The influence of dredged material islands in Upper Laguna Madre, Texas on selected seagrasses and macro-benthos

Abstract

This study was undertaken to gain an insight into the impact that dredged material islands have had on the upper Laguna Madre, Texas and the degree of ecological succession that has taken place since the creation of these man-made structures. Six dredged material disposal sites were intensively studied and 2 sites were less intensively studied during 1976-1977. Two seagrass species, 18 species of bivalve mollusks, and 21 polychaete annelid species were used to compare and contrast the productivity between study sites using a multivariate contrast analysis approach. Of the 18,419 ha in the study area capable of supporting vegetation, 2,238 ha has been directly influenced by dredged material deposition. Analysis of 394 sediment samples revealed 2 sediment types, sand and clayey-sand, in the upper Laguna. Three species of seagrasses were found, shoalgrass (Halodule beaudettei), halophile (Halophila engelmannii), and widgeongrass (Ruppia maritima). Shoalgrass and halophile were dominant seagrasses and occurred throughout the study area except in the deep middle portion of the lagoon. Seagrass growth was generally limited to water depths less than 1.3 m in the unspoiled areas of the Laguna and to water depths less than 1 m around the dredged material islands. This shallow depth restriction on the seagrasses is probably due to the turbid water conditions. This turbidity may be partially due to the effects of dredging, deposition of dredged material, and erosion of existing dredged material islands. Few differences were found between the abundance of vegetation associated with these dredges material islands and the unspoiled vegetated portions of the upper Laguna Madre. The islands deposited in the deeper parts of the lagoon were vegetated while surrounding areas were devoid of vegetation. Dredged material sites less than 10 years old were only sparsely vegetated, indicating the slowness with which seagrasses invade these disturbed areas. A community similarity index indicated the species composition of dredged material sites older than 20 years were similar to that of the unspoiled lagoon area...