Aquatic Ecology 38: 309–315, 2004. © 2004 Kluwer Academic Publishers. Printed in the Netherlands. 309 Dispersion of the horned grebe Podiceps auritus (L.) (Aves) on Lake Myvatn, Iceland, in late summer Thorkell Lindberg Th´ orarinsson 1,2 and ´ Arni Einarsson 1 1 Myvatn Research Station, 660 Myvatn, Iceland (e-mail: myvatn@hi.is); 2 Department of Biology, University of Iceland, 101 Reykjavik, Iceland (e-mail: thorket@hi.is) Key words: Dredging, Sticklebacks, Submerged macrophytes, Tachymeter, Water depth Abstract Studies on habitat selection by waterbirds usually describe large-scale distribution of populations or species but the dispersion within lakes is an understudied aspect. Detailed mapping of horned grebes (Podiceps auritus (L.)) on the North Basin of Lake Myvatn, Iceland, was carried out in late summer 2000 in order to compare their dispersion with water depth and submerged vegetation. Depth and vegetation were measured on sampling sites that were evenly distributed over the North Basin, and the information obtained compared to the distribution of the grebes. Four species of submerged macrophytes dominated in the North Basin; Myriophyllum spicatum, M. alterniflorum, Potamogeton filiformis and P. perfoliatus, mainly at depths < 2.5 m. The grebes seemed to prefer to dive in shallow areas vegetated with stands of Myriophyllum spp. Densities of three-spined sticklebacks Gasterosteus aculeatus were compared in stands of M. spicatum and P. filiformis in order to estimate food availability. No difference was found, so the sticklebacks could not explain the preference of the grebes for areas vegetated with Myriophyllum spp. Nevertheless, this study indicates that shallow areas, vegetated with Myriophyllum spp., are important feeding areas for the horned grebe. Introduction Several studies on the large scale distribution and hab- itat use of horned grebes (Podiceps auritus (L.)) have shown that the species can inhabit a wide range of lake types (see Fjeldså 1973a, b; Faaborg 1976; Kaup- pinen 1993; Heglund et al. 1994; Fournier and Hines 1999). Horned grebes in Iceland and northern Nor- way (ssp. arcticus) are thought to be less specialized in habitat selection and feeding than in other parts of the range, possibly because of the absence of closely related species (Fjeldså 1973a, b). Less attention has been paid to dispersion and space use within habitats (but see Fjeldså, 1973b, c). Detailed mapping of waterbirds has been carried out on the North Basin of Lake Myvatn, Iceland since 1987. In 1967 sediment dredging was started in the North Basin and studies on the dispersion of water- fowl suggest that this can have an impact through loss of suitable habitat (Einarsson and Magnusdottir 1993; Einarsson 1998; Einarsson and Jónsson 1998; Gardarsson et al. 2002). The horned grebe establishes a territory around its nest in early spring (Fjeldså 1973c). The territory is defended for a few weeks in the breeding season, but when the young become independent the adult grebes gather into flocks and moult their flight feathers. In late summer, flocks of horned grebes are found on the North Basin of Lake Myvatn (Einarsson and Jonsson 1998). Individual birds leave these flocks on foraging trips. Like other grebes, the horned grebe is an expert diver and obtains most of its food by diving. Although the horned grebe has been termed an opportunistic feeder that may shift to what is abundant and easily available, the main food in its north-European range is the three-spined stickleback Gasterosteus aculeatus L. (Fjeldså 1973b). Roughly half of the Icelandic horned grebe population nests in the Myvatn area (Nielsen 1998; Einarsson 2000).