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Post-Treatment Monitoring

EPA TMDL information

2009 Report (pdf)

2008 Report (pdf)

2006-2007 Report (pdf)

Water Quality

During 1992-2006, the lake was monitored to assess the increasing dominance of the tui chub. Large cladocerans(water fleas) disappeared, leaving a zooplankton population comprised largely of rotifers (the smallest types of zooplankton). Biomass of benthic invertebrates declined and species composition became limited largely to midge larva (chironomids). In 2001, Diamond Lake experienced a major bloom of Anabaena, which continued annually. In conjunction with the annual blooms, Chlorophyll a values exceeded 60 ug/L, Secchi disk values declined to less than 3 ft, surface (epilimnetic) pH values exceeded 9.5, and the entire bottom (hypolymnion) became anoxic. The lake was listed on the 303(d) list for exceeding water quality standards for pH and nuisance algae. A diagnostic and feasibility study indicated that water quality standards were not being met because of internal loading from about 200 metric tons of tui chub. In addition, the chub also virtually eliminated large cladocerans, which greatly reduced grazing pressure on phytoplankton (algae). Our hypothesis was that removing the tui chub would reduce internal loading of nitrogen and phosphorous and increased grazing pressure associated with the return of large cladocerans would reduce algal biomass and allow the lake to meet water quality standards.

The lake was treated with rotenone in September 2006, resulting in complete eradication of all fish. The treatment caused the release of substantial nitrogen and phosphorus from the decaying fish, which contributed to a sustained dominance of Anabaena under the ice. Following ice-out, an intense bloom of the diatom, Synedra radians, resulted in transparencies of 0.8 m/less than three feet. Diatom abundance decreased through June as the large cladocerans, absent during the period of tui chub dominance, returned. Transparency peaked at 12.5 m/37.5 feet in July 2007 compared to 2.5 m/7.5 feet in July 2006.

The results from the first year following treatment showed that Diamond Lake recovered rapidly in virtually all respects. Although it is important to continue to monitor the changes in the lake over a number of years, the data from the first year are sufficient to reject the hypothesis that Diamond Lake was culturally eutrophic. The nature of the recovery supports the premise that internal loading from the invasive fish supplied nutrients to support excessive algal and cyanobacterial blooms. However, the high level of algal grazing following the remarkable recovery of the large cladocerans also appears to have been necessary to control phytoplankton abundance in 2007.

Water quality standards in Diamond Lake are now being met (removal of nuisance algae, decreased chlorophyll a, and dissolved oxygen near saturation in the surface waters) and a productive trout fishery is being restored to meet fish management goals. Two periods of pH excursion above the standard (8.5) were observed in 2007, however, average summer pH has declined over 1 pH unit from previous years. The results show that removal of invasive fish species can lead to recovery of a sport fishery and may be necessary to improve in water quality in some lakes.


  1. Field Methods:

    Diamond Lake water quality was sampled typically on a bi-weekly basis during 2006 and 2007, except for the winter period when sampling was reduced to periods of safe ice-cover. Sampling was conducted primarily at a fixed station at the centroid of the lake. Water samples were collected in the epilimnion, metalimnion, and hypolimnion when stratified and at a depth of 1 m& when the lake was well mixed. Profiles were collected of temperature, dissolved oxygen, pH, conductivity, and light extinction were collected at every meter. Phytoplankton samples were collected for analysis of community composition, biovolume, and chlorophyll a.Vertical tows were collected using a conical net with a 20 cm reduction collar and 64 µ mesh. The fish community composition and abundance was measured using trap nets and hoop nets, gills nets, and hydroacoustics techniques. ODFW contracted MaxDepth Aquatics to oversee and perform water quality monitoring.

  2. Analytical Methods: Water samples were analyzed for TP, TN, PO4, NH3, NO3, TOC, pH, alkalinity, and silica by the Cooperative Central Analytical Laboratory. Phytoplankton taxonomy was conducted by Aquatic Analysts; zooplankton taxonomy was conducted by ZP TaxonomicServices. Total bacteria counts were performed by Phycotech, Inc using the Dapl method.


  1. Nutrient Concentrations

    Nutrient concentrations generally decline or remain the same. This improves conditions for more favorable algae.

    Total nitrogen in the epilimnion (shown) decreased substantially in summer 2007, but ammonia showed a considerable increase. Concentrations of total phosphorus were similar between years. Ortho-phosphorus also showed little change.

    Total Nitrogen total phosphorus

  1. Anoxia, Supersaturation, and pH

    Concentrations of dissolved oxygen improve for fish and pH declines to meet water quality standards on most days.

    The high rate of primary production contributed to DO supersaturation in the epilimnion and elevated pH values. The removal of the tui chub resulted in reduced primary production and lower rates of decomposition in the hypolimnion, thus eliminating additional sources of internal cycling

    Total Nitrogen total phosphorus

  1. Lake Clarity June 20, 2007Transparency

    Lake transparency increases to record levels!  During June, people could see to the bottom of the lake in all but the deepest spots.

    Transparency measured with both Secchi disk and light extinction showed major improvements following the spring diatom bloom in 2007.

    Transparency reached a record of 12.5 m (37.5 ft) for the lake in June 2007 and stabilized at about 5-6 m (15-18ft) for the remainder of the year.

    Sectional Disk Transparency light extinction

  2. Anabaena & Chlorophyll a

    Blue-green algae decline to very low levels. Concern for lake closures eliminated.

    From 2001, when tui chub reached its carrying capacity in the lake, Anabaena was the dominant taxon/phytoplankton during the summer. Anabaena biovolume increased greatly following the rotenone treatment and remained dominant under the ice in 2006-07. In spring 2007 following ice-out and turnover, the free ammonia caused a bloom of Synedra radians (diatoms) and the decline of cyanophytes (Anabaena/bluegreen algae). During the remainder of 2007, the phytoplankton community composition has been dominated by cryptophytes and diatoms (two types of algae other than bluegreen).

Lake CChlorphylla
Biovolume 2006

Biovolume 2006

Biovolume 2007

Biovolume 2007



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