Biodiversity & Conservation

The nature of intertidal ecosystems (immersion and emersion) means that seagrass beds are exposed to a range of varying environmental factors, such as temperature, desiccation and solar radiation (Massa et al., 2009).

The transport of oxygen to the roots and rhizomes produces an oxygenated microzone around them, which increases the penetration of oxygen into the sediment.

Zostera sp. support numerous epiphytes and periphyton, e.g. leaves may be colonized by microphytobenthos such as diatoms and blue green algae. The brown algae Halothrix lumbricalis and Leblondiella densa are only found on Zostera leaves and Cladosiphon contortus occurs primarily on the rhizomes of Zostera sp.

Algal epiphytes, such as the diatoms Cocconeis scutellum and Cocconeis placentula, on the leaves of Zostera noltii are grazed by small prosobranch molluscs, for example, Rissoa spp., Hydrobia spp. and Littorina littorea.

The sediment supports a diverse infauna, including deposit feeders such as, Arenicola marina, Pygospio elegans, Scrobicularia plana, Macoma balthica, and Corophium volutator; as well as suspension feeders such as Cerastoderma edule (Connor et al., 1997b; Davison & Hughes, 1998).

Zostera noltii density and biomass can be influenced by the presence of high densities of lugworms (Arenicola marina), due to the sediment bioturbation (Philippart, 1994a).

Lugworms (Arenicola marina) are also known to affect the densities of other species associated with Zostera noltii beds, for example, Pygospio elegans (Reise 1985), Corophium volutator and juveniles of various worm and bivalve species (Flach 1992a & b)

Hediste diversicolor are reported to eat the leaves and seeds of Zostera noltii plants (Hughes et al., 2000).

The epifauna and infauna are vulnerable to predation by intertidal fish, and shore crabs (Carcinus maenas) at high tide.

Since the decline of Zostera marina beds Zostera noltii has become the preferred food for dark-bellied Brent geese (Branta bernicla).

Intertidal Zostera noltii beds are heavily grazed by over-wintering wildfowl and are an important food source for Brent geese (Branta bernicla), wigeon (Anas penelope), mute and whooper swans (Cygnus olor and Cygnus cygnus).

Intertidal seagrass beds are improtant spawning areas for transient fishes, with the tidal migration of garfish Belone belone being specifically directed at Zostera noltii beds for spawning. The eggs of the herring Clupea harengus were found at densities twenty times higher in seagrass beds than adjacent intertidal brown algal patches (Polte & Asmus, 2006).

Zostera beds are naturally dynamic and may show marked seasonal changes. Leaves are shed in winter, although Zostera noltii retains its leaves longer than Zostera marina. Leaf growth stops in September/October (Brown, 1990). Leaves are lost, or removed by grazing or wave action over-winter. For example, in the Wadden Sea, Nacken & Reise (2000) noted that 50% of leaves fell off, while Brent geese removed 63% of the plant biomass.
Zostera noltii over-winters as rhizome and shoot fragments, resulting in 'recruitment' of several cohorts in the following spring (Marta et al., 1996). However, Nacken & Reise (2000) noted that the Zostera noltii beds recovered normal shoot density and that grazing wildfowl helped to maintain a balance between accretion and erosion within the bed, without which recovery was inhibited. The rhizome of Zostera noltii has limited carbohydrate storage capability, Marta et al. (1996) and Dawes & Guiry (1992) regarded this species as ephemeral, taking advantage of seasonal increases in nutrients and light especially to grow rapidly in spring and early summer.

Where present, Arenicola marina spawns synchronously either once or twice a year; the precise timing depending on location (Howie, 1959; Clay, 1967; Bentley & Pacey, 1992). Cerastoderma edule spawns between March - August with a peak in summer, Macoma balthica spawns in February - March with another peak in autumn, whilst Scrobicularia plana spawns in summer (Fish & Fish, 1996).
Settlement of spat in intertidal bivalves is generally sporadic (see Cerastoderma edule for review). While Macoma balthica may be protected from low winter temperatures by its depth in the sediment, Cerastoderma edule is vulnerable to low temperatures in winter, especially in severe winters. Therefore, cockle mortality is likely over winter due to low temperatures, lack of food and predation, especially from wildfowl such as the oystercatcher (Haematopus ostralegus). Further mortality is likely in year one cockles due to exhausted energy reserves and predation by the shore crab Carcinus maenas. Epifaunal species, such as Littorina littorea and Hydrobia ulvae may suffer additional wildfowl predation over winter without the refuge provided by Zostera noltii leaves, however, being mobile they are able to seek alternative food sources.

Leaves slow current and water flow rates under the canopy, which encourages settlement of fine sediments, detritus and larvae (Orth, 1992). Seagrass rhizomes stabilize sediment and protect against wave disturbance. Presence of seagrass increases species diversity by favouring sedentary species that require stable substrata (Orth, 1992; Davison & Hughes, 1998).

Zostera noltii provides shelter or substratum for a wide range of species, especially epiphytes and periphyton. Epiphytic species may be grazed by other species (Davison & Hughes, 1998) such as the mobile epifauna, Hydrobia ulvae and Littorina littoreapresent in seagrass beds. The sediment supports a rich infauna of polychaetes, bivalve molluscs and the mud amphipod Corophium volutator. Cockle beds (Cerastoderma edule) are often associated with intertidal seagrass beds. The sediment also includes a diverse meiofauna, for example many species of free-living turbellarians, ostracods and copepods (Asmus & Asmus, 2000b). In addition, intertidal seagrass beds are visited by several fish species when immersed.

