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Empirical and biophysical modeling studies of dispersal barriers for marine plankton

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Title: Empirical and biophysical modeling studies of dispersal barriers for marine plankton
Authors: Norton, Emily
Keywords: connectivity
genetic structure
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Issue Date: 16 Jul 2013
Abstract: Because there are few obvious dispersal barriers in the open ocean, pelagic organisms are typically expected to experience high connectivity over long distances. However, very little work has been done to understand gene flow in pelagic holoplankton on a global scale. In this study, I investigated genetic connectivity among populations of the mesopelagic copepod, Haloptilus longicornis, using both empirical and biophysical modeling approaches. Using the mitochondrial marker, cytochrome oxidase subunit II (mtCOII, 43 locations, N = 1059), highly significant genetic structure was detected among ocean basins (global FST = 0.20; p < 0.00001), with the exception of a lack of genetic structure between South Atlantic and Indian Ocean populations of H. longicornis. Genetic breaks were also detected among subtropical gyre populations within the Atlantic and Pacific Oceans in this species (Atlantic: FCT = 0.21, Pacific: FCT = 0.15, AMOVA, p < 0.00001 for both oceans). I tested for Isolation by Distance (IBD) on the basin and gyre scales in the Atlantic and Pacific, and observed a significant relationship between genetic and geographic distance for basin-wide comparisons (p < 0.01 for Mantel; linear regression: R2 = 0.142 for Pacific, R2 = 0.266 for Atlantic; p < 0.001 for both); however, there was no pattern of IBD for the within-gyre comparisons in any of the four subtropical gyres (p >> 0.05 for all). In combination, these results suggest that the significant result for IBD in the basin-scale comparisons was driven by the equatorial genetic breaks observed in both the Atlantic and Pacific Oceans. In the Atlantic Ocean, the equatorial genetic break was accompanied by low abundance of H. longicornis (0-12oN), and pilot studies indicated that reproductive failure was high in this region. These results indicate that the equatorial region may be poor quality habitat for this species and serve as a biophysical dispersal barrier to migrants traveling among subtropical gyre habitats. In order to better understand contemporary dispersal patterns and the mechanism underlying this dispersal barrier in the Atlantic Ocean, I simulated particle dispersal for 30 years using the Simple Ocean Data Assimilation (SODA) model coupled with a particle-tracking scheme. Particle dispersal was simulated using physical forcing alone and in combination with increased mortality or decreased reproduction in the equatorial region, in order to determine whether a physical or a biophysical dispersal barrier exists in the equatorial Atlantic. Particle dispersal forced with physical oceanographic processes predicted that 26% of particles originating in the southern gyre dispersed to the northern gyre over the 30-year simulation, which is an unrealistically high rate of connectivity given the empirical genetic divergence observed among these populations. When conservative biological processes were imposed in the equatorial region, connectivity among populations in the northern and southern subtropical gyres ceased. These results suggest that the dispersal barrier across the equatorial region must be biophysical in nature, and that it likely prevents contemporary dispersal among subtropical gyre populations. Therefore, populations of H. longicornis in the subtropical gyres of the northern and southern Atlantic are demographically as well as genetically isolated, and may evolve independently in response to local selective pressures. This is the first study on marine holoplankton to compare biophysical modeling predictions with empirical genetic data in order to identify the mechanism underlying a pelagic dispersal barrier.
Pages/Duration: x, 98 leaves
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Appears in Collections:M.S. - Oceanography

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