Finding Biogeographic Boundaries in the Deep Sea: Alaska, Hawaii, and the Emperor Seamount Chain

Les Watling1, Amy Baco-Taylor2, Sarah Bingo3, Glenn S Carter4, Henrietta Dulai5, Scott France6, Becca Lensing1, Nicole Morgan7, E Brendan Roark8, John R Smith Jr9 and Natalie Summers1, (1)University of Hawaii at Manoa, Honolulu, HI, United States, (2)Florida State University, Earth, Ocean, Atmospheric Sciences, Tallahassee, FL, United States, (3)University of Hawaii at Manoa, Oceanography, Honolulu, HI, United States, (4)University of Hawaii, Manoa, Honolulu, HI, United States, (5)University of Hawaiʻi at Mānoa, Earth Sciences, Honolulu, HI, United States, (6)University of Louisiana at Lafayette, Lafayette, LA, United States, (7)Florida State University, Tallahassee, FL, United States, (8)Texas A&M University, College Station, CA, United States, (9)Univ Hawaii, Honolulu, HI, United States
The deep-water (lower bathyal) octocoral and sponge fauna of the North Pacific differs almost completely between the Gulf of Alaska - Aleutian Ridge areas and the Central North Pacific - Hawaiian Ridge areas. Since the Emperor Seamount chain is the only stretch between the two areas where substrata at bathyal depths are common, we hypothesized that a biogeographic break or transition zone would be found in the gap between Nintoku and Koko seamounts, partially based on previous physical oceanographic studies. However, water mass data suggested there was no difference in T and S at bathyal depths anywhere along the chain. We proposed that currents moving east to west through the 500 km-wide gap between Nintoku and Koko Seamounts would act as a “Current Wall” limiting the ability of larvae to move either N or S.

We conducted 10 ROV dives on 7 seamounts, progressing from Suiko in the north to Koko in the south using the Schmidt Ocean Institute’s R/V Falkor and ROV SuBastian. Dives depths were from 2400 – 1800 and 1500 – 1200 m. Octocorals and sponges were imaged and collected (82 octocorals and 22 sponges), and other “morphospecies” collected or counted in the video images, during each dive, and their known distributions tabulated. Hydrographic and geological samples were collected to understand the longer-term history of local water mass characteristics and their variability.

We consistently found species with known northern distributions until we dove at Annei Seamount on the southern side of the gap. The octocorals and sponges at both Annei and Koko seamounts are characteristic of the Central Pacific fauna that was imaged often during the NOAA OER CAPSTONE project. None of those species were seen at Jingu Seamount, a mere 225 km to the north, suggesting that the change in the fauna from North Pacific to Central Pacific occurs over a relatively small distance. At this time, velocity and water mass chemical tracer data are still being analyzed but will be available at the time of the meeting.