Wednesday, December 30, 2009

Aquatic Erratics as Refugia

I've spent the past few days going through data, photos and videos from our 2008 dive surveys at Elwha in preparation for a planned USGS report. I've been struck by the role that relief - in the form of reef or boulders - seems to play in enhancing diversity and supporting life. The example shown here is from an area to the east of the river mouth - a region characterized by fairly expansive plains of mixed sediment (shown in the photos) and decreased densities of large invertebrates and algae. Boulders are also rare to the east of the river mouth, but where they do occur they seem to act as islands for the variety of organisms that are somehow reliant on the hard surface, stability or shelter they provide.

The video is a stark example of this. We came across a large boulder in the middle of this plain of mixed small sediment, which was encrusted with kelps and invertebrates (including the giant barnacle, Balanus nubilis). A little cave under the boulder seemed a perfect shelter for a Giant Pacific Octopus, and sure enough there was one in there. The video is poor, but you can make out the mantle and siphon of this mid-size octopus.

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