Lake Water Quality and Lake Stratification Responses to Seasonal Climates and Extreme Events

For four consecutive years, 2014-2018, a mooring at Deep Haven Deep, Squam Lake, NH has captured water temperature variations at 15-minute intervals from 3 to 26 meters water depth. These data show that the entire water column has unexpectedly high sensitivity in responding to weather extremes, despite strong thermal stratification in summer and ice cover in winter. These results indicate that deep water bodies are far more reactive to atmospheric conditions than generally believed.

Squam Lake (N 43.77-43°, W 71.58-48°) has a maximum water depth of 30 m and a surface area of 2737 ha. It experiences annual summer stratification in the upper 15 meters and reverse stratification throughout the water column after ice cover develops, usually in December. This reverse stratification ends abruptly with ice-out and the beginning of wind-driven mixing. The deepest basin, Deep Haven Deep, receives inflows from three small tributaries that enter the northern end of the lake. Numerous islands, peninsulas and coves limit the fetch and potential for internal seiches. The lake is generally well-oxygenated (5-14 mg/L), oligotrophic (TP < 5 ppb), with high water clarity (secchi depth: 4.3-7.2 m), with bottom anoxia in the lowest 5 m.

Detailed analyses of the multi-year temperature series (Figure 1) indicate that some weather-related events create perturbations throughout the entire water column without disrupting summer stratification. Some whole-water-column responses are apparent even during the ice cover period. Many of these disturbances can be correlated to high stages in local rivers, suggesting that flood waters can create under-lake-ice flows with effects far from the tributary mouth. Other identifiable events correlate to intervals of extremely low winter-time atmospheric temperatures and may be associated with snow-no snow conditions on top of the lake ice. In years with late-development of ice-cover, water temperatures at all levels of the lake can drop to below 1°C, critically low for fish survival.

Comparison of the Squam Lake results with similar work on deep lakes (>20 m) in New England and Iceland reveals that sensitivity in deep lakes is common, rather than unusual, and it opens the scope for use of deep lake sediments as recorders of past weather-related events affecting a local area.