Status of Zostera species in the UK

Basic ecology of Zostera spp.

Importance of Zostera biotopes

‘Seagrass’ is a common name for a large group of higher flowering land plants that have spread into the marine environment in relatively recent geologic times. They are the only group of flowering plants that are truly marine and can function and reproduce under conditions of permanent or cyclic submergence in saline water. Den Hartog (1970) recognized a world total of 49 seagrass species. In temperate waters there are ten species of the genus Zostera and two species of Ruppia. Five seagrass species are found around the British Isles - two species of ‘tassel weed’ (Ruppia maritima and R. cirrhosa) and three species of ‘eelgrass’ (Zostera spp.). This report focuses on the three British Zostera species, which are as follows:

This is the largest of the three species, with leaves up to 1 m long (more usually 20 - 50 cm). It occurs from the shallow sublittoral to the lower littoral zone.

This is a smaller plant than Z. marina (leaf length 15 - 30 cm) and is typically found on the mid to lower shore.

This is the smallest and hardiest species and occurs highest on the shore, often adjacent to saltmarsh communities. Maximum leaf length is about 22 cm.

Further details of the physical attributes of these three species are given in

Appendix 1.

There is some disagreement concerning the taxonomic status of Z. angustifolia. This form was given specific status in 1942 based on differences (from the more common Z. marina) in morphology, reproductive strategy and habitat zonation. Populations conforming to the definition of Z. angustifolia used in Britain have been recorded in continental Europe and on the Atlantic coast of North America (den Hartog, 1970; Rae, 1979; Nienhuis, 1983; de Heij & Nienhuis, 1992; Cleator, 1993). However, outside the UK, most authors have regarded these narrow-leafed intertidal eelgrasses as a phenotypic variant of Z. marina rather than a distinct species (den Hartog, 1970). Many of the morphologic characters used to define Z. angustifolia are known to vary according to habitat and season. Initial results of DNA sequencing work undertaken at the Royal Botanic Garden, Kew, supports the hypothesis that Z. marina and Z. angustifolia are variants of a single species (J. Brenchley, pers. comm.).

For the purposes of this report, Z. angustifolia will be treated as a distinct entity, since it is so regarded in most of the UK-based literature. If this form is confirmed as a species separate from Z. marina, with a distribution largely confined to the British Isles, the need for appropriate monitoring and management in the UK increases due to its relative rarity within Europe. If, as appears more likely, Z. angustifolia is found to be a variety of Z. marina, then this apparent rarity could be considered to be less important (Cleator, 1993). Assessment of the distribution and status of eelgrass species in the UK is hindered by misidentification, which renders some historical records suspect (Kay, 1998). A clarification of the taxonomic status of Zostera species and a re-examination of specimens would contribute to an increased understanding of the distribution and habitat requirements of the three forms, with important implications for their future management.

All three Zostera species can occur as dense swards on intertidal and shallow subtidal muds and sands in sheltered shallow inlets and bays, estuaries and saline lagoons. Zostera can also be found on more exposed areas of intertidal mud and sand flats, as well as shallow subtidal sandbanks. As a result of this habitat flexibility, Zostera species are widely but patchily distributed throughout the British Isles and extensive beds occur in some areas. Within Britain, mixed beds of Z. angustifolia and Z. noltii commonly occur together on the shore but exhibiting distinctive distribution patterns - Z. noltii typically occurs on hummocks that are free-draining at low tide, while Z. angustifolia occupies hollows that retain standing water at low tide (Wyer et al, 1977). In some lagoons, all three Zostera species may be found together, along with Ruppia species. The detailed habitat requirements of Zostera are summarized in Chapter II.

Zostera plants have an extensive network of branched, creeping underground roots that help bind the sand or mud substratum. These horizontal rhizomes bear leafy shoots with abundant green, grass-like leaves. The leaves are flat and linear, with maximum length and width varying according to species (see Appendix 1). Distinct veins run the length of the leaves. The leaves have large air spaces (lacunae) between the cells which act as buoyancy chambers and keep the leaves upright in the water (the name Zostera comes from the Greek ‘zoster’, meaning ‘belt’, referring to the ribbon-shaped leaves). Zostera plants have inconspicuous flowers which lack petals and are aggregated in inflorescences. Male and female flowers are separate but occur on the same plants. Despite these common features, all three Zostera species exhibit considerable morphological plasticity in response to environmental conditions. Growth and reproduction of Zostera are considered in greater detail in Chapter III.

Zostera beds are an important source of food and shelter for the young stages of many fish and crustacean species, some of which are themselves food for commercially-valuable fishery species. They are also important feeding grounds for ducks and geese sought after by wildfowlers.

In addition to the value of the associated fauna, the eelgrass plants themselves play an important role in maintaining the stability of the shoreline. The dense network of rhizomes binds the sediment and reduces erosion in shallow waters. If beds are locally destroyed, their protective capacity can only be replaced by financially costly artificial shoreline reinforcements.

The network of roots and leaves in an extensive Zostera bed provides ecological niches for a wide range of associated fauna and flora, so that these biotopes are important in maintaining coastal biodiversity. Eelgrass beds exhibit high rates of primary productivity and are an important source of organic matter, fuelling detritus-based food chains within the biotope. Organic matter transported out of Zostera beds can also be utilized in other biotopes, in some cases far removed from the point of origin. The role of Zostera beds in coastal and marine ecosystems will be discussed further in Chapter III.

Zostera biotopes were selected for the UK Marine SACs Project not only because of the values noted above, but also because they were in the past a widespread feature of Britain’s nearshore margin, hence they are a significant element of Britain’s natural marine heritage. Although all three Zostera species are still widespread today, they are now considered to be nationally scarce with a patchy distribution (see below). Although there has not been a comprehensive inventory, the existing data and previous records indicate that Zostera beds have made a poor or slow recovery from the impact of wasting disease in the 1920s -1930s. This same situation is generally true for other areas where Zostera was once common, such as along the Atlantic seacoast of North America.

Furthermore as human settlement along Britain’s coast has increased (both in terms of population and scale of physical alteration to shorelines), especially in estuarine areas, the Zostera meadows in these areas have been subjected to increased direct (e.g. dredging, filling) and indirect (e.g. upstream channelization resulting in increased sedimentation) impacts. They are therefore a high priority for monitoring and conservation management.

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