of detritus by microorganisms. Rate 

 of decomposition rather than quantity 

 of detritus determines the rate of 

 detritus utilization by benthic 

 organisms (Tenore and Hanson 1980). 



The river in its interaction 

 with the tides determines the "slope" 

 of the salinity gradient and, there- 

 fore, the area of water within the 

 favorable salinity range for a given 

 species. Steeper slopes mean smaller 

 areas between isohalines; gradual 

 slopes means broad areas between iso- 

 halines. River flow also positions 

 the area of favorable salinites rel- 

 ative to important stationary habi- 

 tat factors such as shoreline, water 

 depth, and bottom type. The size of 

 the area of overlap of these factors, 

 integrated over the nursery season, 

 as well as food concentration, may 

 determine the survival and growth 

 rates of juvenile organisms. 



Freshwater inputs, through ef- 

 fect on mixing, also may influence 

 recruitment of harvestable fish and 

 shellfish by affecting currents that 

 transport estuarine-dependent post- 

 larvae into estuaries from offshore 

 spawning areas. Shoreward-moving 

 bottom currents caused by stratifi- 

 cation and surface currents (and 

 sometimes also bottom currents) 

 caused by density gradients may be 

 a major mechanism for transport of 

 postlarvae into estuaries. Although 

 some evidence for postlarval trans- 

 port by bottom currents exists 

 (Wallace 1940) , postlarval transport 

 by wind (ekman transport) and tides 

 is better documented. 



IMPLICATIONS OF THE MODEL 



A wide body of literature sup- 

 ports the relationship between nu- 



trient inputs and fishery production 

 indicated in the model. This rela- 

 tionship probably is not simple be- 

 cause high nutrient inputs sometimes 

 can cause a shift in taxonomic 

 composition toward algal species 

 that, for one reason or another, are 

 not valuable in the food chain. On 

 the other hand, there probably is a 

 nutrient concentration so low that 

 no useful energy can be obtained by 

 animals from phytoplankton. Never- 

 theless, strong linear correlations 

 between freshwater flow, nutrients, 

 and fishery production have been 

 found (Sutcliffe 1972; 1973). 



The importance of detritus in- 

 puts to fishery production also is 

 well documented. The effect on de- 

 tritus utilization of seasonally or 

 permanently low oxygen in bottom 

 waters due to stratification has not 

 been discussed. Detritus is not al- 

 ways utilized immediately after it 

 arrives in an estuary. Delays of a 

 season or longer between deposition 

 and utilization may occur. Detrital 

 inputs to areas of permanently low 

 oxygen may have little influence on 

 the production of anything other than 

 fossil fuels. 



There are at least three rea- 

 sons why production of fishery spe- 

 cies may correlate with area of fav- 

 orable habitat: (1) growth may be 

 related to the total quality of a- 

 vailable food, and total quantity 

 of available food is the product of 

 food concentration and area; (2) sur- 

 vival and growth rates probably are 

 negatively density dependent; there- 

 fore the larger the favorable area, 

 the higher the survival and growth 

 rates within it; and (3) the small- 

 er the area of favorable habitat, 

 the greater the percentage of 

 juvenile animals found in poor 

 habitat, where lower survival and 

 growth rates would be expected. 



420 



