Depth-Related Changes in Reproductive Strategy of a Cold-Temperate Zostera marina Meadow

Abstract

Biomass allocation and demographic characteristics of a Danish Zostera marina population were determined along a depth gradient of light and exposure to evaluate the reproductive strategy for meadow maintenance and recovery. From May to September, biomass, shoot growth and reproductive allocation were measured monthly at three depths (1.8, 4 and 6 m). Also, seedling survival along the deep edge (6.2 m) of the studied meadow was followed over a 12-month period. Shoot density and biomass showed pronounced differences among depths with up to 12-fold lower shoot density and 6-fold lower biomass at deep compared to shallow sites. Comparatively, little variation was found in leaf formation rates and rhizome elongation rates among depths. However, new shoots formed through vegetative reproduction and surviving until September constituted a significantly higher fraction of total shoot density in shallow water (45.7 %) than in deep water (17.3–21.1 %). Conversely, allocation to sexual reproduction was highest at intermediate and high water depths where the proportion of reproductive shoots to total shoot density at time of maximum density was 21.1 and 10.6 %, respectively, and only 3.9 % at the shallow depth. Seed production was also higher at intermediate depth (1970 seeds m−2) than at the upper (1230 seeds m−2) and lower (760 seeds m−2) distribution limit. Seedling recruitment within the meadow took place at all sampling depths but no seedlings persisted throughout the summer period, whereas 8 % of the seedlings established in bare areas along the deep meadow edge (0.43 seedlings m−2) survived their first year. Overall, the results suggest that the shallow edge of the meadow is primarily maintained by vegetative recruitment whereas the deep edge to larger extent relies on sexual recruitment. The intermediate depth zone may act as a buffer zone supporting the maintenance of shallower and deeper eelgrass through seed supply and vegetative expansion, thereby stabilizing the meadow by increasing its resilience towards disturbances and its recovery potential upon disturbances.