THE STRUCTURE, LIMITS, AND DRIVERS OF DAPHNID DIVERSITY: UNDERSTANDING THE PARADOX OF AZISCOHOS LAKE, MAINE

In 1939, a lake survey conducted by the Maine Department of Inland Fisheries and Game discovered that one of its lakes, Aziscohos Lake, contained an unusually high number of species of the keystone zooplankton genus Daphnia. Nearly 20 years later Dr. John Brooks while completing his authoritative volume “The Systematics of North American Daphnia” revisited the lake and confirmed the 1939 study’s result of six species of Daphnia living within the lake, at the time the highest known coexisting Daphnia assemblage. In 1961, the “Paradox of the Plankton” was posited questioning how sympatric planktonic species can coexist for long periods of time while seemingly avoiding the competitive exclusion principle. As this ‘paradox’ became a focus of inquiry one researcher conducted a 2-year study (1961-1962) on Aziscohos Lake Daphnia to address the question. While the study did not solve the enigma, it provided a detailed record of the dynamics and interactions of the six species. Aziscohos Lake and its zooplankton have not been studied since.

This study is readdressing the dynamics of multi-species Daphnid populations within Aziscohos Lake. Preliminary research has surprisingly shown that five of the original six species still exist within the lake as well as an additional three species which have never been reported in Aziscohos Lake. This nearly century long record of coexistence suggests an equilibrium given the stability of such high levels of diversity. A close examination of the chemical, physical, and biological characters at sites transecting the lake have been employed to better understand the cause and maintenance of this diversity.