Natural tidal processes modified by the existence of fringing reef on La Reunion Island (Western Indian Ocean): Impact on the relative sea level variations

Tide gauge and meteorological data were used to analyze tidal fluctuations and relative sea level (RSL) variability in the microtidal fringing reef of La Saline, at La Reunion, Western Indian Ocean, for 58 days during late summer 2005. Harmonic analysis was applied to evidence tidal constituents and to predict tidal variations in the open ocean and in the fringing reef. The predicted signals were compared to assess the intratidal and supratidal variability as well as the synoptic response of the RSL in the reef. Finally, the non-tidal fluctuations of the signal recorded in the fringing reef were correlated with the external forcing data (wind and wave). The results show that the tide in the reef is reduced and delayed, compared to the tide of the open ocean. The neap-spring tidal cycle in the fringing reef appears more synchronized on the tropical cycle (27.32 days) of the Moon, rather than its synodic cycle (29.52 days). Due to the shallowness of the fringing reef, nonlinear bottom-friction interactions induce an amplification of compound tides, the fortnightly (Msf) and monthly (Mm) long-period tides (LPT), which have a major influence on the relative sea surface elevations variations. They cause relative high and low RSL reaching ±10 cm in the fringing reef, which are significantly different from the sea level in the open ocean. It appears that the contribution of the external forcing to the RSL fluctuations in the fringing reef will largely depend on these high and low sea levels. Our results demonstrate that the orbital moon cycles and the LPT cannot be neglected in upcoming studies of narrow microtidal fringing reefs, involving sea level variations.

► The semidiurnal tide in the fringing reef of La Reunion is reduced in amplitude and delayed, face to tide in ocean.
► Neap-spring cycle in the reef is driven by the tropical moon cycle and punctually modified by the anomalistic moon cycle.
► Presence of nonlinearly generated LPT strongly modulates the relative sea level in the fringing reef.
► The identified processes have a great influence on the water exchange between the reef and the open ocean.