Nitrogen versus phosphorus demand in a detritus-based headwater stream: what drives microbial to ecosystem response?

An irrefutable aspect of global change is the increased mobilization of nutrients (SCHLESINGER 2004), which potentially affects many trophic pathways in aquatic and terrestrial ecosystems. Previous work on the effects of nutrients has focused largely on primary-producer−based pathways (ELSER et al. 2007). However, detrital pathways may be even more sensitive to nutrients because detrital food resources are typically more nutrient poor than those derived from living plants (CEBRIAN & LARTIGUE 2004). The primary ways that nutrient availability affects detrital food webs are (1) changes in cellular nutrient content of basal resources and (2) consequent stimulation of biomass, growth rates, and production of heterotrophic consumers. We observed evidence for both of these effects in a long-term nitrogen (N) and phosphorus (P) enrichment of a detritus-based headwater stream. Specifically, we observed increased N (CROSS et al. 2003, GULIS et al. 2004, GREENWOOD et al. 2007) and P content (CROSS et al. 2003) of allochthonous leaves, wood, fine particulate organic matter (FPOM, fine particulate organic matter) and associated microorganisms, as well as epilithon. We also observed changes in the biomass, growth rates, and production of organisms that spanned levels of organization from microorganisms to metazoans. These changes included increased biomass (GULIS & SUBERKROPP 2003) and production (Suberkropp, unpublished data) of fungi and bacteria; increased growth rates and biomass of epilithon (GREENWOOD & ROSEMOND 2005); increased growth rates, biomass and production of invertebrates (CROSS et al. 2005, CROSS et al. 2006); and increased growth rates of salamanders (JOHNSON et al. 2006). Although we found significant changes in nutrient content of resources and production of organisms, our study design did not allow us to examine separately the effects of N versus P loading. We used 2 lines of evidence from our study to evaluate the potential relative importance of N versus P in driving the changes we observed in multiple resources and consumers. First, dissolved inorganic nutrient concentrations in stream water were used to determine ambient N:P molar ratios (to suggest potential N vs. P limitation, but see DODDS 2003). We also compared predicted N:P ratios (based on background stream water concentrations plus our added enrichment) versus observed N:P in our treatment stream. Dissolved N:P ratios that differ from predicted in either direction would suggest potential selective uptake of N or P by biota. Second, we determined the magnitude of the relative response of C:N, C:P, and N:P of basal resources to enrichment, as well as the trends in the changes in these ratios over 2 years of enrichment.