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MW'MC FACILmES PLAN Nutrients The productivity of fish and their food base hinges on the amount of bioavailable nitrogen and phosphorus in the water. In natural waters of the Pacific Northwest, phosphorus is usually the nutrient that limits primary productivity, which includes algae and zooplankton. This means that, unless extra phosphorus becomes available, there will still be spare bioavailable nitrogen in the water column. Phosphorous The bioavailable form of phosphorus is referred to as soluble reactive phosphorus. There is another portion that is attached to sediment particles that is not immediately available for uptake by aquatic organisms, but has the potential to be released into the water column if dissolved oxygen levels become low. Shallow reservoirs, such as Fern Ridge Reservoir, can have low dissolved oxygen levels and release phosphorus from sediments, especially at night and during early fall when plant material begins dying off. Median values for total phosphorus concentrations are relatively low in the Willamette River upstream of the urban growth boundary at 0.03mg/L, and rise slightly to 0.06 mg/L downstream of the urban growth boundary near the Beltline Road bridge. Nitrogen Nitrogen has three bioavailable forms that include nitrate (NO3-), nitrite (NO2-), and ammonia. The term ammonia refers to two chemical species that are in equilibrium in water: un-ionized (NH3), and ionized (NH4+). Tests for ammonia usually measure total ammonia; that is, NH3 plus NH4+. The toxicity of ammonia is primarily attributable to the un-ionized NH3, as opposed to the ionized form NH4+. In general, the toxicity of NH3 to fish is a function of pH and water temperature. In the presence of NH3, an increase in either pH or temperature can be harmful to aquatic organisms. Nitrate (NO3-) and ammonium (NH4+) are rarely found in Pacific Northwest streams and rivers except immediately downstream of point sources of pollution because chemical and biochemical processes in a river quickly transform them into nitrate. Consequently, most bioavailable nitrogen in the Pacific Northwest is in the form of nitrate for streams, rivers, and groundwater. Nitrate and nitrite data evaluated below are reported as nitrate plus nitrite as nitrogen. This is abbreviated to the form NO3+NO2 (as N). Median values for combined nitrate+nitrite as nitrogen (NO3+NO2 as N) are 0.03 mg/L in the Willamette River upstream of the urban growth boundary, and rise slightly to 0.10 mg/L downstream of the Beltline Road bridge. Median values for combined nitrate+nitrite as nitrogen (NO3+NO2 as N) are 0.03 mg/L in the Willamette River upstream of the urban growth boundary, and rise slightly to 0.10 mg/L downstream of the Beltline Road bridge. A review of the long-term trend of NO3+NO2 data downstream of the urban growth boundary shows that concentrations are increasing with time. This is likely the result of continued development of land immediately upstream of the monitoring location. Stormwater The median value for total phosphorus at all composite stormwater-monitoring sites in Eugene is 0.25 mg/L, with values ranging from 0.09 to 11 mg/L. Concentrations of NO3+NO2 (as N) in Eugene stormwater samples ranged from not detected to 3.7 mg/L; the median was 0.06 mg/L. Sources for NO3+NO2 (as N) are similar to those for phosphorus. 2-22 MWMC_2.0_REV23.DOC