Seagrass beds are characterized by high productivity and biodiversity and are considered to be of great ecological and economic importance (Davison & Hughes, 1998; Asmus & Asmus, 2000b). Primary production is derived from phytoplankton, microphytobenthos and Zostera sp. In addition, organic carbon is derived from the input of detritus into the system (for estimates of g C/m²/year see Asmus & Asmus, 2000b). Asmus & Asmus (2000b) reported that seagrass beds are sediment traps and nutrient sinks, which under storm conditions may become nutrient sources for the surrounding ecosystems, and are, therefore, important for the material flux in the ecosystem. For example, in the Sylt-Rømø Bight, Asmus & Asmus (2000b) estimated that the seagrass beds contributed significantly to material flux within the total intertidal system even though the seagrass beds only covered 12% of the intertidal area.

In periods when Zostera noltii dies off (winter), epiphytic algae and periphyton contribute significantly to the overall community productivity and above ground biomass (Welsh et al., 2000; Philippart, 1995b). Philippart (1995b) estimated that by May on an intertidal mudflat off Terschelling, the Netherlands, periphyton biomass equalled Zostera noltii biomass, declining to 20% of the total above ground biomass by the end of September.

Detritus food chains within the seagrass beds are driven by bacterial decomposition of dead seagrass tissue and other detritus. Dissolved organic matter (DOM) leaching from seagrass and bacterial decomposition supports high numbers of heterotrophic protists. Seagrass detritus is rich in micro-organisms, e.g. 1 g (dry weight) may support on average 9 mg of bacteria and protists, including heterotrophic flagellates and ciliates (Davison & Hughes, 1998). Dead seagrass leaves can be transported by currents to great depths or washed up on the shore; hence supporting detritus based food chains and communities in distant areas of the coast (Davison & Hughes, 1998).

Although primary production is high, secondary production is similar in un-vegetated areas and seagrass beds (Asmus & Asmus, 2000b). Asmus & Asmus (2000b) presented a general food web for intertidal Zostera spp. beds, noting that loss of intertidal seagrass beds resulted in profound changes in the food web of the total ecosystem.

Zostera sp. are perennials but may be annuals under stressful conditions. Seedlings rarely occur in seagrass beds except in areas cleared by storms, blow-out or excessive herbivory (Phillips & Menez, 1988). Seed mortality is very high (Phillips & Menez, 1988; Fishman & Orth, 1996). Seed may be dispersed through the gut of wildfowl (Fishman & Orth, 1996), or float long distances (up to 200 m in some cases) on attached gas bubbles. The generative stalk may be released and can also float long distances. Displaced pieces of shoot or rhizome float and may root if they settle on suitable substrata. Vegetative reproduction probably exceeds seedling recruitment except in areas of sediment disturbance (Phillips & Menez, 1988; Reusch et al., 1998).

Seagrasses are capable of both sexual and asexual reproduction, and therefore beds can be established by either clonal growth or flowering events and seeds (Alexandre et al., 2006). Hootsmans et al. (1987) noted that potential recruitment was maximal (32% of seeds) at 30 °C and 10 psu, no recruitment occurred at 30 psu and they estimated that, in 1983 <5% of Zostera noltii plants in their study area originated from seed. Recruitment of Zostera noltii is, therefore, most likely to occur by dispersal of shoots and rhizomes rather than seed.

Potential recruitment may be hampered by competition with infauna such as the ragworm Hediste diversicolor or blow lug Arenicola marina (Philippart, 1994a; Hughes et al., 2000). Hughes et al. (2000) noted that Hediste diversicolor consumed leaves and seeds of Zostera noltii by pulling them into their burrow, therefore reducing the survival of seedlings.

The distribution of Zostera noltii can be restricted by burrowing and bioturbation of infauna such as Hediste diversicolor and Arenicola marina. Philippart (1994a) concluded that the blow lug populations in the Wadden Sea may have contributed to the decline in the Zostera noltii beds over the previous 25 years. The rhizome mat of the seagrass can inhibit burrowing and colonization of the seagrass bed by burrowing infauna (Hughes et al., 2000; Philippart, 1994a). At low densities, blow lug may be beneficial as they increase nutrient flux and oxygenation in the sediment. Corophium volutator has been reported to inhibit colonization of mud by Salicornia sp. (Hughes et al., 2000) and where present may also inhibit Zostera noltii recruitment.

While Zostera noltii has been reported to recover from seasonal grazing and loss of leaf biomass over-winter (Nacken & Reise, 2000), beds in the Wadden Sea have continued to decline (Philippart, 1994b). The scarce distribution of Zostera noltii in the UK suggests that recovery or recruitment is slow and in the region of 5-10 years (Holt et al., 1997; Davison & Hughes, 1998).

Epifaunal species such as Hydrobia ulvae are widely distributed, mobile, occur at high densities, and have a planktonic life cycle suggesting that they would recruit rapidly. Similarly Littorina littorea is likely to recruit rapidly.
Development of both Arenicola marina and Pygospio elegans starts in the female's tube. Larvae of Pygospio elegans are pelagic, while Arenicola marina larvae migrate up the shore. Recruitment in Arenicola marina is rapid, especially where there are adjacent populations present.
Recruitment in infaunal bivalve populations is sporadic due to variation in larval supply and post-settlement mortality. For instance, although recruitment in Cerastoderma edule is likely to occur annually, significant recruitment to the population may take up to five years.

Zostera noltii is able to recover relatively quickly compared to other seagrass species (Marbà et al., 2004). Nacken & Reise (2000) noted that Zostera noltii beds had returned to the previous abundance within a year following leaf loss and grazing by wildfowl. The majority of species associated with intertidal seagrass beds are not restricted to the biotope (Asmus & Asmus, 2000b), with the exception of Zostera sp. Specific epiphytes, and are likely to be present in the sediment or migrate into the developing bed. Zostera noltii is regarded as a relatively ephemeral species (Dawes & Guiry, 1992).

No text entered.