Deep-sea emergence of red tree corals (Primnoa pacifica) in Southeast Alaska glacial fjords

Robert P. Stone and Jennifer Mondragon

Red tree corals (Primnoa pacifica) were found thriving in shallow-water areas of 2 large glacial fjord systems in Southeast Alaska during scuba surveys. Corals were found at depths as shallow as 9 m in Muir Inlet, Glacier Bay, in 2004 and 2005 and as shallow as 6 m in Tracy and Endicott arms, Holkham Bay, in 2006. These observations represent a depth range extension for the species— red tree corals typically occur at depths greater than 130 m in the Gulf of Alaska and elsewhere where they form extensive thickets. The red tree coral is an important pioneer species in the primary succession of recently deglaciated habitats. The emergence of red tree corals in shallow-water areas of these glacial fjords likely occurs because year round oceanographic conditions (low temperature, higher than normal surface salinity, and low ambient light levels) are similar to those found in their more typical depth range. Glacier Bay was formed beginning in 1794 by rapid glacial retreat, and a deglaciation record has been meticulously documented during the past 130 years. This record provides a chronosequence that we used to study the succession of red tree corals into the bay and it presented an opportunity to evaluate the accuracy of a method previously used to age the corals. In situ access to shallow-water populations provided an invaluable opportunity to examine reproductive and recruitment processes and genetic connectivity of the most important coral species in the North Pacific Ocean. The information from these examinations provides important insights into the ability of this species to recover from disturbance and to colonize protected areas established to mitigate the effects of human activities.

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