Slide 3b shows a similar plot for data in the month of May 
1950 and 1980. The 1950 data give a regression which is 
virtually identical to the steady-state model constructed with 
the 10 years of summer data. The May 1980 data, however, 
diverge significantly from the model. Data points in the upper 
left of the plot represent sharp reductions in oxygen concen¬ 
tration over relatively small salinity increases. This might be 
explained by large differences in the spring freshet which 
typically peaks during April. The average Susquehanna River 
flows in April were 85,000 cubic feet per second (cfs) in 1950 
and 94,000 cfs in 1980. During May of both years, the flow 
averaged about 39,000 cfs. It is not clear that the circulation 
effects of an 11% increase in April flow rates would have 
resulted in the sharp difference between the two plots in Slide 
3b. However, it would be reasonable to expect greater oxygen 
demand in the deeper waters to give such a result. The organic 
matter stimulating greater oxygen consumption in May 1980 could 
have been delivered with the spring freshet, or it could have 
autocthonous material from recent or previous production. From 
this minimal amount of data, we might suspect that there was 
significantly more organic material in the deep layers of the 
Bay in May 1980 than in 1950, under similar river flow regimes. 
Returning to the annual cycle of events in the Bay, let us 
consider the time scale of some of the major processes. The 
spring freshet maks the onset of oxygen decline in the main 
portion of the Bay. The freshet delivers fresh water to the 
system which influences the stratification through the processes 
discussed by Bill Boicourt. The water also delivers organic 
material which can be decomposed in the Upper Bay, thereby 
reducing oxygen concentration. The freshet also delivers 
nutrients to the Main Bay which are utilized by phytoplankton, 
through the processes described by Chris D'Elia, which increases 
the organic matter in the system and the ultimate oxygen 
demand. Thus, the spring freshet can be a pulse source of both 
organic material and the nutrients required to produce new 
organic matter within the Bay. Decomposition then feeds 
nutrients back into the system so that cycles are established 
and keep functioning through the year in the absence of 
additional strong inputs from the watershed. 
The vertical stratification portion of the annual cycle has 
the general form depicted in Slide 2, but it is subject to local 
modifications and to far-field forces affecting advection on 
intermediate time-scales. Bill Boicourt gave examples of local 
mixing, such as wind mixing and mixing due to turbulence over 
the tidal cycle. There is some potential each time the tide 
changes for mixing to occur near the picnocline. Comprehensive 
vertical mixing occurs in the York River Estuary on a 
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