| Summary: | The aim of this study is to evaluate the relative importance of N and P as limiting factor to aquatic fungi growing on leaves in boreal streams in northern Sweden. The study focused on aquatic hyphomycetes (fungi) growing on Betula pendula litter in enclosed containers within a climate controlled room. Water for experiments was collected from five different streams spanning a range of nutrient and total organic carbon (TOC) concentrations and each site had replicate containers with either added nitrogen, phosphorous, nitrogen + phosphorous or ambient. At the end of a three-week incubation period, litter was removed, and microbial activity measured in terms of respiration on leaf discs. In all containers there was a decrease in total N and almost all of the experimentally added inorganic N was consumed by the end of the experiment. In N+P containers most of the added P was used, but only a smaller fraction was used in only P treatments. Concentrations of TOC increased during the experimental period in all containers. There were no significant differences in leaf disc respiration between sites (F=1.483, p=0.571 or sites*treatment (F=2.588, p=0.245). There were, however, significant differences between nutrient treatments (F=4.711, p=0.004).When performing multiple comparisons between different nutrient treatments there was a significant difference in respiration between NP and ambient (p= 0.003) and NP and P (p=0.002) treatments. There were close to significant differences in respiration rate between N and P (p=0.053) and N and ambient (p=0.08) treatments. Overall, results from this experimental study suggest that N is the key limiting nutrient for aquatic hyphomycetes in the system. However, it is not only the availability of nutrients that affect the respiration and decomposition by aquatic microbes such as aquatic hyphomycetes, there are also other factors such as temperature, pH, macroinvertebrates and other environmental factors.
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