Quantifying Dispersal from Hydrothermal Vent Fields in the Western Pacific Ocean
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Quantifying dispersal from hydrothermal vent fields in the western Pacific Ocean Satoshi Mitaraia,1, Hiromi Watanabeb, Yuichi Nakajimaa, Alexander F. Shchepetkinc, and James C. McWilliamsc,d aMarine Biophysics Unit, Okinawa Institute of Science and Technology Graduate University, Onna, Okinawa, 904-0495, Japan; bDepartment of Marine Biodiversity Research and Research and Development Center for Submarine Resources, Japan Agency for Marine-Earth Science and Technology, Yokosuka, Kanagawa, 237-0061, Japan; cInstitute of Geophysics and Planetary Physics, University of California, Los Angeles, CA 90095-1567; and dDepartment of Atmospheric and Oceanic Sciences, University of California, Los Angeles, CA 90095-1565 Edited by Christopher J. R. Garrett, University of Victoria, Victoria, Canada, and approved February 5, 2016 (received for review September 16, 2015) Hydrothermal vent fields in the western Pacific Ocean are mostly among vent populations in the western Pacific basins have not been distributed along spreading centers in submarine basins behind previously addressed. convergent plate boundaries. Larval dispersal resulting from deep- Detailed observations and models for eastern Pacific vents have ocean circulations is one of the major factors influencing gene flow, revealed mechanisms of near-bottom circulation strongly influenced diversity, and distributions of vent animals. By combining a bio- by distinct topographic features of midocean ridges (19–23). Con- physical model and deep-profiling float experiments, we quantify duit-like structures of midocean ridges may shield larvae from cross- potential larval dispersal of vent species via ocean circulation in the axial dispersal and also may enable long-distance dispersal that western Pacific Ocean. We demonstrate that vent fields within back- connects distant vent fields (20). Similar long-dispersal mechanisms, arc basins could be well connected without particular directionality, however, do not apply to species in the western Pacific, where whereas basin-to-basin dispersal is expected to occur infrequently, midocean ridges do not exist. If dispersal were limited to near- once in tens to hundreds of thousands of years, with clear dispersal bottom depths, vent species of the western Pacific would largely be barriers and directionality associated with ocean currents. The contained within a given back-arc basin. southwest Pacific vent complex, spanning more than 4,000 km, may Although most species likely remain near the bottom, some be connected by the South Equatorial Current for species with a strong-swimming larvae (e.g., shrimp and crabs) may disperse higher longer-than-average larval development time. Depending on larval in the water column, possibly ∼1,000 m above the bottom, where dispersal depth, a strong western boundary current, the Kuroshio they can be transported by faster currents (24, 25). Lagrangian Current, could bridge vent fields from the Okinawa Trough to the Izu- measurement methods, using deep-ocean profiling floats pro- Bonin Arc, which are 1,200 km apart. Outcomes of this study should grammed to drift at a specified depth or constant density surface, can help marine ecologists estimate gene flow among vent populations be used to measure dispersal in the water column. This approach has and design optimal marine conservation plans to protect one of the been used for hydrothermal vent surveys as well (26, 27). One ex- most unusual ecosystems on Earth. ample was the Lau Basin Float Experiment (27), which captured boundary currents within the back-arc basin and westward outflow hydrothermal vents | larval dispersal | deep-ocean circulation | from the basin resulting from the South Equatorial Current. For analytical approach various reasons, it is challenging to quantify vent-to-vent transport using only in situ experiments; therefore, one promising approach is ydrothermal vent fields in the western Pacific have received to combine dispersal experiments with ocean circulation models. Hsubstantially less attention than have eastern Pacific vents. Properly analyzed, such observation and modeling data should Western Pacific vents are mostly distributed along spreading yield reasonable estimates of dispersal processes by ocean circulation centers in submarine basins behind convergent plate boundaries, and should help marine ecologists understand biogeography and whereas those of the eastern Pacific occur mainly at midocean ridges. It is estimated that vent-endemic species in back-arc basins Significance were introduced along now-extinct midocean ridges that bridged the eastern and western Pacific Oceans ∼55 million years ago, with Submarine hot springs known as hydrothermal