Water Quality Trends

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


Phytoplankton biomass: Phytoplankton biomass in both the Western and Central Basins decreased between the 1970s and the mid-1980s, and then increased between 1995 and 2011 due to high abundance of cyanobacteria, predominantly Microcystis spp.

Figure 3



Central Basin spring surface chlorophyll a concentration increased from ~3 μg/l in 1985-2000 to >19 μg /l in 2007, even though TP loads remained relatively constant, doubling the CHL:TP ratio during this time period.




Dissolved Oxygen (DO) –  Average hypolimnion Central Basin DO concentrations in August-September for stations with an average depth greater than 20 m increased from less than 2 mg/l in 1987 to over 6 mg/l in 1996, followed by an abrupt decrease to below 3 mg/l in 1998 with concentrations remaining low and quite variable through 2011, the most recent year for which data are available.

Figure 6

While substantial intra-annual variability exists, hypoxic area was generally smallest during the mid-1990s, with larger extents during the late 1980s and the early 2000s.