Dark matters: Contrasting responses of stream biofilm to browning and loss of riparian shading.

Concentrations of terrestrial-derived dissolved organic carbon (DOC) in freshwater ecosystems have increased consistently, causing freshwater browning. The mechanisms behind browning are complex, but in forestry-intensive regions browning is accelerated by land drainage. Forestry actions in streamside riparian forests alter canopy shading, which together with browning is expected to exert a complex and largely unpredictable control over key ecosystem functions. We conducted a stream mesocosm experiment with three levels of browning (ambient vs. moderate vs. high, with 2.7 and 5.5-fold increase, respectively, in absorbance) crossed with two levels of riparian shading (70% light reduction vs. open canopy) to explore the individual and combined effects of browning and loss of shading on the quantity (algal biomass) and nutritional quality (polyunsaturated fatty acid and sterol content) of the periphytic biofilm. We also conducted a field survey of differently colored (4.7 to 26.2 mg DOC L-1 ) streams to provide a 'reality check' for our experimental findings. Browning reduced greatly the algal biomass, suppressed the availability of essential polyunsaturated fatty acids, especially eicosapentaenoic acid (EPA), and sterols, but increased the availability of terrestrial-derived long-chain saturated fatty acids (LSAFA). In contrast, loss of shading increased primary productivity, which resulted in elevated sterol and EPA contents of the biofilm. The field survey largely repeated the same pattern: biofilm nutritional quality decreased significantly with increasing DOC, as indicated particularly by a decrease of the ω-3:ω-6 ratio and increase in LSAFA content. Algal biomass, in contrast, was mainly controlled by dissolved inorganic nitrogen (DIN) concentration, while DOC concentration was of minor importance. The ongoing browning process is inducing a dramatic reduction in the nutritional quality of the stream biofilm. Such degradation of the major high-quality food source available for stream consumers may reduce the trophic transfer efficiency in stream ecosystems, potentially extending across the stream-forest ecotone.

Riparian forests mitigate harmful ecological effects of agricultural diffuse pollution in medium-sized streams.

Agricultural pollution persists as a significant environmental problem for stream ecosystems. Uncultivated buffer zones or reforestation of riparian zones are advocated as a key management option that could compensate the harmful land use impacts. The effectiveness of riparian forests to protect ecological conditions of agricultural streams is yet inconclusive, particularly regarding the benefit of riparian buffers in streams suffering from uninterrupted agricultural diffuse pollution. We studied the effects of riparian land use on periphyton production and diatom, macrophyte and benthic macroinvertebrate communities in medium-sized agricultural streams by a) comparing 18 open field and forested agricultural stream reach pairs that only differed by the extent of riparian forest cover, and b) comparing the agricultural reaches to 15 near-natural streams. We found that periphyton abundance was higher in open reaches than in the forested reaches, but diatom community structure did not respond to the riparian forest cover. Macrophyte and macroinvertebrate communities were clearly affected by the riparian forest cover. Graminoids dominated in open reaches, whereas bryophytes were more abundant in forested reaches. Shredding invertebrates were more abundant in forested reaches compared to open reaches, but grazers did not differ between the reach types. Macrophyte trait composition and macroinvertebrate community difference between the reaches were positively related to the difference in riparian forest cover. The community structure of all three groups in the agricultural streams differed distinctly from the near-natural streams. However, only macrophyte communities in forested agricultural reaches showed resemblance to near-natural composition. Our results suggest that riparian forests provide ecological benefits that can partly compensate the impacts of agricultural diffuse pollution. However, community structure of forested agricultural reaches did not match the near-natural composition in any organism group indicating that catchment-scale management and mitigation of diffuse pollution need to be still advocated to achieve ecological goals in stream management and restoration.