A quick and cost-effective method for modelling water renewal in shallow coral reef lagoons

Type Article
Date 2022-12
Language English
Author(s) Lalau Noemie1, Van Wynsberge SimonORCID1, Soulard BenoitORCID1, Petton SébastienORCID2, Le Gendre RomainORCID1
Affiliation(s) 1 : Ifremer, UMR 9220 ENTROPIE (Institut de Recherche Pour le Développement, Université de la Réunion, Ifremer, Université de la Nouvelle-Calédonie, Centre National de la Recherche Scientifique), 98800, Nouméa, New Caledonia, France
2 : Ifremer, Univ. Brest, CNRS, IRD, LEMAR, 29840, Argenton, France
Source Coral Reefs (0722-4028) (Springer Science and Business Media LLC), 2022-12 , Vol. 41 , N. 6 , P. 1611-1626
DOI 10.1007/s00338-022-02319-7
Keyword(s) Coastal barrier reefs, Hydrodynamic indicators, Coral Reefs, Flush, Green tides, Eutrophication

Water renewal exerts a preponderant role in eutrophication, yet few hydrodynamic indicators exist for performing quick and cost-effective assessments of ecosystem vulnerability. Using field data, we closed the water budget of a shallow coral reef lagoon recently exposed to high levels of nutrient loading that triggered green tides. Then, we tested the relevance of modelling flushing-time, a proxy of water renewal, from oceanic and atmospheric open access data. Water inflows in the lagoon were mainly driven by waves breaking on exposed reefs, but tide and wind also influenced water renewal during low-wave periods. Modelling flushing-time as a function of the wave features (significant wave height, direction, and period), tide, and wind direction provided the most convincing model, with greater contribution of wave height, and adequately reproduced observations for an independent dataset. Using this model to hindcast flushing-time over the period 2000–2019 highlighted that green tides that recently struck the area in January 2018 and June 2019 followed periods of slow water renewal, which therefore contributed to amplify the blooms. The analysis also demonstrated that renewal events even slower than those recorded in 2018–2019 are frequent in this lagoon, which highlights the high vulnerability of this UNESCO World Heritage Site to other pulses of nutrient loading. Since the methodology developed in this study can be easily applied to many other coastal barrier and fringing reefs, it offers a promising perspective for quick and cost-effective assessments of coral reef vulnerability to eutrophication and other ecosystem crises magnified by slow water renewal.

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