Why Are You Here But Not There?

What influences where coral assemblages occur?

One obvious factor is depth. We did not find any black corals on this cruise but we had only one dive to the depth at which they are known to occur. Another factor is substrate. Corals are sessile colonies, meaning they are stationary. Most corals prefer hard surfaces of cobble, boulders or bedrock. The side-scan habitat mapping data we used to determine hard substrates for potential dive sites worked wonderfully.

But what are the other significant factors? Bottom currents? Nutrient-rich or -poor conditions? Oxygen levels? We traversed various habitats such as boulder fields, walls, and patches of bedrock. But to the naked eye one boulder patch looked like another, one wall was like another. In many cases, what appeared to be similar habitat supported completely different species. One boulder patch might have supported Swiftia colonies, while another had Paragorgia with Stylaster and yet another had Lophelia.

In a terrestrial world, factors like precipitation, elevation, soil nutrients, soil chemistry, temperature, disturbance levels and prey or food influence where sessile organisms occur. But what are the similar factors in this deep sea world that would help us predict where specific communities occur? And how frequently can they colonize a suitable hard surface area and how close does the source stock have to be?

So many questions remain to be answered.

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