Quantifying fish-derived nutrient hotspots across reefscapes

Animals aggregated on habitat patches can generate nutrient "hotspots" that enhance biogeochemical cycling and primary production, yet the conditions under which such hotspots emerge in continuous reef habitats remain unclear. This study aimed to determine how different scales of reef structural complexity regulate fish-derived nutrient supply and associated benthic enrichment. We conducted fish surveys and high-resolution photogrammetry across six reefscapes (~2500 m² each) in the Florida Keys, USA. At the 25 m² scale, we quantified large-scale vertical relief and fine-scale complexity using vector ruggedness (VRM), estimated nitrogen (N) and phosphorus (P) supply from fish bioenergetics models, and measured macroalgal tissue %N and %P. We found that fish-derived nutrient supply increased with reef vertical relief up to ~2.8 m, beyond which supply rates saturated. VRM was positively related to nutrient supply, particularly in low-relief areas, indicating scale-dependent effects. Macroalgal nutrient content was non-linearly related to supply, with uptake plateauing above ~250 mg N m^-2 day^-1 and ~35 mg P m^-2 day^-1. Nonlinear patterns were driven by high-relief hotspots, where nutrient supply was several times greater than surrounding reef. These findings show that mesoscale habitat complexity interacts across scales to shape consumer-driven nutrient supply and benthic enrichment. Identifying thresholds in relief and VRM provides new insight into when and where nutrient hotspots form and offers practical guidance for targeting restoration to reef features most likely to enhance productivity.