Impacts of nutrient subsidies on salt marsh food webs
Shannon Murphy, University of Denver, Gina Wimp and Danny Lewis, Georgetown University
Anthropogenic nutrient inputs into native ecosystems cause fluctuations in resources that normally limit plant growth, which has important consequences for associated foodwebs. Such inputs from agricultural and urban habitats into nearby natural systems are increasing globally and can be highly variable, spanning the range from sporadic to continuous. Despite the global increase in anthropogenically-derived nutrient inputs into native ecosystems, the consequences of variation in subsidy duration on native plants and their associated foodwebs are poorly known. Specifically, while some studies have examined the effects of nutrient subsidies on native ecosystems for a single year (a nutrient pulse), repeated introductions of nutrients across multiple years (a nutrient press) better reflect the persistent nature of anthropogenic nutrient enrichment. Using stable isotopes of nitrogen and carbon, we tested the effects of a one-year nutrient pulse with a four-year nutrient press on arthropod consumers in two salt marshes. We found that plant biomass and %N as well as arthropod density fell after the nutrient pulse ended but remained elevated throughout the nutrient press. Notably, higher trophic levels responded more strongly than lower trophic levels to fertilization, and the predator/prey ratio increased each year of the nutrient press, demonstrating that foodweb responses to anthropogenic nutrient enrichment can take years to fully manifest themselves.