Supporting data are given in figure 5-31. The inference can be drawn that as 

 NO 3 concentration even at times of low production is zero at salinites below 

 the freshwater regime (<0.5 ppt), very little of the nitrate in the fresh- 

 water flow ever reaches the estuary. The difference between what is required 

 to support estuarine production and the potential riverine/land run-off supply 

 is so great that it is probably safe to suggest that this source is unlikely 

 to be important to production in Maine estuaries. Specific data for 

 individual estuaries are discussed below. 



Sewage-derived nitrogen can represent a major nitrogen source to an estuary 

 and can even lead to the production of excessive blooms (excessive abundance) 

 of phytoplankton in the summer months (Carpenter et al. 1969). The siting 

 historically, and the present potential development of major population 

 centers on estuaries, make the disposal of sewage nitrogen for these urban 

 centers a potentially serious problem. The New York/New Jersey metropolis on 

 the lower Hudson River estuary, for example, has a population of 16 million, 

 and discharges 160 t (180 tons) of nitrogen daily, enough to have a measurable 

 effect not only on the estuary but also on the productivity of adjacent 

 coastal waters to a distance of 16 miles (25 km) from the mouth of the estuary 

 (Garside et al. 1976.). Except for areas of major population centers, little 

 impact of sewage disposal on nitrogen concentration can be expected in most 

 Maine estuaries except in specific locations (see chapter 3, "Human Impacts on 

 the Ecosystem") . 



Regeneration of nutrients by heterotrophic activity combined with the unique 

 circulation patterns in estuaries may serve as a means by which nutrients, 

 once within the estuarine system, may pass repeatedly through the food chain 

 (Schelske and Odum 1961). This mechanism, often termed a "nutrient trap," has 

 the potential to provide high primary production for a given supply of 

 nutrients. The nutrient trap functions in the following way. Nutrients in 

 the surface layer are consumed by phytoplankton. Phytoplankton are 

 transported downstream by the surface flow and either die and sink or are 

 grazed. Resultant detrital material settles to the bottom layer or to the 

 bottom. Once within the bottom layer the detrital particles in most Maine 

 estuaries are transported upstream (layered tidal inflow) and decompose, or do 

 so in the sediments. In either instance the regenerated nutrients eventually 

 are released to the bottom flow and are transported upstream, during which 

 time they are progressively mixed back into the surface layer. Once in the 

 surface layer they are again available for the growth of more phytoplankton. 

 This mechanism helps enhance the productivity of estuaries. 



The nutrient recycling processes described above function to some degree in 

 all estuaries, yet the biological and physical parameters often are extremely 

 difficult to measure and where measurements have been made, they are only 

 incidental to the study, such as benthic regeneration (Nixon and Oviatt 1972) 

 and zooplankton excretion (Smith 1978), and food habits of the pelagic 

 menhaden, mackerel, and herring (Oviatt et al. 1972). Although such studies 

 are of value in providing some general insight into specific processes or 

 locations, opinions of the relative importance of nutrient regeneration are 

 often diametrically opposed and preclude direct application to Maine 

 estuaries. For example, Martin (1968) considered benthic nutrient 

 regeneration to be capable of supplying all of the nutrient requirements of 

 phytoplankton in Narragansett Bay in August, whereas Carpenter et al. (1969) 



5-58 



