Phosphorus Loading of an Urban Lake by Bird Droppings

ABSTRACT

The role of birds in nutrient cycling in lakes and reservoirs is often not quantified, or thought to be relatively unimportant. However, when bird populations are large relative to the size or volume of the waterbody, a substantial fraction of the nutrient pool may cycle through birds. In this report, we estimate the mass of phosphorus in bird droppings cycling through Green Lake, a productive lake in urban Seattle, between January 1992 and December 1994. Phosphorus loading from bird droppings is compared to the mass of phosphorus entering the lake from other sources measured during a concurrent limnologic study. Waterbirds spent 528,355 bird-days (the number of bird-use days) at Green Lake in 1992, 530,318 bird-days in 1993, and 546,943 bird-days in 1994. The most abundant bird species at the lake were American coots, mallards, gadwalls, and various species of gulls. Of all birds at Green Lake, waterbirds accounted for greater than 99% of the phosphorus loading. Total phosphorus loading by birds was estimated to be 160 kg P year¹ (equivalent to 0.154 g P m⁻² year⁻¹) in 1992,159 kg P year¹ (equivalent to 0.153 g P m⁻² year⁻¹) in 1993, and 167 kg P year¹ (equivalent to 0.160 g P m⁻² year⁻¹) in 1994. Total phosphorus in bird droppings constituted 27% of the total phosphorus loading to the lake from all sources in 1992, 25% in 1993, and 34% in 1994. Based on the behavior of the birds, their high metabolic rate, and the paucity of forage in the surrounding urban area, we estimate 87% of the phosphorus in bird droppings originated from food items in the lake and represented internal cycling. The mass of phosphorus entering the lake in droppings was not correlated with three indices of water quality and trophic state, lake total phosphorus and chlorophyll a concentrations, or Secchi depth. However, birds may potentially increase the productivity of water bodies by changing the form, rate, and pathways of cycling, and physical compartment of phosphorus.

Key Words:

• phosphorus
• nutrient cycling
• urban lake
• waterbirds
• waterfowl
• defecation
• eutrophic
• Green Lake
• Seattle
• Washington