Scottish Seagrass Communities of the Genus Zostera

Introduction

Seagrasses comprise < 0.02% of the angiosperm flora, representing a surprisingly small number of species, of which the principal genus is Zostera spp (Heeminga & Duarte 2000). Zostera occur widely in coastal zones throughout the world and therefore are integral to decisions about coastal management. Zostera can be biogeographically classified as temperate North Atlantic (Heeminga & Duarte 2000). All three species of the genus, Zostera marina L. (eelgrass), Zostera noltii (dwarf eelgrass), and Zostera angustifolia (narrow-leaved eelgrass) are found in Scottish coastal waters and are increasingly becoming threatened (Cleator 1993; UK BAP 2006).

Zostera species are perennial angiosperms restricted to the intertidal and shallow subtidal fringe. The tolerable depth limit for Zostera species is set by light available for photosynthesis. The upslope limit to seagrass extension is imposed by sufficient immersion in seawater or tolerable disturbance by waves. Eelgrass communities will typically zonate in the following littoral regions: Z. marina in the sublittoral fringe; Z. angustifolia in the eu-littoral region; and Z. noltii in the upper littoral zone often adjacent to salt marshes. Seagrasses generally require salinity in excess of 5%o to develop, and a substratum ranging from sandy to muddy. Populations extend mainly by undersediment, vegetative, rhizomatol extension, and may form thick beds. Pollination and seed dispersal (most common in Z. marina) may occur via entrainment into water currents (Cleator 1993; Dawes 1981).

It is recognised that seagrass communities bring many benefits to the coastal environment, that are imperative in the maintenance of biodiversity, the sustainability of industry, and the protection of the coastline. Seagrass communities are, however, also vulnerable and have declined substantially in many coastal areas due to natural and human pressures. During 1930s the Scottish west-coast Z. marina population catastrophically declined due the wasting disease epidemic (Cleator 1993).

It is recognised that the role seagrass communities play in the sustainability of Scottish coastal marine ecosystems and the services they provide to coastal regions is of exceptional significance (UK BAP 2006). Furthermore, for effective conservation, detailed ecological understanding is required. This paper is targeted at environmental managers and key stakeholders within the coastal region. Included in the paper is: a compilation of information about what seagrasses are; highlights the importance of seagrass communities in Scotland; factors controlling their productivity; the potential threats to the seagrass ecosystem; and the required actions in order to achieve the continued existence and restoration of Scottish seagrass ecosystems.

Taxonomy and Distribution Zostera

The limited number of marine angiosperm flora compared to freshwater and terrestrial environments suggests important constraints on colonisation in the marine environment. There are approximately 60 known seagrass species worldwide. In the U.K and Scotland only three closely related seagrass species occur. The family Zosteraceae are monocotyledonous angiosperms encompassing two genera, Zostera and Phyllospadix (Cleator 1993). This paper will consider the three species of the genus Zostera, the principle genera of North Atlantic waters. 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 (Rodwell 2000).

All three species are found in Scottish coastal waters forming Zostera communities, SM1 classification (Rodwell 2000). There are non significant Zostera populations between Berwick and Dunbar and between Montrose Basin and the Moray Firth. This is assumed to be mainly due to a lack of suitable habitat (Cleator 1993). Principle concentrations of Zostera communities are along the western coast of Scotland and within the Moray Firth. There is no information on the total extent Zostera communities in Scotland (Cleator 1993). Obtaining this information is a priority in the development of conservation plans for Zostra. The British Zostera species have the following features in common: they are clonal with creeping rhizomal roots; several alternate, distichous, linear leaves (Davison & Hughes 1998). Flowering stems are lateral or terminal and the inflorences enclosed in a spathe (Rodwell 2000). Reproductive strategy of Zostera species can be related to shore height (Cleator 1993).

Zostera marina

Z. marina demonstrates distinct ecotypic variation and plasticity. European habitats of Z. marina range from arctic waters of the northern Norwegian coast, where it can survive several months of ice cover, to the Mediterranean (Rodwell 2000). Z. marina (Figure 1.1) is now abundant down the western coast of the Scottish mainland and the Outer Hebrides and has therefore recovered substantially from the wasting disease of the 1930s (Figure 1.2) (Rodwell 2000). Z. marina is also found in the Moray forth. Z. marina forms stands with a cover of trailing leaves up to 1m long. Algae, especially Enteromorpha spp., are common associates. They are species that are fully marine, are occasionally are found in estuaries, also occurring in lagoons (Rodwell 2000). They essentially inhabit the sub-littoral fringe, extending from 1-4m (depending on water clarity) below to just above low water of spring tides, and on sand or sandy mud firm substrate. Upper distribution is within the littoral zone is limited by susceptibility to desiccation, and lower limits set by light available for photosynthesis (Heeminga & Duarte 2000). Z. marina demonstrates a range of morphological and reproductive characters dependent on the habitat in which they grow. They demonstrate a decrease in leaf size and density upshore. Narrow-leaved plants from the lower eu-littoral have been confused with Z. angustifolia (Cleator 1993).

