WETLAND LOSS IN THE NORTHERN GULF-OF-MEXICO - MULTIPLE WORKING HYPOTHESES

I examined four hypotheses about causes for the dramatically high coas tal wetland losses (0.86% yr(-1)) in the northern Gulf of Mexico: an e xtensive dredged canal and spoil bank network, a decline in sediments in the Mississippi River during the 1950s, Mississippi River navigatio n and flood protection levees, and salinity changes. Natural factors c ontributing to these habitat changes include eustatic sea-level rise a nd geological compaction, which appear to have remained relatively con stant this century, although variation does occur. These four hypothes es were tested using data on land-to-water changes in 15-min quadrangl e maps inventoried for four intervals between the 1930s and 1990. Land loss rates were directly proportional to changes in wetland hydrology in time and space. A linear regression of the direct losses due to dr edging versus the losses due to all other factors (indirect losses) ha d a zero intercept and a slope that increased with time. The ratio ind irect:direct land loss was highest nearest the estuarine entrance. The coastwide patterns of land loss do not appear to be affected by river ine sediment reductions over the last 60 yr. The effects of changes in wetland hydrology from dredging human-made channels and forming dredg ed spoil banks appear to be the most efficacious hypothesis explaining these dramatic losses. The effects of extensive human-induced changes on this coast have apparently overwhelmed the causal linkages identif ied in the historical re-constructionist view of deltaic gain and loss that emphasizes the role of mineral sediments. A paradigm shift is th erefore proposed that emphasizes a broad ecological view as contrasted to a mostly physical view emphasizing the role of sediment supply in wetland maintenance. In this view, plants are not an ancillary consequ ence of strictly geological dynamics such as sediment supply but are d ominant agents controlling factors relevant to coastal restoration and management efforts.