Calcium Carbonate production and the response of calcifiying organisms to natural and anthropogenic threats on the shallow platform of Zanzibar, Tanzania

Abstract

Biologically produced calcium carbonate (CaCO3) structures are the foundation of coral reefs ecosystems and associated landforms. Produced by the growth and decay of hermatypic scleractinian corals and reef dwelling organisms (e.g. calcifying macroalgae, bryozoans, crustose coralline algae, mollusks, etc.), constructive and destructive processes typically occur on an individual scale. Over time collectively they control the reef framework accretion (on a long-term, reef-spatial scale) and the development of sedimentary deposits (e.g. coral cays and sandbanks). Also, the supply of reef-derived sediments supports other associated shallow water marine ecosystems (e.g. seagrass beds), and the reef framework roughness supports large amounts of biomass on coral reefs, which are among the most diverse ecosystems in the world. However, adverse effects of natural (e.g. marine snow, storms and earthquakes, water temperature rise, coral bleaching, and Crown-of-Thorns) and anthropogenic (e.g. sewage, nutrient loading, overfishing, tourists, mangrove cutting, construction and sedimentation) impacts on benthic community composition affect coral reef functionality and CaCO3 production, among others, over a variety of spatial and temporal scales. The objective of this work is to investigate the biophysical interactions that affect calcifying organisms (hard corals, crustose coralline algae, and calcifying macroalgae) and modify fringing reefs. With an emphasis on overall ecosystem health and the relevance of conservation areas, this study focuses on unprotected fringing reefs and a marine protected area, located in the Archipelago of Zanzibar, Tanzania. We assessed the status of the coral ecosystem and its function to produce CaCO3 and maintain complex reef structures, using a range of multidisciplinary approaches, from ecological census-based methods, biogeochemistry and hydrodynamic measurements to morphological and sedimentological surveys. Traditional ecological census surveys provided a good estimation of the structural complexity, which ranged from (1.18AA /-0.13 to 1.51AA /-0.24 m/m) and community structure of the ecosystem, which pointed hard coral as major CaCO3 producers (8.47AA /-4.37 to 16.90AA /-9.70 KgCaCO3 m-2 yr-1) and the dominant benthic taxa ( 50% coverage). However, sedimentological approaches provide a broad understanding of time-averaged, coral-derived sediment sources shaping the landscape. With the use of biogeochemical approaches, we measured the concentration of suspended particular matter (18.2 to 36.4 mgL-1) and characterized the composition of its organic fraction (particular organic matter). The organic fraction shows that untreated sewage reaches coral reefs adjacent to Stone Town (A 15N, 10.28AA /-0.9a Adegree on surface waters). Also, turbidity and associated shading effects control bathymetrically coral reef formations to 16AA /-2 meters. Moreover, since no baseline studies are available in the literature, we compared our findings with estimates from a local marine protected area, Chumbe Island Coral Park, to better understand potential negative effects of natural and anthropogenic impacts on the natural ecosystem. Despite that Zanzibarian reefs show on average a healthy reef condition, CaCO3 produced on Chumbe Island Coral Park shows doubled production in comparison to CaCO3 produced on more degraded reefs. So far Chumbe Island Coral Park shows the highest CaCO3 budget globally, although scored similar values to other marine protected areas in the Caribbean, which highlighted the importance of conservation areas to keep corals functionality.