Runoff tends to be N-rich. For purposes of this talk, I di¬ 
vide the Patuxent into three different zones as shown in Figure 
3. One's the "upstream" area. One's what I have called loosely 
the "salinity transition" zone, and the other is the "deep stra¬ 
tified" zone. During the high-flow season, terrestrial runoff 
probably accounts for the very high availability of nitrogen and 
relative unavailability of phosphorus. During that period, 
there is nitrogen-rich runoff from urban sources, agriculture, 
and other land sources and there is relatively low demand in the 
water for these compounds. The result is a very nitrogen-rich 
system. 
We also have the input of water from the Susquehanna. Oddly 
enough, it's very important in delivering nitrogen to the Patux¬ 
ent River. The deep water actually "turns the corner" into the 
Patuxent because of the circulation of flow of an estuary, and 
carries lots of nitrogen into the estuary during the wintertime. 
But the winter is not the major growing season. We're in¬ 
terested in controlling the growth of plants during the major 
growing season which is the summer and the low-flow season, when 
the situation is different. 
The major external source during this period is phosphorus- 
rich effluent from upstream sewage treatment plants. The non¬ 
point sources are relatively low during low-flow, and point 
sources predominate. However, there is a "non-point source," if 
you will, of phosphorus coming from bottom sediments that in 
essence "buffers" the concentration of phosphorus at very, very 
high levels in the lower part of the estuary. This, in essence, 
make phosphorus very abundant relative to nitrogen in the estu¬ 
ary during the low-flow period. 
Given the knowledge of ratios and their seasonal variation, 
we did some enrichment studies at Benedict, Maryland. When I 
say "we," I'm talking mainly about Jim Sanders, Walt Boynton and 
Steve Cibik and myself, in which we were looking at the nutrient 
enrichment of outdoor, continuous culture, phytoplankton tanks. 
I will not dwell on the details; but basically, you add nitrogen 
or phosphorus and see what grows. What stimulates growth the 
most is the limiting nutrient. That is, if you add nitrogen and 
you get a big growth response, nitrogen is the limiting nutri¬ 
ent. 
Figure 9 indicates what we found: In the summertime if we 
added dissolved inorganic nitrogen compounds, we got a tremedous 
stimulation of growth in the tanks very soon after an experiment 
started. Natural phytoplankton communities were used that were 
isolated directly from the river. So one would expect that 
their nutritional condition when they were put into that tank is 
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