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

Abstract

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.