FRESHWATER INPUT AND 

 ESTUARINE PRODUCTIVITY 



In a few systems, measurements 

 have been made of the annual input 

 of nitrogen from fresh water and 

 other sources and of the annual pri- 

 mary production in the estuary. If 

 a Redfield (1934) model of stoichio- 

 metry (106C:l6N) is used to calculate 

 the amount of nitrogen required to 

 support the annual production, it ap- 

 pears that, with the exception of 

 some highly eutrophic areas, most 

 of the production must be sustained 

 by recycled nitrogen (Table 5) . It 

 may well be that the input of nutri- 

 ents from fresh water may make some 

 contribution (perhaps an important 

 one) to spring bloom, but for most 

 estuaries, most of the production 

 takes place in the warmer months 

 when recycling is much more important 

 than inputs. I think we will con- 

 tinue to find that this is a gen- 

 eral feature of estuarine systems 

 (Nixon 1981), and it helps to 

 explain why estuarine primary pro- 

 duction and, to a lesser degree, 

 secondary production levels are so 

 similar. It also helps explain 

 why no one has yet developed dia- 

 grams relating nitrogen loading 

 rates to estuarine production as 

 the limnologists have done so suc- 

 cessfully with phosphorus loading 

 in lakes (Vollenweilder 1976; Schin- 

 dler 1981). 



The feature that estuaries have 

 in common, and that sets them off 

 from the sea, is that they are shal- 

 low. They may have large rivers, 

 or small rivers, or no rivers 

 at all; they may have a great deal 

 of salt marsh or very little; they 

 may have grass beds or seaweed beds 

 or phytoplankton, but they all have 

 their zones of decomposition and 

 nutrient regeneration (both pelagic 



and benthic) near the euphotic zone. 

 Moreover, there is usually strong 

 vertical mixing from tides and wind 

 to assure that the coupling of de- 

 composition and production is ef- 

 fective. I think it is worthwhile 

 to put forward the hypothesis that 

 the high production of estuarine 

 waters in general is brought about 

 and maintained by the almost com- 

 plete and rapid coupling of hetero- 

 trophic and autotrophic processes. 

 Moreover, if the relative rates of 

 organic synthesis and decomposition 

 are considered, it seems likely that 

 the upper limit of production is 

 set, for the most part, by the slow- 

 er rate of remineralization. If so, 

 one of the important features of an 

 estuary may be the relative impor- 

 tance of pelagic versus benthic re- 

 mineralization, because the rate of 

 these processes is quite different. 

 The most rapid way to recycle nu- 

 trients is to put the organic mat- 

 ter through pelagic animals, such 

 as microzooplankton. But we need 

 to learn more about the processes 

 of decomposition in the water and 

 in the sediments. Ecologists, like 

 the rest of society, have been pre- 

 occupied with production and growth, 

 with the input and consumption of 

 "new" materials. We need to attend 

 more to what Odum et al. (1977) have 

 called the "regenerative half" of 

 our systems. 



Now, having said all of that, 

 I must admit to being uncomfortable 

 that the discussion so far has cen- 

 tered on short-term measurements and 

 perspectives. We also know very lit- 

 tle about the long-term effects of 

 nutrient input to estuaries. In the 

 short-run, primary production may 

 appear to be supported largely by 

 recycled nutrients, but in the long 

 run, are nutrients being concentrated 

 in the estuaries? Is the recycling 

 rate higher in estuaries with greater 

 input? The similarities of the pri- 



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