WATER QUALITY IS IMPROVING, BUT... 
The State of Florida, local regulatory officials, and bay 
scientists presently believe that water quality of Tampa Bay is 
improving, and that such improvements are the result of active regulation 
and management. There have been tangible improvements since 1974 in many 
key parameters and rooted vegetation is reappearing in shallow waters of 
Hillsborough Bay. How long can these improvements continue; what 
emerging problems could undermine such progress; and what are the natural 
constraints to bay recovery? 
Natural Conditions Affecting Water Quality 
1. Weather 
The bay areas experience one or two days of freezing temperature 
every year or two. Freezes result in fish kills in shallow waters and 
damage mangroves. Heavy leaf drop 1-3 months following freezes results 
in temporarily high detritus and particulate organic levels which are 
probably offset in subsequent years by reduced production in cold-damaged 
forests. Years of above-average rainfall or shorter periods following 
hurricanes result in heavy runoff, causing rivers to freshen throughout 
their length and the bays to have much lower salinity than usual. Heavy 
runoff also increases color and turbidity, and can result in fish kills 
due to salinity shock, periods of reduced oxygen, or both. 
2. Anoxia 
Hillsborough Bay is the only segment of either study area in which 
periods of partial to complete oxygen depletion have been documented. 
Oxygen stress is most severe near the bottom due to benthic respiration, 
phytoplankton self-shading, and the increased light path over channels 
dredged to 42 ft depths. Up to half of Hillsborough Bay’s surface area 
has experienced oxygen stress in particular years, resulting in 
defaunation of benthic invertebrates. Defaunation corresponds to times 
of anoxia, which occur most often in July, August, and September (Santos 
and Simon 1980). The extent to which anoxia in Hillsborough Bay is a 
naturally occurring event is not known, but some anoxic conditions 
probably occurred prior to urbanization due to the combined discharges of 
three rivers in a naturally deep arm of the bay where wind-driven mixing 
is limited. Anoxia occurs in Charlotte Harbor (south of Sarasota Bay) 
due to discharge of the Peace River. Anoxia in the Harbor is considered 
to be a naturally-occurring event because that bay is relatively 
pristine, so part of the oxygen stress in Hillsborough Bay is probably 
natural as wel1. 
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