The response of detrital and autotrophic resources to long-term nutrient enrichment in a detritus-based headwater stream
Greenwood, Jennifer Lynn
MetadataShow full item record
Enrichment of aquatic ecosystems with nitrogen and phosphorus is one of the most significant anthropogenic impacts to surface waters worldwide. Relatively little is known about nutrient effects on detritus-based stream systems relative to autotroph-based systems. This study examined the effects of a long-term nitrogen and phosphorus enrichment on autotrophic and heterotrophic portions of the resource base of a forested detritus-based headwater stream at the Coweeta Hydrologic Laboratory in the southern Appalachian mountains. I assessed the affects of 2 years of nutrient enrichment on the detrital and autotrophic resources: leaf litter, periphyton and bryophytes. Nutrient enrichment strongly affected processing rates of allochthonous leaf inputs. Leaf breakdown rates, microbial activity on leaf material, the nitrogen content of leaf litter, and invertebrate biomass associated with leaf litter, all increased with nutrient enrichment. Increased processing rates also accelerated the flux of nitrogen to invertebrate biomass from leaf litter standing crop. Overall, the effect of enrichment was slightly stronger on a lower quality (lower nitrogen content) leaf, rhododendron, relative to a higher quality leaf type, red maple, suggesting nutrient enrichment may not affect all detrital resources equally. The response of autotrophs to nutrient enrichment was less dramatic, primarily due to strong light limitation. Algal biomass measured as chlorophyll a and algal growth rates increased with nutrient enrichment, with strongest effects in the early spring, when light levels reaching the stream were highest. The diatom-dominated algal species assemblages were not altered by nutrient enrichment and were more related to seasonal effects. Bryophyte biomass also did not change with nutrient enrichment, potentially due to light limitation. Biomass of algal epiphytes on bryophytes, measured as biovolume, showed variable response to nutrient enrichment, and algal community patterns were more affected by substrate type (moss, liverwort or bedrock) than nutrient or light availability. Overall, results from this study show that nutrient enrichment had strong effects on primarily the detrital resources in a headwater stream, with subtle effects on the autotrophic resources. Thus, algal-based measures of nutrient impacts currently used for running waters may be inappropriate for detritus-based stream ecosystems.