Incorporation of marine-derived nutrients from petrel breeding colonies into stream food webs

Summary 1. Stable isotope ratios of aquatic invertebrates, aquatic mosses and leaves of riparian plants were used to determine whether marine‐derived nutrients from breeding colonies of the Westland petrel (Procellaria westlandica) were incorporated into the food webs of small streams in New Zealand. 2. The δ15N signatures of all plants and animals examined were higher by 3.6–4.6‰ in small streams draining catchments with petrel colonies than in nearby streams where petrels were absent. δ13C values of leaves from terrestrial plants were also enriched by about 2‰ where petrels were present, but the carbon ratios of aquatic species were depleted in 13C, rather than enriched, suggesting that any marine signal was over‐ridden by isotopic shifts related to photosynthetic fractionation. 3. A high marine‐nitrogen signal was maintained along the 3 km length of Scotchman Creek with the δ15N values of leptophlebiid mayflies and predatory insects ranging from 7.4–9.5 and 9.2–11.9‰, respectively. 4. Most nutrients derived from petrels are likely to be translocated to streams via the soil, which they enter in the form of excreta, spilled food, feathers, dead chicks, and abandoned eggs. However, because changes in δ15N values are brought about by soil processes such as volatilisation of ammonia, nitrification and denitrification, it is difficult to predict the exact isotope signature of nitrogen entering a stream. Tentative estimates of the proportion of marine‐derived nitrogen in stream biota, calculated using a mass‐balance approach, ranged from 28–38%. 5. Our findings indicate that marine nutrients transported inland by seabirds can be incorporated into the food webs of streams. In pre‐human times when there were many more seabird colonies on mainland New Zealand than exist today, marine‐derived nutrients introduced by birds may have had significant effects on nitrogen cycling and the productivity of New Zealand streams.