Lake Pocotopaug Lake and Watershed Restoration Evaluation ...

6.0 WATER QUALITY6.1 In-Lake Water QualityTemperature and DO profiles at the two in-lake stations during April-November 2001 arepresented in Figure 7. Other water quality data are summarized in Table 10. Values recordedbelow the detection limit are reported as ½ the detection limit. 2001 in-lake values for eachsampling and a summary of all data (previous reports and 2001 data) are provided in AppendixB.Thermal stratification occurs when sunlight warms the upper waters but wind mixing isinsufficient to mix this warmer water all the way to the bottom of the waterbody. This is a naturalprocess, but has distinct implications for lake ecology, as the lower water layer can be a refugeor a detriment depending on how much oxygen is present. Lake Pocotopaug was beginning tothermally stratify in April 2001 and was almost destratified by September. The thermocline waspresent at five meters at LP-1 and at six meters at LP-2 in May. The thermocline dropped to 5.5and 6.5 meters at LP-1 and LP-2, respectively, come July and dropped another 0.5 meters inAugust. These results indicate that Lake Pocotopaug is a typical dimictic water body (twocomplete mixing events in spring and fall separated by summer thermal stratification).Oxygen stratification roughly followed thermal stratification (Figure 7). DO readings below thethermocline were often less than 1.0 mg/L. DO below 1.0 mg/L was recorded above thethermocline at both stations (6 and 7 meters, respectively) in August. Anoxic conditions werealso recorded above the thermocline in June 2000 (VLSG 2001). DO readings of less than 6mg/l, undesirable for many aquatic life forms, were never recorded at depths above 5 m.Turbidity is a measure of water clarity. Turbid waters are indicative of high levels of suspendedparticles that may include algal cells, silt, or resuspended sediments and are usually associatedwith poor water quality. Acceptable standards depend on water body use, but turbidity readingshigher than 10 nephalometric turbidity units (NTU) are indicative of potentially undesirable waterquality. Most “clean” New England lakes exhibit turbidity ranging from 1 to ~5 NTU. Maximumturbidity in Lake Pocotopaug surface water exceeded the 5 NTU threshold at LP-2 (5.2 NTUrecorded in August), but averages were below 5 NTU (2.8 and 2.7 NTU, Table 10).Turbidity was higher near the sediment-water interface. Settling particles accumulating in thedeeper areas of the reservoir were likely responsible, though accidental stirring of finesediments by the sampling procedure could have contributed to the higher readings.The pH varied little in time and space during the 2001 sampling (Table 10). pH ranged from 6.1to 8.5 SU, with the lower values measured near the bottom and the high values near thesurface. Mean pH ranged from 6.6 to 7.4 SU.Lake Pocotopaug Restoration Evaluation 29May 2002