The Balmer: Paschen ratio in the chromosphere and the equilibrium populations of hydrogen angular-momentum states

Abstract

Mexico, the United States, and Canada are home to a diverse array of river systems. As is the case for rivers across the globe, complex mixtures and interactions of stressors have fundamentally altered the structure and function of these ecosystems. Here, we summarize the intensity and extent of major regional stressors to North American rivers and their impacts on important physical, biological, and chemical properties. The large lakes ecoregion is impacted by broad-scale physical alterations of watercourses, which include both point and nonpoint pollution from industry, municipalities, and agriculture. The establishment of invasive aquatic species is another key stressor. The most pressing stressor in polar freshwaters is rapid anthropogenic climate change. Urbanization is a focal stressor in temperate coastal river habitats. Increasing agricultural production, fine-sediment pollution, and nutrient enrichment are major threats to temperate floodplain rivers. Stressors common across temperate upland river systems include increasing populations and urban areas, impoundments and surface-water abstraction, and land-use change. Point-source pollution, nonnative species, and climatic vulnerability are primary stressors affecting tropical and subtropical coastal rivers. Similarly, climate change and human activities that mitigate water scarcity such as impoundments, diversions, crop irrigation, and surface and groundwater withdrawals are principal stressors in xeric and endorheic river systems. Across ecoregions, these and other stressors can interact and cause surprising ecological responses. Further, the development and introduction of new contaminant sources (e.g., genetically modified crops, antibiotics, hormones, pharmaceuticals, and personal health care products) pose new challenges for river conservation.