Reef Complexity Unlocks Fish-Driven Nutrient Hotspots, Boosting Productivity

As lead author Helder notes in the paper, "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 set out to unravel this puzzle, aiming to determine the specific ways in which different scales of reef structural complexity influence the supply of fish-derived nutrients and the resulting enrichment of the benthic community.

To achieve this, researchers conducted extensive fish surveys and high-resolution photogrammetry across six expansive reefscapes, each approximately 2500 square meters, located in the Florida Keys, USA. At a finer resolution of 25 square meters, they meticulously quantified both large-scale vertical relief and fine-scale complexity using a metric called vector ruggedness (VRM). Utilizing fish bioenergetics models, they estimated the nitrogen (N) and phosphorus (P) supply from fish and then measured the %N and %P in macroalgal tissues to gauge nutrient uptake.

The findings revealed compelling nonlinear patterns. Fish-derived nutrient supply increased with reef vertical relief up to approximately 2.8 meters, beyond which supply rates saturated. Vector ruggedness also showed a positive relationship with nutrient supply, particularly in areas with lower relief, indicating that the effects of complexity are scale-dependent. Intriguingly, macroalgal nutrient content exhibited a nonlinear response, with uptake plateauing once nutrient supply exceeded about 250 mg N m^-2 day^-1 and 35 mg P m^-2 day^-1. These nonlinear dynamics were primarily driven by high-relief hotspots, where the supply of nutrients was several times greater than in the surrounding reef areas.

These significant insights demonstrate that mesoscale habitat complexity plays a critical, interactive role across different scales in shaping consumer-driven nutrient supply and benthic enrichment. By identifying key thresholds in both vertical relief and VRM, this research provides new clarity on when and where these vital nutrient hotspots are most likely to form. This understanding offers practical guidance for conservation and restoration efforts, allowing scientists and managers to target specific reef features that are most likely to enhance productivity.