in oceanic waters, is highest (ca. 45%) at the highest salinity 
and decreases rapidly to zero percent at about 10 o/oo. These 
are extremely high cell numbers for phytoplankton and consider¬ 
ing the moderate levels of chlorophyll observed in the James Ri¬ 
ver during this period (ca. 10-13 ug l 1 ), cyanobacteria pro¬ 
bably accounted for most of the primary production. The dif¬ 
ferences in cyanobacterial abundances in the Lower Bay for 
August 1983 depicted in Figures 2 and 3 probably reflects the 
reduced efficiency of inverted microscopy to accurately count 
these small cells. Since there is no historical record for cya¬ 
nobacterial abundances in the Chesapeake Bay plankton, it is 
difficult to speculate if these high concentrations of cyano¬ 
bacteria observed with epifluorescent microscopy represent a 
"normal" condition or long-term response to nutrient enrichment. 
The presence of high numbers of coccoid cyanobacteria has 
implications for other aspects of the plankton ecosystem. Phyto¬ 
plankton constitute the base of the food web in the Chesapeake 
Bay; they are the primary source of fixed carbon which supports 
all the higher trophic levels. This process is illustrated in 
Figure 4 which depicts the quintessential marine food chain 
starting with a large diatom consumed by a copepod which in turn 
is consumed by a planktivorous fish consumed by a top 
carnivore. With a large phytoplankton at the base, only one 
intermediate level is needed before a fish gets its food. 
However, in a plankton ecosystem where coccoid cyanobacteria 
dominate, this is not the case. Figure 5 depicts the trophic 
relationships thought to dominate in the North Pacific Gyre 
where coccoid cyanobacteria dominate the phytoplankton, and I 
believe it is a realistic representation of trophic processes in 
the Lower Chesapeake Bay during the summer months. Here we see 
cyanobacteria being consumed by protozoan flagellates which, in 
turn, are consumed by a ciliate. This ciliate may then be 
consumed by a copepod. Compared to the Figure 4, where there 
was only one step between a phytoplankter and a copepod, there 
are now potentially two levels. Since each added level in a 
food chain or web necessarily reduces the amount of energy 
reaching the higher trophic levels, the presence of substantial 
numbers of picophytoplankton at the base of the food web may 
reduce production or alter the species composition at the higher 
levels. 
Protozoan grazing of cyanobacteria in the Chesapeake Bay 
has been demonstrated (Haas, 1982) and Hans Paerl has described 
protozoans grazing the interior of a M. aeruginosa colony in a 
tidal freshwater habitat. 
107 
