SHORT NOTE Brenda Konar Æ Katrin Iken Competitive dominance among sessile marine organisms in a high Arctic boulder community Received: 10 March 2005 / Revised: 31 July 2005 / Accepted: 5 August 2005 / Published online: 22 September 2005 Ó Springer-Verlag 2005 Abstract In most hard substrate environments, space is a limiting resource for sessile organisms. Competition for space is often high and is a structuring force within the community. In the Beaufort Sea’s Boulder Patch, crustose coralline red algae are major space occupiers. This research determined if coralline algae were com- petitively dominant over other sessile organisms. To test this hypothesis, overgrowth was documented in terms of ‘‘winners’’ and ‘‘losers’’ on the contact borders between different species. Crustose corallines occurred in over 80% of the observed interactions but were only winners in approximately half of them. Most frequently, bry- ozoans, tunicates, and sponges were superior competi- tors over crustose corallines, while at the same time these invertebrate groups were among the least abundant space occupiers. Introduction Coexistence of species is largely driven by an interacting system of disturbance and competition (Airoldi 2000). While disturbances clear space, competitive success ultimately determines who inhabits the space. Compe- tition for space is a major structuring force in marine benthic communities (Bertness and Leonard 1997). Substrate competition by encrusting marine organisms usually occurs by direct overgrowth at their contact zones (Sebens 1986). These competitive border interac- tions are commonly evaluated with species being ranked as winners or losers based on their abilities to overgrow the competing taxa. Although losers of interactions are overgrown by winners, this does not always result in death of the overgrown organism (Jompa and McCook 2002). Coralline algae, in particular, can survive while they are overgrown and sometimes even continue to grow (Sebens 1986; Dethier and Steneck 2001). While the outcome of border interactions can vary depending on species, depth, timing, and location, some taxa are typically competitively dominant over others (Nandakumar 1996; Airoldi 2000; Barnes and Dick 2000; Barnes 2002). In temperate and polar waters, ascideans, sponges, and bryozoans have been shown to be strong space competitors (Nandakumar 1996; Mau- ghan and Barnes 2000; Barnes and Kuklinski 2004). The Boulder Patch is an isolated hard-bottom kelp community surrounded by the soft sediment habitat of the high Arctic Beaufort Sea. Space seems to be a lim- iting resource as most rock substrate is covered by sessile organisms, usually crustose coralline algae. Based on these observations, we tested whether crustose coralline algae were competitively dominant over other sessile species, and thus excluded or diminished other space occupiers. We included upright and crustose organisms in this analysis as both are space occupiers in this sys- tem. Our objectives were to determine (1) if space was limiting, (2) the dominant space occupier, and (3) which taxa were dominant competitors. Study site This study was conducted at the Boulder Patch in Stefansson Sound, Beaufort Sea Patch (147°40¢W, 70°20¢N; see Dunton 1990 for map details). Sediment load is high in this area because of run-off from the Sagavanirktok Delta, but overall sediment accumulation in the benthos is limited due to strong currents (Dunton and Schonberg 2000). Light intensity and duration for the benthic community can be reduced due to water column sediments and the polar winter, respectively (Dunton 1990). Ice scour is relatively low because the area is protected from thicker ice by offshore barrier islands. Water temperatures range from À1.9°C during B. Konar (&) Æ K. Iken School of Fisheries and Ocean Sciences, University of Alaska Fairbanks, Fairbanks, AK 99775-7220, USA E-mail: bkonar@guru.uaf.edu Tel.: +1-907-4745028 Fax: +1-907-4745804 Polar Biol (2005) 29: 61–64 DOI 10.1007/s00300-005-0055-8