Wind Induced Resuspension in a Shallow Tropical Lagoon

In shallow environments, particle resuspension can induce large ecological effects. Under some certain conditions of fetch, wind velocity, bathymetry and bed roughness, resuspension is generated by wind induced waves. During December 1991, a shallow station (1 m depth) in the north shore of a tropical lagoon (Côte dʹIvoire) was investigated in order to study the impact of wind induced resuspension on the ecosystem. In this area, Austral Trade winds are dominant almost all year long, and their velocity shows a marked diel pattern. During the survey, three sequences were distinguished: a period of Austral Trade winds (with possible resuspension), a period of Boreal Trade winds (no wind induced waves at the station) and a period of transitional Trade winds. Only Austral Trade winds with a speed >3 m s-1 allowed particle resuspension. For chlorophyll, mineral seston and ammonia, significantly higher values were noted during the windy sequences. Conductivity and water colour varied in relation to tides. Granulometric and mineralogical analyses showed that only the 0-3 cm superficial level of the sediment was involved in resuspension. This process induced several effects: (1) an increase of suspended matter concentration in the water and thus a light attenuation due to a higher turbidity, (2) a distribution in the whole water column of nutrients from the pore water, (3) a modification of the sediment granulometric characteristics and (4) an increase in the food available for planktonic filter feeders since algal cells were periodically resuspended in the whole water column. Wind induced resuspension occurred in 10% of the Ebrié lagoon. In this area, the daily alternate of resuspension-sedimentation sequence is then a major factor controlling the productivity of a system which is potentially highly productive (high nutrient load, favourable climatic conditions) yet characterized by high turbidity. These observations can be generalized to comparable systems in the tropical area.