COMMUNITY STRUCTURE AND BOTTOM-UP REGULATION OF HETEROTROPHIC MICROPLANKTON IN ARCTIC LTER LAKES

Microplankton community structures and abundance was assessed in lakes at the Toolik Lake LTER site in northern Alaska during the summers of 1989 and 1990. The microplankton community included oligotrich ciliates, but rotifers and zooplankton nauplii comprised greater than 90% of total estimated heterotrophic microplankton biomass. Dominant rotifer taxa included Keratella cochlearis, Kellicottia longispina, Polyarthra vulgaris, Conochilus unicornis and a Synchaeta sp. Microplankton biomass was lowest in highly oligotrophic Toolik Lake (< 5 mugC l-1 at the surface) and highest (up to 55 mugC l-1) in the most eutrophic lakes, experimentally fertilized lakes, and fertilized limnocorrals, consistent with bottom-up regulation of microplankton abundance.