Abstract
We investigated the influence of freshwater discharges on biogeochemistry and benthic communities of La Saline fringing reef, the largest coral reef system of La Reunion Island (France, Indian Ocean). Sampling was conducted in February-March 2012 along a land-ocean continuum, including rivers, groundwater, reef waters, and oceanic waters. Our results indicated the discharges of basalt groundwater (enriched in nitrates) in the southern part of the reef, and the discharges of sand groundwater and river waters (enriched in organic matter) in sites located northward. These discharges mainly impacted fore reef surface waters, while groundwater resurgences were more diffuse and not clearly detected in depth. We highlighted the relationship Si(OH) 4 /NO 3 − to identify basalt groundwater discharges. We also put forward the relationships humic-like fluorescence/NH 4 + and tryptophan-like fluorescence/PO 4 3− to identify sand groundwater discharges and wastewater pollutions, respectively. We observed the eutrophication of the southern part of the reef through higher phytoplankton biomasses, a massive presence of fleshy algal formations and a low coral coverage. The spatial homogeneity of the benthic community structures onto the entire slope contrasted with the spatial heterogeneity of chemical variables in water. This reflected the different spatial and temporal scales involved in biological and chemical processes in coral reef ecosystems.
Highlights
Coral reefs are ecosystems of great biological productivity and diversity (Fisher et al, 2015) that are present mostly in shallow coastal waters of oligotrophic tropical and subtropical oceans (Kleypas et al, 1999; Hoegh-Guldberg et al, 2017)
This study highlights the discharges of basalt groundwater in the southern part, and in a lesser extent, the discharges of sand groundwater and Hermitage River waters in sites located northward, in both back and fore reef areas of La Saline ecosystem
These discharges mostly impacted fore reef area in surface waters through water transfer from the back reef zone allowed by physical processes (“surface pathway”)
Summary
Coral reefs are ecosystems of great biological productivity and diversity (Fisher et al, 2015) that are present mostly in shallow coastal waters of oligotrophic tropical and subtropical oceans (Kleypas et al, 1999; Hoegh-Guldberg et al, 2017) They are recognized as being vulnerable to anthropogenic pressures that are exerted both through the direct exploitation of reef resources and Freshwater Impact on Coral Reef the multiple human activities on land, which lead to impacts at the global and local levels (Dubinsky and Stambler, 1996; Pandolfi et al, 2011; Ban et al, 2014; Hughes et al, 2017). Because reef waters are often oligotrophic and N-limited, SGD may lead to their eutrophication, which is characterized by the shift from corals to algal communities (Koop et al, 2001; Dailer et al, 2010; Zubia et al, 2018)
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