Tides, tidal currents and their effects on the intertidal ecosystem of the southern bay, Inhaca Island, Mozambique

Sediment characteristics and tidal currents were studied in the 1500 ha intertidal area south of Inhaca Island, Mozambique. The tide is semi-diurnal with a range at spring of about 3 m. The area connects directly to the ocean through the Ponta Torres Strait and (indirectly) through several narrow tidal channels ending up in Maputo Bay. Velocities of up to 0.75 m s(-1) were measured in the Ponta Torres strait. After Low Water, Indian ocean water starts entering the tidal flats, while entrance from the Maputo Bay side is delayed by one to several hours. With only one tide gauge and one current meter at hand, we found indications of a higher mean sea level on the oceanic side (probably due to ocean wave set up) and a mean flow in the Ponta Torres Strait directed towards Maputo Bay. A storm from the SW had a particularly strong influence on the sea level of Maputo Bay, and for a period of a few days the current flowed constantly towards Maputo Bay. Measurements in the innermost Saco channel indicated higher velocities during ebb than during flood. The resulting residual current will increase sedimentation in the bay, a process which is confirmed by aerial photographs and could be linked to the sanding of the coral reefs and the mangrove extension in the area. Extensive sandbanks are situated around both inlets as a result of the higher current velocities in these areas, thereby inhibiting the settlement of finer sediments and organic matter. Where the two flows meet, formation of mudflats and seagrass beds are favoured. Mudflats are found mainly below High Water Neap, exposed only during spring tides. This longer inundation period favours the settlement of finer material and organic matter. The area and location of the substrates have great influence on the richness and abundance of the benthic fauna, as grainsize and organic matter have been correlated with richness and abundance in other studies.