FORAMINIFERAL RESPONSE TO HABITAT DISRUPTION: ARROYO COLORADO, TEXAS

In order to assess the possible effects of pollutants on foraminifera along the Texas Gulf Coast, sediment cores were taken in four areas considered to be at risk: southern Laguna Madre, Nueces Bay, the Arroyo Colorado, and Laguna Atascosa. Geochemical analyses indicate that the sediments are relatively uncontaminated. Foraminiferal analyses, however, reveal otherwise undetected inconsistencies and changes with time. First, though foraminiferal assemblages in the surface sediments of Laguna Madre, Nueces Bay, and Laguna Atascosa are typical, they are atypical in the Arroyo Colorado. In Texas estuarine environments, Ammonia often comprises a large proportion of the assemblage. But in the Arroyo Colorado, an estuarine river, the assemblage is almost all Elphidium to the exclusion of Ammonia. A second observation is that down-core in the Arroyo Colorado cores, Ammonia becomes abundant as numbers of El-phidium decrease. The assemblage at the base of the Arroyo Colorado cores is thus more typical for the Gulf Coast, while that in the surface sediments is not. This down-core change in the foraminiferal assemblage is coincident with a change in sediment grain size, both most likely due to the dredging of the Harlingen ship channel in the late 1940s. The reasons why Ammonia, an opportunistic genus that thrives in a wide range of environmental conditions, was unable to tolerate a habitat disturbance and is nearly absent in the surface sediments of the Arroyo Colorado are unclear.