Use of lakes by black duck broods in relation to biological, chemical, and physical features

Characteristics of 32 freshwater lakes in central and western Nova Scotia were quantified to determine the relative influence of various biological, chemical, and physical factors on habitat selection by black ducks (Anas rubripes Brewster) during brood-rearing. Acidity and trophic status varied greatly among the waterbodies, of which 20 were used by black ducks for rearing their young. Duck brood density was positively related to lake trophic status. The highest brood densities occurred on hypertrophic waterbodies with a large anthropogenic input of nutrients. Lakes with black duck broods had significantly higher concentrations of phosphorus and nitrogen, darker water with more dissolved organic carbon, and within the littoral zone, greater macrophyte cover, greater density and biomass of pelagic invertebrates, gentler slopes, and a softer, more organic substrate. Similar trends were observed within a subset of 17 acidic lakes (pH ≤ 5.5), 8 of which supported black ducks. The density of black duck broods was significantly correlated with 17 of 20 variables, most notably total phosphorus (r= + 0.81). Partial correlation (removing the effect of total phosphorus) revealed that brood density was significantly correlated with the abundances of pelagic (r= + 0.77) and benthic (r= + 0.68) invertebrates, macrophyte cover (r= + 0.52), and substrate score (r= + 0.57), but not with other chemical variables, including pH. Our results suggest that quality brood-rearing habitat is distinguished by a combination of factors, especially available nutrients, macrophyte cover, and invertebrates, subject to constraints imposed by physical characteristics of the littoral zone of the lake. The abundance of invertebrates, the primary food of young black ducks, emerged as the most important biological factor influencing the density of black duck broods.