The Recoverability of Zostera Species

Although Zostera species are fast-growing and relatively short-lived, they can take a considerable time to recover from damaging impacts - if recovery is possible at all. Holt et al. (1997) estimated that Zostera species recoverability is within the range of five to ten years but, in many cases, recovery may take longer. This is borne out by the slow or apparent lack of recovery from the 1920s to mid-1930s wasting disease epidemic.

In the management of a marine SAC with Zostera biotopes, it is important to consider the factors required to facilitate the recovery, maintenance and expansion of the Zostera beds.

1. Factors that may limit or facilitate recovery

The factors and processes that control the successful development and consolidation of eelgrass beds are not yet fully understood and further research is required.

When Zostera plants reproduce sexually, seed production can be high. Despite reports of generally high germination success in the field (Churchill, 1983), Olesen & Sand-Jensen (1994a, b) maintained that colonization of new areas is probably restricted by the limited dispersal and the subsequent successful development of seedlings into patches. They reported that seedling development into patches is often unsuccessful or slow.

Within European waters, an improvement in light penetration and an increase in photosynthetically active radiation (PAR), usually resulting from a reduction in turbidity, are considered to be the main pre-conditions for successful recovery (Giesen et al, 1990a; Jonge & Jonge, 1990). Reductions in nutrient inputs, which contribute to reductions in eutrophication and turbidity, are also considered to be beneficial (Olesen & Sand-Jensen, 1994b).

The table below summarizes the major factors believed to influence the capacity of Zostera beds to recover after disturbance or destruction.

 

Factors that may affect Zostera bed recovery

Factors that may limit bed recovery

Factors that may facilitate bed recovery
  • Removal of habitat

Artificial transplantation

Unstable substrata

Stable substrata

Fragmenting and destabilized Zostera beds, caused by factors such as changes to coastal processes, physical damage or stochastic weather events

Stable Zostera beds

Reduced rhizome growth, seed production, germling success and seedling development into patches

Increased rhizome growth, seed production, germling success and seedling development into patches

Reduced light penetration, caused by increased turbidity, eutrophication, some forms of pollution, or epiphyte smothering

Improvements in light penetration, caused by reductions in turbidity, eutrophication, pollution, epiphyte and algal smothering

Nutrient enrichment

Reductions of, or limited increases to, nutrient inputs

Declines in epiphyte grazer populations

Healthy and stable epiphyte grazer populations

Unusual increases in wildfowl grazing pressure

Wildfowl grazing activities may prevent excessive sediment build up in Zostera beds

Competition with non-native species, Spartina sp. and Sargassum muticum

Absence of non-native species, Spartina sp. and Sargassum muticum

Environmental stress, (e.g. extreme temperatures or pollutants), which may increase the susceptibility to wasting disease infection

Absence of environmental stresses and low populations of L. macrocystis, the causative fungal pathogen for wasting disease

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