Collaborative research: Defining ecosystem heterotrophic response to nutrient concentrations and ratios

溪流和河流是全球碳处理的重要场所。碳以枯叶和枯木（碎屑）的形式从陆地进入溪流，并被细菌和真菌（异养微生物）定植。这种微生物和碎屑碳的复合物为鱼类、两栖动物和其他消费者提供食物网的燃料。碎屑碳被生物体消耗，但也被细菌和真菌转化为二氧化碳。这一重要的碎屑燃料途径存在的问题是，人类来源的营养输入（氮和磷）刺激微生物驱动的二氧化碳释放和碎屑分解，导致重要生态系统功能可用碳减少。在本研究中，将使用实验室孵育，河边渠道和全流营养添加来确定引起这种生态系统变化的氮和磷的浓度和比例。了解这些阈值浓度和比率可以指导水生生态系统的管理，以维持野生动物保护和人类利用的水质。研究结果可以帮助预测过量的营养物质如何影响溪流中碎屑碳的命运，这与全球碳预算有关。过量的营养负荷是美国一半以上淡水和沿海生态系统受损的主要原因。尽管已有数百项科学研究评估了自养（如藻类）对营养负荷的反应，但很少有研究测试了营养对腐质、异养微生物和它们所支持的较大水生生物的影响。拟议的研究将促进我们对水生生态系统如何通过可能导致碳损失的碎屑燃料途径受到营养物质的影响的理解，这一观点目前在与营养物质富集相关的管理决策中缺乏。该项目将为四名早期职业研究人员提供关键的研究支持，并为美国东南部三所机构的本科生和研究生提供经验和培训。该项目将包括公共和K-12教育部分，重点关注营养过剩如何影响水生食物网和水质。

Streams and rivers are important sites of global carbon processing. Carbon in the form of dead leaves and wood (detritus) from the land enters streams and is colonized by bacteria and fungi (heterotrophic microorganisms). This complex of microorganisms and detrital carbon fuels food webs that support fish, amphibians and other consumers. Detrital carbon is consumed by organisms but is also converted to carbon dioxide by bacteria and fungi. Problematic to this important detritus-fuelled pathway is that human-derived nutrient inputs (nitrogen and phosphorus) stimulate microbially-driven release of carbon dioxide and decomposition of detritus, resulting in less available carbon for important ecosystem functions. In this study, laboratory incubations, streamside channels and whole-stream nutrient additions will be used to determine the concentrations and ratios of nitrogen and phosphorus that elicit such ecosystem changes. Knowledge of these threshold concentrations and ratios can guide management of aquatic ecosystems to maintain water quality for wildlife conservation and human utilization. Study findings can aid predictions of how excess nutrients affect the fate of detrital carbon in streams, which is relevant to global carbon budgets. Excess nutrient loading is the primary source of impairment to more than half of the freshwater and coastal ecosystems in the U.S. While hundreds of scientific studies have assessed autotrophic (e.g. algal) response to nutrient loading, few have tested nutrient effects on detritus, heterotrophic microorganisms and the larger aquatic organisms they support. The proposed research will advance our understanding of how aquatic ecosystems are affected by nutrients via detritus-fuelled pathways that potentially lead to carbon loss, a perspective that is currently lacking in management decisions related to nutrient enrichment. This project will provide critical research support to four early-career investigators, as well as experience and training for undergraduate and graduate students at three institutions in the southeastern U.S. The project will include public and K-12 educational components that will focus on how excess nutrients affect aquatic food webs and water quality.