Long-term changes in area distribution of eelgrass ( Zostera marina) in Danish coastal waters

Abstract

For five shallow-water eelgrass stands in Denmark, changes in area distribution over the period 1940/50s–1990s were assessed from aerial photographs. In areas where the wasting disease had decimated eelgrass meadows in the 1930s, this included determination of the time scale of recolonization. In addition, we aimed at evaluating whether fluctuations in eelgrass area distribution were related to various natural and human-induced disturbances, namely storm events, ice-cover, water temperature and nutrient loading. We found that populations affected by the wasting disease exhibited a time lag of more than 10 years before substantial recolonization began, probably reflecting long distances to seed-producing populations and extreme climatic events in the period. After the initial time lag, eelgrass area distribution increased rapidly, and a substantial recovery had taken place in the 1960s. All eelgrass populations showed marked inter-annual fluctuations. Declines were often rapid with reductions of about 60% occurring in less than 6 years. Recoveries occurred over similar time scales and documented that recolonization may take place relatively fast when suitable environmental conditions are present. Fluctuations in eelgrass area distribution tended to be larger in enclosed, protected bays as compared to open coasts, probably because enclosed sites are often more eutrophic. Changes in shallow-water eelgrass area distribution did not correlate with the available long-term records of natural and human-induced disturbance parameters. Thus, while deep-water eelgrass populations have declined markedly over the last century in response to eutrophication, long-term changes in shallow-water populations are less equivocal and seem more stochastic.