Spatial patterns of resource transfer in coupled food webs: The uphill flow of aquatic subsidies via moose on Isle Royale, USA

Resource transfer (flux) links aquatic and terrestrial ecosystems, but studies of long-term spatial dynamics of resource flux across aquatic-terrestrial boundaries are rare. We analyzed the magnitude and spatial dynamics of aquatic-derived resources that moose (Alces alces) transfer to terrestrial systems in Isle Royale National Park (Lake Superior, USA).  We used excretion models, foraging parameters, and long-term moose carcass locations (n=3616) collected between 1958-2006. We found that excretory nitrogen (N) of aquatic origin transferred to terrestrial systems via moose approaches 4.5 kg N ha^-1 yr^-1 in summer core areas used by moose, which would constitute ~25% of the net annual N mineralization typical for Isle Royale forests.  Potential littoral zone phosphorus (P) loss due to moose herbivory reaches 0.2 kg P ha^-1 yr^-1 in aquatic foraging areas, constituting ~60% and ~222% of annual terrestrial P runoff and annual atmospheric P deposition, respectively. The area of terrestrial resource dispersion is greater than the extent of aquatic resource loss and there is less than 0.1% likelihood that the clustering of high density moose carcass sites could be the result of random chance.  Wolves (Canis lupus) and moose can create concentrated areas of resource flux due to clustered predation and foraging patterns respectively.  To the extent that wolves influence moose populations, habitat use, and carcass distribution, they also impact aquatic-terrestrial resource subsidies transferred via moose.  Moose likely increase local terrestrial N cycling when foraging on N-rich aquatic macrophytes, but the net effect of aquatic foraging on aquatic P cycling requires more scrutiny.