INTRODUCTION

Calanoid copepods are important members of limnetic communities in high-elevation lakes, sometimes dominating the crustacean zooplankton fauna (Reed and Olive 1958; Patalas 1964; Anderson 1971, 1974, 1980; Stoddard 1987; Larson et al. 1994). Abiotic factors such as lake elevation (Reed and Olive 1958; Patalas 1964; Anderson 1971, 1974), water temperature (Allan and Goulden 1980; Byron et al. 1984; Walters et al. 1987), lake area and depth (Anderson 1974; Stoddard 1987), and water chemistry (Sprules 1975; Byron et al. 1984; Stoddard 1987) influence calanoid distribution and abundance. These factors directly affect survival and reproduction or indirectly affect species abundance by altering food quality and quantity or the intensity of biotic interactions (DeMott 1989; Gliwicz and Pijanowska 1989).

Vertebrate predators, especially fish, can prey selectively on larger, more visible prey and cause a shift in community composition from large-bodied to small-bodied forms (Zaret 1980; Gliwicz and Pijanowska 1989). Impacts of fish predators on zooplankton can vary along a gradient of increasing predation intensity, with the greatest effects occurring at the highest predation intensities (Brocksen et al. 1970; Stenson 1972; Gliwicz and Prejs 1977; Langeland 1978; Dodson 1979; Zaret 1980; McQueen et al. 1986; Kerfoot 1987; Post and McQueen 1987).

Calanoid copepods are frequently among the larger-bodied taxa of zooplankton in high-elevation lakes (Anderson 1971, 1974; Stoddard 1987; Starkweather 1990), are often highly pigmented (Hairston 1978; Starkweather 1990), and so may be susceptible to fish predation. Anderson (1972) reported disappearance of the large diaptomid D. arcticus from a lake in the Canadian Rockies following repeated introductions of trout. Several studies have documented the presence of large diaptomids in fishless oligotrophic lakes and their absence in lakes with fish in the same geographic area (Anderson 1974; Northcote and Clarotto 1975; Dodson 1979; Stoddard 1987; Starkweather 1990; Donald et al. 1994). Trout stocked in fishless lakes in British Columbia led to reduction in body size of D. kenai but had little effect on its density (Northcote et al. 1978). Anderson (1980) reported that cutthroat trout (Oncorhynchus clarki), a common species in high-elevation lakes in the west, often co-occurs with large diaptomids.

Large and small calanoids also may be vulnerable to predation from invertebrate predators, especially Chaoborus (Dodson 1970; Sprules 1972; Fedorenko 1975a,b; Northcote et al. 1978; Giguere 1979; Anderson 1980) and Gammarus (Anderson 1980). Large diaptomid copepods, in turn, can prey on other zooplankton including smaller copepods (Anderson 1967, 1970; Dodson 1970, 1974; Sprules 1972; Maly 1976; Giguere 1979; Paul et al. 1995) and rotifers (Williamson and Butler 1986; Williamson 1987; Paul et al. 1995). Zaret's (1980) predation submodel II predicts that high densities of vertebrate predators would eliminate large predatory copepods thus allowing smaller herbivorous copepods to increase in abundance. Such "top- down" effects in food webs may be most evident in oligotrophic systems such as high lakes (Gliwicz and Prejs 1977; Dodson 1979; McQueen et al. 1986; Neill 1987; Carney 1990), particularly when vertebrate predation intensity is high and persistent (Dodson 1979; Gliwicz and Pijanowska 1989).

In this chapter we investigate factors influencing distribution, abundance, and patterns of co-occurrence of large and small diaptomid copepods in high-elevation lakes in North Cascades National Park Service Complex (NOCA), Washington, USA. Specifically, we investigated 1) the effect of abiotic factors on distribution and abundance of large (Diaptomus kenai and D. arcticus) and small (D. tyrrelli) diaptomid copepods, and 2) whether the abundance of large and small diaptomids copepods differed between lakes with high trout densities, lakes with low trout densities, and lakes with no trout.