THE RELATIVE IMPORTANCE OF BIOTIC AND ABIOTIC VECTORS IN NUTRIENT TRANSPORT

The mass of nutrients (nitrogen, phosphorus, potassium, and calcium) imported during 1984 and 1985 to the North Inlet Estuary, Georgetown County, South Carolina, by precipitation and runoff was compared with that imported by a colonial-nesting wading bird, the white ibis (Eudocimus albus). From late March through late June of both years, breeding ibises imported nutrients to the North Inlet Estuary study site from freshwater bottomland forest swamps, where they fed on crayfishes (Procambaridae). Although 1984 was a relatively wet year, in 1985 the ibis breeding season was preceded by a severe winter-spring drought. In 1984 ibises nested in higher numbers, had higher per-pair breeding success, and imported 11 times more nutrients than in 1985. Nutrient input from atmospheric sources was substantially lower in 1984 than in 1985. The 1984 ibises imported 9% as much nitrogen, 33% as much phosphorus, 0.4% as much potassium, and 0.3% as much calcium to the estuary as did atmospheric sources. In 1985 nutrient input from ibises amounted to only 0.2% of the nitrogen, 2.9% of the phosphorus, and much less than 1% of the potassium and calcium imported by atmospheric sources. Our results show that nutrient inputs to estuaries from colonial-nesting wading birds can be substantial when compared with those from atmospheric sources and can vary considerably among years. They also suggest that nutrient regimes in estuaries with large assemblages of wading birds may differ significantly from those lacking such colonies.