23 Mar 2021

23 Mar 2021

Review status: this preprint is currently under review for the journal BG.

Functional consequences of Caribbean coral reef habitat degradation 

Alice E. Webb1, Didier M. de Bakker2,3, Karline Soetaert4, Tamara da Costa1, Steven M. A. C. van Heuven5, Fleur C. van Duyl2, Gert-Jan Reichart1,6, and Lennart J. de Nooijer1 Alice E. Webb et al.
  • 1Department of Ocean Systems, NIOZ Royal Netherlands Institute for Sea Research, Den Hoorn, The Netherlands
  • 2Department of Marine Microbiology and Biogeochemistry, NIOZ Royal Netherlands Institute for Sea Research, Den Hoorn, The Netherlands
  • 3Wageningen Marine Research, Wageningen University and Research, Den Helder, The Netherlands
  • 4Department of Estuarine and Delta Systems, NIOZ Royal Netherlands Institute for Sea Research, Yerseke, The Netherlands
  • 5Groningen University, Faculty of Science and Engineering, Groningen, The Netherlands
  • 6Department of Earth Sciences, Utrecht University, Utrecht, The Netherlands

Abstract. Coral reefs are declining worldwide. The abundance of corals has decreased alongside the rise of filter feeders, turf and algae in response to intensifying human pressures. This shift in prevalence of functional groups alters the biogeochemical processes in tropical water ecosystems, thereby influencing reef biological functions. An urgent challenge is to understand the functional consequences of these shifts in order to develop suitable management strategies that aim at preserving the biological functions of reefs.

Here, we quantify biogeochemical processes supporting key reef functions (i.e. net community calcification (NCC) and production (NCP), and nutrient recycling) in situ for five different benthic assemblages currently dominating shallow degraded Caribbean reef habitats. To this end, a custom made tent was placed over communities dominated by either one of five functional groups: coral, turf and macroalgae, bioeroding sponges, cyanobacterial mats or sand, to determine chemical fluxes between these communities and the overlying water, during both day and night. Measured fluxes were then translated into responsible biogeochemical processes by solving a system of differential equations describing the contribution of each process to the measured chemical fluxes. Estimated processes are low compared to those known for reef flats worldwide. No real gain in primary habitat is recorded, with negative or very modest net community calcification rates by all communities. Similarly, net production of biomass through photosynthesis is relatively low during the day and remineralisation of organic matter at night is relatively high in comparison, resulting in net heterotrophy over the survey period by most communities. Estimated recycling through nitrification and denitrification are high but denitrification does not fully counterbalance nutrient release from aerobic mineralisation, rendering all substrates sources of nitrogen. A multivariate pairwise analysis revealed that there is no significant difference between processes occurring on any of the assemblages, suggesting functional homogenisation between distinct substrate types. We infer that the amount and type of organic matter released by abundant algal turfs and cyanobacterial mats on this reef, likely enhances heterotroph activity, and stimulates the proliferation of less diverse copiotrophic microbial populations, rendering the studied reef net heterotrophic and the overall biogeochemical ‘behaviour’ similar regardless of substrate type.

Alice E. Webb et al.

Status: open (until 15 May 2021)

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Alice E. Webb et al.

Alice E. Webb et al.


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Short summary
The biogeochemical behavior of shallow reef communities is quantified to better understand the impact of habitat degradation and species composition shifts on reef functioning. The reef communities investigated here barely support reef functions that are usually ascribed to conventional coral reefs and the overall biogeochemical behavior is found to be similar regardless of substrate type. This suggests a decrease in functional diversity which may therefore limit services provided by this reef.