Z. marina also demonstrates variation in phenology in relation to the position of the plants on the shore. Leaves of Z. marina of the eu-littoral are found senesce during autumn and early winter, while permanently submerged plants are perennial. Vegetative growth occurs in the subtidal populations, and growth by seed production is rare. Flowering can occur in eu-littoral varieties, however is not known for any Scottish populations. Vegetative growth in perennial Z. marina occurs in two phases. The summer leaves die in the autumn and smaller winter leaves grow in their place. During spring these die back and are replaced by the larger summer leaves (Cleator 1993).

Generally, shoots have leaves 3-7 leaves. Leaf width varies from 2mm in juveniles and up to 10mm in large individuals. The leaf bundles are terminal shoots on horizontal rhizomes. For each new leaf produced, a rhizome segment (internode) is formed and two bundles develop from nodes between the segments. Roots are thin and are covered with root hairs up to 20cm long. Internodal length varies from 5-40mm (Borum et al. 2004).

Figure 1.1. Zostera marina (eelgrass). Sublittoral, marine angiosperm. Species is identified by terminal shoots on the horizontal rhizome (Dawes 1981).

Figure 1.2. Geographical distribution of Zostera marina in Scottish coastal waters. Eelgrass is predominantly found along the Scottish western seaboard (Adapted from Rodwell 2000).

Zostera angustifolia

Z. noltii and Z. angustifolia typically occur on the firths of the Scottish east coast (Figure 1.3): The Cromarty Firth supports what is most probably the largest total area of dwarf eelgrass and narrow leaved eelgrass in Britain (approximately 1200 ha) (Rodwell 2000). Z. angustifolia is largely absent form the Scottish west coast and the population was never affected by the wasting epidemic. There are populations being found along the southern shore of the Firth of Forth, near Dunbar, and north east Fife in the Eden estuary and at Tayport. Further populations are found in the west coast at Mull and the Solway Firth. Outside the British Isles, Z. angustifolia has only been recorded in Denmark and Sweden (Cleator 1993). Form stands with a cover of slender, trailing leaves up to 25cm long and 2-3mm wide. It may occur mixed with Z. noltii and with various green algae (Rodwell 2000). This species can adopt an annual strategy. The rhizomes and leaves both die. In the Moray Firth species suffer heavy die-back during the autumn and early winter from natural shedding. Die back may also occur form stochastic events such as storm damage, and wildfowl grazing. Seed production, germination and recruitment from seedlings appears to be more important in Z. angustifolia than other Zostera species (Cleator 1993). Z. angustifolia of the Tyne are perennial and propagate by vegetative means (Rae 1979). They have been confused with Z. marina. As Z. angustifolia do not rely on animals for pollination, flowers are minute, green and petalless, with male and female flowers separate. They are in spikes and enclosed partly within the leaf sheath. Pollination occurs underwater (Cleator 1993). Seeds are 2-4mm long and when developed detach with flowering shoots. The seeds either settle on the substrate or dispersed along with floating shoots. Grazing wildfowl may also disperse the seeds.

This species is most characteristic of the lower and middle eu-littoral zone, extending well above the low tide mark, where sites are never deeply submerged at high tide nor ever fully dry at low tide. Found in water filled hollows in the littoral region exposed at low tide, Z. angustifolia prefers firm or sloppy muds and muddy sands (Rodwell 2000).

Zostera noltii

In Scotland, the geographical distribution of Z. noltii parallels that of Z. angustifolia (Figure 1.3), with the exception that significant populations lie from Argyll south (Cleator 1993). Outside the British Isles, Z. noltii can be found from the southern Norwegian coast down to the Mediterranean. Z. noltii (Figure 1.4) forms dense beds, with a cover of trailing, narrow and flexible leaves up to 20cm long. This seagrass species is characteristic of muddy sand in the upper littoral zone, where Z. marina is absent due to its intolerance to desiccation. It may occur also in a patchy mosaic with Z. angustifolia. Communities may grade into salt marsh members in the upper fringes of the populations extension. They can tolerate more brackish waters than the other species, however will not inhabit the subtidal. Z. noltii is also out competed in the subtidal zone (Rodwell 2000). Z. noltii resists desiccation during low tide when they are fully exposed to air. This is achieved as there flexible leaves flat on the substratum, retain a film of water keeping the vegetation moist during the tidal cycle (Borum et al. 2004).

Figure 1.3. Geographical distribution of Zoster angustifolia and Zostera noltii in Scottish coastal waters. These two species are characteristic of the eastern firths, with a significant population in the Cromarty Firth (Adapted from Rodwell 2000).

Figure 1.4. Zostera noltii (dwarf eelgrass). A species of the middle and eu-littoral zone.

The species is best identified by the many small shoots with narrow leaves attached by short branches to the horizontal rhizome (Borum et al. 2004).

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Figure 1.4. Zostera noltii (dwarf eelgrass). A species of the middle and eu-littoral zone.

