Abstract
Abstract Following coral mortality in tropical reefs, pioneer communities dominated by filamentous and crustose algae efficiently colonize substrates previously occupied by coral tissue. This phenomenon is particularly common after mass coral mortality following prolonged bleaching events associated with marine heatwaves. Pioneer communities play an important role for the biological succession and reorganization of reefs after disturbance. However, their significance for critical ecosystem functions previously mediated by corals, such as the efficient cycling of carbon (C) and nitrogen (N) within the reef, remains uncertain. We used 96 carbonate tiles to simulate the occurrence of bare substrates after disturbance in a coral reef of the central Red Sea. We measured rates of C and dinitrogen (N2) fixation of pioneer communities on these tiles monthly over an entire year. Coupled with elemental and stable isotope analyses, these measurements provide insights into macronutrient acquisition, export and the influence of seasonality. Pioneer communities exhibited high rates of C and N2 fixation within 4–8 weeks after the introduction of experimental bare substrates. Ranging from 13 to 25 μmol C cm−2 day−1 and 8 to 54 nmol N cm−2 day−1, respectively, C and N2 fixation rates were comparable to reported values for established Red Sea coral reefs. This similarity indicates that pioneer communities may quickly compensate for the loss of benthic productivity by corals. Notably, between 40% and 85% of fixed organic C was exported into the environment, constituting a vital source of energy for the coral reef food web. Our findings suggest that benthic pioneer communities may play a crucial, yet overlooked role in the C and N dynamics of oligotrophic coral reefs by contributing to the input of new C and N after coral mortality. While not substituting other critical ecosystem functions provided by corals (e.g. structural habitat complexity and coastal protection), pioneer communities likely contribute to maintaining coral reef nutrient cycling through the accumulation of biomass and import of macronutrients following coral loss. A free Plain Language Summary can be found within the Supporting Information of this article.
Highlights
Coral reefs display some of the highest rates of gross primary production (GPP) in the marine environment, despite a low ambient nutrient availability and little sustained exogenous nutrient inputs (Hatcher, 1990)
Ranging from 13 to 25 μmol C cm−2 day−1 and 8 to 54 nmol N cm−2 day−1, respectively, C and N2 fixation rates were comparable to reported values for established Red Sea coral reefs
Our findings suggest that benthic pioneer communities may play a crucial, yet overlooked role in the C and N dynamics of oligotrophic coral reefs by contributing to the input of new C and N after coral mortality
Summary
Coral reefs display some of the highest rates of gross primary production (GPP) in the marine environment, despite a low ambient nutrient availability and little sustained exogenous nutrient inputs (Hatcher, 1990). Whether coral mortality occurs naturally on a small scale or in the form of mass mortality events after severe (anthropogenic) stress, the surface area of bare hard-bottom substrates, that is, the carbonate structures and skeletons from dead corals, is available for (re)-colonization These substrates are quickly covered with biofilms and colonized by diverse benthic organisms, including heterogeneous assemblages of filamentous algae, crustose coralline algae (CCA), and sessile invertebrates and coral larvae (reviewed in Norström, Nyström, Lokrantz, & Folke, 2009). A study by Davey, Holmes, and Johnstone (2008) indicates that skeletons of artificially bleached corals were active sites of N2 fixation While these laboratory experiments indicate an important role of diazotrophs on apparently ‘bare’ substrates after coral mortality, in-depth investigations targeting the significance of pioneer communities in their natural environment on the C and N dynamics of a reef are still lacking. C and N elemental, as well as stable isotope analyses complemented community-wide flux measurements to (a) quantify rates of primary production and N2 fixation of benthic pioneer communities, (b) investigate the effects of temporal variability of key environmental variables and (c) gain insights into element acquisition (storage in biomass) and regeneration (export or supply to the environment) of pioneer communities in coral reefs after disturbance
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