Nutrient effects on a detritus-based stream ecosystem.

To determine the effects of nutrient enrichment in a detritus-based stream, we have continuously enriched a headwater stream with N and P for 4.5 years and quantified the response at different scales of stream structure and function: microbial, metazoan, and ecosystem. The effects of enrichment in the treatment stream (Watershed 54, WS54) were compared to conditions in the reference stream (Watershed 53, WS53) and one year of comprehensive baseline sampling was conducted in both streams prior to enrichment. Our first objective was to assess the effects of nutrient enrichment on growth and production of organisms. Our second objective was to determine the effects of nutrients on the fate of carbon resources, which were altered via increased microbial metabolism and invertebrate consumption. Most data were analyzed using Randomized Intervention Analysis (RIA)(Carpenter et al. 1989), which was based on pre-treatment differences between the two streams to differences following nutrient addition. This study was the first to determine the long-term effects of nutrient enrichment in a system in which organisms derive most of their carbon from detritus or detritus-based prey. Nutrient addition profoundly affected the treatment stream, with effects propagating from microbes to ecosystem-level effects. Below are specific results. EFFECTS ON ORGANISMS Nutrient addition resulted in an increase in microbial biomass and production. We observed dramatic increases in leaf-associated fungal production and biomass and significant effects of nutrient addition on leaf-associated bacterial production, but not on bacterial biomass. Whereas production of fungi exhibited an overall average 211% increase in response to nutrient addition in year 2 of enrichment, bacterial production only increased by 60% compared to the control stream. The mean number of fungal species detected per sample date before enrichment and in the reference stream was 12-13 but this increased to 18.5 species during enrichment. Primary producers showed some response to nutrient addition, despite light limitation. Contrary to our predictions, we observed a significant increase in algal biomass as measured by chlorophyll a, but the response was largely limited to high light months. There was a significant effect of enrichment on biofilm AFDM, but without the seasonal effect seen with chlorophyll a. There was no significant effect on cell densities of algae. There has been no apparent change in algal community composition as a result of enrichment. Production and biomass of invertebrates increased in response to nutrient addition. There has been a significant effect of enrichment on invertebrate biomass measured monthly and invertebrate production in the treatment stream was 153% higher compared to trends in the control stream after 1 year. Leaf litter standing crop in the stream is declining. Long-term data indicate a significant regression between benthic leaf litter standing crop and invertebrate production. Data from yr. 1 indicate a much higher production in the nutrient addition stream than would be predicted from leaf litter standing crop. Our longer term predictions for invertebrate biomass in the treatment stream include a decline in production compared to the response we observed after 1 yr of enrichment, based on the decline in benthic leaf litter. Growth rates of invertebrates were enhanced by nutrient addition. We measured increased growth rates of invertebrates with short aquatic life cycles, but did not observe increased growth rates on invertebrates with longer life cycles. Chironomidae, the dominant collector/gatherers, grew ca. 30% faster in the enriched stream than in the reference stream among all size classes. Tallaperla maria, a long-lived leaf-eating stonefly, did not respond to enrichment in terms of daily growth rates. We have data on the C, N, and P content of consumers showing that some taxa have much lower body P content than others (i.e., Tallaperla maria 0.5%, Chironomidae . 1.1%). Lower body P content generally suggests a lower P requirement for growth. Thus, Tallaperla maria, and other taxa with lower body P, may not be nutrient limited. Rates of organic matter breakdown were accelerated by nutrient addition. We observed increased breakdown of rhododendron and red maple leaves as well as wood. Rates of leaf decay exhibited a ca. 70% increase in the treatment stream relative to the control after 1 year of enrichment, and a ca. 200% increase in the 2nd year of enrichment. Breakdown rates with nutrient enrichment for both leaf types were outside of the 99% confidence interval of long-term means. Rates of decay of wood veneers exhibited 477 and 795% increases in the treatment stream relative to the control after 1 and 2 years of enrichment, respectively. Organic matter standing crop and export were affected by nutrient addition. We have observed dramatic reductions in leaf litter standing crop in the treatment stream, as well as increased export of fine particulate organic matter (significant effect on seston yield via RIA). There was no effect of nutrient addition on dissolved organic carbon. Whole-stream metabolism was affected by nutrient addition. Measurements of whole-stream gross primary productivity (GPP) and respiration (R) were highly variable seasonally. Contrary to our prediction that there would be a greater response in R than in GPP, effects on GPP were greater than on R. This result, however, was based on a very strong positive response of GPP to nutrient addition in April when GPP rates were highest, particularly in yr 2, and less response of GPP at other times of the year. The positive effect of nutrient addition on R was more consistent throughout the year. Food quality of detritus, as measured by C:N and C:P increased with nutrient enrichment. We have found that P content of leaf litter is much higher in the enriched stream than in the reference stream (i.e., lower C:P and higher %P). We have not seen any remarkable changes in %N or C:N of leaf litter in the enriched stream, suggesting that microbes demand P more than N. Respiration on detrital resources increased in response to nutrient addition. Yes, we observed increased respiration on both leaves (50-70% stimulation) and wood (300-350% stimulation) in response to nutrient addition