are discussed below in terms of the depth of the entrance of each estuary and 

 the expected offshore nutrient distribution (Garside, unpublished ) . 



Land surface runoff and river flow are potentially significant sources of 

 nutrients to estuaries. When large areas of cultivated land lie within an 

 estuary's watershed substantial quantities of nutrients from fertilizers 

 applied to agricultural land can be carried to the estuary by surface runoff. 

 This nutrient source is not likely to be very important in Maine estuaries, 

 because only a small proportion of the land in estuarine watersheds is used 

 agriculturally. Nevertheless nutrients in river flow from natural systems 

 (wetlands and lakes) cannot be ignored and should be examined on the basis of 

 measured nitrogen concentration in river water. 



The United States Geological Survey (USGS) maintains flow gage and water 

 quality monitoring stations on a number of major rivers in the State. 

 Nitrogen concentrations were measured as nitrate (NO^ , the most oxidized form 

 of organic nitrogen) and Kjeldahl nitrogen. The latter term refers to all 

 forms of nitrogen determined by the Kjeldahl technique and includes ammonia 

 (NH3 or NH4 , the most reduced forms of inorganic nitrogen) and both dissolved 

 organic nitrogen (DON) and particulate organic nitrogen (PON) if the sample 

 was not filtered before analysis. Ammonia and nitrate nitrogen are directly 

 available to phytoplankton (Dugdale and Goering 1967), but most other forms of 

 organic nitrogen first must be broken down by bacteria before they can be used 

 for plant growth. 



Of the rivers for which data are available (Androscoggin, Kennebec, Penobscot, 

 and St. Croix) all have higher Kjeldahl nitrogen in winter than in summer, 

 although some have low values during certain winter months and high values 

 during some summer months. It is difficult to be precise in this analysis and 

 much of the scatter in the data may be attributable to methodology. However, 

 if the general summer/winter trend is significant it is probable that high 

 Kjeldahl nitrogen when temperatures are low represents high DON rather than 

 NHo . Temperatures sufficient to allow bacterial conversion of organic 

 nitrogen to ammonia probably also would allow the bacterial oxidation of 

 ammonia to nitrate. 



During summer months, it is not possible to make such clear distinctions, 

 since heterotrophic activity can supply ammonia rapidly (Garside et al. 1978). 

 Summer concentrations of nitrate or ammonia plus DON are small in most 

 instances. Nitrate concentrations do not exceed 13 ^H during the summer 

 months and are often 1 yM to undetectable and ammonia concentrations are not 

 likely to double the total nitrogen concentration. 



For the Sheepscot estuary using a mean summer daily flow of 2.7 x lO^ m /d 

 (USGS 1967 to 1976 data) and assuming 10 mM (based on data for the Kennebec 

 watershed) this source would supply 37.5 kg/N day. Assuming that 

 phytoplankton assimilate 1 g C/m /day and have a carbon to nitrogen 

 assimilation ratio of 6:1 (by atoms) this production would require 0.194 g N/nr 

 /day (Garside et al. 1978). Thus, the total river supply would support 

 primary production over an area of about 0.2 km^ . The Sheepscot estuary south 

 of Wiscasset has an area of about 50 km , so that land runoff can support only 

 about 0.4% of the total estuarine production. 



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10-80 



