The effect of nutrient enrichment on the aquatic resource subsidy to terrestrial spiders.

Increased watershed development can elevate stream nutrient concentrations (Vitousek et. al. 1997), but little is known how this enrichment affects food web dynamics of aquatic systems and their surrounding terrestrial habitats. Stream enrichment increases leaf decomposition rates, alters benthic invertebrate community structure, and increases invertebrate secondary production (Cross 2004; Greenwood 2004). However, stream systems are linked to their surrounding terrestrial system via aquatic insect emergence; an aquatic subsidy of terrestrial food webs (Jackson and Fisher 1986; Sanzone et. al. 2003) and a possible pathway for changes in aquatic systems to impact terrestrial systems. Because resource subsidies typically flow down a productivity gradient, an increase in the productivity of a donor system may increase this resource subsidy and alter the food web dynamics of the recipient system (e.g., Sanzone 2001, Polis et. al. 1997). Since July 2000, we have been continuously enriching a first order headwater stream and have facilitated increased aquatic invertebrate production (Cross 2004), which may increase the aquatic insect subsidy to the terrestrial system. Therefore, changes in food web dynamics caused by nutrient enrichment may not be limited to aquatic systems, but may also cascade through the terrestrial system with unknown effects. To better understand how aquatic and terrestrial systems are linked, I will test whether stream nutrient enrichment increases aquatic subsidies to terrestrial systems. Using a 15N stable isotope tracer I will test three predictions. Prediction 1: Aquatic insect emergence subsidizes near-stream terrestrial spiders more than upland spiders. Prediction 2: Aquatic insect subsidies increase spider abundance, richness, and biomass near streams when compared to upland habitats. Prediction 3: Stream nutrient enrichment increases this aquatic subsidy