census, individual 

 fresh marsh or in 

 freely among the 

 taking advantage 

 conditions. The 



water gradient. We take the organization 

 for jranted, but brackish areas are always 

 between fresh and salt areas. The marshes 

 next to the uplands are usually fresher 

 than marshes in the interior of the basin 

 because they receive rain runoff; salt 

 marshes are more naturally dissected by 

 channels than fresh marshes because they 

 receive stronger tidal energy, and so 

 forth. 



Similarly, biotic assemblages are 

 organized along these gradients. We have 

 seen that one of the chief consumer groups 

 in the marsh, the waterfowl, partitions 

 itself within the different marsh zones 

 according to the tolerance of individual 

 species for salt and preference for 

 available foods, marsh ponds, and water 

 depths. But these preferences are only 

 average ones. On any single aerial bird 

 flocks may be found in 

 salt marsh. They inove 

 different marsh zones, 

 of favorably changing 

 increased waterfowl 

 density when marshes changed from sawgrass 

 to annuals, mentioned in the previous 

 chapter, is an example of the mobility of 

 the fauna among marsh zones. The possible 

 displacement of muskrats toward saline 

 marshes by the invading nutria is another. 



Nektonic organisms provide 



particularly good examples of the use of 

 multiple subsystems within the coastal 

 basin (Figure 56). Many year-round 

 residents of the estuary are euryhaline 

 and move freely throughout the basin. 

 Such species as the bay anchovy, mullet, 

 alligator gar, rainwater killifish, and 

 tidewater silverside are found from salt 

 to freshwater, many of them in the small 

 creeks that border the marshes. Others, 

 like the threadfin shad, the blue and 

 channel catfish, and the river shrimp move 

 down basin during the fall and winter as 

 brackish areas freshen. The marine- 

 spawned croaker, menhaden, and blue crab 

 use the whole estuary as a nursery area, 

 penetrating all the way through salt and 

 brackish zones to fresh marshes in their 

 migrations. 



Extra-Basin Couplings 



FISH & SHELLFISH 



WHirr iiHRiMP 



TEAL & PINTAIL 



WAGING BIRDS 



BLACKBIRDS 

 & CRACKLES 



ent 



The marine-spawned, estuarine-depend- 

 fish and shellfish mentioned above 



Figure 66. Patterns of estuarine use by 

 nektonic organisms in the Barataria basin, 

 Louisiana (Chambers 1980). 



are, from an economic point of view, the 

 most important group of consumers that 

 frequent the coastal marshes. Typically 

 they spawn on the continental shelf, move 

 into estuaries as juveniles, and return to 

 the Gulf of Mexico as adults to continue 

 the cycle. Nearly all the commercially 

 important nektonic species on the gulf 

 coast are estuarine-dependent (Gunter 

 1957). Within the estuary marsh habitat 

 is crucial for these species. For example. 

 Turner (1977) showed that both along the 

 gulf coast and worldwide, the commercial 

 shrimp harvest is directly related to the 

 marsh area in the inshore nursery. The 

 relationship is to the total marsh area - 

 not just salt marsh; the relationship of 

 yield to the inshore open water area is 

 poor. 



The brown shrimp life cycle is typi- 

 cal for these estuarine-dependent species 

 (Figure 67). Early in their juvenile 

 stage they can be found deep in the marsh 

 in small bayous and ponds. As they in- 

 crease in size, they move slowly out into 



80 