The species is best identified by the many small shoots with narrow leaves attached by short branches to the horizontal rhizome (Borum et al. 2004).

Like Z. angustifolia, Z. noltii may also undergo a marked die-back during the winter and may suffer complete foliage loss. Unlike Z. angustifolia spring regrowth is mainly attributable to regeneration from the rhizome opposed to germination from seed set (Loques et al 1990). It has been observed in Z. noltii populations in the Moray Firth that seed production does not generally play an important part in its life history (Rae 1979).

Z. noltii has small leaf bundles with 2 to 5 narrow leaves attached to a horizontal rhizome. Each rhizome holds many shoots on short branches separated by rhizome segments (Rodwell 2000).

Taxonomic confusion

There is considerable confusion over the taxonomic status of Z. angustifolia. As mentioned, Z. marina exhibits ecoplasticity in growth forms, and is found in both subtidal and intertidal beds. It may therefore be confused with Z. agustifolia, as this species exhibits the same characteristics as the intertidal Z. marina (Cleator 1993). Some workers believe Z. angustifolia to be a phenotypic variant of Z. marina rather than a distinct species. DNA sequencing work supports the hypothesis that Z. marina and Z. angustifolia are variants of single species(Hughes & Davison 1998). However for the purpose of this report, Z. marina and Z. angustifolia will be regarded as distinct entities.

Growth and Reproduction

Vegetative growth

All species of Zostera share a similar architecture, being clonal, rhizomatous plants. As previously mentioned, Zostera partitions a large proportion of biomass in the rhizome. The rhizome network extends horizontally below the sediment surface. Zostera exhibit modular growth comprising repeating, clonal units. Each unit is composed of a set of modules: a rhizome; a bundle of leaves attached to the rhizome; a root system; fruits and flowers depending on season and variety. The rhizomes are composed of internodes, and at each internode of the horizontal rhizome a vertical leaf shoot sprouts. The rate of formation of seagrass leaves, rhizomes and roots, and, therefore, the spread of the clone, depends on the activity of meristems (Borum et al. 2004).

Clonal growth is a fundamental component of the production and space occupation of seagrasses, particularly in the colonisation of new habitats and recovery from perturbations to the environment and die-back. In addition to rhizome elongation, occupation of habitat space is also accelerated by the branching growth of the rhizome. An additional point is that, rhizome propagules can break off the parent plant and be carried away by currents and may generate new plants if deposited on a suitable substratum (Olesen & Sand-Jensen 1994).

Sexual reproduction

Seed dispersal in plants is generally presumed to provide for longer distance dispersal and colonisation capabilities than vegetative propagation. Colonisation of new, unvegetated areas, or recolonisation of disturbed areas that may be spatially separated from existing beds, will depend on seed dispersal capabilities (Orth et al. 1994). A variety of mechanisms have evolved to take advantage of the water to facilitate sexual reproduction. In all three species, flowers and seeds are generally produced between early May and early September (Tubbs & Tubbs 1983). Zostera flowers are highly adapted to optimize pollination efficiency in an aquatic environment (Ackerman 1986). The male flowers release long strands of pollen into the water. The buoyancy of these pollen filaments enables them to remain at the depth at which they were released for several days, so increasing the likelihood of the pollen filaments encountering female flowers. After fertilisation, the seed develops within a green membranous wall which photosynthesises, producing a small bubble of oxygen that is trapped inside the seed capsule. Eventually this forces the capsule wall to rupture, releasing the mature seed (Orth et al. 1994).

Current and wave mediated transport at the time of release may disperse seed and bedload transport may redistribute them once the seed have settled on the bottom. Rare storm events may be important for distributing seeds. Reproductive shoots with seeds that are detached from the grass bed (rafting) has been suggested as an important dispersal mechanism. Waterfowl have been suggested as a possible vehicle for seed dispersal. Seeds generally take about 4-5 months to germinate, giving ample time for a number of dispersal mechanisms to affect where the seed will be distributed. A significant fraction of the seeds release may be consumed by grazers such as crabs. Seeds are generally not viable beyond their first generation so no seed bank exists (Borum et al. 2004; Orth et al. 1994).

As mentioned earlier, Z. anngustifolia relies upon distribution via a sexual means as a reproductive strategy, while Z. noltii tends to favour vegetative reproduction. Flowering is controlled by temperature, with Z. marina requiring a temperature greater than 15ºC to flower. This suggests that flowering does not play an important role in the life-history of Z. marina populations in northern latitudes (Tutin 1942; Cleator 1993).

Germination is severely affected by physical conditions. In Z. marina, germination experiments have revealed that optimum seed germination conditions can be achieved at very low salinities, at 30 degrees and 1%o (Cleator 1993). This is a little surprising, as Z. marina occurs exclusively in fully saline conditions. It is also possible that a period of low temperature (<15°C) is required to initiate germination. While it has been demonstrated that optimum germination is achieved at low salinities, experiments with Z. marina demonstrated that optimum seedling survival is obtained at full strength seawater. This is supported by the fact that Z. marina populations are less frequently distributed in estuaries (Hootsman et al. 1987).

