Habitat preferences of British Zostera species
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.
||Coarse sediments, sand - fine gravel
Mud or muddy sand
Typically on mixtures of sand and mud.
||Subtidal, typically to 4 m.
Intertidal, mid- to low-tide mark. Rarely
to 4 m.
Intertidal, never found below low-tide
||Intolerant of dessication
Occurs intertidally, but typically in
Most resistant to dessication. Occurs
higher on the shore than the other species
||Avoids brackish water
Typically in conditions of variable
Can occur in variable salinities
Substratum type, water movement and stability of Zostera
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.
Light, depth and water clarity
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.
Temperature and dessication
It appears that Zostera can tolerate sea surface temperatures
ranging from about 5 - 30o 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 20oC without showing signs of
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.,
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.