Recovery of floral and faunal communities after placement of dredged material on seagrasses in Laguna Madre, Texas

摘要

The objectives of this project were to determine how long alterations in habitat characteristics and use by fishery and forage organisms were detectable at dredged material placement sites in Laguna Madre, Texas. Water, sediment, seagrass, benthos, and nekton characteristics were measured and compared among newly deposited sediments and nearby and distant seagrasses each fall and spring over three years. Over this period, 75% of the estimated total surface area of the original deposits was either re-vegetated by seagrass or dispersed by winds and currents. Differences in water and sediment characteristics among habitat types were mostly detected early in the study. There were signs of steady seagrass re-colonization in the latter half of the study period, and mean seagrass coverage of deposits had reached 48% approximately three years after dredging. Clovergrass Halophila engelmannii was the initial colonist, but shoalgrass Halodule wrightii predominated after about one year. Densities of annelids and non-decapod crustaceans were generally significantly greater in close and distant seagrass habitats than in dredged material habitat, whereas densities of molluscs were not significantly related to habitat type. Nekton (fish and decapod) densities were almost always significantly greater in the two seagrass habitats than in dredged material deposits. Benthos and nekton communities in dredged material deposits were distinct from those in seagrass habitats. Recovery from dredged material placement was nearly complete for water column and sediment components after 1.5―3 years, but recovery of seagrasses, benthos, and nekton was predicted to take 4―8 years. The current 2 to 5 year dredging cycle virtually insures no time for ecosystem recovery before being disturbed again. The only way to ensure permanent protection of the high primary and secondary productivity of seagrass beds in Laguna Madre from acute and chronic effects of maintenance dredging, while ensuring navigation capability, is to remove dredged materials from the shallow waters of the ecosystem.