A cyanobacterial-based food web may have significant imp¬ 
lications for nutrient cycles in estuaries. For example, dia¬ 
toms are generally large cells which may sink rapidly out of the 
euphotic zone to the sediments. Furthermore, diatoms are grazed 
primarily by copepods which produce fecal pellets containing 
significant amounts of carbon, nitrogen, and phosphorus. These 
pellets sink rapidly to the sediments where they are likely to 
be remineralized to forms of nitrogen and phosphorus which, 
under suitable conditions, leave the sediments and are once 
again available to the phytoplankton. This may be a long-term 
cycle and because of the net up-estuary flow of bottom water in 
estuaries, the nutrients tend to be retained in the estuary 
rather than being flushed out. In contrast, cyanobacteria are 
small and do not sink out of the euphotic zone. Furthermore, 
they are grazed primarily by protozoans which excrete soluble 
nitrogenous waste products which are immediately available for 
phytoplankton use. This rapid recycling of nutrients may, in 
fact, help to maintain cyanobacterial blooms. However, by being 
retained in the surface waters, nutrients are more likely to be 
flushed out of the system by down-estuary surface flows. Thus, 
nutrient cycles may differ with respect to the principal agents 
of recycling, and the sites and rates of recycling between 
phytoplankton communities dominated by coccoid cyanobacteria and 
those dominated by larger phytoplankton. 
In conclusion, it appears that the tidal freshwater Potomac 
River still contains sufficient nutrients to support a major 
bloom. It is apparent that in future work in this region, more 
attention must be accorded to geochemical and biological pro¬ 
cesses which regulate the flux of nutrients between the sediment 
and water column. The recent emphasis in sediment processes re¬ 
lated to nutrient cycling illustrates the need to consider the 
total ecosystem when considering plankton processes. Experiments 
on phytoplankton-nutrient interactions may be performed in bot¬ 
tles, but it is important to remember that phytoplankton don't 
live in bottles; they exist as part of a complex ecosystem with 
air-water and sediment-water interfaces and are subject to meteo¬ 
rological as well as hydrologic influences. 
In the more saline portions of the Chesapeake Bay, I think 
we are only beginning to realize the potentially important role 
played by cyanobacteria in planktonic processes. Their contri¬ 
bution to primary production during the warmer months and their 
impact on food chain and nutrient recycling processes are likely 
to be substantial. It is unfortunate that the recently insti¬ 
tuted Chesapeake Bay phytoplankton monitoring programs in both 
Maryland and Virginia, which are designed to provide baseline 
data on phytoplankton abundance and composition into the 21st 
century, are not using enumeration techniques which are capable 
of accurately counting cyanobacteria. 
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