In Z. noltii conditions for seed germination are identical to that given for Z. marina, although seedling development was enhanced by reduced salinity, reflecting the tolerance for low salinity associated with this species (Tutin 1942).

Growth rates

The time interval required to recover after severe die-back and the ability to form meadows depends largely on the rate at which the meadow can extend and produce. Furthermore, growth rates are important in the planning of efforts to restore diminished Zostera communities. Generally leaf elongation per shoot is rapid, in the order of a 1-3cm per day. The growth rates of Zostera rhizomes are greater in the smaller Z. noltii species than in Z. marina (Table 1.1). Furthermore, branching of the rhizome is more profuse in Z. noltii than in Z. marina. As Z. noltii extends its rhizome at a rapid rate and branches to a greater extent than Z. marina, it will be able to spread in two dimensions more quickly. Smaller species like Z. noltii produce leaves much faster (13.71 days) than that of Z. marina. These high rates of production and growth allow Z. noltii to play a pioneering role in coastal seagrass communities, and recover and sustain during high levels of disturbances. Z. marina has intermediate rates of growth compared to other seagrass members. It has the potential to spread through vegetative extension, however it is estimated meadow ranges may take up to a decade to develop, compared with a year for Z. noltii (Borum Et al. 2004).

Seagrass growth rates are negatively related to size of the seagrass meadow, as greater resource expenditure is required. Furthermore, growth of the Zostera species can be directly affected by abiotic factors such as climate, nutrient availability, and sediment quality (Heemniga & Duarte 2000). Growth rate of the rhizome is negatively associated with shoot density. Horizontal rhizome elongation and branching rate decrease with increasing shoot density. This mechanism proves important in avoiding shoot mortality in crowding conditions, i.e intraspecific competition (Borum et al. 2004).

Table 1.1. Biological attributes of Zostera marina and Zoster noltii

(Borum et al. 2004). Taken as an average from well selected European

samples. nd: no data

Zostera species store resources carbon and nutrients in their belowground organs that they use during periods of resource scarcity. Z. marina has the largest resource capacity in the rhizome and, therefore, can withstand greater heterogeneity in ambient resource availability (Wetzel & Penhale 1979).

Abiotic factors

Seagrasses are obligate marine dwelling phototrophic plants and are the only angiosperms that are adapted to a marine existence. They require the same fundamental resources as terrestrial angiosperms, however, the conditions for carbon acquisition are considerably different. Generally, the four fundamental habitat requirements are; sufficient illumination, suitable substrate for root anchorage, sufficient immersion in seawater, and a moderate level of wave exposure (Heeminga & Duarte 2000). Some of the factors regulating the growth and distribution of Zostera that will be described can also be affected by humans. The three Scottish species of Zostera differ slightly in their typical depth, substratum and salinity preferences (Davis & Hughes 1998). These are summarised below (Table 1.2).

Table 1.2. The typical habitat requirements for the three Scottish species (Davis & Hughes 1998).

Light, depth and water clarity

Transmission of light through the water column is several orders of magnitude less than through air. Light intensity thus rapidly decreases with water depth. The photosynthetically active radiation (PAR) is limited to a depth of 200m in the clearest oceans. PAR may also be attenuated by particulate and soluble material within the water column. In coastal waters turbidity is generally quite high as there is a greater load of suspended particulate matter due the physical action of the waves and tide. Consequently, light transmission is significantly less than in pure water and therefore the depth limit for PAR in coastal seas mostly vary from less than a metre to several tens of metres. Thus the seagrass distribution in the coastal zone is limited by photosynthesis to quite a narrow depth range. The majority of seagrass stands are confined to depths less than 20m (Heeminga & Duarte 2000). A general relationship between the colonisation depth of seagrasses (ZC, in m) and light attenuation coefficients (K, in m -1) of the water was derived by Duarte (1991). The data indicate that seagrasses roughly can spread to depths receiving more than 11% surface irradiance. However, this relationship is not clearly defined as K may not be stable in coastal environments as turbidity may fluctuate greatly due differences in biotic and abiotic factors. The relationship between K and depth limits is not applicable to all species in all coastal environments as there is variability amongst these. The light regime may be sight specific and influence the distribution of seagrass species differently. Estuaries and river plumes that are loaded with terrestrial borne sediment and in eutrophic waters that permit extensive phytoplankton productivity are examples of conditions which give rise to increased turbidity, and reduced light penetration (Hemminga & Duarte 2000).

Around the British Isles, Z. marina typically occurs down to 4 m but may extend deeper in

some locations (Rodwell 2000). 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 (Whelan & Cullinane 1985).

Substratum type, water movement and stability of Zostera beds

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 (Cleator 1993). 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 (Davis & Hughes 1998). This is because the physical action of waves can tear up plants from the substrate. Furthermore, increased erosion from strong waves and currents can expose the roots and rhizomes causing the plants to detach from the surface. In addition high rates of siltation and deposition can bury seagrasses (Portig et al. 1994). In addition, for rhizome elongation and firm attachment, a soft substrate is required (Heeminga & Duarte 2000).

