Ecological History of Lake Pontchartrain, an Estuarine Community

Abstract

During the past 5,000 years since stabilization of the level of the Gulf of Mexico, the Lake Pontchartrain basin has undergone drastic environmental changes. About 2,000 years ago when the Missis- sippi River established its course in the New Orleans area one of the early deltaic distributaries impounded an arm of the Gulf, creating the Lake Pontchartrain basin. Since this time the basin has undergone rapid filling from a variety of sources, as well as gradual freshening. Associated with these changes, as revealed by the fossil record, the sedentary benthic invertebrates have included a progression of species reflecting the respec- tive environments from the marine tidal flat community of 5,000 years ago (Dosinia and Mercenaria), through the brackish-water assemblage of 2,000 years ago (Crassostrea and Thais), to the present-day near- fresh community (Rangia, Mytilopsis, etc.). Along with filling and freshening the lake has apparently come under greater influence of weather conditions which now induce sudden and severe changes in the hydrographic environment. This physical instability is also reflected in the nature of the fauna. Although over 300 species are currently recog- nized from the lake, only about 4 maintain large endemic populations as year-around residents, a clam (Rangia cuneata), crab (Rithropano- peus harrisii), copepod (Acartia tonsa), and fish (Anchoa mitchilli). Most of the remaining abundant species are migratory and spawn else- where, invading the lake as seasonal transients. Their population levels and the species composition of the fauna, in general, vary dramatically from one season to another and from year to year, apparently reflecting to a large degree the influence of the physical factors on larval migration and establishment. Impoundment has apparently been followed by a great increase in the availability of humus from decaying marsh grass, a retardation of flushing, an increase in siltation, and a general rise in tur- bidity. As a result the community appears to have progressed toward a heterotrophic condition in which the consumer species are greatly de- pendent upon organic detritus originating, in large measure, outside the lake. Detritus feeders are now among the most successful species com- prising the community of this lake.