AN APPLIED PALEOECOLOGY CASE STUDY: BAHIA GRANDE, TEXAS PRIOR TO CONSTRUCTION OF THE BROWNSVILLE SHIP CHANNEL

Bahia Grande is a large former lagoon located within Laguna Atascosa National Wildlife Refuge in Cameron County, Texas to the west of the city of Port Isabel. When the Brownsville Ship Channel was built in 1936, Bahia Grande was cut off from the marine water of Laguna Madre. Since that time, Bahia Grande has been primarily dry with only ephemeral fresh water coming from heavy rainfall events, resulting in a severe decline in biological productivity. Notably, no occurrences of living marine mollusks have been reported since 1936. A restoration project led by the U.S. Fish and Wildlife Service has proposed to cut new channels between Bahia Grande and the Ship Channel to restore its connection to Laguna Madre. This is a large-scale project with major implications for the surface water quality and associated biota in the region. Unfortunately, not very much is known about the fauna, environmental conditions, or geologic controls of Bahia Grande prior to 1936 so it is difficult to predict how the lagoon will be affected by the restoration and whether the restoration will be comparable to the original environment. This study uses paleoecological data in an attempt to better understand the historical conditions in Bahia Grande.

A total of 51 shallow cores were taken from a sampling grid in Bahia Grande and several surrounding lagoons. Molluscan shell material from the samples was identified to species level. The resulting data matrix was analyzed with multivariate statistical methods to search for any patterns in the distribution of species. The results indicate that, prior to 1936, Bahia Grande had an established fauna with relatively low diversity. Mulinia lateralis is the most common and widespread bivalve species. Anomalocardia auberiana and Macoma tenta are also relatively high in abundance. Gastropods are dominated in nearly all samples by ectoparasitic Odostomia sp. Other frequently occurring gastropods include Bittium varium and Acteon punctostriatus. The presence of these taxa suggests that Bahia Grande was a hypersaline lagoon prior to 1936. Diversity and abundance are higher in samples located in the middle of the lagoon, indicating that water depth is likely a primary factor in the distribution of mollusks.