In connection with a study of pollution in Biscayne Bay, at the southeastern tip of the Florida peninsula, special attention has been given to the hydrography of the area. Generally, in studies of the ecology of tidal estuaries, the exchange of waters is of great significance in determining
the character of the environment. Without a proper hydrographic investigation of the estuarine character of the area, it would be difficult to establish logical conclusions from the biological or bacteriological data. Another reason for this hydrographic study was the fact that very few
investigations of water circulation in shallow, bar-built estuaries, such as Biscayne Bay, have been performed. Due to the complexity of Biscayne Bay, all rigorous model estuaries would fail to apply to it. Therefore the hydrographic aspects present a specially difficult problem. Nevertheless,
by means of carefully planned observations a reasonably clear understanding of the water exchange in Biscayne Bay has been reached. Biscayne Bay is classed as a positive, shallow, tidal bar-built estuary, and the area studied is described in detail. Factors affecting the water circulation
are discussed and special attention given also to the discharge of Miami River. When the summer pollution situation of 1954 is compared with that of 1949 it is found that, due to the abnormally high summer river water discharge of 1954, pollution is less severe than would have been expected,
except for the area close to and south of the mouth of the Miami River. The general tidal conditions in Biscayne Bay are discussed as a basis for the tidal study of salinity and of the per cent saturation of dissolved oxygen. The horizontal salinity and dissolved oxygen distributions are
discussed and their relationship studied. The horizontal distribution of per cent saturation of dissolved oxygen at different tidal phases is given. Corresponding studies of surface currents and surface salinities are explained. The ratio between the tidal prism volume and the mean river
water discharge is found to be around 24. Finally the flushing number has been computed for the part of Biscayne Bay between Miami River and Government Cut and a most probable value of 0.3 has been derived.
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