Carbon

Bicarbonate is the most abundant form of inorganic in seawater (~pH 8) being around 150 times that of CO2 (at 15°C). It is known that bicarbonate use is a common feature in this plant group for photosynthesis. However, it has been observed that under normal conditions, inorganic carbon is limiting to photosynthetic rate, thus indicating an apparently inefficient acquisition system. In comparison to macroalgae, the photosynthesis system of seagrasses is evolutionary inferior. Hence, it has been suggested that seagrasses may profit from an increased atmospheric CO2 associated with increasing use of fossil fuels (Beer & Koch 1996).

Nutrients

Nitrogen and phosphorous are the most limiting nutrients to the growth of seagrass beds. However, per unit mass, nitrogen and phosphorous requirements for seagrasses are approximately 4 times less than for phytoplankton, meaning Zostera can withstand nutrient poor environments. Generally, seawater is a nutrient poor environment. Nutrient uptake occurs through the leaves in the water column. Nitrogen is most readily absorbed in the form of ammonium. In addition to leaf absorption, nutrient uptake occurs in the rhizome through interstitial water in the sediment. Most sediments are nutrient rich due to the mineralization of organic matter (Heeminga & Duarte 2000). Seagrass beds encourage the accumulation of organic matter within the sediment, both from deposition of suspended material and incorporation of detritus. In the laboratory, Roberts et al. (1984) found that moderate nutrient enrichment of sediment stimulated the growth of Z. marina shoots. However, it has been observed in many cases that excessive nutrient enrichment of seagrasses is detrimental to growth (Davis & Hughes 1998). Most Scottish populations of Z. noltii and Z. marina are found in organic sediments (Cleator 1993).

Salainity

Zostera can tolerate a wide range of salinities ranging between 5%o and 35%o (Hemminga & Duarte 2000). In natural populations, it has been observed that the area of contact between seagrasses and salt-tolerant freshwater macrophytes is found at around 10%o in a number of estuaries (Cleator 1993). Salinity affects the osmotic pressure in the cells, with seagrass tissues suffer osmotic stress at low and high salinities leading to cell necrosis and death (Heeminga & Duarte 2000). Furthermore, the dilution causes also a decreased content of inorganic carbon in the water. Species of Zostera differ in their abilities of salinity tolerance and will be a contributing factor in the littoral zone inhabited. Z. noltii typically occurs on intertidal flats and can tolerate great changes in salinity from a few %o to more than 30%o within a few hours. As mentioned previously, field studies indicate that germination in Z. marina occurs over a range of salinities and temperatures (Churchill, 1983; Hootsmans et al., 1987).

Desiccation

Within the Zostera genus, tolerance of exposure to air is species specific. Desiccation is an important factor in determining the shallow, upper growth limits for mature plants of Z. marina. Cleator (1993) suggests that this may be due to the rigidity of the base of the plant, exposing a short length of blade to the air during very low tides. Exposure to air can result in cells necrosis due a loss in osmotic potential, and photodamage by high UV irradiance (Dawson & Dennison 1996). Due to this Z. marina species are confined to distribution in the sublittoral fringe. Z. angustifolia is less susceptible to desiccation than Z. marina. This is partly due the ability of the shoots of Z. angustifolia to be flexible, which lie flat when unsupported by water at low tide (Cleator 1993). While Z. marina is regarded as a sublittoral species, it has been observed that Z. noltii can be particularly widespread in the intertidal zone (Hemminga & Duarte 2000). These areas may completely dry for a period during the tidal cycle. This intertidal seagrass species can resist desiccation as they form a dense, continuous population, with their leaves lying flat on the substrates surface retaining water. They further have the ability to withstand exposure to high levels of UV radiation (Jimenez et al. 1987).

The Importance of Zostera Communities

Seagrass meadows support many commercially important resources (shellfish) and provide ecological services (maintenance of biodiversity, supporting coastal water quality, protection of the coastline), that are directly used or beneficial to humans. Furthermore, it is recognised that seagrasses can act as indicators of environmental conditions providing a useful to tool in environmental monitoring. These attributes are largely attainable to the physical structure of the plants themselves and the underwater meadows they form, their biological activity, and that of the

associated fauna and flora. An E.U directed study (Borum et al. 2004) estimates that the “value of the services provided by seagrass ecosystems produced a minimum estimate of 15837 € ha-1 y-1, which is two orders of magnitude higher than the estimate obtained for croplands.” The large knowledge about the biology and ecology of seagrasses gained during the last third of the 20th century has driven increased awareness of the economic value of seagrasses to humans.

Primary productivity

They are amongst the most productive of all biomes, and estimated global net productivity of seagrasses is approximately 0.6 × 1015 gC yr-1 (Duarte & Chiscano 1999). The majority of photosynthate is allocated the rhizome, and furthermore, biomass is not readily decomposed. This result in seagrasses acting as global carbon sinks, estimating to hold approximately 15% of oceanic carbon (Duarte & Chiscano 1999). They are globally significant in the sequesterisation of global carbon. Furthermore, associated periphytic and benthic algae associated with seagrasses are important primary producers (Borum et al. 2004).

