the surface layer. Nevertheless, samples taken in September, 1978, at the 

 International bridge (S=2.18 ppt) and on the southern side of St. Croix Island 

 (S=27.05 ppt) contained 3.5 and 3.7 mM NO3 . Nitrogen to phosphorus ratios 

 were 2.5:1 and 4.1:1, respectively. Nitrogen would thus be expected to be the 

 limiting nutrient. Both freshwater (3.5 jjM) and oceanic (3.7 pM) sources of 

 nitrate are implied, although the oceanic source is quantatively likely to be 

 more important. The high offshore nutrient concentration in surface water in 

 the northeastern coastal regions of Maine (as compared to areas in 

 southwestern Maine) results from mixing of nutrients into the surface layer 

 from deep water in the Gulf of Maine (see chapter 4, "The Marine System") and 

 is probably the major factor in supplying relatively high concentrations of 

 nutrients to the estuary. 



Flushing of the St. Croix is moderate and most of the exchange of water in the 

 estuary is tidal (figure 5-32), partly because of the high tidal range. 

 Nutrient resident times in the system are sufficient for uptake. No specific 

 data for the estuary are available. 



Organic Matter Cycle 



Nearly all organic matter found in the natural environment has been derived 

 from or is a constituent of living organisms. Metabolically active compounds 

 are passed rapidly from species to species during feeding, assimilation, 

 growth, and death. Other organic components include toxic substances that may 

 affect growth profoundly in minute concentrations and relatively inert 

 compounds that may be transformed to active compounds by abiotic factors (wave 

 action and salinity changes). 



To illustrate the potential significance of dissolved organic substances in 

 oceans, when compared to the sizes of living organisms, the organic carbon 

 content of an open ocean water sample is plotted in figure 5-33 as a function 

 of particle size. Although all living cells are contained in the size 

 fractions greater than approximately 0.5pm, they account for <5% of the total 

 mass of carbon in the water column. In fact, it may be conjectured that any 

 marine organism larger than a 10 p m phytoplankton cell is totally 

 insignificant in the organic cycle in the oceans. 



There are three size groups of organic matter in sea water: particulate, 

 intermediate, and dissolved. Particulate matter is any microbial organism or 

 nonliving material that is large enough to be ingested by a filter-feeding 

 organism or to support bacterial attachment. The intermediate class of 

 material is too small to catch and digest but too large to assimilate through 

 the cellular membrane. Dissolved matter can be assimilated through cellular 

 membranes. In the following discussion "biologically active" organic matter 

 represents the larger and smaller size groups. Intermediate size particles 

 are ill defined and probably are converted and transformed primarily by 

 physical and chemical processes. 



The primary source of organic material in nearshore areas and estuaries is 

 terrestrial vegetation (introduced by fresh water runoff), emergent wetland 

 plants, benthic diatoms, and attached algae. In the open ocean, phytoplankton 

 is the most important source of organic matter. Concentrations of organic 

 material are much more variable in estuaries, because of localized inputs, 

 than in the open ocean. 



5-65 



10-80 



