Patch dynamics of eelgrass Zostera marina

Eelgrass has declined extensively during the last decades following eutrophication of coastal regions of western Europe and the USA.Recent efforts have been taken to reduce nutrient loading, with the hope of restoring the former widespread populations.We studied fine-scale patch dynamics of eelgrass Zostera marina L, in permanent plots outside the continuous vegetation in a protected embayrnent in Limfjorden, Denmark, to evaluate means of recruitment and rates of expansion and mortality of patches.The size distribution was dominated by small patches which were formed by seedlings at high rates during spring (0.16 to 0.76 m-').Mortality was high and only low proportions (0 to 24 %) of the studied cohorts remained as individual patches 1.5 and 2.5 yr later.Patch mortality was restricted to small patches containing <32 shoots with a mean age of < 5 yr.The sharply declining mortality with increasing patch age and size is presumably due to improved anchoring, mutual physical protection and physiological integration among the shoots.The lateral expansion of established patches by centrifugally growing horizontal rhizomes averaged 16 cm yr-l and was independent of patch size With this lateral growth the possible annual areal expansion will be faster in systems with many small patches (260% in 0.1 m' patches) and slower in systems with few large patches (19% in 10 m2 patches).Successful recovery of the eelgrass vegetation in large systems is, therefore, dependent on seed production and dispersal and subsequent seedling establishment and patch growth.