Z. marina beds have been recorded as producing approximately 1 tone of detrital material per Km2 (Bach et al. 1986). In addition, it has been documented that Zostera beds have a fast and continuous turnover aboveground biomass. During the growing season it was observed that that biomass was replaced every 21.4 days (Kentula & McIntire 1986). This continuous export of leaves and shoots constitutes a large loss of biomass from the system and represents a major contribution to detrital food chains. Dead seagrass leaves can be transported by waves and currents into other biological communities, potentially impacting trophic systems in distant regions. Den Hartog (1987a) describes enrichment of upper littoral zone habitats by stranded seagrass material. Furthermore, seagrass detritus has been recorded at depths of nearly 8000m in the water column (Davis & Hughes 1998).

In perennial Zostera populations, the biomass usually follows a distinct seasonal pattern showing minimum values in winter and peak values in the mid-summer following a period of extensive growth.

Associated flora and fauna

Scottish seagrass habitats support various coastal organisms, some of which are threatened. Seagrass meadows consist of a dense leaf canopy and an extensive rhizomatol and root system which provides a substratum for attachment unavailable in unconsolidated bottoms, stabilizes the sediment, and reduces transmitted irradiance, providing a mosaic of microhabitats (Heeminga & Duarte 2000). Due to this structural complexity, seagrass meadows can function as a refuge from predators (Heck & Thoman 1981). This habitat structure, along with high primary production rates, provides the foundation for an abundant food supply (Orth et al. 1984). The water slowing effect that the seagrass meadow has also encourages the deposition of planktonic larvae.

Zostera habitats are important nursery grounds for crustaceans and fish (example, flatfish) due to available substrate attachment, refuges available and reduced motion of seawater (Heeminga & Duarte 2000).The seagrass meadows are important feeding habitats for certain associated water fowl and vast numbers of birds exploit some beds. For example, enormous numbers of wigeon (Anas penelope) are known to gather and feed on Zostera intertidal beds in the Cromarty Firth (Rodwell 2000). The decline of Zostera in the 1930s is held to be responsible for a major reduction in the numbers of Brent Geese (Branta bernicla), which feed on the seagrass during their winter migrations (Den Hartog 1987a). The Cromarty Firth is outside the overwintering range of the brent goose.

The Scottish Zostera habitat also plays a key role in the life-cycles of a number of rare species including Cladophora retroflexa and C. battersii (Rodwell 2000). The Z. marina populations of loch Indaal, Islay, support algal populations which are extremely rare, including the red algae Polysiphonia harveyi only recently recorded in the British Isles (Maggs & Hommersand 1990). Seagrasses also represent a valuable habitat for some shallow water nekton. Two species of pipefish, Entelurus aequoraeus and Syngnathus typhie are almost obligate dwellers of seagrass bed communities (Cleator 1993). It has been suggested that the paucity of some associated seagrass flora and fauna may be due to the 1930s wasting disease epidemic. There is some evidence that Z. marina beds may act as corridor habitats for species migrating north from warmer water. This is based on evidence that a species of wrasse (Labrus turdus), normally associated with the Mediterranean, has been sighted in British coastal waters (Fowler 1992). Due to the great variety of biota associated with the seagrass communities, they are a target for commercial fishing including shellfish harvesting, and clam dredging (Cleator 1993).

Seagrasses as filters improving water quality

Seagrass beds act as natural filters, as the dense leaf canopies dampen water movement promoting the deposition of suspended particles and planktonic larvae. This particle trapping capacity is enhanced by seagrass dwelling, filter feeders. As a result, seagrasses can decrease the turbidity of coastal waters, aiding water quality and light transmission through the water column, which also enhances productivity (Borum et al. 2004).

Seagrasses in coastal protection

In addition to trapping sediments and particles within the leaf canopy, the rhizome matrix, and roots within the seagrass beds stabilise sediment and inhibit the relocation of sediment by the physical action of the tide and waves. This is because the seagrass beds act to reduce water motion and the rhizomatic network acts to bind sediment. In this way sediments vegetated by seagrasses can inhibit erosion of the coastline. The raising of coastlines has been noted by Cleator (1993). These stable raised areas in the sublittoral fringe would cause wave energy to dissipate offshore, thus sheltering the adjacent beech.

Natural Disturbance

Seagrass beds like any biological community have to cope with perturbations, and are particularly susceptible to changes that modify light and sediment quality. The seagrass environment is particularly prone to physical disturbances, whether by waves or from storm events, and as a result the seagrass ecosystem is highly dynamic.

