FRESHWATER INPUTS AND ESTUARINE PRODUCTIVITY 



Scott W. Nixon 



Graduate School of Oceanography, University of Rhode Island 

 Kingston, Rhode Island 



ABSTRACT 



The processes responsible for 

 the high level of production char- 

 acteristic of estuarine systems 

 are not yet well understood. There 

 are at least five major hypotheses 

 which have been put forward at 

 various times to account for estu- 

 arine production, including the 

 fertilizing effect of nutrients in 

 fresh water, advection of nutrient 

 rich offshore water, the trapping 

 of nutrients in estuarine circula- 

 tion, the outwelling of nutrients 

 from salt marshes and other wet- 

 lands, and the rapid recycling of 

 nutrients within the estuary. The 

 remarkable similarity of primary 

 and (to a lesser degree) secondary 

 production levels among estuaries 

 with widely varying fresh water in- 

 puts, hydrodynamics properties, and 

 geographical and geological charac- 

 teristics suggests that a more gen- 

 eral feature of estuarine systems 

 is most important in enhancing pro- 

 duction in these areas. While river 

 inputs may contribute to the spring 

 bloom, most of the production in 

 many estuaries appears to take 

 place during the warmer months and 

 to be supported by recycled nutri- 

 ents. Two characteristic features 

 of estuarine systems are their shal- 

 low depth and relatively strong 

 mixing. Both of these features con- 

 tribute to a relatively complete 

 and rapid coupling of heterotrophic 

 and autotrophic processes in estua- 

 ries. Because remineralization 

 appears to be a slower process than 



the formation of new organic matter, 

 it may be that heterotrophic process- 

 es play an important role in regula- 

 ting the primary production of es- 

 tuaries, and that the similarity of 

 carbon fixation rates in various 

 estuaries arises because of some 

 common limit on the rate of nutrient 

 recycling. More attention also 

 needs to be given to the problem 

 of understanding the relationship 

 between short-term processes, such 

 as annual production or regenera- 

 tion, and longer-term processes, 

 such as the continuous input of 

 nutrients from rivers and anthro- 

 pogenic sources. 



THE NATURE OF THE PROBLEM - WHY 

 ARE ESTUARIES SO PRODUCTIVE? 



Estuarine systems are usually 

 characterized by levels of primary 

 production per unit which are con- 

 siderably higher than those typical 

 of offshore waters (Table 1). In 

 many cases, the higher production 

 of estuarine and nearshore waters 

 has been attributed either direct- 

 ly or by implication to the ferti- 

 lizing effect of river inputs of 

 nitrogen, phosphorus, or silica. 



The river appears to be the 

 principal source of nitrogen 

 and phosphorus in the estuary 

 (Forge River, Moriches Bay, 

 L.I.), (Barlow et al. 1963). 



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