August. This estuary is probably nitrogen-limited during most of the summer 

 months and probably is not as productive as the Sheepscot estuary. 



Penobscot estuary . The Penobscot, like the St. Croix, has no clearly 

 defined seaward boundary, but the depth is only 13 m near its mouth and the 

 inflow of enriched sea water is relatively small because of it. Unlike the 

 St. Croix, the offshore waters of the Penobscot are not particularly rich in 

 nutrients during the summer. Measurements of water taken in September, 1978, 

 were as follows: 



S ppt NO3 MM N:P 



These data indicate that the estuary is potentially nitrogen-limited. The 

 fresh-water source contains 2.9 MM NO3 and the oceanic source 1.4 mM NO3 . The 

 relatively high nitrate concentration at Bucksport indicates some local 

 internal source, possibly sewage waste. 



The estuary is not very rapidly flushed, especially the lower portion south of 

 Verona. Due to a lack of a rich nutrient supply, it is probably not very 

 productive compared to the Sheepscot. 



Pleasant, Narraguagus, and Union estuaries . No specific data are 

 available for any of these estuaries, but they are probably similar to the 

 Machias. Because of their small size, tidal exchange in these estuaries is 

 probably important. 



Machias estuary . The Machias River estuary is fed by the Machias and East 

 Machias Rivers and has an entrance depth of 6 m. Two samples were taken in 

 September, 1978, on the Machias below the falls, (S=8.51 ppt, N03=3.6 yM) and 

 at the confluence of the Machias and East Machias Rivers (S=25.73 ppt, N03=4.4 

 4'!) . The N:P ratios were 3.6:1 and 3.4:1, respectively. Nitrogen is also 

 potentially limiting in this estuary. The nitrate content of fresh water was 

 4.7 mM and even though the sill is shallow the oceanic source is probably a 

 more important source of nutrients than fresh-water inflow. Ocean waters in 

 this region of coastal Maine are well mixed and rich in nutrients. According 

 to the model the estuary flushes fairly rapidly above the confluence of the 

 two rivers and tidally below them. 



Cobscook Bay . Cobscook Bay is fed by the Dennys River. The river flow is 

 extremely small compared to the tidal exchange. Shroeder (1977) describes the 

 area as having intense vertical mixing. Tidal exchange is so extreme that 

 although a constant supply of nutrients is ensured, no phytoplankton 

 population specific to the bay is expected, rather it is expected to be 

 typical of the adjacent coastal waters. The rapid exchange of highly 

 productive waters in the estuary should ensure high benthic and fixed algal 

 production. This would be an important consideration if this exchange were to 

 be restricted as a consequence of tidal power development. 



St. Croix estuary . It is difficult to define the seaward boundary of this 

 estuary but a sill depth of 10 m probably restricts the offshore supply to 



5-64 



