Nitrogen enrichment in macroalgae following mass coral mortality

Abstract

Scleractinian corals are engineers on coral reefs that provide both structural complexity as habitat and sustenance for other reef-associated organisms via the release of organic and inorganic matter. However, coral reefs are facing multiple pressures from climate change and other stressors, which can result in mass coral bleaching and mortality events. Mass mortality of corals results in enhanced release of organic matter, which can cause significant alterations to reef biochemical and recycling processes. There is little known about how long these nutrients are retained within the system, for instance, within the tissues of other benthic organisms. We investigated changes in nitrogen isotopic signatures (δ15N) of macroalgal tissues (a) ~ 1 year after a bleaching event in the Seychelles and (b) ~ 3 months after the peak of a bleaching event in Mo’orea, French Polynesia. In the Seychelles, there was a strong association between absolute loss in both total coral cover and branching coral cover and absolute increase in macroalgal δ15N between 2014 and 2017 (adjusted r2 = 0.79, p = 0.004 and adjusted r2 = 0.86, p = 0.002, respectively). In Mo’orea, a short-term transplant experiment found a significant increase in δ15N in Sargassum mangarevense after specimens were deployed on a reef with high coral mortality for ~ 3 weeks (p < 0.05). We suggest that coral-derived nutrients can be retained within reef nutrient cycles, and that this can affect other reef-associated organisms over both short- and long-term periods, especially opportunistic species such as macroalgae. These species could therefore proliferate on reefs that have experienced mass mortality events, because they have been provided with both space and nutrient subsidies by the death and decay of corals.

Influence of upwelling on coral reef benthic communities: a systematic review and meta-analysis

“…The pooled effect of upwelling from all studies resulted in an overall increase in fleshy macroalgal cover. This is unsurprising, given that macroalgae are well documented to be opportunistic, able to efficiently utilize heightened water column nutrient concentrations therefore outcompeting slower growing hard coral species [ 10 , 63 ]. This trend was not observed across all geographic locations, however.…”

Baseline dynamics of Symbiodiniaceae genera and photochemical efficiency in corals from reefs with different thermal histories

“…This physiological response leads to coral bleaching and may result in mortality, which can have detrimental effects on reef ecosystems and coral-dependent communities ( e.g. , phase shifts to algal-dominated states ( Ostrander et al., 2000 ; Vaughan et al, 2021 ); homogenization of fish populations Richardson, Graham & Hoey, 2020 ).…”