Food consumption and faecal deposition of plant nutrients by black swans (Cygnus atratus Latham) in a shallow New Zealand lake

Abstract

Deposition of faeces by black swans (Cygnus atratus Latham) feeding on benthic algae in a shallow New Zealand lake was determined by collection of faeces from the lake bottom and from the shore. The two methods showed good agreement after adjustment for the weight loss on immersion. The mean daily faecal output per swan was 52 g dry weight. The nitrogen content of the faeces averaged 2.3% of dry weight, and was dominated by soluble organic nitrogen (59% of total N). Phosphorus averaged 0.44% of dry weight, with 66% of it being particulate, and 30% soluble reactive phosphorus. Although faecal inputs of total phosphorus were sufficient to generate concentrations of 15–30 mg m−3, the faecal contributions of both N and P were only a minor component of the fluctuations observed in the lake, and were also small in relation to the total nutrient pool in the water and benthic algae. Waterfowl faeces appear to have low ratios of N to P, which will favour dominance of the phytoplankton by cyanobacteria in lakes where the faecal component of nutrient loads is large. The few data available suggest that the nitrogen content of waterfowl faeces is largely independent of that in their food. Food consumption, calculated by using cellulose as an indigestible 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 highest 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.