Phosphorus Loading Targets for Central Basin Hypoxia

The following provides a quick snapshot of key findings.  A more in-depth discussion and references to the scientific literature can be found here.

We developed and tested a model for establishing the relationship between phosphorus loads and Central Basin hypoxia. The model was calibrated over 19 years (1987-2005) using chlorophyll a, zooplankton abundance, phosphorus, and DO concentrations, and was compared to key process rates, such as organic matter production and sedimentation, DO depletion rates, and estimates of hypoxic area.

calibration

The model was then used to develop response curves for hypolimnetic DO concentration, hypoxic-days (number of days per year with hypolimnetic DO below 2 mg/l), hypolimnetic DO depletion rates, and hypoxic area as a function of loading of total phosphorus (TP) and dissolved reactive phosphorus (DRP) into the Western and Central Basins. The resulting response curves incorporate uncertainty associated with interannual variability in weather and resulting lake stratification from the 19 calibration years.

If the desired outcome is to return to hypoxic areas of the mid-1990s (ca. 2,000 km2) which coincided with the recovery of several recreational and commercial fishes, the lake’s TP load would have to be approximately 4,804 MT/year.  This is a 46% reduction from the 2003-2011 average loads and 56% below the current target.

TP curve

If this same hypoxic goal was used to set new targets for DRP loading, the lake-wide load would have to approach 598 MT/year, a value roughly equivalent to loads in the early 1990s.  Because DRP load has increased so dramatically since that time, this represents a 78% reduction from the 2005-2011 average DRP load

DRP Curve

Importantly, these response curves indicate that a focus on DRP requires about half of the reduction of the TP target, which is consistent with the higher bioavailability of DRP.