THE ROLE OF MIGRATORY WATERFOWL AS NUTRIENT VECTORS IN A MANAGED WETLAND

Dense aggregations of waterfowl, often caused by loss of native wetlan ds and increased waterfowl numbers, can result in the destruction of w etland vegetation and agricultural crops, increase the risk of infecti ous disease outbreaks, and decrease water quality. Problems related to water quality may be particularly severe in arid regions of the south western United States, where water quality and quantity are contentiou s issues. Over 40,000 Lesser Snow Geese (Chen caerulescens caerulescen s) and Ross' Geese (Chen rossii) winter annually at Bosque del Apache National Wildlife Refuge, New Mexico. Daily feeding bouts by geese mov e large quantities of nutrients from farm fields where they feed to ma naged wetlands where they roost. Using energy and mass balance models, population estimates, daily and seasonal migration patterns, and feed ing behaviors, we estimated the mass and ratio of nitrogen to phosphor us of nutrients loaded by geese into the wetlands of the Bosque del Ap ache National Wildlife Refuge. Loading rates peaked in late November 1 995 at more than 300 kg nitrogen per day and over 30 kg phosphorus per day. As feeding behaviors changed through the winter and bird densiti es declined, loading rates fell. Our estimates suggest that in the win ter of 1995-1996, bird-borne nutrients supplied nearly 40% of the nitr ogen and 75% of the phosphorus entering the primary wetland used for g eese roosting. High loading rates by geese are a consequence of their colonial roosting behavior; over 90% of the geese roost on 10% of the wetland area. The effects of nutrient loading could be reduced by incr easing flushing rates or dispersal of roosting waterfowl. The loss of natural wetlands, however, limits options for expanding wetland area a vailable to waterfowl, and it may be difficult to increase flushing ra tes in this arid region. Thus, management of waterfowl and the nutrien ts they move will continue to be an important issue for wetlands of wi ldlife refuges.