Recolonization of intertidal Zostera marina L. (eelgrass) following experimental shoot removal

Abstract

The recovery of eelgrass ( Zostera marina) from physical disturbances is understudied and no attention has been given to the likely differences in damage recovery rates between the continuous lower intertidal perennial meadows and higher intertidal eelgrass patches. In the present study, 4 m 2 plots were treated by removing all eelgrass shoots and followed over a 3 year period until they attained control (untreated) plot shoot densities. The number and types of shoots (vegetative, reproductive, and seedling) were counted semi-monthly with comparisons made between experimental and control plots. In the second year of the study additional plots were denuded and sown with Z. marina seeds to evaluate the importance of seedlings in the recovery process. Our results suggest that recovery in both low and high intertidal plots was due exclusively to rhizome growth from adjacent perennial eelgrass. This recovery began immediately within the permanent eelgrass meadow and was complete within 24 months. Denuded transition zone patches took almost twice as long to recover to control densities. Natural seedling production appeared to play no part in recovery. When larger numbers of seeds were planted into denuded plots in the winter, only a small portion of them produced seedlings in the spring and none appear to survive through the summer. These experimental eelgrass recovery rates were similar to recovery rates determined from aerial photographs of an eelgrass meadow damaged by a boat grounding near the study site. We suggest that transition zone perennial patches are more vulnerable to natural and anthropogenic disturbance events than lower intertidal eelgrass meadows.