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Historical Changes in Connecticut Lakes Over a 55-Year Period

Botany Department, Connecticut College, New London, CT 06320.

^* Corresponding author (pasiv{at}conncoll.edu).

ABSTRACT

Changes in the chemical and physical conditions of 42 Connecticut lakes are compared between three time periods, the late 1930s, the mid- to late 1970s and the early 1990s. On average, lakes have decreased in Secchi disk depth by 1.2 m and doubled in total phosphorus concentration, many in a unidirectional manner. As a result, the suite of lakes can be characterized as having shifted from an oligo-mesotrophic condition (1930s) to a late mesotrophic condition (1990s). Since the 1970s, lakes have increased in base cation concentrations an average of 70 µeq/L, many as the result of an increase in sodium. Increases in sodium were generally coupled with increases in chloride ions. Many of the lakes positioned in watersheds that have become more residential since the 1930s and/or 1970s have also increased in alkalinity. Despite the overall increase in base cations, chloride, and alkalinity, about 25% of the waterbodies that have remained situated in primarily forested watersheds in crystalline rock regions have decreased in total cation concentrations; about half of these lakes have also significantly decreased in alkalinity since the 1930s. The changes are discussed in relation to the degree of urbanization of the watersheds over the same time period.

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