Mexico at that time more than to 

 streamflow. During the fall and winter 

 months (October to February) , streamflow 

 is at its lowest point, and tidal ranges 

 arp highest during this time. Tidal 

 range is lowest during spring, perhaps 

 as a result of being masked by peak 

 river flows. Salinity patterns reflect 

 the influence of streamflow, with the 

 lowest salinity during the spring floods 

 and the highest salinity from July to 

 September. 



Like the rest of coastal Louisiana, 

 Atchafalaya Bay is influenced by synop- 

 tic weather types. The effect of 

 various weather patterns on water levels 

 at Eugene Island is shown in Figure 60. 

 Generally the winds are northerly during 

 the winter and southerly during the 

 summer (see Description of the Region) . 



Primary Production 



Major habitats in the Atchafalaya 

 hydrologic unit in 1978 included estua- 

 rine open water, fresh marsh, and 

 cypress-tupelo forest. Since 1955 there 

 has been a 140% increase in area of 

 fresh marsh, with only a small increase 

 (5%) in cypress-tupelo forest, and a 

 small (4%) decrease in estuarine open 

 water area. The greatest change over 

 the 23-year period was the complete 

 conversion of the salt marsh ecosystem, 

 largely to fresher marsh habitats. 



Primary production in the turbid 

 estuarine bays of the Atchafalaya basin 

 has not been well documented, although 

 chlorophyll a concentrations have been 

 measured. These ranged from 3-6 mg/m 3 

 in the delta to 15 mg/m 3 in adjacent 

 Four League Bay (J. Day, Louisiana State 

 University, Center for Wetland Re- 

 sources; unpub . data). These values are 

 low, compared with chlorophyll values in 

 the Barataria basin (see Barataria 

 Hydrologic Unit) . No studies have been 

 made of primary production in the 

 cypress-tupelo forest in the Atchafalya. 

 Recent studies of cypress swamps in 

 Florida and the Barataria basin have 

 shown increasing primary production with 

 increased hydrologic flows and asso- 

 ciated nutrients (Brown 1978; see 

 Cypress-tupelo Swamp Habitat). The 

 frequently flooded cypress-tupelo forest 



receives a load of suspended sediment 

 during spring floods and may be more 

 productive than the cypress-tupelo 

 systems of the Barataria basin. 



The most intensively studied marsh 

 ecosystems in the Atchafalaya hydrologic 

 unit are the marshes developing on 

 emerging islands in the delta. Initial 

 emergent vegetation on an accreting mud 

 flat are arrowhead ( Sagittaria sp.) and 

 common water nymph ( Na jas guadalupensis ) 

 (B. Johnson, Louisiana State University, 

 Coastal Ecology Laboratory; pers.comm.) . 

 Early island formation is mediated by 

 the physical processes of flooding, 

 sediment deposition scour, and erosion. 

 Biotic factors such as root stabiliza- 

 tion of the substrate only become 

 important later in the deltaic develop- 

 mental sequence. 



After additional sediment deposi- 

 tion, willow ( Salix nigra ) becomes 

 established on the elevated levees, and 

 further successional changes in the 

 island are influenced by the roots and 

 stems. The vegetation slows the water 

 velocity, increasing the sediment depo- 

 sition and altering the chemical condi- 

 tions of the sediments. Cattail ( Typha 

 latif olia ) and arrowhead become estab- 

 lished at slightly higher mean eleva- 

 tions in and behind the protective wil- 

 low communities (Johnson et al . 1981). 



As the vegetative community in the 

 backwater of the island develops, the 

 proportion of fine silts and organic 

 carbon in the substrate increases rela- 

 tive to sand, because the community is 

 sheltered from high velocity spring 

 flood waters. In addition to the spring 

 flood, the normal tidal inundation of 

 the substrate regulates marsh develop- 

 ment . 



As the marsh becomes established, 

 the structural complexity of the commu- 

 nity increases, both above and below the 

 mean water level. This results in in- 

 creasing diversity of species in the 

 marsh. The increase in community di- 

 versity appears to be inversely related 

 to flooding stress (Johnson et al. 1981). 



These pioneer marshes, once estab- 

 lished, are not particularly productive, 



142 



