Nutrient cycling at the landscape scale: The role of diel foraging migrations by geese at the Bosque del Apache National Wildlife Refuge, New Mexico

Wildlife refuges are sites of dense aggregations of wintering waterfowl. Re fuge managers are concerned about local water-quality effects and the amoun t of birdborne nutrient load that might cause eutrophication in roosting po nds and(or) be exported to downstream systems. We initiated this research e ffort to test the hypothesis that daily feeding migrations by geese represe nted a significant source of nutrients to wetland systems at the Bosque del Apache National Wildlife Refuge in the middle Rio Grande River valley of N ew Mexico. We documented the role of geese in translocating nutrients throu gh a combination of time budget and mass balance models for birds, bioassay s of phytoplankton growth, and stable isotope methods that trace the source of nitrogen to food webs. Geese increased the nutrient loading rates in so me wetland ponds by up to 40% for total nitrogen and 75% for total phosphor us. Bioassays revealed that nitrogen was consistently limiting to primary p roduction by algae in the ponds. Chlorophyll levels increased in proportion to bird densities. Fish and crayfish from the ponds intensively used by bi rds had very low stable nitrogen signatures relative to those from the Rio Grande River. This result derived from the low N-15 in alfalfa and corn, wh ich were the primary forage for birds, and accumulated through food webs in proportion to bird use of individual ponds as roosting areas. In general, the wetland ponds functioned efficiently in retaining the birdborne, alloch thonous nutrient load and only modest amounts of the total nitrogen or phos phorus were exported downstream.