The nutrient cycle is linked to seasonal changes in coastal Maine waters. 

 When the water column is mixed, in winter, nutrients are distributed 

 throughout it. The low levels of phytoplankton growth at this time of year 

 (see "Phytoplankton" below) often result in available nutrients not being 

 utilized. 



When the thermocline develops, in spring, (usually May in Maine) phytoplankton 

 proliferate and use up the nitrogen in the surface layer. Since only limited 

 exchange takes place across the thermocline, no further nitrogen can be 

 supplied to the surface layer. Phytoplankton cells that die and fecal pellets 

 produced by zooplankton are sufficiently dense that they sink out of the 

 surface layer. This process transports the nitrogen that supported the spring 

 bloom out of the surface waters, so that within a month surface nitrogen 

 concentrations are undetectable. With the exception of processes described 

 below no new nutrients are available for the rest of the summer and 

 phytoplankton productivity remains low. This lack of nutrients in summer and 

 early fall also may limit macroalgal production. When the thermocline begins 

 to disappear, usually in September, nutrients, including nitrogen, are 

 returned to the surface layer. 



The nutrient data given in Apollonio and Applin (1972) from a transect from 

 Cape Newagen (region 2) to 20 miles (32 km) offshore show the typical seasonal 

 cycle of coastal waters described above. In December, 1969, the inshore 

 waters out to 33 feet (10 m) were almost homogeneous in nitrate content (8 y M 

 NO 3 ) over the entire depth (165 feet; 50 m) , while deeper waters offshore 

 retained some salinity stratification and surface waters contained about 6 pM 

 NO 3 . By January, 1970, overturn was complete over the entire transect; very 

 little density structure remained and the upper 300 feet (100 m) of the water 

 column contained 8+1 pM NO 3 . This condition persisted with a lens of 

 higher nutrient (10 to 12 uM NO3 ) water below 400 feet (120m) until early May, 

 when the seasonal thermocline had become established in the upper 33 feet (10 

 m) of the water column. The upper 16 feet (5 m) of the water column had <1 yM 

 NO3 remaining, and 5 yM concentrations were found at 33 feet (10 m) . By July 

 and August the thermocline had deepened to 66 to 100 feet (20 to 30 m) and the 

 upper 66 feet (20 m) had <1 yM NO3 . In September the thermocline was 

 beginning to weaken, with ImM NO 3 concentrations reaching the surface, and 

 the 5 mM NO3 concentration, which had been as deep as 165 feet (50 m) during 

 the summer, was then at about 66 feet (20 m) . By October, overturn was well 

 under way and surface nitrate concentrations reached 5 to 7 pM. During the 

 following May, stratification began to occur. 



Within this general picture of the seasonal distribution of nutrients, a 

 number of processes can occur that enhance the supply of nutrients to the 

 surface layer and consequently result in higher primary productivity. 

 Estuaries often supply nutrients to coastal waters. This comes about partly 

 because of the natural circulation processes of estuaries (see chapter 5, 

 "Estuarine System") and partly because nutrients are introduced to estuaries 

 in sewage wastes. The latter process is not likely to be significant anywhere 

 in Maine at the present time, with the possible exception of the area where 

 sewage is dumped into Casco Bay (region 1), adjacent to the Fore and 

 Presumpscot Rivers. An outflow of nutrients to coastal water due to natural 

 circulation processes would be expected to be most significant in the vicinity 

 of the mouth of the Sheepscot River, since it has the deepest entrance in the 

 characterization area. In fact, it is extremely difficult to find evidence of 



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