Williamson Tanner J, Cross Wyatt F, Benstead Jonathan P, Gíslason Gísli M, Hood James M, Huryn Alexander D, Johnson Philip W, Welter Jill R
Department of Ecology, Montana State University, Bozeman, MT, 59717, USA.
Department of Biological Sciences, University of Alabama, Tuscaloosa, AL, 35487, USA.
Glob Chang Biol. 2016 Jun;22(6):2152-64. doi: 10.1111/gcb.13205. Epub 2016 Mar 8.
Although much effort has been devoted to quantifying how warming alters carbon cycling across diverse ecosystems, less is known about how these changes are linked to the cycling of bioavailable nitrogen and phosphorus. In freshwater ecosystems, benthic biofilms (i.e. thin films of algae, bacteria, fungi, and detrital matter) act as biogeochemical hotspots by controlling important fluxes of energy and material. Understanding how biofilms respond to warming is thus critical for predicting responses of coupled elemental cycles in freshwater systems. We developed biofilm communities in experimental streamside channels along a gradient of mean water temperatures (7.5-23.6 °C), while closely maintaining natural diel and seasonal temperature variation with a common water and propagule source. Both structural (i.e. biomass, stoichiometry, assemblage structure) and functional (i.e. metabolism, N2 -fixation, nutrient uptake) attributes of biofilms were measured on multiple dates to link changes in carbon flow explicitly to the dynamics of nitrogen and phosphorus. Temperature had strong positive effects on biofilm biomass (2.8- to 24-fold variation) and net ecosystem productivity (44- to 317-fold variation), despite extremely low concentrations of limiting dissolved nitrogen. Temperature had surprisingly minimal effects on biofilm stoichiometry: carbon:nitrogen (C:N) ratios were temperature-invariant, while carbon:phosphorus (C:P) ratios declined slightly with increasing temperature. Biofilm communities were dominated by cyanobacteria at all temperatures (>91% of total biovolume) and N2 -fixation rates increased up to 120-fold between the coldest and warmest treatments. Although ammonium-N uptake increased with temperature (2.8- to 6.8-fold variation), the much higher N2 -fixation rates supplied the majority of N to the ecosystem at higher temperatures. Our results demonstrate that temperature can alter how carbon is cycled and coupled to nitrogen and phosphorus. The uncoupling of C fixation from dissolved inorganic nitrogen supply produced large unexpected changes in biofilm development, elemental cycling, and likely downstream exports of nutrients and organic matter.
尽管人们付出了很多努力来量化气候变暖如何改变不同生态系统中的碳循环，但对于这些变化如何与生物可利用氮和磷的循环相关联，我们却知之甚少。在淡水生态系统中，底栖生物膜（即藻类、细菌、真菌和碎屑物质的薄膜）通过控制重要的能量和物质通量，成为生物地球化学热点区域。因此，了解生物膜如何响应气候变暖对于预测淡水系统中耦合元素循环的响应至关重要。我们沿着平均水温梯度（7.5 - 23.6°C）在实验性溪边渠道中培育生物膜群落，同时通过共同的水源和繁殖体来源密切维持自然的昼夜和季节温度变化。在多个日期测量生物膜的结构属性（即生物量、化学计量、群落结构）和功能属性（即代谢、固氮、养分吸收），以便将碳流的变化与氮和磷的动态明确联系起来。尽管溶解性限制氮浓度极低，但温度对生物膜生物量（2.8至24倍变化）和净生态系统生产力（44至317倍变化）有强烈的正向影响。温度对生物膜化学计量的影响出人意料地小：碳氮比（C:N）不随温度变化，而碳磷比（C:P）随温度升高略有下降。在所有温度下，生物膜群落均以蓝藻为主（占总生物体积的>91%），固氮率在最冷和最暖处理之间增加了高达120倍。尽管铵态氮吸收随温度增加（2.8至6.8倍变化），但在较高温度下，高得多的固氮率为生态系统提供了大部分氮。我们的结果表明，温度可以改变碳的循环方式以及碳与氮和磷的耦合方式。碳固定与溶解性无机氮供应的解耦在生物膜发育、元素循环以及可能的下游养分和有机物质输出方面产生了巨大的意外变化。
