Pattagansett Lake is no different than many other lakes in temperate climates having more than just shallow depths:  it stratifies thermally during warmer months.  Stratification means that lake water separates into discrete layers by temperature with the warmest water at the surface and the coolest at the bottom. In late spring the temperature differences become large enough that the water layers become resistant to mixing by wind.  Limnologists (scientists who study lakes) refer to the upper, warm layer as the epilimnion, the middle layer of rapid temperature change as the thermocline or metalimnion, and the deeper, cooler area as the hypolimnion.  Oxygen dissolved in lake water is required by many forms of aquatic life, including fishes, aquatic insects and other invertebrates such as zooplankton (mostly microscopic crustaceans), and even the microscopic plant life called phytoplankton, which produce much of the dissolved oxygen.  These are aerobic species.  Some forms of life, including specific forms of bacteria, do not require oxygen and are referred to as anaerobic species.  Oxygen solubility in water varies by temperature, with more oxygen found in cooler water.  Of course, this varies by other factors, both physical and biological.  So, shouldn’t the deeper, cooler waters of the lake have more oxygen?  Not necessarily.  With stratification, no atmospheric oxygen gets introduced into deeper water.  And any oxygen produced there by phytoplankton is rapidly used up by certain chemical processes and bacteria and other decomposers feeding on organic materials until only anaerobic bacteria can survive below the thermocline.  Thus, in summer a considerable volume of Pattagansett Lake water below the thermocline has little (hypoxic) or no (anoxic) dissolved oxygen and aerobic life cannot live there. The Pattagansett Lake Association participates in Connecticut Lake Watch.  From May through September at approximately weekly intervals we take recordings of lake water transparency.  For this, we use a simple device called a Secchi disk.  This is an 8-inch disk having four alternating black and white sections. The Secchi disk is lowered using a measured cord and the depth at which it disappears from view is noted.  Water transparency gives an indication of the productivity of a lake as those receiving high amounts of nutrients (nitrogen and phosphorous in particular) produce heavy blooms of phytoplankton and are less transparent.  Heavy inputs of sediments also reduce transparency.  We also use a scientific water chemistry meter to record temperature, dissolved oxygen, pH, and conductivity (a measure of dissolved ions) at 1-meter intervals from the surface to 5 m deep, which is the length of our instrument’s cord.  Some results from our work in 2021 follow:

The greatest Secchi disk visibility in early spring is followed by summer lows as phytoplankton abundance peaks.  An increase in transparency occurs from mid-August to mid-September, followed by another decline, which could indicate a secondary, late summer bloom of phytoplankton. Overall, our Secchi disk transparency ranged from 1.5 to 3.4 m (about 5 to 11 feet).  Based on water transparency and nutrient inputs, Pattagansett Lake is classified as a mesotrophic lake, a classification midway between clear, low production oligotrophic lakes (ex: Beach Pond in Voluntown) and more turbid, high production eutrophic lakes (ex: Amos Lake in Preston).

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