concluded that benthic regeneration in Chesapeake Bay was insignificant as a 

 nutrient source for overlying primary production. 



It cannot be concluded that any single nutrient regeneration process is 

 applicable to any of the estuaries of coastal Maine. In some estuaries one 

 process will be dominant and in others, others may dominate. Thus, predicting 

 the importance of regeneration or even more specifically which components of 

 regeneration are important in Maine estuaries cannot be undertaken. Some 

 specific statements based on actual measurements in the Sheepscot estuary are 

 made below but more information is needed before any more general or specific 

 statements on nitrogen regeneration in Maine can be made. 



Nevertheless, insofar as certain parts of the biosphere are potentially 

 important in the recycling of nutrients in estuaries their vulnerability to 

 perturbation must be considered. Changes in uses of estuaries that may have 

 direct impacts on populations with no apparent commercial or recreational 

 value also may have indirect impacts on total estuarine productivity, by 

 virtue of the key position of such populations in maintaining the estuarine 

 nutrient cycle. 



The Role of Flushing 



The importance of land runoff and ocean water in supplying nutrients to an 

 estuary has been discussed above. Land runoff and tidal exchange are 

 important in the flushing of estuarine systems and help determine the 

 distribution and residence time of nutrients in estuaries. The concepts of 

 flushing are discussed more fully above under "Hydrography," but the 

 following considerations apply specifically to nutrients in Maine estuaries. 



Fresh water inflow drives estuarine circulation. A large freshwater inflow 

 results in lower salinities further down the estuary and more rapid flushing 

 of the estuary. Small freshwater inflow results in higher salinities further 

 up the estuary and longer residence times for water in the estuary. Since 

 most of the nutrients in many Maine estuaries are supplied by the ocean rather 

 than freshwater input, low freshwater inflows result in nutrient-rich ocean 

 water influencing a larger part of an estuary. Additionally, longer 

 residence time of water helps assure that the processes of uptake and 

 regeneration have sufficient time to occur and that more of the nutrients 

 entering an estuary are retained and regenerated. The reverse is true in 

 estuaries with disproportionately higher freshwater flows. Consequently, 

 fresh water inflow (especially as a fraction of the volume of the estuary) in 

 Maine is inversely related to the nutrient supply and productive potential of 

 an estuary. 



Tidal exchange, the fraction of the volume of an estuary that is moved in and 

 out of the estuary on each tide, also is important in this regard. Tidal 

 exchange contributes only slightly to the circulation but in smaller estuaries 

 it is especially important for flushing. On each tide, fresh water enters the 

 estuary, principally at the surface. It mixes with water in the estuary and a 

 slightly larger volume of water leaves the estuary on the falling tide. If 

 these tidal volumes are large compared to the low tide volume of the estuary, 

 the estuary will be well flushed and nutrients may not be resident in the 

 estuary for sufficient time to contribute to the productivity of the system. 



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