stem of the Bay, we're looking at the largest problem in terms 
of volume of water to clean up first. We really need to be 
focusing more on the tributaries, the areas that are smaller in 
water volume but where I think we have the best chance of doing 
something in the near-term. The salinity transition zones of 
tributaries are important in the tributaries. 
Figure 4, derived from the "Heinle Report" to EPA, illus¬ 
trates that the nutrient enrichment sequence as shown has occur¬ 
red on the Patuxent. There are two points to note. First, down 
stream of the Benedict Bridge is where you start to see the 
higher salinities in the river. Even in the old data, 1936 to 
1939, in the estuarine portion (downstream of Benedict Bridge), 
we had historically high concentrations of phosphorus in the 
water column in the summertime. In more recent data, 1968 and 
after, we see again the same kind of a pattern, very high concen 
trations of phosphorus in the summertime. Although in general, 
phosphorus levels are quite high the year-round. Second, there 
have been drastic increases in phosphorus upstream of the bridge 
in fresher waters. 
How might those concentrations affect algal growth? In the 
business of nutrients, the rule of thumb is, if it's there, it's 
probably not as important as it is if it's not there. A limit¬ 
ing nutrient is one that controls the growth of plants by its 
scarcity. By virtue of the fact that it's not present in 
abundance, it controls plant growth. Plants need nutrients to 
grow. We put fertilizers on our gardens to supply more of a 
growth-limiting nutrient to increase the concentrations. 
The high summertime phosphorus levels downstream suggest 
that there is something going on in that area of the estuary 
that is putting phosphorus into the water column in excess of 
algae demand and from a source that we might not be able to 
control very effectively with a traditional management strategy. 
Next in the sequence of enrichment effects that I 
illustrated, turbidity increases as phytoplankton grows in 
response to nutrients. The increase in turbidity is indicated 
by Secchi depth which shows how far you can lower a small white 
disk before it disappears. Obviously, the deeper you can lower 
it, the clearer the water. 
Well, in the "good old days," you could lower the Secchi 
disk deeper in the southern part of the river, of the Patuxent 
River, before it would disappear. In more recent data, we see 
that the Secchi depths are considerably less than they used to 
be as illustrated in Figure 5. This is some of the most 
important evidence that was adduced in the EPA Chesapeake Bay 
study that has led to a lot of the further refinements in our 
knowledge. 
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