Current Patterns and the Distribution of Benthic Habitats in a Coastal Lagoon of Mauritius |
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Authors: | D Daby |
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Institution: | (1) Faculty of Science, University of Mauritius, Réduit, Mauritius |
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Abstract: | The oceanography around Mauritius (in the Western Indian Ocean) remains largely unstudied, hence there is an acute scarcity
of marine environmental data for management purposes. Rigorous water depth and current measurements were made on a system
of grids inside Le Morne lagoon (in the south western part of Mauritius) in March–April 2000 to generate semi-quantitative
models of general flow pattern in the form of contour maps using SURFER 6 computer programme. A simultaneous survey on composition
of bottom cover was conducted to examine possible relationships with current speed. A separate investigation recorded surface
and bottom currents prevailing amongst various habitat types to demonstrate the nature of the resulting damping effect on
surface current speed. Significant correlations generated from data analysis were discussed as a basis for real biophysical
relationships. Some of the limitations in the current analysis and some of the seemingly contradictory results are acknowledged
and addressed in the light of the general assumption that the structure of the lagoon is conditioned by current speeds. Much
stronger current speeds just outside the reef (e.g. >0.5 m s−1) than inside the lagoon (e.g. <0.32 m s−1) indicated a substantial slow-down of water current by the reef barrier. Inshore bottom currents were weaker than at the
surface and current speed correlated well with water depth. Bottom and surface current directions were generally similar,
i.e. going northward during flood tide and southward during ebb. The lagoon would be classified as ‘restricted’, exhibiting
well-defined tidal circulation, which is modified by wind forcing. Dense fields of branching Acropora corals slowed down surface current speed by as much as 87%, but the relationship between current speed and bottom cover appears
to be variable, depending on the specific location within the lagoon under consideration. The contour plots of the flow pattern
model generated reasonably high qualitative modelling of spatial current speed pattern in the lagoon, with stronger currents
generally along the reef areas, at the reef passes and in the deeper zones. However, these plots did not match closely those
displaying distribution of bottom cover, thus confirming results obtained from pair-wise correlation tests, namely the lack
of a significant relationship between current speed and bottom cover. Most of the correlations would appear to represent biological
relationships, with different types of communities enabling or excluding other types. Thus, the biophysical structure of the
lagoon would be driven a priori by the distribution and abundance of corals rather than current speeds, which contradicts the above hypothesis. Recurrent
natural hazards subject the benthic communities to a state of ‘perpetual knock-down and recovery’. Recovery, however, can
be seriously impaired by the chronic ongoing degradation of the coastal marine environment of Mauritius. An urgent review
of its coastal zone management and protection strategy would be desirable for the island. |
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Keywords: | biophysical interactions coastal management disturbance tropical reef lagoon complex |
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