Wigeongrass (Ruppia maritima), a submerged aquatic plant inhabiting estuarine wetlands, is an important winter food for waterbirds along the Texas Gulf Coast. We examined availability of wigeongrass at Mad Island Wildlife Management Area, Texas, USA by estimating aboveground biomass from October through January, 1998–1999 and 2001–2002. We also used an exclosure experiment to determine the extent to which herbivory by waterbirds was responsible for depletion of wigeongrass. Aboveground biomass of wigeongrass declined an average of 189 g m⁻² and 71 g m⁻² between October and January each year. Aboveground biomass declined at a higher rate among plots exposed to herbivory compared to exclosures, and the loss of biomass attributable to foraging by waterbirds was 19%. In 1998, counts of gadwalls (Anas strepera), American wigeons (A. americana), and American coots (Fulica americana) using study ponds peaked in November and then followed a declining trend similar to availability of wigeongrass, suggesting that as wigeongrass was depleted herbivorous waterbirds moved to other habitats where food was more available.