inflows. Brown and pink shrimp har- 

 vests are negatively correlated with 

 fall (September-October) harvests. 

 These analyses corroborate the Sabine 

 Lake finding by White and Perret 

 (1974) that the seasonality of dis- 

 charges as well as total annual quan- 

 tities have an important influence on 

 recruitment of shrimp. 



CONCEPTUAL MODEL 



The flow of fresh water into es- 

 tuaries may influence fishery produc- 

 tion, either directly or indirectly, 

 in at least five ways: (1) transport 

 of nutrients; (2) transport of detri- 

 tus; (3) transport and deposition of 

 sediments; (4) reduction of salinity; 

 and (5) mixing and transport of water 

 masses. The pathways by which fresh 

 water may influence fishery produc- 

 tion are shown in the simple energy- 

 flow model in Figure 1 . The model 

 is a conceptual integration of a set 

 of hypotheses supported in part by 

 the material presented previously. 



In the model, direct river in- 

 puts are sediments, nutrients, and 

 fresh water. Sediments build and 

 maintain tidal wetlands (marsh), 

 counteracting both natural and an- 

 thropogenic processes that destroy 

 tidal wetlands. Nutrients in river 

 water stimulate productivity of wet- 

 land vegetation. The physical force 

 of runoff flushes decaying wetland 

 vegetation into tidal creeks and 

 open waters, where it is processed 

 by microorganisms into food for ben- 

 thic animals, which are fed on by 

 juvenile fish and invertebrates. 

 Nutrients released to open water 

 stimulate productivity of phyto- 

 plankton and seagrasses, which also 

 provide food for juvenile fish and 

 shellfish, either directly or through 

 the grazing or detrital chains. 



Freshwater inputs establish 

 chemical potential energy that some- 

 times drives the mixing of the estu- 

 ary. The relationship between fresh- 

 water inputs and mixing is a complex 

 function that is dependent on tidal 

 amplitude (both astronomical and wind- 

 driven) and the area and geomorpholo- 

 gy of the estuary (Hansen and Rat- 

 tray 1966). Simmons (1955) sug- 

 gested the ratio of freshwater flow 

 over one tidal cycle to tidal prism 

 (tidal amplitude multiplied by es- 

 tuarine area) as a rough index of 

 type of mixing in coastal plain 

 estuaries. Ratios in the range of 

 1:1 indicated stratified conditions. 

 This means poor vertical mixing, with 

 bottom waters that are usually low 

 in oxygen. Ratios on the order of 

 1:10 suggest a partially-mixed estu- 

 ary. The moderate vertical salinity 

 gradients in partially-mixed estu- 

 aries can be a more important force 

 than tides in driving estuarine cir- 

 culation in regions where tidal am- 

 plitudes are low. Ratios approach- 

 ing 1:100 indicate a vertically homo- 

 geneous estuary with no salinity 

 differences between surface and bot- 

 tom. Although well-mixed vertically, 

 this type of estuary may be less 

 well-mixed horizontally than the 

 partially-mixed estuary because of 

 the lack of the vertical salinity 

 gradient necessary to drive densi- 

 ty currents. The mixing character- 

 istics of an estuary may vary season- 

 ally if seasonal variation in fresh- 

 water inputs is large. The mixing 

 ratio should be used only as a rough 

 guide because it does not consider 

 estuarine geomorphology , an impor- 

 tant determinant of mixing type. 



Three pathways shown in Figure 

 1 are dependent on mixing character- 

 istics. Mixing characteristics de- 

 termine the availability of oxygen 

 to bottom waters, which in turn de- 

 termines the rate of decomposition 



418 



