Eelgrass Zostera marina in subarctic Greenland: dense meadows with slow biomass turnover in cold waters

Eelgrass Zostera marina L. is the most dominant seagrass species throughout the temperate northern hemisphere, and knowledge on its distribution and production in relation to climatic conditions is relevant for predicting the future of the meadows. We synthesized past and present information on eelgrass near the northern distribution limit in Greenland, and quantified the biomass, production and reproductive potential of eelgrass populations in 4 widely separated locations of Godthabsfjorden at 64 degrees N. Eelgrass observations in Greenland date back to 1813; most of the meadows recorded in the past still exist, and new ones were identified. The meadows are relatively small and geographically isolated, typically occurring in inner branches of the fjord system where summer water temperatures are higher (13 to 15 degrees C) than in the outer part (<10 degrees C). The shoot density (871 to 2045 shoots m(-2)), aboveground biomass (90 to 327 g dry wt [DW] m(-2)) and rhizome extension rates (8 to 29 cm yr(-1)) match levels further south. By contrast, the annual production of 7 to 13 leaves shoot(-1) is considerably lower and leads to slow leaf biomass turnover (1.6 to 2.6 yr(-1)) in these northern populations. Even though flowering was common, mature seeds were found only once, after a warm summer, and no seedlings were observed. An analysis of published data on eelgrass leaf biomass and production across the entire species distribution range revealed that leaf biomass is not affected by either latitude or air temperature, whereas annual leaf formation rates are significantly lower in cold areas at high latitude compared to warm areas at low latitude. The results suggest that distribution and production in Greenland are currently limited by low temperature and likely to increase in a warmer future.