number. The few Antillean forms that do occur in Florida Bay are found normally only 
along the edge of Florida Bay where it mixes with waters of the Straits of Florida and 
the Gulf of Mexico. Generally, the Antillean faunal elements are confined to waters 
having stable salinity and a high degree of clarity. Many of the common animals of the 
study area are extremely rare or absent in day time collections. The flora and fauna of 
the study area are regulated seasonally by temperature and salinity changes. However, 
the region has been divided into major habitats based on substratum characteristics as 
well. The inshore mudbanks and supratidal marl prairies support an impoverished biota 
able to survive the extremes of heat, cold and desiccation that prevail in those areas. 
The majority of the animals are burrowing forms that live in the upper layers of marl 
mud in the intertidal zone, under debris in the shore drift or construct deep burrows to 
water in the supratidal area. The turtle grass beds of the Florida Bay mudbanks form 
the largest single community restricted to a somewhat uniform substratum material. 
Greatest numbers of species, both plant and animal, were found in the stable high 
salinity region between Sandy Key and East Cape Sable. Areas of very high and very 
low salinity showed marked reduction in numbers of species. If turbidity became heavy 
in either hypersaline or very low salinity waters the numbers of species were reduced 
even further. The plant and animal populations were always greatest when salinity 
values were between 30 and 45 %o. The numbers of species, and numbers within 
species declined with declines in salinity. An offshore movement of animals from bays 
and the estuary, begun each year by falling salinity during the rainy season, was 
hastened by a somewhat abrupt decrease in temperature characteristically beginning in 
November and lasting through January. The widest variety of species in Florida Bay 
was usually found during November-December as migrant animals from further north 
along the Florida coast were joined by species from local inshore areas. The normal 
November decrease in average temperatures was usually preceded by an oxygen 
depletion period in the shallow swamp ponds and lakes. This “bad water period" 
concentrated small fish from the swamps along the bay margins where they were 
heavily preyed upon by birds and larger fishes. Temperature fluctuations in southern 
Florida were seldom severe enough to cause mass mortality. There was, however, an 
annual offshore movement of Coot and Whitewater Bay species in November, December 
and January. Many of the animals participating in this movement were mature 
individuals of eurohaline species such as the blue crab, Callinectes sapidus and the 
striped mullet, Mugil cephalus, that are sea spawners in spite of wide temperature and 
salinity tolerances as adults. Other species, exemplified by the pink shrimp, Penaeus 
duorarum, use the bays as nursery grounds and leave these regions upon attainment of 
a certain size. Generally these begin leaving the estuaries prior to the onset of 
November cold weather but the offshore movement is accelerated by cold temperatures 
in shallow water. The offshore movements were most obvious in dry years when the 
bays fill with a variety of species and least obvious in years of heavy runoff that 
create low salinities in the back bays that exclude most marine species. Mass mortality 
of fishes due to cold was observed during December, 1957 when water temperatures of 
14-16°C were recorded. A similar cold wave in February, 1958 caused no detectable 
kill and indicated a tendency to acclimatize on the part of the sensitive species. These 
studies have made it possible to describe probable past alterations in the ecology of the 
study area and to predict changes in the plant and animal communities under differing 
temperature, rainfall and runoff conditions. As a result, recommendations can be made 
as to the quantities of water and the runoff pattern likely to result in the greatest gain 
to the plant and animal community. These findings can probably be applied to other south 
Florida estuaries and, with due consideration to differences in rainfall patterns and 
temperature variation, can be useful in predicting the effects of man made changes in 
major estuarine systems of the southeastern United States and the Gulf of Mexico. 
Most of the significant changes in estuaries of this area are brought about by alteration 
in salinity and turbidity patterns. These changes are hastened by such developments as 
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