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We examined how short-term (19 days) nutrient enrichment influences stream fungal and diatom communities, and rates of leaf decomposition and algal biomass accrual. We conducted a field experiment using slow-releasing nutrient pellets to increase nitrate (NO3-N) and phosphate (PO4-P) concentrations in a riffle section of six naturally acidic (naturally low pH due to catchment geology) and six circumneutral streams. Nutrient enrichment increased microbial decomposition rate on average by 14%, but the effect was significant only in naturally acidic streams. Nutrient enrichment also decreased richness and increased compositional variability of fungal communities in naturally acidic streams. Algal biomass increased in both stream types, but algal growth was overall very low. Diatom richness increased in response to nutrient addition by, but only in circumneutral streams. Our results suggest that primary producers and decomposers are differentially affected by nutrient enrichment and that their responses to excess nutrients are context dependent, with a potentially stronger response of detrital processes and fungal communities in naturally acidic streams than in less selective environments. [ABSTRACT FROM AUTHOR]

Most ecosystems are subjected to multiple stressors derived from natural and anthropogenic sources and community responses to human disturbance in naturally stressful habitats may differ from those in more benign habitats. We examined the influence of a natural (geology-driven acidity) vs. human-induced stress (land drainage) and their interaction on the composition and concordance of stream diatom, bryophyte and invertebrate communities. To account for differing drainage impacts in circumneutral (sedimentation) and naturally acid (reduced pH and increased metal concentrations) streams we investigated concordance in three groups of streams: reference (circumneutral and naturally acidic reference), circumneutral (reference and drained) and naturally acidic (reference and drained) streams. We expected diatoms to respond more strongly to anthropogenic acidification and more weakly to sedimentation compared to bryophytes and invertebrates. We expected overall strong concordance among the three taxonomic groups, but especially so in reference streams. All three organism groups had distinct species composition in naturally acidic vs. circumneutral streams. Concordance between communities was overall strong, especially so in the reference streams. All groups responded to drainage disturbance in both types of streams. Invertebrates were slightly less responsive to increased acidification in the naturally acidic streams but were more affected by sedimentation in the circumneutral streams than were the other two groups. The natural stressor affected communities more than the anthropogenic stressors. Naturally stressed communities were affected by an anthropogenic stressor as much as those in more benign habitats, although the additional stressor was similar to the initial stress (further reduction of stream pH). Naturally acid streams may need special concern in bioassessment because models based on circumneutral reference sites will likely produce biased predictions for these streams. [ABSTRACT FROM AUTHOR]

Studies on the interactive responses to multiple simultaneously acting stressors have focused on individual or population-level responses in laboratory microcosms, while field-based studies on community-level responses are rare. We examined the influence of a natural (non-anthropogenic acidity) vs. human-induced stress (land drainage) and their interaction on species richness and spatial turnover (ß diversity) of stream diatom, bryophyte, and benthic invertebrate communities. Our four stream categories were: circumneutral reference, circumneutral impacted, naturally acidic, and naturally acidic impacted streams. We expected the most sensitive species to be present only in the circumneutral reference streams. Therefore, species richness should be highest in these streams and lowest in the naturally acidic streams additionally stressed by forest drainage. Alternatively, communities in acidic streams may consist of the most tolerant taxa that are unaffected by further stressors, species richness in these streams remaining unaffected by drainage. We also expected spatial turnover to be highest in the circumneutral near-pristine streams and lowest in the drainage-impacted acidic streams. In all three taxonomic groups, a diversity was lower in the naturally acidic than in circumneutral streams. The additional impact of the anthropogenic stress on species richness varied between groups, having no effect on diatoms, antagonistic effect on bryophytes, and additive effect on invertebrates. We also found differences in how each stressor modified ß diversity of each taxonomic group. For diatoms, ß diversity showed an overall tendency to decrease with increasing stress level, while bryophyte ß diversity responded mainly to forest drainage. Benthic invertebrate ß diversity did not differ between treatments. Our results suggest that non-additive effects among stressors need special attention to improve the understanding and management of multifactor responses in streams. Our results also argue for the primacy of a multi-taxon approach to environmental impact detection, and for the inclusion of a wide array of ecological responses, particularly community turnover, in bioassessment programs to detect responses that may go unnoticed by conventional richness-based measures. [ABSTRACT FROM AUTHOR]

Acid mine drainage (AMD) is a significant environmental issue worldwide. On the West Coast of the South Island, New Zealand, many AMD-affected streams occur within close proximity to naturally acidic streams, enabling us to compare the response of communities in naturally and anthropogenic acidified systems. We investigated epiphytic diatom communities in 39 streams along an AMD gradient that included naturally acidic and circum-neutral reference streams. There was a wide range in taxonomic richness in reference streams and those moderately impacted by AMD (8-33 taxa). Taxonomic richness was greatly reduced in severely impacted streams (1-5 taxa) at a threshold of pH 3.4 and was dominated by Pinnularia cf. acidophila (69-100% relative abundance). Community composition differed between circum-neutral reference streams and moderately and severely impacted streams. However, naturally acidic and moderately impacted streams had similar diatom communities primarily composed of Eunotia and Frustulia species. Our results indicate that diatom communities are strongly structured by pH and able to tolerate moderate conductivity and metal concentrations. This is a challenge for researchers and water managers attempting to incorporate diatoms into AMD monitoring in regions with naturally acidic streams. [ABSTRACT FROM AUTHOR]