1. The factors underlying population and community dynamics are almost invariably multivariate, and ecosystems worldwide are affected by many anthropogenic stressors. Inorganic dissolved nutrients and deposited fine sediment are common stressors in agricultural streams and can be expected to influence benthic algae and cyanobacteria. 2. We determined population- and community-level responses of phototrophic periphyton along twin-stressor gradients of dissolved nutrients and deposited fine sediment after 20 days of exposure in stream mesocosms. Multiple linear regression and an information-theoretic approach were used to select the best predictive models for each response variable, accounting for potential subsidy-stress responses for each stressor (where at low stressor levels, an ecological variable responds positively up to an inflection point, beyond which the effect is negative) and for multiple-stressor interactions. This approach enabled us to test the subsidy-stress hypothesis for each stressor and to investigate whether the two stressors operated singly or in concert (additively or with an interaction) on periphyton attributes. 3. Focussing further on three ecological guilds of periphyton (low profile, high profile, motile), we also tested the specific hypotheses, that sediment augmentation would produce a proportional increase in motile and a decrease in high-profile forms, and that nutrient augmentation would produce proportional increases in both motile and high-profile forms. 4. Unimodal subsidy-stress patterns along the sediment gradient were found only for densities of two common Nitzschia species. By contrast, unimodal patterns along the nutrient gradient occurred frequently at both population and community levels, supporting the subsidy-stress hypothesis for nutrient enrichment. Some single-stressor responses were observed, but the twin stressors sediment and nutrients mainly acted in a simple, additive way, probably because their modes of action are different. As a notable exception from this rule, potentially harmful cyanobacteria (Phormidium and Oscillatoria species) responded synergistically to the stressors. Our hypothesis about the representation of ecological guilds across the sediment gradient was fully supported, whilst the hypothesis regarding nutrients received partial support. 5. Categorisation of periphyton taxa into physiognomic guilds revealed predictable response patterns to the stressors. The prevalence of motile taxa in stream communities may be suitable for detecting sedimentation impacts and early signs of nutrient enrichment. Finally, based on periphyton response shapes along the nutrient gradient, thresholds could be identified at which community variables changed abruptly.
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