Climate-driven increases in nutrient runoff promote filamentous macroalgal mat development despite benthic grazer presence

<title>Abstract</title> <p> Climate change is expected to alter precipitation patterns, terrestrial runoff, and nutrient delivery to shallow coastal ecosystems, potentially increasing the frequency and intensity of macroalgal blooms. Opportunistic filamentous green macroalgae are well adapted to exploit episodic nutrient enrichment, yet it remains unclear whether benthic grazers can effectively constrain macroalgal development under future nutrient-enriched conditions. To address this question, a six-week flow-through mesocosm experiment was conducted using intact sediments, with and without nutrient enrichment and natural populations of the polychaete <italic>Hediste diversicolor</italic> . Nutrient enrichment significantly increased macroalgal biomass, whereas grazer abundance and biomass remained similar among treatments. The strongest algal development occurred in nutrient-enriched sediments despite the continued presence of <italic>H. diversicolor</italic> , suggesting that increased nutrient availability promoted algal growth beyond the capacity of grazers to effectively constrain mat development. In contrast, macroalgal growth was limited under ambient nutrient conditions, indicating that grazing may contribute to suppressing algal establishment when nutrient availability is low. These findings suggest that even moderate increases in nutrient availability may favour the development of filamentous macroalgal mats despite the presence of abundant benthic grazers. As climate-driven increases in runoff are expected to increase the frequency of nutrient pulses in coastal ecosystems, opportunistic macroalgae may increasingly benefit from conditions that reduce the effectiveness of biological control. </p>