Wasting disease

Wildfowl grazing

Epiphyte grazing

Potentially the greatest natural threat to eelgrass beds is the periodic outbreak of wasting disease, which appears to principally affect sublittoral beds of Z. marina. Between the 1920s and mid-1930s, formerly extensive eelgrass beds on both sides of the Atlantic experienced significant declines in the first recorded major outbreak of the disease. By the end of this outbreak, wasting disease had been reported throughout western Europe. The narrow-leaved form of Zostera (presumably Z. angustifolia) was less affected by the disease, while Z. noltii did not appear to be affected at all (Rasmussen, 1977).

The symptoms of wasting disease are the appearance of rounded, dark brown spots on the leaves, which coalesce until the leaf is completely blackened. The leaves die and detach from the main plant, the regenerative shoots decay and after two or three seasons of this defoliation, the rhizomes discolour and die. The final stages of this disease can be devastating, with up to 90% of the plants being lost and the bed being laid bare.

The indirect effects of the disease were also severe. A variety of characteristic and representative species declined or disappeared, some fisheries declined and a number of beaches and sandbanks, previously protected by eelgrass, experienced increased erosion. The food supplies for overwintering wildfowl (wigeon, Brent geese and swans) were reduced, forcing the birds to migrate to different feeding grounds

Recovery did not begin until the mid-1930s and has generally been slow. A further decline in the Dutch Wadden Sea was reported in the 1970s (Den Hartog & Polderman, 1975; Polderman & den Hartog, 1975; van den Hoek et al., 1983). In the early 1980s, wasting disease reappeared on the east coast of North America (Tubbs, 1995; Short et al., 1986; Short et al., 1988). Between 1987 and 1992, symptoms of wasting disease appeared in several populations in north-west Europe, including estuaries on the southern coast of England and the Isles of Scilly (Fowler, 1992).

The causes of wasting disease have been debated since the 1920s - 1930s epidemic and several causative factors were suggested, including a number of fungal, bacterial or protozoan pathogens. After the 1980s outbreak, research in America identified the pathogen as the fungus Labyrinthula macrocystis. Muehlstein et al. (1988, 1991) showed that Labyrinthula does not generally cause disease in low salinities, explaining why UK populations of the intertidal Z. angustifolia and Z. noltii appear to have been relatively unaffected by wasting disease.

It is likely that the causative factor, presumed to be L. macrocystis, persists at low, harmless levels within Zostera marina populations between epidemics. The reasons for the disease outbreaks are not fully understood (Giesen et al., 1990a, b), but it is possible that Zostera plants only succumb when stressed by other environmental factors such as low levels of insolation, increases in water temperature, or pollution (Short et al., 1988). The disease may occur periodically, in an unredictable long-term cycle whose triggering factors remain to be identified.

Several studies in Britain have monitored changes in eelgrass populations in relation to grazing by overwintering wildfowl, particularly wigeon and Brent geese. Zostera is an important food source for wildfowl, providing a concentrated and nutritious food supply that quickly replenishes energy reserves expended during migration. As overwintering wildfowl numbers can fluctuate from year to year, often related to weather patterns, the grazing pressure on Zostera can be highly variable. When migrant birds arrive at their overwintering site, they generally preferentially feed on eelgrass and only switch to algae when the Zostera resource becomes exhausted. Wyer et al. (1977) suggested that Z. noltii is the most important of the three species. It retains its leaves well into the winter, unlike the other two species which begin shedding their leaves in the late autumn. As Z. noltii is found highest up the shore, the low water grazing period is longer.

Wigeon nip off the eelgrass, blade by blade, without much waste. Brent geese tear up parts of the plant and the material they do not consume floats away on the surface. However, when they stop feeding directly on the eelgrass beds due to the rising tide, they may later locate and feed on this floating ‘reserve’ material (Butcher, 1941a). Swans tear up large quantities, with the rhizomes attached, but do not consume all the plant material disturbed. Madsen (1988) found that geese feed preferentially on above-ground material and only shift to the below-ground material at lower Zostera densities. However, in Strangford Lough, Portig et al (1994) found that the impact on the below-ground biomass occurred as soon as birds arrived, as the Zostera occurs on thixotropic mud which liquefies on disturbance, making it easier for the birds to paddle and dig for rhizomes.

Grazing wildfowl can consume a high proportion of the available standing stock of Zostera. Portig et al. (1994) found that in Strangford Lough, 65% of the estimated biomass (~1100 tonnes fresh weight) of Zostera was consumed by grazing wildfowl but that up to 80% was disturbed by their feeding activity. The above-ground biomass (~330 tonnes fresh weight) was reduced by 93% while the below-ground biomass (~770 tonnes fresh weight) was reduced by 74%. Tubbs and Tubbs (1983) reported that Brent geese grazing resulted in the cover of Z. marina and Z. noltii being reduced from 60-100% in September to 5-10% between mid-October and mid-January. Jacobs et al. (1981) estimated that grazing wildfowl consumed 50% of the total standing stock of Z. noltii at Terschelling in the Dutch Wadden Sea. Madsen (1988) reported that in the Danish Wadden Sea, dark-bellied Brent geese consumed 91% of the Zostera biomass in consecutive years.

At Lindisfarne, Northumberland, Percival (1991) reported that grazing pressure did not affect the percentage cover of Zostera until late winter and that most of the loss appeared to be due to other factors, particularly wave action during storms. It appears that Zostera can recover from ‘normal’ levels of wildfowl grazing (Charman, 1979; Madsen, 1984; O’Brian, 1991; Ranwell, 1959; Tubbs & Tubbs, 1982), but if a bed is stressed by other factors it may be less able to withstand grazing pressure. An example of this was reported by den Hartog (1994b) who found that Brent geese may have removed the few remaining healthy plants that survived after beds of Z. marina / Z. angustifolia in Langstone Harbour had been overwhelmed by growth of the alga Enteromorpha.

It was noted elsewhere that epiphyte grazers such as Hydrobia ulvae can contribute to the health of Zostera plants by removing the algae which foul the eelgrass leaves. Any factors (natural or anthropogenic) which reduce grazer populations or cause increased proliferation of algae may therefore have an indirect adverse impact on the Zostera bed. The factors most likely to cause such changes are pollution incidents (causing grazer mortality) or excessive nutrient enrichment (causing eutrophication). These processes are most likely to occur as a result of human activities and will therefore be discussed more fully in the section on impacts by human activities.

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