Some Effects of 30 Years of Weir-Management on Coastal Marsh Aquatic Vegetation and Implications to Waterfowl Management

Aquatic vegetation was compared between weir-managed and unmanaged brackish marsh ponds at Marsh Island, Louisiana, parts of which have been weir-managed since 1958. Coverage by aquatic vegetation was greater in weir-managed ponds than in unmanaged ponds, but weir-management affected individual species differently. Eleocharis parvula was more common in unmanaged ponds than in managed ponds during the early 1960s but has not occurred in any ponds sampled since then. Ceratophyllum demersum and the dominant plants Myriophyllum spicatum and Ruppia maritima were more common in weir-managed ponds than in unmanaged ponds, although the magnitude of the difference varied greatly among sample dates. Because Myriophyllum spicatum and Ruppia maritima are important and preferred duck foods in Louisiana brackish marsh, it was concluded that weir-management improved habitat quality for migrant and resident ducks at Marsh Island. Weir-managed ponds contained more aquatic vegetation than unmanaged ponds in 1988, even though the crest of weirs had lost 15 cm of elevation relative to rising sea level and the accreting marsh surface. This suggests that aquatic vegetation can be increased with a greater degree of water exchange than previously believed, which is desired to reduce interference to juvenile fish and crustacean movement. Weir-managed and unmanaged ponds did not differ in pond depth, which suggests that weir-management did not affect sedimentation