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Corals are in decline worldwide due to local anthropogenic stressors, such as nutrient loading, and global stressors, such as ocean warming. Anthropogenic nutrient loading, which is often rich in nitrate, inhibits coral growth and worsens corals’ response to warming while natural sources of nitrogen, such as ammonium from fish excretion, promotes coral growth. Although the effects of nutrient loading and ocean warming have been well-studied, it remains unclear how these factors may interact with biotic processes, such as corallivory, to alter coral health and the coral microbiome. This study examined how nitrate vs. ammonium enrichment altered the effects of increased seawater temperature and simulated parrotfish corallivory on the health of Pocillopora meandrina and its microbial community. We tested the effects of nitrogen source on the response to corallivory under contrasting temperatures (control: 26 °C, warming: 29 °C) in a factorial mesocosm experiment in Moorea, French Polynesia. Corals were able to maintain growth rates despite simultaneous stressors. Seawater warming suppressed wound healing rates by nearly 66%. However, both ammonium and nitrate enrichment counteracted the effect of higher temperatures on would healing rates. Elevated seawater temperature and ammonium enrichment independently increased Symbiodiniaceae densities relative to controls, yet there was no effect of nitrate enrichment on algal symbiont densities. Microbiome variability increased with the addition of nitrate or ammonium. Moreover, microbial indicator analysis showed that Desulfovibrionaceae Operational taxonomic units (OTUs) are indicators of exclusively temperature stress while Rhodobacteraceae and Saprospiraceae OTUs were indicators of high temperature, wounding, and nitrogen enrichment. Overall, our results suggest that nitrogen source may not alter the response of the coral host to simultaneous stressors, but that the associated microbial community may be distinct depending on the source of enrichment.
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This content will become publicly available on December 5, 2025
Interactions between consumer access and nutrient enrichment have divergent impacts on two common Pacific corals
As coral reefs face accelerating threats at global scales, examining how different coral species respond to local disruption from top-down and bottom-up forces can inform management at local scales. This may provide additional time for reducing global stressors. We conducted a full factorial experiment examining the effects of corallivory, herbivory, and fertilizer addition on 2 corals:Acropora pulchraandPorites rus, the former of which is faster-growing but more susceptible to predation, disease, and heat-induced mortality. Fertilizer addition enhanced corallivory but did not affect net coral growth. Consumer exclusion enhanced the net growth ofA. pulchraby 18.1 times regardless of fertilizer treatment, while the net growth ofP. rusdid not differ among caging and fertilizer treatments.A. pulchrawas rarely overgrown by algae regardless of treatment, while herbivore exclusion and fertilizer enrichment produced opposing effects on overgrowth ofP. rus. In uncaged treatments, fertilizer enrichment led to greater herbivory but also 1.8 times greater algal overgrowth ofP. rusrelative to unfertilized treatments. However, in caged treatments, algal overgrowth ofP. ruswas 1.9 times higher in unfertilized versus fertilized treatments. Our results suggest that interactions between corallivory, herbivory, and fertilizer enrichment can have alternative effects on different coral species, with a hardier coral experiencing more negative impacts of fertilizer enrichment than a more sensitive coral, which was, in turn, more strongly suppressed by predation. Local stressors that disrupt top-down and bottom-up processes may increase the vulnerability of even the most robust corals, and it is these corals that have been predicted to become more common under future ocean scenarios.
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- PAR ID:
- 10591812
- Publisher / Repository:
- Marine Ecology Progress Series
- Date Published:
- Journal Name:
- Marine Ecology Progress Series
- Volume:
- 750
- ISSN:
- 0171-8630
- Page Range / eLocation ID:
- 53 to 63
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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