Tides, circulation and water masses in Maputo Bay

Abstract

Maputo Bay is a large and relatively shallow bay in the sothern part of Mozambique. It is strongly affected by tidal circulation and rich in marine life. It is important or finishing, tourism recreation and shipping, but also as a recipient for sewage from the city of Maputo. In this thesis, hydrography, circulation and water masses in Maputo Bay are discussed on the basis of sea level, velocity, temperature and salinity observations from two sites near the entrance of the bay. Observation were made during 35 days in March-April 1993. The maximum tidal velocities exceed 1 m/s, whereas the maximum salinity reached 35.3 on the oceanic side. The minimum salinity was approx. 33, indicating a non-negligibleb supplu of freshwear. The temperatures varied by about 1ºC on the diumal time-scale and decreased from 26.5ºC to 24.5ºC during the period of observations. From harmonic analysis we found a purely semidiurnal the with neap and spring tidal ranges of o.7 and 2.5 m, respectively. The current velocity lags approx. 90-120º behind sea level. A distinct salinity pattern was seen; the lateral salinity gradients were 0.05-0.1 psu/km and the eastem side was more saline than the western. Buy study salt and volume fluxes as functions of salinity, we found a net volume fluxes (residual circulation) of 300 m³/s basically outwards in the salinity interval from 34.7- 34.9 and inwards from 34.9 to 35.2. The tidal fluxes were of the order 10 m³/s, whereas the net circulation was just a few cm/s. The salinity data indicates that the residual circulation clockwise, with an inflow on the eastenrn side and an outflow on the wastern. During the period of observations, we found a freshwater supply of 200-300 m³/s. The residence time for the Maputo Bay water vs the oceanic was estimated to approx. 2 weeks.