Hydrodynamics of the Toliara Reef Lagoon (Madagascar): Example of a Lagoon Influenced by Waves and Tides

摘要

In meso-tidal lagoons, the coral reef barrier can be temporarily submerged at high tide and partially exposed at low tide, which causes highly specific lagoon dynamics. To understand those dynamics, three field-measurement campaigns were conducted in the Toliara Lagoon (Madagascar), where the tide and the waves increase that effect. The method combined measurements taken from fixed moorings and measurements from a small, moving, inflatable boat along the transects through the reef passages. A tidal analysis methodology adapted to this lagoon was used to reconstitute the currents through the passages and to determine the spatial and temporal current variability. Then, the lagoon water dynamics were studied. The tidal dynamics of the lagoon appear to depend significantly on flows through the passages, although they were also affected by water passing across the reef. Water entered the lagoon through the passages during the tidal flow and left it during the ebb. The tidal-prism flushing time (estimated using the tidal prism method) was 1 day during spring tide and 5 days during neap tide. At the same time, the average flow during a tidal cycle appears to be directly linked to the waves breaking over the reef. Indeed, the average cross-reef flow was inflowing and was mainly driven by the ocean swell. That inflow was balanced by an outflow through the passages. Hence, the average bulk flushing time was estimated at 13 days during the wet season and 4 days during the dry season.