The three British species of Zostera differ slightly in their typical depth, substratum and salinity preferences (Stewart et al., 1994) . These are summarized below and discussed in greater detail in the succeeding sections.

Substratum type and water movement are considered together because of the close linkage between sediment grade and the degree of exposure to tides and currents. Finer sediments will generally occur in situations of lower water movement. As illustrated in the location maps, site descriptions and MNCR biotope classifications from Chapter I, all three Zostera species require sandy to muddy substrata and sheltered environments, such as enclosed bays or coastal areas with a gentle longshore current and tidal flux. Dense swards of Zostera are typically found on muds and sands in sheltered inlets and bays, estuaries and saline lagoons. In more unstable, higher energy (wave or current exposed) sites, the beds tend to be smaller, patchier and more vulnerable to storm damage.

Olesen & Sand-Jensen (1994a) reported that in Danish waters, new Z. marina beds took at least five years to become established and stable, and that the survival and viability of the bed was strongly influenced by its size. Small patches with less than 32 shoots showed high mortality, but as the sizes and ages of the patches increased, mortality declined. Once established, a dense bed of Zostera plants reduces current flow, leading to increased deposition of suspended sediment and organic detritus (much of the latter derived from the plants themselves). This enhanced deposition rate, together with the sediment-binding effect of the rhizome network, reduces erosion and acts to stabilize the substratum. Conversely, if an established, continuous bed becomes fragmented for any reason, the bed will tend to become less stable and more vulnerable to the normal forces of erosion. Channels may form, the cover may become patchier and if the trend continues, isolated patches will develop which are more likely to be washed away. It would appear that there is a threshold of loss, below which destabilization and further losses of beds can occur (Holt et al., 1997).

Ranwell et al. (1974) proposed that Zostera requires a sediment regime that closely balanced between the forces of erosion and accretion. They monitored sediment levels during transplant trials in Norfolk and found that during the summer growth period, the intertidal Zostera patches accumulated about 2 cm of silt, so that the patches became slightly raised. However, sediment accreted during the summer was lost when the leaves died back in the autumn and winter. Such a balance is potentially important for Zostera plants. For example, in subtidal beds of Z. marina, sedimentation can cause the level of the bed to rise, resulting in plants growing closer to the surface and increasing the likelihood of the plants being partially exposed at low tide and subject to higher temperatures and dessication in summer. At the other extreme, Portig et al. (1994) suggested that where sediment deposition exceeds sediment erosion, eelgrass beds could potentially become smothered.

Zostera beds tend to be reasonably stable once established, especially where the beds are formed by the perennial Z. marina. The more exposed intertidal beds of Z. angustifolia and Z. noltii are in general more susceptible to environmental fluctuations and episodic events such as severe storms or floods. The shallower-growing plants may also be more susceptible to the side-effects of human activities, such as increased sedimentation from large-scale dredging or other coastal engineering projects.

These three interlinked factors will influence the depth to which Zostera is found. Like all plants, Zostera requires a particular light regime to photosynthize and grow. The amount of sunlight that filters through the water column (irradiance) is reduced as water depth increases, and is also affected by the clarity of the water. Turbidity affects Zostera growth by significantly reducing light penetration, thus restricting the amount of photosynthetically active radiation available to the submerged plants. Increases in turbidity are a commonly cited factor in the decline of eelgrass beds, particularly those of Z. marina (e.g. Giesen et al, 1990a, b).

Around the British Isles, Z. marina typically occurs down to 4 m but may extend deeper in some locations (Stace, 1997). In the very clear waters of Ventry Bay, south-west Ireland, Z. marina occurs in a continuous bed from 0.5 m to 10 m, and in patches to a maximum depth of 13 m (Whelan & Cullinane, 1985), probably the deepest-growing Zostera in north-west Europe. Off the north-west American coast, the maximum depth at which eelgrass has been recorded growing is 6.5 m. However, in the extremely clear water off the Californian coast, eelgrass has been found growing at depths of more than 30 m (Teal, 1980).

Jimenez et al. (1987) found that Z. noltii is better adapted to high light intensities than Z. marina (= Z. angustifolia ?) and this is probably one of several adaptations that allows Z. noltii to occur higher up the shore than Z. angustifolia.

It appears that Zostera can tolerate sea surface temperatures ranging from about 5 - 30^o C, with an optimum growth and germination range of 10 - 15 ^oC (Yonge, 1949). Young concluded that the northern distribution of the genus was controlled by this breeding temperature ‘window’ and that the southern limit was set by the direct effect of heat upon the plant. Den Hartog (1970) stated that Z. marina generally tolerates temperatures up to 20^oC without showing signs of stress.

Although Z angustifolia and Z. noltii are more adapted to intertidal conditions and can tolerate a broader temperature range than Z. marina, their upper shore habitat renders them more exposed to extremes of cold or heat when exposed at low tide or in very shallow bays. Den Hartog (1987) suggested that cold winters can result in significant losses. In extreme winter conditions, the formation of ice amongst the sediments of exposed intertidal eelgrass beds can lead to the erosion of surface sediments and the uprooting of rhizomes, as well as direct frost damage to the plant. Critchley (1980) reported that intertidal Zostera beds at Bembridge, Isle of Wight were damaged by frost. Covey & Hocking (1987) observed that in the Helford River, during exceptionally cold weather in January 1987, ice formed in the upper reaches of the mudflats and led to the defoliation of Z. noltii (the rhizomes survived).

With regard to dessication, Z. noltii is typically found on areas of intertidal sediments that drain well while Z. angustifolia dominates areas where water is retained (Duncan, 1991; Fox et al., 1986). Zostera marina grows mainly in the shallow sublittoral, and is less resistant to desiccation. Tutin (1938) suggested that this may be due to the rigidity of the base of the plant, which results in a short length of stem being exposed to the air during very low tides. Thirty minutes exposure on a warm, sunny day can kill the base of the leaves. Zostera angustifolia is less susceptible to desiccation because its flexible shoots lie flat at low tide when unsupported by water and it tends to grow in waterlogged sites. Zostera noltii occurs higher up the shore than the other two Zostera species and is the best adapted to coping with aerial exposure and desiccation. In well-drained sites Z. noltii may dry out completely twice a day.

Mature Zostera plants have a wide tolerance to salinity changes. McRoy (1966) reported optimum salinities of 10 - 39 parts per thousand, while den Hartog (1970) reported tolerance of 5 parts per thousand. in the Baltic. Subtidal populations of Z. marina that are not subjected to lowered salinity produce few or no reproductive shoots (Giesen et al., 1990b). Laboratory studies indicate that maximum germination in Z. marina occurs at 1 part per thousand salinity (Hootsmans et al., 1987). This low salinity figure is surprising as Z. marina occurs almost exclusively in fully saline conditions. However, field studies indicate that germination in Z. marina occurs over a range of salinities and temperatures (Churchill, 1983; Hootsmans et al., 1987).

Nutrient uptake by Zostera from the water column occurs through the leaves and from the interstitial water via the rhizomes. Nitrogen is usually the limiting element and is most easily absorbed as ammonium. In sandy sediments, phosphate may become a limiting factor due to its adsorption onto sediment particles (Short, 1987).

In the laboratory, Roberts et al. (1984) found that moderate nutrient enrichment of the sediments stimulated the growth of Z. marina shoots. Tubbs & Tubbs (1982) observed that an increase in Zostera beds paralleled an increase in the nutrient input to the Solent. However, excessive nutrient enrichment has been cited as a factor in the decline of Zostera beds in many parts of the world. This issue is considered in further detail in Chapter V.

Next Section                 References