FOOD-CONSUMPTION AND FECAL DEPOSITION OF PLANT NUTRIENTS BY BLACK SWANS (CYGNUS-ATRATUS LATHAM) IN A SHALLOW NEW-ZEALAND LAKE

Deposition of faeces by black swans (Cygnus atratus Latham) feeding on benthic algae in a shallow New Zealand lake was determined by collect ion of faeces from the lake bottom and from the shore. The two methods showed good agreement after adjustment for the weight loss on immersi on. The mean daily faecal output per swan was 52 g dry weight. The nit rogen content of the faeces averaged 2.3% of dry weight, and was domin ated by soluble organic nitrogen (59% of total N). Phosphorus averaged 0.44% of dry weight, with 66% of it being particulate, and 30% solubl e reactive phosphorus. Although faecal inputs of total phosphorus were sufficient to generate concentrations of 15-30 mg m(-3), the faecal c ontributions of both N and P were only a minor component of the fluctu ations observed in the lake, and were also small in relation to the to tal nutrient pool in the water and benthic algae. Waterfowl faeces app ear to have low ratios of N to P, which will favour dominance of the p hytoplankton by cyanobacteria in lakes where the faecal component of n utrient loads is large. The few data available suggest that the nitrog en content of waterfowl faeces is largely independent of that in their food. Food consumption, calculated by using cellulose as an indigesti ble faecal marker, was 104 g dry weight swan(-1) d(-1), a figure that appears low in relation to those for other swan species. Even the high est published figure for food intake by a swan is only about one half of the corresponding average metabolically-adjusted figures for geese, and we caution against the uncritical use of bioenergetic models for determining rates of food consumption and defaecation.