Density-driven water exchange controls seasonal declines in jellyfish (Aurelia aurita) abundance in a shallow fjord system

The common jellyfish Aurelia aurita s.l. occurs in many coastal and shelf sea environments around the world, including fjord systems such as Kerteminde Fjord/Kertinge Nor (Denmark). As observed for the majority of temperate A. aurita populations, jellyfish appear in this fjord system each spring, go through gradual declines in their initially high abundance during summer/autumn and disappear by winter. While food limitation has been suggested to cause distinct seasonal declines in the local jellyfish abundance, the role of hydrodynamic forcing in the shallow, semi-enclosed fjord systemis still unclear. Based on a 2 yr field campaign, we provide a dynamic description for the density-driven water exchange in Kerteminde Fjord/Kertinge Nor to quantify the dispersal of jellyfish from the local population to adjacent coastal waters. The predicted mean residence time (+/- SD) of 45 +/- 1 d for the water masses in Kertinge Nor is in good agreement with an observed residence time of 41 +/- 7 d during which jellyfish abundance in the fjord system was reduced to e(-1) = 37%. These results indicate that major fractions of 86.7 +/- 8.9% and 82.6 +/- 7.9% of observed declines in jellyfish abundance in the fjord system were controlled by the density-driven water exchange during 2013 and 2014, respectively. Our findings emphasize the relevance of hydrodynamic processes in context with jellyfish blooms by ensuring dispersal of jellyfish source populations from semi-enclosed, potentially food-limited regions to open coastal waters.