Scientific Studies Supporting Development of A Dissolved Oxygen TMDL

The State of California is instituting a total maximum daily load (TMDL) requirement for oxygen demand on the San Joaquin River (SJR) in Central California that includes controlling the amount of plankton biomass entering the tidal reach of the SJR from the upstream region of the SJR. Current understanding of non-point source nutrients inputs and algal biokinetics in the SJR is limited and scientific studies are being conducted to support TMDL implementation. The Upstream Dissolved Oxygen TMDL Project combines traditional monitoring with modeling and directed scientific studies to address outstanding questions concerning how algae grow and accumulate in the SJR. Flow, phytoplankton biomass, nutrient concentrations, and other water quality parameters are being measured throughout the upstream SJR study area and a mass balance on phytoplankton and nutrients has been developed. It can be demonstrated that phytoplankton biomass is accumulating in the main-stem of the SJR and that the majority of biomass production occurs in-situ, rather than as the result of biomass inputs from sources such as farm-ponds and agricultural drains. Phytoplankton growth in the SJR is rapid, with maximum net observed growth rates of approximately 1.2 per day. Phytoplankton growth has a seasonal signal, with growth rates peaking in July. Factors controlling growth rate and yield are still under investigation, but it is clear that biokinetics are not limited by soluble macronutrients (nitrogen and phosphorous) which occur well in excess of kinetic half-saturation concentrations. Although TMDL requirements are based on controlling loads, phytoplankton biokinetics are a function of nutrient concentrations, therefore evaluating water quality at the watershed level is an important step in developing a phytoplankton biomass control plan. Nonparametric methods are being used to rank drainages throughout the study reach and to develop water quality indexes with the objective of providing stakeholders a practical, scientific tool for setting remediation priorities on a watershed scale. Similar indexes for load analysis are also being investigated.