Ecosystem function of subarctic Zostera marina meadows: influence of shoot density on fish predators and predation rates

Recent studies have shown a decline in predation intensity in seagrasses with higher latitude, but knowledge about arctic and subarctic regions is scarce. Moreover, changes in trophic structure are likely to vary with latitude, and to impact predation. Thus inclusion of high-latitude sites and the quantifyication of predation at multiple trophic levels are critical for our understanding of predator-prey dynamics in seagrasses at a global scale. In this in situ study, we investigated predator communities and predation rates in 2 subarctic seagrass meadows in Northern Norway. We measured the effect of seagrass density on abundances of small-bodied fish and predation rates on small (amphipods/mesograzers) and medium-sized crustaceans (shrimps/mesopredators) using a standardized live tethering technique. Results varied strongly between both study sites. Juvenile fish occurred in higher diversity and density in high seagrass density in one meadow, while sticklebacks dominated the other meadow, irrespective of habitat complexity. Predation rates varied strongly between prey type; on average, 41 % of shrimps and 16% of amphipods were consumed, while a standardized non-live prey was consumed at a rate of only 3%. In one meadow, predation on shrimp was strongly reduced (from 60% outside the meadow to 3% in high-density patches), indicating the importance of habitat complexity in reducing predation rates at this site. The results demonstrate that predation rates differ between trophic levels, and that habitat effects on predation depend on local meadow characteristics. We found that subarctic seagrass provides habitat for a diverse fish community, and specifically one meadow hosted more juveniles of commercial species compared to bare habitats.