These ideas cannot be translated quantitatively into rates of nutrient supply 

 or potential productivity but can be used for the following comparative 

 purposes. The important volumes to consider are the fresh water inflow volume 

 (Vf), the low tide volume (Vl), and the tidal exchange (intertidal) volume 

 (V'j') . The relative proportions of these will determine qualitatively the 

 magnitude and nature of estuarine flushing. Two parameters: 



F = 100Vp/(Vp + Vt + Vrr, ) (F = percentage of total volume changed by fresh- 

 water flow) 



T - 100Vx/(Vp + Vl +Vx ) (T = percentage of total volume changed by tidal 

 flow) 



describe the percentage of the total volumes involved in fresh water and tidal 

 flushing, respectively. A third parameter: 



L = 100\^ / (Vp + VL+Vp)(L = percentage of total volume remaining in 

 estuary at low tide) 



brings the sum of F, T, and L to 100 and is a function of the unflushed 

 volume . 



These parameters can be calculated for Maine estuaries and plotted. The 

 position of an estuary on the plot indicates, relatively, the type of flushing 

 (and hence nutrient supply) that might be expected (figure 5-32). Thus, the 

 Presumpscot is well flushed; most of the volume is represented by fresh water 

 flow and tidal exchange. Neither the supply of nutrients nor their residence 

 time might be expected to favor high uptake and productivity. Comparatively, 

 the Sheepscot has a relatively slow flushing rate, and what exchange of water 

 does occur is mostly tidal. Such an estuary might be expected to have a 

 larger supply of nutrients and a longer residence time allowing for their 

 uptake and retention. Reference will be made to these parameters in the 

 following discussion of nitrogen availability in the coastal estuaries of 

 Maine. 



Fore estuary . Only one source of data (Normandeau Associates 1974) is 

 available on nutrients in the Fore River estuary. These data (May 1974) show 

 a nitrate distribution with values of about 6 pM NO3 at the 1-295 bridge in 

 the estuary and 1.8 yM at the mouth of the estuary in Casco Bay. This report 

 also gives values for ammonia concentrations, which show maximum values in 

 the upper half of the estuary (10 pM) , and particulate nitrogen which is about 

 0.6 yM and is distributed rather uniformly. Much of the ammonia is probably 

 derived from sewage; certainly a source internal to the estuarine system is 

 implied by the high value of 10 pM in the upper half of the estuary. 

 Nevertheless, these concentrations are insufficient to cause eutrophication, 

 and flushing precludes the accumulation of nutrients. 



Presumpscot estuary . The Presumpscot has the shallowest entrance (2 m) of 

 all the estuaries in the study area that remain flooded at low tide. The 

 Presumpscot only has access to surface coastal sea water and therefore would 

 not be expected to have a rich nutrient source. 



Samples collected in September, 1978, at the route 9 and route 1 road bridges 

 had salinities of 2.3 and 31.59 ppt and nitrate concentrations of 6.7 and 



5-60 



