The State of Maryland has quite a good mathematical water 
quality model for the Patuxent River. The model includes one 
very important feature, which is sediment-nutrient release. I 
would agree — and I don't have time to develop the argument 
very much here — that while mathematical models are pretty good 
in telling us approximately how much of a pollutant is delivered 
to a given area of an estuary, they're not very helpful for dis¬ 
tinguishing whether nitrogen or phosphorus is the critical 
element to control. Much of the controversy regarding N and P 
in the Chesapeake centers on what I think is an over-reliance on 
such models by managers. 
Monitoring studies have given us excellent information on 
dissolved inorganic nitrogen (DIN) elemental ratios. If one 
looks at the nutrient concentrations at Benedict, again remember¬ 
ing that what's there in least supply is likely to be the limit¬ 
ing nutrient. Figure 8 illustrates an excess of dissolved in¬ 
organic phosphous (DIP) in the summer and very little DIN, and 
vice versa in the winter. Nitrate, nitrite, and ammonium are 
forms of nitrogen. And phosphate is a form of phosphorus. In 
Figure 8, you see that there is a very high peak of nitrogen in 
the wintertime and a very high peak of phosphorus in the late 
summer. When you take the ratio of dissolved inorganic nitrogen 
to phosphorus, we get something analogous to a fertilizer ratio 
used by farmers and gardeners. We see that the river is ex¬ 
tremely nitrogen-rich in the winter (DIN:DIP > 90:1) and very 
nitrogen-poor in the summer (DIN:DIP < 5:1). What does that 
mean? 
It means that the relative abundance of nitrogen in the win¬ 
tertime is much greater than it is in the summertime and that 
the relative availability of phosphate in the summertime is much 
greater than it is in the wintertime. 
As I said, commercial fertilizer constitutes a good anal¬ 
ogy. Plants need nutrients in certain ratio. The 10-10-10 or 
other ratio you see on a fertilizer bag tells whether it is 
ideal for vegetables, lawns, et cetera. Since algae are plants 
also, they as well need an ideal supply ratio of nitrogen to 
phosphorus. 
It turns out that the ratio of nitrogen atoms to phosphorus 
that the average planktonic alga needs is somewhere between 10 
and 20 to 1. So as we can infer from Figure 8, in the summer¬ 
time phosphorus is relatively abundant and nitrogen is relative¬ 
ly scarce, and vice versa in the wintertime. 
Secondarily treated sewage effluent is extremely P-rich. 
The ratio of nitrogen to phosphate in sewage is very, very low, 
typically 5 or 6 to 1. So sewage is very phosphorus-rich rela¬ 
tive to plants' needs. 
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