the period of peak phytoplankton production has no counterpart in nitrogen 

 cycling. 



Nitrogen . The sources of nitrogen for phytoplankton growth and the 

 processes by which nitrogen is supplied are complex and may vary among 

 estuaries in Maine. Data are insufficient to support definitive statements 

 about specific estuaries, so the following generalizations on nutrient supply 

 are based on the authors' experience with estuarine systems in Maine. 



The supply of nitrogen in any estuary depends to a large extent on the unique 

 circulation patterns of that body of water (see "Hydrography" above; Dyer 

 1973). The three major external sources of inorganic nitrogen to an estuary 

 are inorganic nitrate nitrogen that is introduced in the bottom flow of saline 

 water from the ocean, nitrate (and possibly ammonia) that enters from 

 agricultural land runoff in the freshwater flow, and sewage-derived nitrogen 

 (both nitrate and ammonia). Although it is not a source of "new" nitrogen, 

 regeneration within an estuary also can be an important source of nitrogen for 

 plant growth. The principal export of nitrogen from an estuary is in the 

 surface outflow, in the form of inorganic nitrogen, detritus, phytoplankton, 

 zooplankton, and migrations of fishes and birds. Annually there is probably 

 little accumulation of nitrogen in Maine estuaries, so that the export to 

 local waters will equal the inputs to the individual estuaries. This export 

 may represent an important source of nutrients to local coastal waters. 



In most estuaries, salinity increases from surface to bottom and much of the 

 input of coastal water is contained in a net nontidal upstream bottom flow. 

 The nutrient cycle in nearshore waters follows the seasonal pattern described 

 in chapter 4, "The Marine System." Nutrient distribution is related directly 

 to density structure of the waters. Surface water is not as rich in nutrients 

 as the denser waters below it (figure 5-30). The amount of nutrients in the 

 saline water inflow depends on the depth from which that saline water comes 

 (source waters). This is illustrated for the Kennebec and Sheepscot Estuaries 

 in figure 5-30. An estuary that is shallow or has a shallow sill at its 

 seaward end obtains sea water from near the surface. The Kennebec is an 

 example of such an estuary. If the depth from which water is drawn into the 

 estuary is above the seasonal thermocline very little import of nutrients will 

 take place. Conversely, an estuary that is deep at its mouth will draw 

 enriched water from below the thermocline; the Sheepscot is an estuary of this 

 type. 



Under certain circumstances, it is possible to determine the nutrient content 

 of this oceanic water supplied to an estuary. In fall as day length 

 decreases, estuarine phytoplankton production and nutrient uptake decline 

 (Garside et al. 1978). Consequently, nutrient and salinity distribution are 

 similar (as determined by the proportion of fresh and saline water and the 

 concentration of nutrients). In summer, lower nutrient concentrations are 

 expected because of the high uptake of nutrients by primary producers. In 

 winter, the thermal structure breaks down in coastal waters and all estuaries, 

 regardless of depth, receive similar concentrations of nutrients as the 

 nutrient concentration of offshore waters becomes more uniform (see Apollonio 

 and Applin 1972). However, in early fall during the brief period when primary 

 production is low but the offshore thermocline structure remains, the 

 distribution of nutrient concentrations with respect to salinity can be used 

 to identify the nutrient concentration in the coastal water that has been 



5-54 



