spring-tide, neap-tide cycle. Therefore, time scales 
influencing stratification range from 1 year for major fresh 
water inflows, to monthly for places like the York River, to 5 
to 10 days for mixing driven by meteorological events. 
Advective processes, those which move water without 
necessarily mixing it, also occur on various time-scales. 
Internal waves can move water vertically on scales of minutes to 
hours. These were first suggested and possibly observed by 
Biggs and Flemer during close time-series oxygen measurements 
made in Chesapeake Bay. Local wind forcing can advect surface 
waters across the Bay. The lower layer flows upwind to maintain 
hydrostatic equilibrium shifting the tilt of the picnocline on 
time-scales of a few days. Data collected by Tom Malone and his 
group illustrate this phenomenon in Slide 4. The upper panels 
show both the picnocline and the oxycline tilting downward to 
the west on 20 August 1984. 
By 23 August, as shown in the lower panels, local winds have 
shifted the tilt downward to the east. This process has been 
implicated in "jubilees" during which crabs and fish come 
inshore to escape low-oxygen water moving into the shallows. 
To summarize, there are some historical data with which we 
can compare recent information and suggest that changes have 
occurred adversely effecting biological resources in Chesapeake 
Bay. Although we belive low-oxygen concentrations in deep water 
is a normal feature of summer in Chesapeake Bay, indications are 
it has increased in spatial and temporal extent. Deterioration 
with respect to oxygen is most likely caused by increased 
material both entering the Bay and being produced in it in 
response to inorganic nutrient inputs. Recent careful studies 
have shown that summer anoxia can be interrupted by local mixing 
events, and that, once formed, anoxic water may be advected from 
place to place by local and far-field meteorological events. 
Such variability must be accounted for as we assess the 
long-term trend. It will also hamper our ability to detect 
improvements in conditions in response to management actions 
implemented for both point and non-point source nutrient inputs. 
I would like to stop there; and if there are any questions, 
I'd be happy to try to answer them. Yes? 
Question: Would you care to venture an opinion as to 
whether or not the trend that you've seen is related to our use 
of the Bay as opposed to being what might naturally happen in a 
body of water under relatively pristine conditions, 
nevertheless, natural history for a body of this type? 
95 
