Quantification of cormorant litter and nutrient deposition to Great Lakes island ecosystems

Ornithogenic nutrients derived from waterbirds such as the double-crested cormorant (Phalacrocorax auritus, Lesson) have been linked to habitat change within nesting colonies. For the islands of Lake Erie, where increasing cormorant populations and subsequent habitat change have spurred management activity, estimates of the quantity and chemical characteristics of avian-derived contributions are lacking. To evaluate the quantity and chemical characteristics of ornithogenic litterfall beneath a double-crested cormorant colony on a western Lake Erie island we investigated the mass of material and nutrient composition (PO43 −, NO3− and NH4+) reaching the forest floor under three nest densities (Low: 1-96 nests ha− 1; Medium: 97-255 nests ha− 1 and High: > 255 nests ha− 1). As expected, litterfall (total mass) input differed among nest densities with the most substantial input (225.05 g/m2 week− 1) measured under High nest density conditions. Nutrient concentrations also showed increases with nest density to a point, where mean PO43 − and NH4+ concentrations showed no differences between Medium and High nest density sites. As well, NO3− concentrations were highest under Medium density, with no differences in this nutrient observed between Low and High density. Collectively, litterfall nutrient composition was similar to those linked to habitat changes in other waterbird colonies. Similarities in the concentrations of several nutrients between Medium and High nest density categories suggest that management actions aimed at reducing allochthonous nutrient contributions should try to sustain nest density at or below 96 nests ha− 1.