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^ The depletion of oxygen below the thermocline (where the lake stratifies) <br />By measuring fl~ree water quality indicators and comparing them to user perceptions of lake condition, lakes fall into <br />categories representing norms for metro lakes. These categories, or grades, look like this: <br />ATP C;LA S>~ Percentile' grade ' _ Perceived COt~Ciitio-- <br /><23 <10 >10 <10 A C stal clear, beautiful <br />23-32 10-20 7-10 10-30 B Little algae, minor problems <br />32-68 ZO-48 4-7 30-70 C Definite algae, impaired use <br />68-152 48-77 2-4 70-90 D High algae, un-enjoyable <br />>152 >77 <2 >90 F Severe algae, enjoyment impossible <br />TP =total phosphorus (parts per billion or ppb); CLA =chlorophyll (ppb); SD =Secchi disk transparency <br />(feet). <br />Percentile is a ranking of metro lakes, like a grading curve. <br />The grades are letter assignments and not meant to indicate `passing' or `failing.' <br />Perceived Conditions are based on user perceptions. <br />The water quality trend in Lake Owasso is that of decreasing nutrient levels, decreasing algae abundance and <br />increasing water clarity. By examining seasonal average condition -the measurement of phosphorus, chlorophyll <br />and Secchi disk averaged between May and September - it is apparent that the water quality of Lake Owasso has <br />undergone significant changes over the past tlu-ee decades. <br />