Nitrogen enrichment in macroalgae following mass coral mortality

Eleanor J Vaughan,Valeriano Parravicini,Gareth J Williams,Nicholas A J Graham,Samantha J Howlett,Shaun K Wilson

doi:10.1007/s00338-021-02079-w

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.

Highlights

Tropical coral reefs are highly productive ecosystems, but as they are typically surrounded by oligotrophic waters, they require constant recycling and retention of waterborne nutrients and organic matter (Galloway et al 2004)
For the Seychelles data, four separate two-way analysis of variance (ANOVAs) were used to assess the effect of time period, reef state and their interaction on (a) total coral cover, (b) branching coral cover, (c), d15N and d) %N across all 13 reefs where Sargassum were consistently collected. Based on this analysis and subsequent post hoc Tukey tests, we found that predominant changes in these response variables were observed on ‘‘coral mortality’’ reefs, with little response on ‘‘regimeshifted’’ reefs
%N in Sargassum tissues was higher in samples collected from ‘‘coral morality’’ reefs in 2017 than in 2014 (p \ 0.0001, Fig. 1c; significant interaction between year and state F1,124 = 5.0, p = 0.03), there was no temporal difference in N content in samples collected from ‘‘regimeshifted’’ reefs (p = 0.20)

Summary

Introduction

Tropical coral reefs are highly productive ecosystems, but as they are typically surrounded by oligotrophic waters, they require constant recycling and retention of waterborne nutrients and organic matter (Galloway et al 2004). There are a wide range of physical and biological processes on coral reefs which can retain these essential energetic resources within local biogeochemical cycles for extended periods of time. These processes can sustain rapid rates of biological activity such as primary productivity, as well as many other key ecosystem functions For instance, are capable of nitrogen fixation (Moulton et al 2016), and other primary producers, such as phytoplankton and macroalgae, readily take up and store nutrients and dissolved organic matter (DOM) in their tissues (Fong et al 1994). This DOM is recycled either through tissue breakdown or through consumption by higher trophic level organisms such as herbivorous fishes, which in turn recycle significant amounts of nutrients through excretion (Burkepile et al 2013)

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Results

Conclusion