WA10 TMDL and Monitoring Programs for the San Joaquin River Basin (USA)
D135-136
8:00 AM - 12:00 PM, Wednesday
() Nutrient monitoring in support of the dissolved oxygen TMDL in the San Joaquin River, California.
Dahlgren, R.1, Van Nieuwenhuyse, E.2, 1 University of California, Davis, CA, USA2 U.S. Bureau of Reclamation, Sacramento, CA, USA
ABSTRACT- Episodes of hypoxia occur in the lower San Joaquin River, California and are typically most acute in the late summer and fall. The oxygen deficit can stress and kill aquatic organisms and often inhibits the upstream migration of fall-run Chinook salmon. The primary objective of this paper is to discuss the role of nutrient monitoring in the context of the dissolved oxygen TMDL. Our monitoring program consists of biweekly sampling (since October 1999) from 13 sites in the San Joaquin River watershed to provide a record of temporal/spatial dynamics in water quality. In addition, a source-search protocol examined 30 sites during the summer irrigation season to more precisely determine constituent loads from various subwatersheds. Readily available forms of nitrogen (NH4 + NO3 = 1.8 mg N/L; range=0.7 to 2.8 mg N/L) and phosphorus (soluble-reactive PO4 = 0.12 mg P/L; range = 0.06 to 0.24 mg P/L) in the lower San Joaquin River are much higher than those levels suggested to limit algae production (N limitation <0.1 mg N/L; P limitation <0.01 mg P/L). Therefore, mineral N and soluble-reactive P concentrations would have to be decreased by more than an order of magnitude to effectively decrease algae production in this eutrophic aquatic ecosystem. Current nitrate concentrations in the lower San Joaquin River are about 7 times greater than those reported during the first water quality survey in 1908. While nitrogen and phosphorus availability are important determinants of algae standing crop in many aquatic ecosystems, the high nutrient and turbidity levels in the San Joaquin River lead to light-limited growth. During a representative sampling in the summer irrigation season, the three east-side tributary streams (Merced, Tuolumne, Stanislaus) delivered 55, 51 and 34% of the flow, total P and total N loads, respectively. The remaining loads originate primarily from agricultural return flows.
Key words: nutrients, eutrophication, water quality monitoring, total maximum daily load