Storms are episodic and often intense in nature and can effect some of the fluctuations observed in the shape and size of seagrass beds (Orthe & Moore 1983). Fowler (1992) reported a degradation of Z. marina beds on the Isles of Scilly following serious winter storms the previous year. Strom events are known the fragment habitats, and could account for the patchy nature of segrass communities. Small-scale, recurrent events like the lapping of waves may be important factors in shaping the community structure, which are characteristically patchy. The waves and currents are capable of redistributing sediment resulting in either burial of the plants or exposure of the roots and rhizome, and tearing and uprooting the plants (Kirkman & Kuo 1990).

The various activities of other organisms can cause fluctuations in the population dynamics of seagrass beds. Mainly the actions of herbivory and competition affect the distribution and growth of the seagrass communities. Grazing by waterfowl is a form of natural disturbance which can cause considerable fluctuations in the seagrass population structure over time. Water fowl graze on leaves and occasionally rhizome on the shallow water intertidal zone. In Langstone Harbour on the Solent, a decrease in leaf cover of Z. marina and Z. noltii from 60-100% in September to 5-10% between November and January was related to grazing by Brent Geese (Tubs & Tubs 1983).

The most severe, natural induced change occurred in the 1930s, when almost the entire North Atlantic population of eelgrass (Zostera marina) was destroyed by the wasting disease in a few years (Cleator 1993). The wasting disease is caused by the marine slime mould, Labyrinthula spp. Currently there is no conclusive evidence as how the epidemics is stimulated, however, it has been suggested that stress and disturbance may reduce the resistance of the plants which leave it vulnerable to infection (Orth et al. 2006). Some causative agents may be low irradiance; temperature; alterations in current flow; and pollution. The symptoms of the disease are black lesions on the leaf surface which quickly spread, causing the leaf to die within a few weeks. After two or three seasons of constant defoliation, the rhizomes will become discoloured and perish (Den Hartog 1987b). The disease has been mainly associated with Z. marina. Studies have proven the Z. noltii is susceptible to infection. However, Z. noltii grows in the upper intertidal environment where Labyrinthula is known to be less virulent and natural levels of infection are correspondingly low (Den Hartog 1987b; Cleator 1993).

Natural disturbance may be an important subsidy in maintaining community diversity as the community is prevented in reaching a stable state with community member dominance. This is characteristic in the dynamics of the seagrass community structure.

Anthropogenic threats

The human impact on seagrass ecosystems has been considerable. Human influence at the land-sea margin has contributed to extensive losses of the vulnerable seagrass communities increasingly over the past 20 years. Globally, the estimated loss of seagrass from direct and indirect human impacts amounts to 33,000 km2, or 18 % of the documented seagrass area (Borum et al. 2004).

A large proportion of the Scottish population live on or adjacent to the coast. As a result pollution, development and recreational activities are increasingly affecting the coastal environment with significant impacts (Cleator 1993; Davison & Hughes 1998). Zostera populations are considerably vulnerable to changes in the environment in relation to eutrophication, sedimentation, and increased turbidity. The impacts on the fauna associated with seagrass habitats may be considerable. Furthermore, the global warming phenomena may induce long-term instability in the seagrass communities (Table 1.3).

With increased human usage of the coastal environment for transportation, food production and recreation, comes directly associated impacts. Large scale coastal engineering projects undertake in the Netherlands have been implicated in the demise of Zostera in the Waddon Sea, mainly due to the increase in turbidity and water depth (Giesen et al. 1989). Coastal infrastructure such as pipelines and cable to provide gas, water and electricity e.t.c., may impact during the construction and maintenance. Coastal tourism is a rapidly growing industry and will transform the coastal environment. This involves urbanisation of the coastline which promotes erosion.

Ship activity also causes disturbance to seagrass through anchoring damage, which can be rather extensive at popular mooring sites (Walker et al. 1989), as well as fisheries operation, particularly shallow trawling and smaller scale activities linked to fisheries, such as clam digging and use of push nets over intertidal and shallow areas and, in extreme cases, dynamite fishing (Cleator 1993; Kirkman & Kirkman 2000). There has been considerable concern over the use of suction dredging equipment to harvest cockles from the seagrass beds of Auchencairn Bay on the Solway Firth (Cleator 1993).

Excessive nutrient enrichment of the coastal environment has lead to a global deterioration in water quality, which is identified as a major loss factor for seagrass populations worldwide (Heeminga & Duarte 2000). Anthropogenic nitrogen is largely responsible for the majority of nitrogen input into watersheds and input from the atmosphere. Nutrient enrichment may increase production in Zostera (Tubbs & Tubbs 1983), within the nutrient limiting subtidal environment. However, seagrasses are well designed to cope with nutrient limitation (Hemminga & Duarte 2000), and other primary producers such as micro- and macro-algae, are both more efficient in nutrient uptake (Duarte 1995). Coastal eutrophication promotes phytoplankton blooms, which have been shown to reduce biomass and depth penetration of Zostera (Hauxwell et al. 2006). Eutrophication results in a shift from a nutrient limiting to a light limiting environment. Algal overgrowth, may intercept a large percentage of incident light before it reaches the leaf canopy preventing photosynthesis. Increased eutrophication may uncouple Zostera growth dynamics from the seasonal pattern of surface irradiance (Hauxwell et al. 2006). These effects, however, can be counteracted by gastropod grazing on the epiphytes. Epiphyte grazing could therefore be important in the maintenance of healthy seagrass beds in eutrohphicated seagrass communities (Davison & Hughes 1998; Philippart 1995). It has also been suggested that nutrient enrichment may cause an accumulation of ammonium internally (Cleator 1993).

