increase during active delta growth to a 

 inaxiinuin when the distributary is abandoned, 

 and then decrease as marshes subside and 

 degrade back to open water bodies. The 

 length of the interface between the marsh 

 and adjoining water bodies (the marsh 

 edge) is small in young delta lobes 

 because the new marsh is fairly solid. 

 After abandonment, however, the marsh edge 

 increases as marshes open up and more and 

 more tidal streams interfinger through 

 them. 



This is reflected in the ratio of 

 marsh edge length to marsh area (m/m^) in 

 different marsh zones. There are no 

 measurements of this ratio available for 

 the delta, but in the neighboring chenier 

 plain's fairly solid fresh and intenriedi- 

 ate marshes the ratio is 15 and 17, 

 respectively. As tidal energy increases, 

 the ratio increases to 39 in brackish 

 marshes and 50 in salt marshes (Gosselink 

 et al. 1979). Applying these ratios to 

 the delta hydrologic units, the mean edge 

 length per unit area of marsh, weighted 

 for the area of different marsh zones in a 

 hydrologic unit, increases with the age of 

 the unit (Figure 28). However, because 

 younger units have more marsh, the total 

 length of the marsh edge (the product of 

 the ratio and the marsh area) is greatest 

 in the recently abandoned Barataria and 

 Terrebonne units (III and IV, Figure 28). 



V II III IV I VI 



HYDROLOGIC UNIT 



Figure 28. Marsh edge length:area ratio 

 and total marsh edge length for delta 

 hydrologic units. The units are arranged 

 in order of increasing age (data from 

 Chabreck 1972). 



How are these differences in the 

 physical characteristics of hydrologic 

 units related to biological productivity? 

 Two measures of productivity d.rQ net 

 primary production and the inshore shrimp 

 harvest (Figure 29). Total net productiv- 

 ity is lowest in the active deltas and 

 highest in the Pontchartrain hydrologic 

 unit - mostly a function of the size of 

 the unit. Primary production per unit 

 area, however, is highest in the Barataria 

 and Terrebonne basins. Inshore shrimp 

 yield is also highest in the same basins. 

 Since these basins are in the early 

 destructional phase, these data support the 

 hypothesis of Gagliano and Van Beek 

 (1975). 



Regressions of biological productiv- 

 ity on salinity, marsh area, and edge 

 length (Table 6) should be taken with 

 caution because they are based on data 

 from only six hydrologic jnits. Neverthe- 

 less, they make for interesting specu- 

 lation. Average net primary production 



o 



crnoo 



D O 



O - 



Px 



a - 



3 r 



E O 



__ Whif 

 Sttrlmp 



V II III IV I VI 



NET 



PRIMARY 



PRODUCTION 



V Hill IV I VI 



NET 



PRIMARY 



PRODUCTION/m' 



II iiirv I VI 



INSHORE 

 SHRIMP 

 CATCH 

 1955-74 



Figure 29. Net primary production and 

 fishery yield of Mississippi River Deltaic 

 Plain hydrologic units. Production 



calculated fran average production of each 

 habitat type and its area in the hydro- 

 logic unit. Shrimp data from Barrett and 

 Gillespie (1975). Basins are, in order of 

 increasing age: I - Pontchartrain-Lake 

 Borgne, II - Balize, III - Barataria, 

 IV - Terrebonne, V - Atchafalaya, VI - 

 Verm il ion. 



31 



