The supply of phosphate in Tampa Bay, including Hillsborough Bay, 
far exceeds the demand by phytoplankton. According to Fanning and Bell 
(1985) no other estuary they know of has as high a phosphate 
concentration as the Tampa Bay system (average = 14uM). They attribute 
high phosphate concentrations to leaching of Florida’s phosphate beds, 
fertilizer drainage from agricultural lands, and industrial and sewage 
inputs. 
On the other hand, nitrogen supplies are probably the single most 
important limiting nutrient to primary production in Hillsborough Bay. In 
particular, evaluating inputs of ammonia is an important first step in 
assessing the nitrogen budget. Ammonia is the nitrogen form most readily 
assimilated by phytoplankton (Darley 1982, Pennock 1987), and the 
dominant inorganic nitrogen form released from the bottom (Nixon et al. 
1976, COT 1986b). Our measurement showing that sediment recycling 
supports a large fraction of the nitrogen needed for phytoplankton 
production (34%) in Hillsborough Bay are similar to the findings of 
Fisher et al. (1982) for each of ten shallow marine systems (mean = 
35+8.7%). Annually averaged 14 C productivity data for those ten systems 
ranged from 0.15 to 2.0gCm‘ 2 d _1 compared to 1.98gCm" 2 d _1 (Johansson et 
al. 1985) for Hillsborough Bay. Regardless of the system’s level of 
phytoplankton productivity, sediment recycling appears to supply roughly 
one-third of the water column nitrogen demand. Hillsborough Bay’s 
sediment recycling rates lend support to the Fisher, Carlson and Barber 
(1982) observation that system production and benthic nutrient recycling 
are functionally interconnected processes. However, it is important to 
realize that system productivity in estuarine systems, such as 
Hillsborough Bay, may primarily be driven by point and non-point nutrient 
sources, and not by sediment recycling. Increases in point and non-point 
nutrient inputs sustain greater levels of primary production and 
eventually create additional sediments of high organic content. The 
nutrients contained within these sediments are recycled and, in turn, 
further enhance the inorganic nutrient pool available to primary 
producers. 
As part of the FDER wasteload allocation study, McClelland (1984) 
produced a nutrient box model for Tampa Bay. He calculated that all 
non-point and point sources, including storm water runoff, only amounted 
to one-third of the nitrogen released from the sediments. Adding these 
inputs to the benthic fluxes still only account for about 50% of the 
nitrogen needed to support the observed primary production. Other sources 
of nitrogen are supplied by in situ water column regeneration and 
possibly sediment resuspension. Also, some nitrogen could be lost from 
the system by bacterial denitification. These are among several processes 
that have not been addressed in Hillsborough Bay or the Tampa Bay system 
in general. 
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