vents host unique a potential origin at the East Pacific Rise (1, 2). More recent ecosystems of endemic animals that do not depend on pho- tosynthesis. Quantifying larval dispersal processes is essential studies suggest the possibility that Indian Ocean ridge systems once to understanding gene flows and diversity distributions of vent connected Atlantic and Pacific vent fields (3). Spreading centers in – endemic species, as well as to protect vent communities from back-arc basins are active for typically 5 10 million years (4, 5). anthropological disturbances (e.g., deep-sea mining). In this Thus, life spans of back-arc spreading centers are significantly study, we assess the potential frequency of larval exchange longer than population lifetimes of vent animals observed in the between vent fields throughout the entire western Pacific via eastern Pacific (∼1 million years) (6). ocean circulation processes, so that population geneticists can Recent genetic studies have addressed the matter of genetic make quantitative comparisons. We show that western Pacific differentiation among vent populations (7–11). Genetic data imply vents in distant basins are potentially connected with strong that back-arc basin populations are well-mixed genetic pools (12, directionality. This article makes a valuable contribution to a 13). In contrast, vent populations in distant basins (∼3,000 km difficult and important area of deep ocean processes. apart) are genetically distinct, suggesting that occasional migrations may have occurred over the course of several hundred thousand Author contributions: S.M. and J.C.M. designed research; S.M., H.W., Y.N., and A.F.S. per- formed research; S.M., H.W., Y.N., A.F.S., and J.C.M. contributed new reagents/analytic tools; generations (14). There is one example of a widespread species S.M., H.W., Y.N., and A.F.S. analyzed data; and S.M. and J.C.M. wrote the paper. Bathymodiolus septemdierum ( complex) occurring in all western The authors declare no conflict of interest. Pacific back-arc basins (15). To interpret gene flows of vent species, This article is a PNAS Direct Submission. it is necessary to understand larval dispersal by ocean circulation, as Freely available online through the PNAS open access option. – well as tectonic history (16 18). However, quantitative data re- 1To whom correspondence should be addressed. Email: [email protected]. garding dispersal processes in the western Pacific are still woefully This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10. inadequate, leaving many unanswered questions. Dispersal patterns 1073/pnas.1518395113/-/DCSupplemental. 2976–2981 | PNAS | March 15, 2016 | vol. 113 | no. 11 www.pnas.org/cgi/doi/10.1073/pnas.1518395113 Downloaded by guest on September 29, 2021 gene flow among vent populations in the western Pacific Ocean. We Temporal variability of flow is often measured with correlation assessed potential larval dispersal from hydrothermal vent fields in timescales, representing characteristic periods during which flow the western Pacific on varying spatial scales, from intra- to interbasin remains more or less consistent in speed and direction. Correla- vent communications, by integrating information from a deep-ocean tion timescales could be qualitatively inferred from float tracks profiling float experiment and predictions derived from an ocean during the first several months of this study (Fig. 1B). Float de- circulation model. ployments separated by ∼30 d or longer demonstrate different dispersal patterns, although some consecutive releases are similar. Results and Discussion In other words, the Eulerian correlation time is less than 1 mo. Dispersal in a Back-Arc Basin. As a base case, we focused on dispersal We calculated the Eulerian correlation time (e-folding time) from processes from a vent field in the Okinawa Trough. The Okinawa time series of model flow fields. The estimated correlation time is Trough is an active back-arc spreading basin behind the Ryukyu about 2 wk, which is longer than that of the ocean surface (several arc-trench system, where the Philippine Sea Plate subducts beneath days) (31), reflecting less energetic circulation. Current observa- the Eurasian Plate (Fig. 1). The current rifting started about tion data from northern East Pacific Rise (32) appear to have a 2 million years ago (28). Depths of vent fields in the Okinawa similar Eulerian correlation time. Trough registered in the InterRidge vents database (29) vary be- tween 560 and 1,850 m, with a mean depth of 1,100 m. Dispersal Probability. Because of the unpredictable nature of dis- To assess spatial and temporal scales of dispersal at Hatoma persal, a large number of cases (degrees of freedom) are necessary to have sufficient statistical power. In the model domain, nearly Knoll (1,520 m) in the southern