11-10-2003 Council Packet - Laserfiche WebLink

Home Browse Search

In-lake phosphorus concentrations are variable with summer surface total phosphorus (TP) ranging from 41 to 63 ug L. Total phosphorus goal from the District’s 1997 509 plan is 50 ug/L; the goal set out in the HHPLS study report i>> practically the same, 50 to 55 ug/L. Over the six years (1997 to 2002) of water quality monitoring on Stubbs Bay, the average TP concentration is 53 ug/L. With the exception of 2001, the phosphorus concentrations have been increasing steadily, at about 4.6 ug/L per year. Over the same period, Chloropyll-a concentrations ranged from 23 to 39 ug/L, and averaged 32 ug/L. Chlorophyll-a concentrations are relatively high compared to that which would be expected from the total phosphorus concentrations. The average chlorophyll-a concentration of 32 ug/'L indicates a summer frequency of nuisance algae blooms (defined as Chlorophyll-a exceeding 20 ug/L) of 75 percent, and severe nuisance (Chlorophyll-a exceeding 30 ug/L) of 45 percent. Summer average Secchi depth ranged from 0.6 to 0.9 m, and averaged 0.76 for 1997 through 2002. The concerns that have been raised regarding Stubbs Bay indicate that the public desires a more stringent goal and are probably related to the recent negative trend in water quality. 1.2 Stubbs Bay Diagnostic Results • Stubbs Bay experiences a high internal load as evidenced by high bottom phosphorus concentrations. Stubbs Bay ranks about 8'*' highest bottom concentrations when compared to the 43 MCWD lakes and bays monitored in 2002 • Estimates of summer internal phosphorus load ranged from 320 to 790 kg per year. For reference the internal load calculated for Jennings Bay was 570 kg per year. • Internal loading is further evidenced by high lake concentrations which coincide with spring and fall overturn, when phosphorus-riej bottom water mixes with the surface water. • External tributary loads vary from 34 kg in 2000 to 600 kg in 2002; on average the external tributary load is 270 kg. The average inflow TP is 250 ug/L. Most of the water and phosphorus loads are derived from the Classen Lake Creek. A second tributary located at the northwest comer of the bay discharges mostly during spring snowmelt, and represents a small part of the total load. T «ll504y2%SIMM's HrpuaiBay lUpact^K Stubbs Bay Diagnostic Report