Nitrates were measured at Dix Island within a half-mile of the mouth of the 

 estuary on 17 August 1970. Measurable nitrate was present at all depths and 

 all nutrient concentrations at the surface were much higher than those near 

 the bottom. In fact, surface nitrate concentrations were similar to those in 

 the Sheepscot (3 MM) but deep water values are much lower (1 pM) in the 

 Kennebec than at a comparable location in the Sheepscot. 



Sheepscot estuary . The Sheepscot has the deepest entrance of any Maine 

 estuary ( 50 m) and as a consequence has access to nutrient-rich deep water 

 throughout the year. Based on samples taken in September, 1976, Garside and 

 coworkers (1978) identified the oceanic source water as Maine Intermediate 

 Water with a nitrate content of 9.2 uM (figure 5-31). No nitrate from the 

 freshwater flow reaches the lower part of the estuary so that the estuary is 

 almost totally dependent on the oceanic source for its supply of nutrients. 

 These authors tentatively concluded, based on a limited time series analysis 

 of ammonia, that the regeneration of nitrogen as ammonia during darkness and 

 its subsequent uptake in the light represents a major potential supply of 

 nitrogen in the estuary. In fact, recent calculations (Garside, unpublished ) 

 suggest that probably only 20% of the measured primary production could be 

 supported by nitrate supplied in the inflow of deep coastal water. This 

 suggests that regeneration is essential to the maintenance of high 

 productivity in the Sheepscot and probably all Maine estuaries. 



The large subtidal volume of this estuary, relative to its freshwater flow and 

 tidal exchange, suggests that its circulation rate is slow. Garside and 

 coworkers (1978) have shown that the Sheepscot always has a two-layered 

 structure as far north as Wiscasset, where it becomes vertically well mixed. 

 Thus, deep coastal water and nutrients will be supplied to the entire lower 

 estuary and will have a residence time sufficient for their uptake and 

 retention. Garside and coworkers (1978) have reported that nutrient 

 concentrations throughout the lower estuary at all times of the year were 

 sufficient so that phytoplankton productivity would not be expected to be 

 limited by lack of nutrients. Nitrate concentrations had minimum values in 

 the 2 to 3 yM range, and similar concentrations were found for ammonia. 

 Nutrient concentrations increased with depth, with 6 to 8 pNi nitrate in the 

 deep inflowing water. Other nutrient measurements have been conducted in the 

 Sheepscot estuary (Maine Department of Marine Resources, unpublished ; Maine 

 Yankee Power Company 1970 to 1976) . The former data are in general agreement 

 with those presented above. The latter are not directly comparable as they 

 represent minimum values. 



Damariscotta estuary . The Damariscotta estuary is shallow (7.5 m) near 

 its mouth and the inflow of nutrient-rich sea water is reduced. In September, 

 1978, one sample was taken at the Damariscotta/Newcastle Bridge; its salinity 

 was close to that of sea water. Salinity was 32.33 ppt and the nitrate 

 concentration was 1.7 PM with a N:P ratio of 0.9:1. Thus, nitrate potentially 

 limits productivity in this estuary, since the oceanic surface water, that is 

 the source water, also would be low in nutrients. The high salinity at the 

 town of Damariscotta at the head of the estuary indicates that, except during 

 high spring fresh water inflow, the estuary probably functions more like a 

 shallow, poorly flushed arm of the sea. McAlice (1970 to 1977) presents 

 nitrate, phosphate, and silicate data for the period from December, 1969, to 

 May, 1971, and nitrate concentrations were near zero from mid-May to early 



5-63 



10-80 



