaweeds. This hypothesis was supported by a controlled experiment showing an increase in δ 15 N in Ulva thalli after 9 days of culture in seawater enriched with algal leachate. Such endogenous nitrogen recycling supports the growth of opportunistic Ulva , which dominate summer strandings and negatively affect coastal water quality assessments under the European Water Framework Directive. These findings highlight the importance of accounting for seaweed strandings in strategies and management actions aimed at improving coastal water quality. • δ¹ ⁵N in benthic Ulva shows seasonal variation: lower in spring, higher in summer. • DON and NH₄⁺ increase in intertidal and interstitial waters near summer algal wracks. • δ¹ ⁵N Ulva values increase with the extent of algal wracks and DON concentrations. • N assimilation by Ulva from decaying algae is supported by an in vitro experiment. • Decomposition of algal wracks: an endogenous nitrogen recycling for benthic Ulva.