Benthic microalgae (BMA) biomass and community structure in freshwater lotic systems are often limited by inorganic nutrient availability. We examined nutrient limitation of benthic algae biomass and community structure using nutrient diffusing substrates in four rivers along a land use gradient in the Lake Biwa basin, Japan. Ambient in-stream nutrient concentrations were correlated to catchment land use, with the highest nitrogen and phosphorus concentrations being found in rivers draining more forested catchments. Nutrient limitation of primary producer biomass and nutrient-driven changes in community structure were evident in all four rivers, regardless of in-stream nutrient concentrations and surrounding land use. BMA biomass (measured as chlorophyll log-response ratio) exhibited the greatest nutrient limitation in rivers with higher in-stream nutrient concentrations. The relationship to catchment land use was less clear, with the highest nutrient limitation being observed in the catchment with moderate amounts of agricultural and forested land use. Nutrient additions resulted in a shift from dominance by Bacillariophyceae (diatoms) to a mixed Bacillariophyceae-Chlorophyceae (chlorophyte) community in all rivers and this shift was most pronounced in the forested catchments. Changes within the diatom community with nutrient additions were also observed, although the shifts in diatom community within a river in response to nutrient additions were much smaller than the differences in diatom community composition among rivers. Diatom taxa classified as highly motile increased with nutrient additions in all rivers. Our results suggest that primary producer community in rivers may be sensitive to nutrient inputs even in areas with elevated nutrient concentrations and catchments dominated by agricultural land use. There is likely widespread nutrient limitation in rivers of Japan, across both in-stream nutrient and land use gradients and any increases in nutrient loading will likely stimulate benthic algal growth. Our findings highlight the importance of looking at both biomass and species composition to assess ecosystem-level impacts of elevated nutrient levels.

淡水系统中的底栖微藻 (BMA) 生物量和群落结构通常受到无机养分可用性的限制。我们在日本琵琶湖盆地沿土地利用梯度的四条河流中使用养分扩散底物检查了底栖藻类生物量和群落结构的养分限制。河流中的环境营养浓度与集水区土地利用相关，在排放更多森林集水区的河流中发现了最高的氮和磷浓度。初级生产者生物量的养分限制和群落结构中养分驱动的变化在所有四条河流中都很明显，无论河内养分浓度和周围土地利用如何。BMA 生物量（以叶绿素对数响应比测量）在河流中养分浓度较高的河流中表现出最大的养分限制。与集水区土地利用的关系不太清楚，在具有适度农业和林地利用的集水区观察到最高的养分限制。营养添加导致所有河流从芽孢杆菌科（硅藻）的主导地位转变为芽孢杆菌科-绿藻科（叶绿素）混合群落，这种转变在森林覆盖的集水区最为明显。还观察到硅藻群落随养分添加的变化，尽管河流中硅藻群落响应养分添加的变化远小于河流之间硅藻群落组成的差异。被归类为高度运动的硅藻类群随着所有河流中营养物质的添加而增加。我们的研究结果表明，河流中的初级生产者群落可能对养分投入敏感，即使在养分浓度升高的地区和以农业用地为主的集水区也是如此。日本河流中可能存在广泛的养分限制，包括河流内养分和土地利用梯度，养分负荷的任何增加都可能刺激底栖藻类的生长。我们的研究结果强调了观察生物量和物种组成以评估营养水平升高对生态系统水平的影响的重要性。我们的研究结果表明，河流中的初级生产者群落可能对养分投入敏感，即使在养分浓度升高的地区和以农业用地为主的集水区也是如此。日本河流中可能存在广泛的养分限制，包括河流内养分和土地利用梯度，养分负荷的任何增加都可能刺激底栖藻类的生长。我们的研究结果强调了观察生物量和物种组成以评估营养水平升高对生态系统水平的影响的重要性。我们的研究结果表明，河流中的初级生产者群落可能对养分投入敏感，即使在养分浓度升高的地区和以农业用地为主的集水区也是如此。日本河流中可能存在广泛的养分限制，包括河流内养分和土地利用梯度，养分负荷的任何增加都可能刺激底栖藻类的生长。我们的研究结果强调了观察生物量和物种组成以评估营养水平升高对生态系统水平的影响的重要性。