Abstract
Maintaining and enabling evolutionary processes within meta‐populations are critical to resistance, resilience and adaptive potential. Knowledge about which populations act as sources or sinks, and the direction of gene flow, can help to focus conservation efforts more effectively and forecast how populations might respond to future anthropogenic and environmental pressures. As a foundation species and habitat provider, Zostera marina (eelgrass) is of critical importance to ecosystem functions including fisheries. Here, we estimate connectivity of Z. marina in the Skagerrak–Kattegat region of the North Sea based on genetic and biophysical modelling. Genetic diversity, population structure and migration were analysed at 23 locations using 20 microsatellite loci and a suite of analytical approaches. Oceanographic connectivity was analysed using Lagrangian dispersal simulations based on contemporary and historical distribution data dating back to the late 19th century. Population clusters, barriers and networks of connectivity were found to be very similar based on either genetic or oceanographic analyses. A single‐generation model of dispersal was not realistic, whereas multigeneration models that integrate stepping‐stone dispersal and extant and historic distribution data were able to capture and model genetic connectivity patterns well. Passive rafting of flowering shoots along oceanographic currents is the main driver of gene flow at this spatial–temporal scale, and extant genetic connectivity strongly reflects the “ghost of dispersal past“ sensu Benzie, 1999. The identification of distinct clusters, connectivity hotspots and areas where connectivity has become limited over the last century is critical information for spatial management, conservation and restoration of eelgrass.
Highlights
Eelgrass (Zostera marina L.) is one of the most widely distributed species of seagrass in the northern hemisphere and the dominating species of the temperate North Atlantic (Short, Carruthers, Dennison, & Waycott, 2007)
Our assessment includes a temporal comparison based on oceanographic dispersal modelling of extant and historical distribution data of Z. marina for the region, and we investigate the hypothesis that the large observed decline has resulted in decreased connectivity and lower genetic diversity
We combine and cross- validate genetic and hydrodynamic modelling approaches in order to infer the importance of dispersal in shaping population structure and to compare trade-offs and synergies offered by the integration of the two approaches as applied to management and mitigation
Summary
Eelgrass (Zostera marina L.) is one of the most widely distributed species of seagrass in the northern hemisphere and the dominating species of the temperate North Atlantic (Short, Carruthers, Dennison, & Waycott, 2007). The oceanographic features of the area are unique, with a strong outflow of surface water from the Baltic Sea into the Kattegat, creating an asymmetric circulation along the coasts with strong effects on connectivity, creating a barrier between the Kattegat and Skagerrak (Jonsson, Corell, André, Svedäng, & Moksnes, 2016; Jonsson, Nilsson Jacobi, & Moksnes, 2016; Leppäranta & Myrberg, 2009) Biophysical dispersal models, such as Lagrangian trajectory models (Cowen & Sponaugle, 2009; Grech et al, 2016; Selkoe et al, 2010), can model such directional dispersal based on biologically realistic assumptions, for example, time of propagule release, drift duration and depth, and may be superimposed on layers of habitat preference. We combine and cross- validate genetic and hydrodynamic modelling approaches in order to infer the importance of dispersal in shaping population structure and to compare trade-offs and synergies offered by the integration of the two approaches as applied to management and mitigation
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