HISTORICAL CHANGES IN CONNECTICUT LAKES OVER A 55-YEAR PERIOD

Changes in the chemical and physical conditions of 42 Connecticut lake s are compared between three time periods, the late 1930s, the mid- to late 1970s and the early 1990s. On average, lakes have decreased in S ecchi disk depth by 1.2 m and doubled in total phosphorus concentratio n, many in a unidirectional manner. As a result, the suite of lakes ca n be characterized as having shifted from an oligo-mesotrophic conditi on (1930s) to a late mesotrophIc condition (1990s). Since the 1970s, l akes have increased in base cation concentrations an average of 70 mu eq/L, many as the result of an increase in sodium. Increases in sodium were generally coupled with increases in chloride ions. Many of the l akes 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 w atersheds in crystalline rock regions have decreased in total cation c oncentrations; about half of these lakes have also significantly decre ased in alkalinity since the 1930s. The changes are discussed in relat ion to the degree of urbanization of the watersheds over the same time period.