Evidence suggests that nitrogen is a potentially limiting nutrient in 

 estuarine and coastal marine waters in Maine during the summer (active growth) 

 months. The atomic ratio of nitrogen to phosphorus (N:P ratio) in marine 

 waters is generally 15:1 (Armstrong 1965). The uptake ratio for the two 

 elements during phytoplankton growth is higher than this, so that nitrogen 

 could be expected to be exhausted before phosphorus and, thus, nitrogen would 

 be the potentially limiting nutrient. In the discussions of nitrogen supply 

 in Maine estuaries below, the N:P ratios are given. As these N:P ratios are 

 generally much lower than 15:1 in Maine estuaries, nitrogen is considered 

 limiting. 



A similar conclusion can be drawn by examining the distribution of the two 

 elements within a given area. In the New York Bight, for example, 

 concentrations of the two elements decrease seaward from the mouth of the 

 Hudson River estuary because of dilution and biological uptake. A plot of 

 nitrogen concentration versus phosphorus concentration indicates that nitrogen 

 would be exhausted first and ultimately would limit production (Garside, 

 unpublished ) . It is apparent that nitrogen can be expected to limit 

 productivity even in marine and estuarine areas that are heavily polluted by 

 sewage wastes. 



Ambient nutrient concentrations do not represent the nutrient status of a 

 natural phytoplankton population necessarily, because phytoplankton growth is 

 a dynamic process. That is, the ambient concentration of a nutrient is a 

 static property, whereas the dynamic process of phytoplankton growth may 

 depend more on the rate of supply of nutrients rather than the instantaneously 

 measured concentrations. In this regard, two further arguments can be made to 

 support the contention that nitrogen limits phytoplankton growth in marine and 

 estuarine waters of coastal Maine. 



The resupply of nutrients in a body of water often depends on regeneration as 

 a result of in situ heterotrophic activity, such as zooplankton grazing. 

 Bacterial processes are not necessary because merely breaking the cell wall is 

 sufficient to release phosphate. Phytoplankton cells themselves contain the 

 necessary enzymes to bring about phosphate release. By contrast, organic 

 nitrogen compounds are far less labile and usually are regenerated by 

 bacterial activity at a slower rate. Consequently, the resupply of phosphorus 

 generally can be expected to be greater than that of nitrogen, making the 

 latter more likely to limit phytoplankton growth. For a more detailed 

 discussion of nutrient regeneration see Raymont (1963). 



A second aspect of phosphorus supply is inferred from the observation that in 

 temperate estuaries phosphorus supplies near the bottom are higher during 

 summer than they are in any other location at any time (Taft and Taylor 1976). 

 Many workers have noted that estuarine muds contain large quantities of bound 

 phosphorus. Jitts (1959) showed that river mud adsorbs more phosphate as the 

 ratio of iron to organic matter increases. Taft and Taylor (1976) support 

 the hypothesis that phosphorus probably precipitates as ferric phosphate 

 during winter oxygenated conditions and is released during summer months. The 

 release mechanism involves the development of anoxic conditions in the 

 sediment as organic detritus from primary producers decomposes. Ferric 

 phosphate is reduced to ferrous phosphate, which is soluble and mobilized from 

 the sediment. This storage/release mechanism that provides phosphate during 



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