Natural and Management-Induced Reduction in Monimolimnetic Volume and Stability in a Coastal, Meromictic Lake

Abstract

ABSTRACT Lower Mystic Lake is a coastal meromictic lake with a northern and southern basin. Although the southern basin has remained meromictic, the northern, shallower basin became holomictic in the early 1980s. A secondary chemocline forms above the primary chemocline, so that mixing during autumnal overturn does not always reach to the depth of the primary chemocline. The secondary chemocline may develop and intensify during periods when wind action and seiche activity are not expected, and may persist as a distinct feature for over a year. The source of the solutes in the secondary chemocline appears to be the primary chemocline and the monimolimnion. The Amelia Earhart Dam, constructed in 1966, prevents any further influx of seawater that was necessary to maintain the meromictic stratification. As a result, depth of the primary chemocline has increased at an irregular rate since the last influx of seawater. Between 1966–1994 the chemocline was eroded downwards 12 m, causing a 95% reduction in monimolimnetic volume. Of the total loss of volume between 1968 and 1994, approximately 4% was caused by management pumping water from the monimolimnion, that is, about 29% of the volume of the chemocline and monimolimnion as they existed in 1977, just prior to the major effort in treatment. The remainder of the monimolimnion was removed by wind action, convective circulation and other natural means. Meromictic stability showed a linear decrease between 1974 and 1994. During that time period, average annual loss of meromictic stability due to natural processes amounted to approximately 64 g-cm cm−2 yr−1 for the south basin alone, and 84 gm-cm cm−2 yr−1 for the lake as a whole. Using stability measurements it was projected that the monimolimnion would most probably mix completely into the lake by approximately 2013–2014. Overall, completion of the Amelia Earhart Dam had a greater effect on the loss of the monimolimnion than efforts to remove this stratum by pumping as it prevented replacement of water eroded from the chemocline with fresh seawater.