Table 1.3. Impacts of direct and indirect human forcing on seagrass ecosystems.

Z. marina is known to biaccumulate organic pollutants including tributyltin, the active ingredient of anti-fouling paint. Due to these bioaccumulative properties of Zostera, pollutants have the potential to persist and be passed along the food chain. Furthermore, pollutants can biomagnify potentially in some commercially important primary consumers (Cleator 1993).

Additonal potential sources of impact on seagrass communities are; agricultural inputs and runoff from non-point source pollution; deforestation leading to soil erosion and siltation and turbidity at estuaries; and municipal waste. All of these sources are considered to be more acute on the east coast especially in the Montrose Basin, the Firth the Forth, the Cromarty and Dornoch Firths (Cleator 1993).

Large scale impacts such as the increasing rate of global climate change seen in this century, and predicted to accelerate into the next, will significantly impact the Earth's oceans, with large potential impacts to seagrasses (Short & Neckles 1999). Most critical are increases in seawater temperature resulting from the greenhouse effect, and the resultant 10-15cm rise in sea level with the secondary impacts. The response of seagrasses to climate change is expected to affect the many processes that determine seagrass growth and reproduction.

Increased CO2 is expected to increase the rate of photosynthesis. Seagrasses ecosystems are often limited in inorganic carbon, and the increase in atmospheric CO2 concentration by 25 % over the 20th century, may have led to an increase in light-saturated seagrass photosynthesis by as much as 20 % (Short & Neckles 1999). Global warming is also projected to increase the frequency and intensity of storm events, possibly leading to increased coastal erosion and sediment resuspension with siltation, with greater turbidity and poorer light climate. Any effect of an increase the increase in temperature over the 20th century has not been observed. However, a continual increase may effect the metabolic processes, nutrient uptake, flowering and seed germination (Borum et al. 2004; Short & Neckles 1999).

Mitigation of Impact

It is important in the synthesis of rational conservation plans that increased understanding of Zoster biology, and accurate distributional data are acquired. Furthermore, it is imperative that the significance of Zostera communities is emphasised in environmental impact assessments, and developments likely to impact Zostera are carefully monitored (Hemminga & Duarte 2000).

A wide range of management tools are available to mitigate the potential impacts associated with seagrass habitat degredation. The fulfilment of water quality objectives under the E.U Water Framework Directive (WFD) will have benefits for coastal benthic flora, directly serving to mitigate the potential impacts associated with degredation of coastal water quality (Borum et al. 2004). Mitigation of eutrophication can be achieved by control and treatment of urban and industrial sewage to reduce the loading with nutrients, organic matter and chemicals. The installation sewage treatment plants with efficient nutrient removal may be costly (Borum et al. 2004). Agricultural non-point source pollution may be remedied by the installation of vegetated buffer strips on agricultural land to barrier surface runoff into water courses (Hefting et al. 1998). Other objectives of conservation measures should entail the regulation of land use in catchment areas to reduce nutrient runoff and siltation due to soil erosion; regulation of land reclamation and coastal constructions; the regulation of fisheries and clam digging in the vicinity of seagrass beds (Duarte 2002; Borum et al. 2004).

The monitoring of seagrass distribution and abundance is valuable in that they are instrumental vehicles to increase awareness of the important role of seagrasses in the ecosystem, and highlight the threats seagrass ecosystems are exposed to, and the importance to preserve the seagrass meadows to maintain the biological balance and the biodiversity of the coastal ecosystem (Duarte 2002; Borum et al. 2004). Effective monitoring can be achieved principally by the use of remote sensing data of Scottish coastal areas. From remotely sensed data, biological parameters such as biomass, leaf area index (LAI), and abundance can be directly extrapolated based on the reflectance values of a given location in the image feature space, and ground truthing. These indicators respond to environmental variables such as water quality, hence an additional value in seagrass monitoring. Given a high temporal frequency of data, effective monitoring can be installed (Borum et al. 2004).

Effective protection can only be achieved by legislative acknowledgement of the importance of seagrass ecosystems. Currently, many concentrated seagrass populations are within Sites of Special Scientific Interest and it is an imperative of the Scottish Environmental Protection Agency that coastal management practices aim to conserve seagrass communities, actions be taken to mitigate potential impacts and that coastlines are used in a sustainable manor (UK BAP 2006). With the continual application of these conservation objectives, if coastal activities can be achieved with minimal impact, if a coherent monitoring system can be established, and if the awareness of the public on issues surrounding the sustainability of Scottish seagrass resources is increased, then effective conservation of these significant habitats may be achieved.

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