Overfishing and nutrient pollution interact with temperature to disrupt coral reefs down to microbial scales

TLDR

Coral losses worldwide highlight the need to understand drivers of reef decline, with overfishing and nutrient pollution reducing resilience by intensifying coral–algal competition, lowering recruitment, growth, and survivorship, and potentially disrupting coral microbiomes. We conducted a 3‑year field experiment that simulated overfishing and nutrient pollution to assess their effects on reef ecosystems. The experiment showed that these stressors increased turf and macroalgal cover, destabilized microbiomes, raised pathogen loads, more than doubled disease incidence, and raised mortality up to eightfold; elevated temperatures amplified these impacts, concentrating 80 % of mortality in warm seasons, while nutrients also heightened bacterial opportunism and mortality in parrotfish‑bitten corals, demonstrating that overfishing and nutrient pollution kill corals by sensitizing them to predation, heat, and bacterial opportunism.

Abstract

Abstract Losses of corals worldwide emphasize the need to understand what drives reef decline. Stressores such as overfishing and nutrient pollution may reduce resilience of coral reefs by increasing coral–algal competition and reducing coral recruitment, growth and survivorship. Such effects may themselves develop via several mechanisms, including disruption of coral microbiomes. Here we report the results of a 3-year field experiment simulating overfishing and nutrient pollution. These stressors increase turf and macroalgal cover, destabilizing microbiomes, elevating putative pathogen loads, increasing disease more than twofold and increasing mortality up to eightfold. Above-average temperatures exacerbate these effects, further disrupting microbiomes of unhealthy corals and concentrating 80% of mortality in the warmest seasons. Surprisingly, nutrients also increase bacterial opportunism and mortality in corals bitten by parrotfish, turning normal trophic interactions deadly for corals. Thus, overfishing and nutrient pollution impact reefs down to microbial scales, killing corals by sensitizing them to predation, above-average temperatures and bacterial opportunism.