Flexi Says: Signs of stress or damage in freshwater ecosystems include changes in both the quantity and quality of water, and reductions in the biodiversity of the ecological community. Changes in quantity include periods of low or no water flow, and smaller peak flows. Changes in water quality include excess nutrients, increased pollution, higher temperatures, and lower dissolved oxygen. Reductions in biodiversity include decreases in population numbers or loss of species. All of these measures can be significant if they go beyond the historical natural variability of the system.