Environmental Factors Contributing to the Disaggregation of a Colonial Cyanoprokaryote and Its Influence on Picoplankton Abundance within Lake Joyce, Virginia

A colonial cyanoprokaryote, Aphanocapsa holsatica and autotrophic picoplankton abundance were monitored weekly over a two year period in Lake Joyce, Virginia. Significant differences were observed in both the cyanoprokaryote and picoplankton abundance over the study period and an inverse relationship was observed between these two plankton groups. Disaggregation of colonies was shown to contribute to picoplankton populations where water temperature and precipitation input apparently trigger colony dispersion. This relationship is suggested to occur in other aquatic habitats. Results of this work and its implications for ecosystem dynamics are discussed. INTRODUCTION Picoplankton is defined as plankton between 0.2 and 2.0μm in size (Sieburth et al., 1978) and may include a variety of both heterotrophic and autotrophic organisms (Marshall, 2002). Numerous studies have shown picoplankton as an abundant and productive component within a variety of oceanic, estuarine and freshwater environments (Li, et al., 1983; Fahnensteil and Carrick, 1992; Marshall and Nesius, 1993; Affronti and Marshall, 1994). However, questions remain as to the relationship of picoplankton in aquatic food web dynamics (Stockner and Shortreed, 1989; Fogg, 1995; Marshall, 2002). To answer these questions, more detailed information is required on factors which influence picoplankton composition dynamics. With this information, a better understanding of the availability of picoplankton as a link or sink for nutrients can be determined. The objectives of this study are: 1) identify variation in both autotrophic picoplankton and colonial cyanoprokaryotic abundance using a high frequency sampling regime and 2) identify the effects, if any that water temperature and storm water runoff have on colonial cyanoprokaryotic and autotrophic picoplankton population dynamics in Lake Joyce, Virginia. Lake Joyce, Virginia (36° 54' 44" Lat., 76° 7' 19" Long.) is a 60ha freshwater lake whose overflow empties via Pleasure House Creek and the Lynnhaven River into the lower Chesapeake Bay. The Virginia Department of Environmental Quality ( 1994) has described this water body as an unstratified, hypereutrophic system whose average depth is 1.1 m. The major nonpoint source input is from urban storm water runoff. Lake Joyce is representative of other lakes in the Norfolk/Virginia Beach area where its general usage includes fishing, boating, and water skiing. METHODS During this study, three replicate surface grab samples (125 mL) were collected weekly at one station in Lake Joyce over a 24 month period (May 29, 2000 to May 20, 200 VIRGINIA JOURNAL OF SCIENCE Plcoplankton Abundance