Davison and Hughes (1998) conducted a review of information on eelgrass beds, including their sensitivity to a number of water quality factors (see Annex B for summary).

Nutrient enrichment: Whilst in some cases local increases in sediment nutrient content may have favourable consequences for Zostera growth (presumably where sediment nutrient content is poor), Davison and Hughes (1998) conclude that nutrient enrichment is more often cited as a major cause of decline, or the lack of recovery, of Zostera beds. Five different harmful effects have been identified, which are not mutually exclusive and several or all of them may apply in any given situation:

high internal nitrate concentrations may cause metabolic imbalance in Zostera;

Zostera may be more susceptible in conditions of nutrient enrichment to infection by

wasting disease (Labrinthula macrocystis);

• increased growth of epiphytic algae as a result of eutrophication is correlated with seagrass loss;
• increased growth of blanketing or floating algae as a result of eutrophication may cause severe shading of Zostera;
• phytoplankton blooms resulting from nutrient enrichment can increase water turbidity, reducing ability of Zostera to photosynthesise;

High nitrate concentrations (daily levels of 3.5 and more µM NO₃^- - N) have been implicated in the decline of mature Zostera marina (Burkholder et al 1992), due, it is suggested, to high internal concentrations causing a metabolic imbalance. Zostera marina was found to be more sensitive than Ruppia maritima, and the effect was exacerbated by heavy epiphyte growth.

Levels of phenolic compounds were found to be lowered in Zostera under conditions of nutrient enrichment, possibly due to a reduction in available carbon within the plant. Phenolic compounds play an important role in providing Zostera with defence against infection, including wasting disease. Burkholder et al 1992 found that plants from enriched mesocosms succumbed to infection by Labrinthula macrocystis, while plants in the control mesocosm remained healthy. >Wasting disease= is quoted as the single most important naturally occurring cause of Zostera decline. It should be noted that Labyrinthula (the fungus which causes wasting disease) does not appear to cause disease in conditions of low salinity.

Eelgrass leaves provide a substratum for the growth of many species of epiphytic algae. Data suggest that epiphyte density on seagrasses (and macroalgae) is a key factor in determining the maximum depth at which such plants can successfully grow; Burt et al (1995) found that light availability to seagrass plants was reduced by between 2 and more than 80% by epiphyte growth, independent of water depth, with the greatest decrease occurring during the main growing season. Epiphytes may also smother the Zostera plants unless kept in check by the grazing activities of gastropods and other invertebrates. Healthy populations of epiphytic grazers are therefore beneficial to the maintenance of Zostera beds.

Other studies have correlated seagrass loss with increased growth of blanketing or floating, as well as epiphytic algae, often as a result of nutrient enrichment (see Davison and Hughes 1998). Blanketing algae such as Enteromorpha, Ectocarpus confervoides and Ceramium rubrum may cause severe shading of Zostera.

Turbidity: Highly turbid water inhibits Zostera growth by reducing the amount of light available for photosynthesis. Phytoplankton blooms, resulting from nutrient enrichment, can increase turbidity and have been shown to reduce the biomass production and the depths to which Zostera marina can grow (Dennison, 1987).

Non-toxic contamination other than nutrient enrichment: The effects of other water quality parameters are less researched. It appears that Zostera can tolerate sea surface temperatures ranging from about 5 to 30EC, with an optimum growth and germination range of 10 to 15EC (Yonge, 1949 in Davison and Hughes 1998). High temperatures (above 15EC) appear to be required for flowering and germination of seeds of Zostera marina (Davison and Hughes 1998). Subtidal populations of Zostera marina which are not subject to lowered salinity produce no or few reproductive shoots (Giesen et al 1990), with laboratory studies indicating that maximum germination of Zostera marina occurs at 1 ppt salinity. However, other field studies indicate that germination in Zostera marina occurs over a wide range of salinities and temperatures (Churchill 1983 and Hootsmans et al 1987). In extreme winter conditions, the formation of ice amongst the sediments of exposed intertidal (and shallow subtidal) eelgrass beds can lead to the erosion of surface sediments and the uprooting of rhizomes, as well as direct frost damage to the plant. Plants may be killed or defoliated by severe frosts.

Toxic contamination: Contamination of coastal waters by heavy metals or antifoulants has not been shown to significantly affect Zostera plants, but agricultural herbicides are known to be harmful. Eelgrass beds do not appear to be highly sensitive to chronic oil pollution, but major oil spills can inhibit growth of plants (Davison and Hughes 1998). In both cases, the associated fauna and flora seem to suffer more damage than the eelgrass itself, in particular where dispersants are also used, which may have repercussions on the Zostera later, owing to reductions in populations of epiphytic grazers and consequent shading of the Zostera.

Heavy metals (mercury, nickel and lead) and a number of organic substances (napthalene, pentachlorophenol, Aldicarb and Kepone) have been found to reduce nitrogen fixation in Zostera roots, which may affect Zostera viability. Zostera marina was found to accumulate Tributyl Tin (TBT), but other studies found that TBT had not caused any observable damage to Zostera plants in the field (Davison and Hughes 1998).

Research on the triazine herbicide Irgarol, used in antifouling paints, on Zostera marina showed that this herbicide is present in the roots and shoots. Triazine herbicides are specific inhibitors of photosynthesis and sublethal effects have been detected (P. Donkin, pers. comm. in Davison and Hughes 1998). The terrestrial herbicide Atrazine has been implicated in declines of Zostera marina in Chesapeake Bay. In another study, exposure to 100 ng/l of this herbicide over 21 days resulted in growth inhibition and 50% mortality of Zostera marina (Delistraty and Hershner 1984). The effects of pesticides on seagrass plants have also been found to be an important cause of their decline in southern England (Asmus & Asmus 1999).

Next section             References