We've seen some striking changes in the Patuxent River over 
time in other regards. Nutrient-stimulated increase in tur¬ 
bidity alone is not the sole manifestation. Turbidity could 
also be due to increased sediment loads, for example. However, 
present evidence suggests that there was an increase in 
phytoplankton growth and that an accumulation of algae biomass 
decreased water clarity. 
If we compare older data with newer data on the Patuxent 
River, we have seen a year-round increase in phytoplankton 
counts at the Benedict area, the "salinity transition" zone of 
the river. Concomitant with that, if we compare minimum dis¬ 
solved oxygen concentrations in 1938 with 1978 June and August 
data, we see a strikingly lower dissolved oxygen content in deep 
water in recent years as depicted in Figure 6. 
To be sure, the main stem of the Bay also influences down¬ 
stream oxygen concentrations. However, in the Benedict-Sheridan 
Point area, there's a particularly pronounced oxygen sag. This 
has been identified by the State of Maryland as a critical zone, 
an area that we see as being a particularly impacted area, and 
one that I am optimistic that we can improve. 
Conventional dogma states that fresh waters are phosphorus- 
limited and marine waters are nitrogen-limited. If one wants to 
make any inroads to controlling the oxygen depletion/nutrient 
problems in the Bay, ideally one wants to make the nutrient 
that's in shortest supply in even shorter supply. 
So in freshwater, where phosphorus is usually in shorter 
supply than nitrogen what one generally does is remove phospho¬ 
rus. In marine waters we haven't had very much experience in 
terms of nutrient-control strategies; but we do know that gen¬ 
erally speaking nitrogen is in least availability and that 
nitrogen (N) is the element of major concern for management. 
Because estuaries lie in between freshwater and sea water 
areas, the question arises, naturally, what limits the growth of 
algae in estuaries, and what would we want to control? 
How can one determine which nutrient is a limiting nutrient? 
Studies to do so have fallen into four general categories as 
described in Figure 7. 
Enrichment studies are probably the most direct way of 
finding out which nutrient element limits plant growth, because 
one merely takes a parcel of water and adds nutrients to see 
what grows in response to nutrient enrichment. 
Elemental ratios of nutrients dissolved in the water are 
extremely important in giving a general idea of what's available 
in excess. But there are also some problems associated with 
that approach. 
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