In studies of North Carolina estuaries, Paerl (1982, 1983) 
has demonstrated that cyanobacteria require quiescent or stable 
water conditions in order to form surface scums or blooms. In 
the tidal fresh water region of rivers, water column stability 
is enhanced by a reduction in river flow, by high solar radia¬ 
tion which leads to thermal stratification, and by decreased 
winds. The effect of these factors on bloom formation in the 
tidal Potomac is emphasized in a study by M.P. Sullivan (1985) 
at the Metropolitan Washington Council of Governments. He re¬ 
viewed 36 years of hydrometeorological data for the Potomac Ri¬ 
ver and developed an environmental index which included all of 
the aforementioned factors. As shown in Figure 1, the index, 
which ranges from a highly unfavorable -6 (high flow, high wind, 
low solar radiation and temperature) to a highly favorable +6 
(low flow, low wind, high temperature and solar radiation), cor¬ 
relates well with average summer algal biomass determined from 
twelve summer blooms since 1965. Bloom severity is quantified 
as average surface water chlorophyll-a values from the upper 70 
kilometers (km) of the tidal Potomac River. These data il¬ 
lustrate that hydrometeorological processes play a significant 
role in regulating bloom phenomenon and must be considered along 
with nutrient loading data in evaluating historical trends of 
blooms. Sullivan further suggests that the particularly severe 
and unexpected cyanobacteria! bloom of 1983 may have resulted 
primarily from a highly unusual combination of low wind, high 
solar radiation and temperature, and low river flow which oc¬ 
curred during that summer. According to his probability analy¬ 
sis from 36 years of data, the 1983 index value of ca. 5.5 will 
occur only about 3 times in a hundred years. 
The magnitude of the 1983 Potomac River bloom illustrates 
that the system still harbors sufficient nutrients to support an 
abundant algal biomass. The most likely source of nutrients for 
the 1983 bloom appears to have been the sediments and not direct 
point source additions. The Expert Panel, convened to study the 
1983 bloom, hypothesized that a high pH, resulting from the 
depletion of carbon dioxide from the water column following high 
daily rates of photosynthesis, increased the flux of phosphorus 
from the sediments to the water column where it was available 
for phytoplankton growth (Expert Panel, 1985). Research this 
past year by Sybil Seitzinger at the Philadelphia Academy of 
Natural Sciences, who measured the rate of phosphorus release 
from isolated, intact sediment cores from the Potomac River 
subjected to various levels of pH, supports this hypothesis. 
Can we expect surface scums of cyanobacteria to occur down¬ 
stream in high salinity regions of the estuary if nutrient and 
other environmental conditions in those regions become favorable 
for bloom formation? Recent research by Paerl et al. (1984) in 
North Carolina and Kevin Sellner working in the Potomac River 
102 
