Ecosystem and physiological scales of microbial responses to nutrients in a detritus-based stream: results of a 5-year continuous enrichment

Our study examined the response of leaf detritus–associated microorganisms (both bacteria and fungi) to a 5-yr continuous nutrient enrichment of a forested headwater stream. Leaf litter dominates detritus inputs to such streams and, on a system-wide scale, serves as the key substrate for microbial colonization. We determined physiological responses as microbial biomass and activity expressed per unit mass of leaves and system-level responses by quantifying leaf litter standing crop monthly and expressing responses per unit area of streambed. Physiological (mass-specific) trends differed from system-level (area-specific) trends. Physiological responses to enrichment were generally positive. With the exception of bacterial biomass, nutrients increased all metrics expressed per unit mass leaf litter in the treatment stream relative to the reference (fungal biomass and production, bacterial production, microbial respiration). This positive physiological response to nutrient enrichment was associated with lower leaf litter standing crop in the treatment stream, resulting in less substrate for microbial colonization. Consequently, during most years on a system-level scale, only fungal production and microbial respiration were positively affected by nutrients, whereas fungal biomass was negatively affected. Thus, from a whole-stream perspective, nutrients led to a lower quantity of leaf detritus with greater variation, resulting in net reductions of associated fungal biomass and greater intra-annual variability in both fungal biomass and respiration. Our results demonstrate profound effects of nutrients on heterotrophic pathways that mediate detritus processing in stream ecosystems. Similar effects on heterotrophic microbes and detrital resources may be a widespread consequence of anthropogenic nutrient enrichment. Heterotrophic microorganisms have been shown to be stimulated by increases in inorganic nutrient concentrations. However, long-term effects of nutrients on their biomass and activity, consequences for the detrital substrates they colonize, and effects on microbial production at system-level scales are unknown. Previous studies have shown that nutrient enrichment stimulates leaf decomposition, fungal biomass, and spore production associated with decomposing leaves in streams, particularly in systems characterized by relatively low nutrient concentrations (Howarth and Fisher 1976; Grattan and Suberkropp 2001; Ferreira et al. 2006; but see Newbold et al. 1983; Royer and Minshall 2001). However, these studies have typically followed the short-term decomposition dynamics of a group of leaves or leaf pieces placed in a stream. Much less is known about the long-term fate of leaf litter standing crop and the activity of leaf-decomposing microorganisms in the common situation of a stream being enriched with nutrients over several annual cycles. In contrast to lotic ecosystems that depend on autochthonous primary production, in which nutrient enrichment has been shown to increase biomass at the base of the food web (Peterson et al.