Moist-soil wetlands that are seasonally flooded provide important habitats for waterfowl in the Mississippi Alluvial Valley (MAV). These wetlands often contain tall and dense vegetation that may constrain waterfowl use before natural openings form. During winters 2006–2009, I estimated abundances of waterbirds, seeds and tubers, and invertebrates in response to autumn, prelooding treatments of light disking, mowing, and no manipulation (control) of vegetation in 26 moist-soil wetlands in the MAV. Seeds and tubers were most abundant in control and mowed plots in late autumn. Decomposition was least and invertebrate abundance was greatest in control plots during winter. Dabbling ducks were most abundant in mowed and disked plots during winter. Lightly disked plots contained ~30% fewer seeds and tubers than mowed and control plots. In late winter, ~260 kg[dry]/ha of seeds and tubers remained among mowed, disked, and control plots. Therefore, autumn mowing of robust moist-soil vegetation can be used to create an interspersion of emergent vegetation and open water attractive to waterfowl and conserve waterfowl foods. Additionally, I identified 6 seed taxa that may not be used for food by dabbling ducks (i.e., Amaranthus spp., Cyperus odoratus, Eleocharis spp., Ipomoea spp., Jacquemontia tamnifolia, Sesbania herbacea) and estimated that removing these and other taxa not reported in diet literature in the MAV resulted in a ~31% reduction in estimated moist-soil food availability for ducks. In other experiments, I estimated that waterfowl reduced experimentally placed Japanese millet (Echinochloa frumentacea) to ~10 kg/ha and other natural seeds and tubers to ~170 kg/ha in experimental plots in mid-winter. However, waterfowl did not abandon wetlands or stop foraging when seed reduction ceased, suggesting residual abundances of seeds and tubers represented a food availability threshold (FAT). Using the median FAT value of 220 kg/ha from both experiments and removing 31% of seed mass that may not be consumed by dabbling ducks, results in a ~70% decrease in moist-soil seed availability in the MAV. Conservation planners should consider reducing the current estimates of seed and tuber availability and recommend increasing active management or implementation of additional managed, moist-soil wetlands in the MAV.

The Cienega de Santa Clara is a 5600 ha, anthropogenic wetland in the delta of the Colorado River in Mexico. It is the inadvertent creation of the disposal of brackish agricultural waste water from the U.S. into the intertidal zone of the river delta in Mexico, but has become an internationally important wetland for resident and migratory water birds. The marsh is dominated by Typha domengensis with Phragmites australis as a sub-dominant species in shallower marsh areas. The most important factor controlling vegetation density was fire. The second significant (P<0.01) factor controlling NDVI was flow rate of agricultural drain water from the U.S. into the marsh. Reduced summer flows in 2001 due to canal repairs, and in 2010 during the YDP test run, produced the two lowest NDVI values of the time series from 2000 to 2011 (P<0.05). Salinity is a further determinant of vegetation dynamics as determined by greenhouse experiments, but was nearly constant over the period 2000 to 2011, so it was not a significant variable in regression analyses. Evapotranspiration (ET) and other water balance components were measured in Cienega de Santa Clara; we used a remote sensing algorithm to estimate ET from meteorological data and Enhanced Vegetation Index values from the Moderate Resolution Imaging Spectrometer (MODIS) sensors on the Terra satellite. We used Landsat NDVI imagery from 1978-2011 to determine the area and intensity of vegetation and to estimate evapotranspiration (ET) to construct a water balance. Remote sensing data was supplemented with hydrological data, site surveys and literature citations. The vegetated area increased from 1978 to 1995 and has been constant at about 4200 ha since then. The dominant vegetation type is Typha domingensis (southern cattail), and peak summer NDVI since 1995 has been stable at 0.379 (SD = 0.016), about half of NDVI(max). About 30% of the inflow water is consumed in ET, with the remainder exiting the Cienega as outflow water, mainly during winter months when T. domingensis is dormant.