Experimental studies on factors limiting colonization by Daphnia pulex Leydig of coastal montane lakes in British Columbia

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Abstract
Laboratory and field experiments were conducted to account for the absence of a widespread and locally abundant zooplankter D. pulex from oligotrophic montane lakes in coastal British Columbia [Canada]. Water quality, food concentration and invertebrate predation were examined as potential limiting factors. Laboratory exposure of neonate and adult D. pulex to montane lake water and lowland poor water, both manipulated to the same range of seston densities, revealed significant effects of food levels and water sources. Elevated food levels consistently produced marked increases in survival, growth and reproduction compared with natural, low seston densities, Enhancement effects were always lower in water from montane lakes. Laboratory predation by Chaoborus trivittatus larvae over a range of natural prey densities showed heavy predation only in the absence of normal alternative prey. Large-scale field experiments in 10,000 l enclosures in a montane lake produced high invasion success by D. pulex after artificial fertilizer additions and modest invasion success in the absence of Chaoborus predation. Invasion was unsuccessful at natural seston densities when Chaoborus was present. Food limitation, coupled with unspecified water quality effects and low levels of Chaoborus predation, apparently limits D. pulex invasion potential, despite numerous nearby sources of dispersers.