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Temperature and water exchange in a semi-enclosed lagoon, Bamburi, Kenya
Kirugara, D. (2002). Temperature and water exchange in a semi-enclosed lagoon, Bamburi, Kenya. CORDIO-East Africa: [s.l.]. 3 pp.
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| Abstract |
In the context of high sea-surface temperature anomalies causing widespread coral mortality, this study investigates the heat balance of a semi-enclosed coral reef lagoon system and any additional contribution of UV radiation as a trigger for bleaching. The study lagoon is situated north of Mombasa Island, along the Nyali- Bamburi-Shanzu coastline, at 4°0’01" S, 39°0’44" E (Figure 1) and has a surface area of 3.75 km2 and 12.5 km2 during spring low and high tides respectively. It consists of three topographic features: the shallow back-reef lagoon, the 300 m wide, 7.5 km long reef crest that is exposed during low tide and shelters the lagoon from oceanic swells and the relatively deep central longitudinal channel that collects all lagoon water at spring low tide. The mean depth of the lagoon does not exceed 0.7 m and the width varies between 1.5 km and 2.0 km at MSL. The main channel system connects the lagoon southwards to Nyali lagoon through a 250 m wide, 5.8 m deep point. Towards the northern end of the lagoon is a shallower (2.5 m) channel system connecting the lagoon to the mouth of Mtwapa creek (Kirugara et al., 1998). The lagoon water circulation has been described previously and modeled by Kirugara et al. (1998). Lagoon water exchange is driven by the wave-induced flow that is dependent on the degree of the reef submergence by tide and wave conditions, the characteristics of the incoming swell and the difference between the oceanic and lagoonal tidal levels. During spring tides, more than 80% of lagoon water is exchanged at each tidal cycle. In neap tides, unlike many other coastal marine areas, the continuous pumping of water over the reef (wave-induced flow) into the lagoon maintains a higher lagoon sea level compared with the typical oceanic level. This forces a simultaneous exit of lagoon water through the channels modulated by the tidal regime ensuring that more than 60% of lagoon water is exchanged during every tidal cycle. These mechanisms ensure the efficient flushing of lagoon waters during 1 - 2 tidal cycles. The study monitors the following biophysical factors, integrating them over space and time: salinity and temperature inside the shallow lagoon and adjacent oceanic waters at 10 m to 30 m depth, solar radiative heat flux at the sea surface, and attenuation of this radiation into the lagoon and ocean, possibly differentiating UV and PAR. With this information an attempt will be made to estimate the heat balance for the lagoon on a long-term basis, taking into consideration the different monsoon periods. |
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