ISU Study Explains Current Water Quality

LIMNOLOGY OF CLEAR LAKE

· Clear Lake is a formerly oligotrophic (nutrient poor) lake that has increased in total phosphorus concentration from around 60 ppb in the early 1970s to around 190 ppb in 2000. Total phosphorus appears to be increasing in Clear Lake at an average rate of about 4 ppb/year. At this rate, Clear Lake would move from a eutrophic (nutrient rich) lake to a hyper-eutrophic lake (highly nutrient rich), attaining 340 ppb by 2040. Total phosphorus has already tripled in the last 30 years and appears to be climbing under current watershed management scenarios.

· Concurrently, water clarity has been cut to nearly a third of what it was in the early 1970s, and probably around 10% of the clarity the lake had near the turn of the century. Water clarity in 1974 was about 0.9 meters (nearly 3 ft.), but is now around 0.35 meters (about 1 ft.).

· Agricultural, urban and construction activities have caused around 85,000 tons of sediment to be added to the lake each year. This has caused the lake to lose depth at a rate of about 4 mm/year. Assuming a constant rate of sediment addition to the lake, Clear Lake would be completely filled-in in 700-800 years.

· In Clear Lake, lake sediment resuspension by wind-induced waves and recreational boat traffic may contribute to daily, often substantial, nutrint flux with total phosphorus concentrations doubling and ammonia concentrations reaching levels toxic to fish.

· Bacteria were studied intensively in Clear Lake and were consistently found at lower levels than the levels EPA has established as safe for recreation. Concentrations of fecal coliforms, E. coli, and fecal enterococci were highest near shore and showed patterns that should allow remedial measures to trace bacteria sources and eliminate or reduce these inputs.

     The fisheries of Clear Lake are an important component of recreation as well as an important indicator of water quality. Although bullhead and carp have been common in the lake for 50 years, they are now the dominant fish, existing at densities of 150-300 lb/acre and 100-200 lb/acre, respectively. They have probably filled the void left by the bass, crappie and bluegill because of their great tolerance of degraded water quality conditions (e.g., sediment, Cyanobacteria, low oxygen, ammonia). Since resistant fish like carp degrade water quality, successful improvements will need to include management of fish populations to reduce carp dominance. This can be accomplished by (1) improving water quality to enhance vegetation and decrease substances degrading fish habitat, and (2) managing bottom-feeding fish (primarily carp and bullhead) to reduce their abundance. As part of the ISU study, IDNR removed fish from Ventura Marsh to determine what affect it would have on water quality. The results were remarkable. Water clarity increased from less than a foot to over 4 feet (to the bottom of the marsh), phosphorus levels dropped, and nutrient filtering aquatic vegetation returned. The experiment showed that controlling carp populations can be one way of improving water quality.