Ecosystem processes and community structure in running waters of the boreal forests of Quebec, Canada, are strongly influenced by climate and channel geomorphology. Here we present an overview of a project examining longitudinal trends as small streams gradually coalesce into large rivers, summarizing our results in a series of budgets and predictive equations describing changes in organic carbon dynamics and community structure. There were significant trends with stream order for 70% of the 73 components, processes, and ratios examined. Of 46 independent components examined, 63% showed a significant trend with stream order. As stream size increased from 1st to 9th order there was a decrease in total carbon inputs (i.e., precipitation, throughfall, primary production, and allochthonous materials) followed by a gradual increase due to greater primary production in streams >6th order. The standing stock of carbon decreased exponentially downstream, and total carbon outputs (i.e., respiration, leaching, methane evasion, and insect emergence) increased slightly downstream. Nevertheless, some ecosystem-level processes, as well as community structure, showed equivocal trends, which were apparently due to the hierarchical scale of examination and the relative degree of physicochemical vs. biological control of the processes and communities. The data, when placed in a watershed perspective, showed that total carbon inputs were evenly distributed by steam order throughout the 19 871-km 2 Moisie River drainage network. Most carbon was stored in the small 1st to 3rd order streams, whereas the majority of organic carbon was metabolized in the 7th to 9th order rivers. Fluvial transport of organic carbon to the Gulf of St. Lawrence was nearly three times that of the measured total annual input, suggesting that inputs of dissolved organic carbon in groundwater were more important than previously expected.Ecosystem-level measurements of carbon retention and utilization also showed significant trends with stream order. The spiraling length for carbon increased exponentially from 8-15 km in small streams to 426 km in the 9th order river. There was a concomitant decrease in reach retention with stream order, while the rate coefficient of respiration and rate of downstream movement increased with order. The stream metabolism index, a measure of ecosystem efficiency, increased from 1st to 7th order, thereafter decreasing as streams became larger. These trends with stream order were related to physical gradients in channel dimensions, hydrology, riparian influences, and sunlight. We conclude that these subarctic lotic ecosystems have numerous strong relationships with stream order and that the dynamics can be described by a relatively small set of predictive equations.

Continuous enrichment of an arctic river with only 10 parts per billion phosphate-phosphorus caused an immediate growth of attached algae for more than 10 kilometers downstream, showing that phosphorus alone limited photosynthesis. As a result of the increased photosynthesis, there was an increase in bacterial activity in films on rocks on the bottom of the stream. The major source of energy became the photosynthetic carbon fixed in the stream rather than the organic material entering from the surrounding tundra, and the overall metabolism of the stream shifted from heterotrophy to autotrophy. An increase in the size and developmental stage of some of the dominant aquatic insects illustrates the food limitation in this nutrient-poor habitat.

The access of almost all 270 million U.S. residents to reliable, safe drinking water distinguishes the United States in the twentieth century from that of the nineteenth century. The United States is a relatively water-abundant country with moderate population growth; nonetheless, current trends are sufficient to strain water resources over time, especially on a regional basis. We have examined the areas of public water infrastructure, global climate effects, waterborne disease (including emerging and resurging pathogens), land use, groundwater, surface water, and the U.S. regulatory history and its horizon. These issues are integrally interrelated and cross all levels of public and private jurisdictions. We conclude that U.S. public drinking water supplies will face challenges in these areas in the next century and that solutions to at least some of them will require institutional changes.

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