Effects of ice scour on the structure of sublittoral marine algal assemblages of St. Lawrence and St. Matthew Islands, Alaska

Abstract

Ice scour and gouging play a major role in the distribution and abundance of macroalgae and sessile invertebrates in the northern Bering Sea. Annual ice scouring and gouging create a gradient of disturbance, at one extreme removing the entire biota from horizontal rock surfaces in shallow water (< 12 m), with a transition to a rock slope area where the effects of disturbance are less intense and more intermittent in nature. This appears to restrict adult perennial algae such as Laminaria dentigera, L. groenlandlca, L. yezoensis, and Alaria crispa to valleys and crevices protected from ice scour Recruitment of young Laminariales was highest on the less severely scoured sloping rock surfaces between the adult populations and the ice scoured rock tops, possibly due to the gradient of disturbance. Laminaria spp. were abundant in shallow water, but below the depth of ice scouring (> 12 m), Laminaria spp. were less common, and Agarum cribrosum, whlch was rare in shallow water, became common. Contrary to other studies, the lower distribution of A. cribrosum is not related to grazing sea urchins, as urchins and other large herbivores are absent. The algal zone extended to a depth of ca 28 m, where sessile invertebrate cover became high. The marine benthos of St. Lawrence Island was characterized by a high cover of Laminaria spp., few understory algae, and sessile invertebrates such as barnacles, hydroids, mussels, sponges, and bryozoans, while St. Matthew Island had a high cover of Laminaria dentigera, abundant understory red algae such as Ptilota asplenioides, Cirrulicarpus gmelinii, and encrusting coralline algae, and fewer sessile invertebrates.