countercurrent and the nutrient 

 modulating actions of tidal 

 marshes, bottom sediments, and 

 submerged vascular plants. 



It is difficult to argue with the 

 proposition that estuaries are high- 

 ly productive because of all the 

 things that make them estuaries, 

 but it would be nice to know a bit 

 more about the relative importance 

 of the different features of estu- 

 aries in this regard. Similarly, 

 it may finally be true that each 

 estuary represents a unique set of 

 processes coming together in a spe- 

 cial way to result in a particular 

 level of production. But before 

 having to treat each estuary separ- 

 ately, I think the most useful 

 course to begin with is to press hard 

 for a general model or concept. If 

 various unified views can be stated, 

 and shown to fail, then we can al- 

 ways fall back on the diversity and 

 uniqueness of nature for an expla- 

 nation. 



SOME OBSERVATIONS 

 ABOUT FRESHWATER INPUT 



There are at least three char- 

 acteristics of river input that are 

 of interest in trying to assess the 

 importance of this feature in gen- 

 eral estuarine productivity, includ- 

 ing the magnitude of water flow, 

 the concentration of nutrients in 

 the water, and the seasonal varia- 

 tion of each. 



THE MAGNITUDE OF FLOW 



It is clear that the amount of 

 fresh water being discharged along 

 the coast is extremely variable 

 (Figure 3) . The influence of this 



discharge on the salinity of the 

 estuarine receiving waters, however, 

 is more complicated, because that 

 parameter also reflects the volume of 

 the estuary, the tidal prism, and the 

 mixing and flushing characteristics 

 of the system. In general, however, 

 all of these features seem to combine 

 with freshwater inputs to produce 

 estuaries with lower mean salinities 

 along the southeast and gulf coasts 

 of the United States (Figure 4) . It 

 is very difficult to know if this 

 apparent trend is real or if it 

 arises simply because of the location 

 of sampling stations on the various 

 estuaries. If it is real, it would 

 seem reasonable to expect that the 

 influence of fresh water might be 

 greater in these estuaries and that 

 they might therefore be quite dif- 

 ferent from more northern systems in 

 their productivity if freshwater in- 

 put is important in this regard. 



NUTRIENT CONCENTRATIONS 



As far as I am aware, there are 

 remarkably few reliable measurements 

 of the major nutrients in rivers 

 flowing into estuaries and even fewer 

 which include a total inventory of 

 all of the major forms of the nutri- 

 ents over an annual cycle. While 

 documents summarizing the ionic 

 composition of many substances of 

 geochemical interest are available 

 (e.g. Livingston 1962), the apparent 

 variability and scarcity of data on 

 nutrient chemistry seem to have ef- 

 fectively prevented anyone from put- 

 ting together a credible geographical 

 summary. The problem is further 

 complicated by the development of 

 agriculture or the location of large 

 urban areas along the lower reaches 

 of many rivers, so that the anthro- 

 pogenic contribution to the riverine 

 nutrient load can be very large and 

 extremely variable from estuary to 

 estuary. The resulting expectation 



38 



