For example, the baseflow of some streams in southern Ontario has been affected by land clearing, drainage, wetland filling, and soil erosion so much that headwater streams have lost some of their natural ecological function. These changes have affected entire watersheds by increasing water temperatures, sediment and nutrient levels, and storm flows. Water quality Phosphorus and nitrogen are essential nutrients required by algae and aquatic vascular plants, the primary producers in aquatic ecosystems. In nutrient-limited systems, slight increases in the concentration of these nutrients stimulates greater productivity at all trophic levels. In most cases, aquatic systems are limited by phosphorus.
Eutrophication is more likely where nutrients come from several sources.
Some reduction in dissolved oxygen occurs naturally as water flows downstream, but human activity has a marked effect. Water quality was degraded as the water moved downstream through agricultural and urban land. At Site 2, water quality was slightly degraded by an urban area. At Site 4, at the mouth of a tributary that drains intensively farmed land, levels of dissolved oxygen were extremely low and some species of fish could not survive. Figure 7-1 Dissolved oxygen in a B.C. stream A growing concern for stream and river quality is the entry of animal manure.