The Effects of a Dredge Excavation Pit on Benthic Macrofauna in offshore Louisiana

Over two years after the original creation of a sand excavation pit 8 km off the Louisiana coast, benthic macrofauna communities and sedimentary characteristics are still effected. Macrofaunal communities inside the pit had lower abundance, biomass and diversity than communities outside the pit. This difference however, was only significant compared to some of the stations outside the pit. Results from multi-dimensional scaling and cluster analysis indicate that macrofaunal communities were less than 31 % similar inside the pit to communities outside the pit. The polychaete, Mediomastus ambiseta, was the most abundant species outside the excavation pit, but the species was only counted once inside the pit. The most dominant species, which made up over 90% of organisms inside the pit, was the pioneer polychaete, Paraprionospio pinnata. Three to five species were found at each station inside the pit as opposed to nine to 27 species at stations outside the pit. Differences in species compositions between inside and outside the pit were due to a loss of species rather than replacement by different species. All species inside the pit were also found outside the pit. Sediment inside the pit contained more silt and clay compared with outside, however no differences in water quality were detected. Sediment in the erosional zone outside the pit and on a dredge disposal mound was coarser than all other sediments sampled. Because the macrofaunal community inside the pit has not recovered within 38 months, it is likely that it will require more time before it resembles the surrounding conditions.