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SEAFLOOR ZONES

Zone 1: Natural physical disturbances of the seafloor in shallow water under fifteen meters in depth make it undesireable for sessile (attached) organisms. Ice is the dominating influence, not predation. The formation of anchor ice on the seafloor can smother and kill sessile and motile organisms. As the anchor ice becomes buoyant enough to float free, it can lift entrapped organisms off the bottom and imprison them in the sea ice ceiling. In addition, the seafloor is scoured by drift ice (and icebergs) pushed in by winds and currents. This shallow zone appears devoid of invertebrate life with only motile organisms found. There is an absence of competition for food and space and little predation; the animals present are predominately detritus feeders. Common occupants are seastars and sea urchins and other motile organisms like fish, sea spiders, and nemertean worms may be found.



Weddell seal feces and the occasional seal pup carcass (as shown here) provide a rich food resource in shallow water under fifteen meters in depth (Zone 1). Seastars and urchins swarm in to feed and long nemertean worms join in as well.

Zone 2: In water from fifteen meters depth down to thirty-three meters depth, much less anchor ice forms and the seafloor is below the scouring effect of drift ice. The reduced impact of anchor ice affords anemones an opportunity to attach to the seafloor. The invertebrate fauna has numerous sessile (attached) organisms and is predominated by cnidiarians: anemones, soft coral, hydroids, hydrozoans, etc. Also found are ascidians (tunicates), fish, seastars, sea urchins, nemertean worms, and sea spiders. This picture shows the demarcation between Zones One and Two: anchor ice above and anemones below.



Zone 3: Where anchor ice ceases to form in water below thirty-three meters depth, the seafloor community changes dramatically. Sponges dominate this zone below thirty-three meters and there is a rich and diverse community of invertebrates. This zone is physically stable being below the depth of anchor ice formation. This stability encourages the development of a complex, diverse community regulated primarily by predation (rather than physical factors as in Zones 1 and 2). The seafloor is covered with a mat of sponge spicules laid down by many generations of sponges. Seastars are the most conspicuous motile animal in this zone and some of them are sponge predators.

1: Ecological Monographs 44(1):105-128, 1974; 2: Antarctic Ecology, Volume 1. MW Holdgate, ed. NY: Academic Press, 1970. pp244-258; 3: Science 163(3864):273-274, 1969

Text ©Peter Brueggeman. Photographs ©Norbert Wu. Photographs may not be used in any form without the express written permission of Norbert Wu. Norbert Wu no longer grants permission for uncompensated use of his photos under any circumstances whatsoever; want more info?