Population dynamics of Holopedium gibberum in a Pacific Northwest drinking water reservoir: effects of temperature, food, and competition

Abstract

<italic>Holopedium gibberum</italic> is one of the few cladocerans to create a gelatinous sheath. For this reason it can clog drinking water filtration facilities. In order to better understand the population dynamics of <italic>Holopedium</italic> in a reservoir in the Cascade foothills, <italic>in-situ</italic> single variable treatments were used to investigate the effects of temperature, food, and turbidity. The fatty acid composition of <italic>Holopedium</italic>, <italic>Daphnia</italic> and the seston was tested to examine likely food sources and the possibility of direct competition. It was found that temperature limits the <italic>Holopedium</italic> population during the spring phytoplankton bloom and low food quality limits the population during the late summer. Competitive exploitation between <italic>Holopedium</italic> and <italic>Daphnia</italic> is possible. The composition of several fatty acids in the cladocerans was correlated, indicating shared resources. <italic>Daphnia</italic> followed the seasonal trends in the seston fatty acid composition more closely than <italic>Holopedium</italic>, indicating that the overlap in resources was not complete. <italic>Holopedium</italic> appears to be using the food resources most commonly found above the thermocline and may be able to out-compete <italic>Daphnia</italic> under conditions when chrysophytes or allochthonous fatty acid sources dominate in the seston, however, <italic>Daphnia</italic> appears to utilize a wider range of food resources.