In southern estuaries like Tampa Bay, thermal impacts associated 
with the heated water discharge initially caused the most concern among 
regulatory agencies. Thermal effects are the most visible of power plant 
impacts; steam rises from the water surface on cool days; rafts of foam 
float on the discharge water; and once abundant grassbeds are greatly 
reduced in size or disappear altogether. Several studies have been 
conducted to determine the ultimate impact of thermal discharges at power 
plants on Tampa Bay and in other nearby estuaries. Studies of the 
benthic fauna at TECO’s Big Bend Station on eastern Hillsborough Bay 
(Mahadevan, Culter and Yarbrough 1980) have indeed found that thermal 
effects are severe in the vicinity of the plant discharge. These effects 
are manifested by low overall faunal densities, an abundance of 
opportunistic and pollution indicator species and dissimilarities with 
unaffected open bay stations. These effects, however, were limited to 
the main discharge canal (Figure 3). Mild adverse effects represented by 
a higher incidence and abundance of opportunistic species and lower 
species diversity were limited to a 1 km area outside the plant discharge 
canal. Impacts caused by plant cooling water discharges were difficult 
to discern from the wide seasonal and year-to-year fluctuations in the 
benthic community (Figure 4). Overall, the studies concluded that 
adverse thermal effects were minimal. 
Impingement of fishes and macroinvertebrates on the travelling 
intake screens was also studied at Big Bend (TECO 1980b). These studies 
found that an average of 132 fishes and 125 macroinvertebrates were 
impinged per unit per day. Dominant species trapped on the screens were 
sand seatrout, bay anchovies, horseshoe crabs, and pink shrimp. Based on 
these studies it was concluded that impingement at Big Bend was 
negligible in comparison to the total population and the sport and 
commercial catch. These impingement rates were deemed to be acceptable 
at Big Bend. 
Studies designed to quantify the levels of entrainment of fish 
eggs and larvae through condenser cooling systems have been conducted at 
three of the five power plants on Tampa Bay. These studies found that 
entrainment levels were high at all three plants (Table 2). At Big Bend, 
for example, an estimated 86 billion (8.6 x 10*°) fish eggs and 26 
billion (2.6 x 10*°) fish larvae were entrained per year by the plant 
(Phillips, Lyons, Daily and Sigurdson 1977). The majority of these were 
eggs and larvae of forage species such as bay anchovies, silver perch, 
gobies and blennies. Annual entrainment by all five power plants located 
on Tampa Bay has been estimated to be 2.74 x 10 11 fish eggs and 8.30 x 
10j0 fish larvae which ultimately results in the annual removal of 2.84 x 
10 10 (nearly 3 billion) harvestable adults from the Tampa Bay commercial 
and recreational fishery (TBRPC 1985). Regulatory agencies ruled that 
entrainment levels at Big Bend were unacceptable and that offstream 
cooling or some alternate technology needed to be evaluated. 
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