COASTAL TIDALLY-DRIVEN CIRCULATION AND THE ROLE OF WATER EXCHANGE IN THE LINKAGE BETWEEN TROPICAL COASTAL ECOSYSTEMS

Water circulation and exchange processes in a shallow, semi-enclosed t ropical bay were studied in southern Kenya (Gazi Bay) through measurem ents of tidal elevations, salinity, temperature, dissolved oxygen and current velocities at stations established in mangrove creeks, seagras s beds and coral reef zones. Occurrence of wide shallow entrance, lack of topographic controls (sills) and the orientation of the Bay entran ce with respect to dominant tidal water circulation patterns, accounts for the high rates of exchange (60-90% of the volume per tidal cycle) between the inshore and offshore waters. High flushing rates are coup led with short residence times in the order of 3-4 h. The dominant wat er circulation driving force is the semi-diurnal tide, causing a stron g reversing current in the mangrove creeks (0.6 ms(-1)) and low magnit ude current in the seagrass and coral reef zones (<0.30 ms(-1)). Tidal asymmetry, characterized by stronger ebb flows than flood flows in th e mangrove creeks, partly promotes the net export of organic matter to the seagrass beds. The brackish and turbid water plume in the mangrov e creeks and south-western region of the Bay is trapped along the coas t and in the mangrove swamp, and does not reach the coral reef. The fr eshwater influx via rivers and direct rainfall in the Bay accounts for a volume of 305 000 m(3), of which 20% is lost as a result of enhance d evapotranspiration, which is also responsible for a salinity maximum zone (38) in the upper region of the Bay covered by mangroves. (C) 19 97 Academic Press Limited.