Biodiversity & Conservation

The rhizomes and roots of Ruppia maritima help to stabilize, and oxygenate, the sediment surface, while the stems and leaves provide additional substratum for a variety of algae and invertebrates. Although the functional groups within the ecosystem probably remain fairly constant the abundance and diversity of species within each group varies with the habitat, especially the salinity regime (e.g. Verhoeven & van Vierssen, 1978). Ruppia maritima and Ruppia cirrhosa provide primary production and substratum within the biotope. Few organisms, except wildfowl, feed on the Ruppia spp. directly, however, decomposition of leaves and stems, especially in autumn and winter, support a detrital food chain within the biotope and probably also provide primary productivity to deeper water and drift line communities (Verhoeven & van Vierssen, 1978; Zieman et al., 1984; Kantrud, 1991).

Additional, primary productivity is provided by microbial (e.g. diatoms) and macroalgal epiphytes growing on the leaves of Ruppia spp., and a floating mat of filamentous algae (e.g. Chaetomorpha sp. and Cladophora spp.) and, when present, stoneworts (e.g. Chara aspera and Lamprothamnium papulosum).

Ruppia spp. leaves may be used as substratum by algal epiphytes as above and faunal epiphytes such as bryozoans and hydroids (e.g. Electra crustulenta, Conopeum seurati, and Cordylophora caspia).

The leaves of Ruppia spp. and the algal mats may provide temporary substratum for juvenile anemones and bivalves (e.g. Anemonia sulcata, Mytilus edulis, Cerastoderma glaucum) and the larvae and pupae of aquatic insects (e.g. the shore fly, Ephydra riparia) (Verhoeven & van Vierssen, 1978; Verhoeven 1980a; Boström & Bonsdorf, 2000). Aquatic insects probably utilize any available aquatic macrophytes as substratum.

The epiphytes and algal mats may be grazed by gastropods (e.g. Rissoa spp., Hydrobia spp. or Potamopyrgus spp.), amphipods (e.g. Gammarus spp.) and isopods (e.g. Jaera spp., and Idotea spp.) and probably mysids (e.g. Neomysis integer).

Verhoeven & van Vierssen (1978) and Verhoeven (1980b) suggested that isopods and amphipods may feed directly on Ruppia spp., however, their most important role in the food chain was the breaking down of decomposing leaves into fine particles of detritus suitable for suspension and deposit feeders in the detrital food chain.

Suspension feeders filter both phytoplankton and detritus (organic particulates), for example Corophium spp., Cerastoderma glaucum, Mya arenaria, hydroids, bryozoans, and polychaetes (Hediste diversicolor, Polydora spp.).

Surface and infaunal deposit feeders include polychaetes (e.g. Arenicola marina, Manayunkia aestuarina and Pygospio elegans), amphipods (e.g. Corophium volutator), bivalves (e.g. Macoma baltica), and chironomid larvae.

Small invertebrates are preyed on by small mobile predators that use the Ruppia beds for shelter, for examples insect larvae, mysids, shrimp and sticklebacks (e.g. Gasterosteus aculeatus and Spinachia spinachia).

Generalist predators use, but are not closely associated with, the Ruppia beds, e.g. the shore crab Carcinus maenas, the eel Anguilla anguilla, and the goby Pomatoschistus microps.

Several species of wildfowl feed directly on Ruppia spp., although the exact species will vary with location, season and salinity, e.g. the tufted duck Aythya fuligula, the coot Fulica atra, the wigeon Anas penelope, the mute swan Cygnus olor.

Mysids, shrimp and crabs probably act as scavengers within this biotope.

Detailed lists of species and their position within the habitat for several locations in western Europe (Finland, the Netherlands, and France) are given by Verhoeven and his co-author (Verhoeven & van Vierssen, 1978; Verhoeven, 1979, 1980a, b).

Ruppia maritima is thought to be an annual while Ruppia cirrhosa is perennial (see recruitment) (Verhoeven & van Vierssen, 1978). Annual Ruppia species die back complete in winter, over-winter primarily as seed (druplets), that germinate in April (early spring) (Verhoeven & van Vierssen, 1978; Kantrup, 1991). Perennial species over-winter as leaf bearing rhizomes that bud in early spring with occasional development from seed (Verhoeven & van Vierssen, 1978; Kantrup, 1991). Seasonal change includes:

• in early spring (April-May) Ruppia species grow rapidly, annual species producing a more luxuriant growth than the perennial species;
• Ruppia species produce their greatest biomass by August-September;
• where Ruppia forms mixed stands with the sago pondweed Potamogeton pectinatus, the pondweed may become dominant by June-July, with Ruppia species dominating by August-September;
• Ruppia spp. flowers about 5-6 weeks after the onset of spring growth, with pollination occurring about 1-2 weeks later;
• epiphytic microflora (diatoms and algae) steadily colonize the plants during the growing season, and epiphytes cover the plant entirely by autumn;
• wildfowl graze the beds throughout the year, the exact species depending on season, and
• most of the plant material dies in late summer (September) and is removed by autumn winds and resultant wave action, and may form floating plant masses or drift algae and hence support a greater abundance of detritivores.

Ruppia spp. beds inhabiting temporary pools or ditches or other ephemeral habitats may dry out during the summer months and be killed. However, such harsh conditions favour annuals that produce large amounts of seed. Long term changes in the salinity regime are likely to result in changes in the abundance of Ruppia sp. and the associated species; e.g. an long term decrease in salinity may favour the growth of the sago pondweed Potamogeton pectinatus, however an increase in salinity may favour Ruppia cirrhosa.

The leaves and stems of Ruppia spp. provide substratum and refuge for several species, while the rhizome and root system stabilize the sediment, and the transport of oxygen from the leaves to the roots oxygenates the sediment in the vicinity of the roots (the rhizophere) changing the local redox potential, sediment chemistry and oxygen levels. In low salinities Ruppia spp. forms mixed stands with other macrophytes such as Potamogeton pectinatus or Zannichellia spp. whereas in variable to fully saline water it may form mixed stands with Zostera spp. and contain more estuarine or fully marine species. Hypersaline conditions may favour Ruppia cirrhosa, which tolerates up to ca 108psu over Ruppia maritima (Verhoeven, 1979). Species diversity varies with salinity, being maximum in near full seawater or freshwater conditions but reaching a minimum in the physiologically harsh brackish conditions most favoured by Ruppia spp. Ruppia spp. inhabit a variety of salinity regimes and varied habitats from near saline estuaries, to brackish ditches and man-made channels to saltmarsh and wetlands, including both long term and temporary pools, therefore habitat complexity and species composition can vary markedly. However, Verhoeven and his co-author recognised the following elements of the community:

• the Ruppia spp. and other associated aquatic macrophytes or macroalgae;
• mats of filamentous algae, e.g. Chaetomorpha spp., Cladophora spp., and Ulva spp., that harbour high densities of invertebrates e.g. Chironomid larvae, amphipods, copepods and juvenile bivalves (Verhoeven & van Vierssen, 1978; Verhoeven 1980a; Boström & Bonsdorf, 2000);
• epiphytic species attached to the plants e.g. diatoms, filamentous diatoms, hydroids, bryozoans;
• temporary epiphytic species, e.g. larval or juvenile anemone, bivalves, and aquatic insects;
• species depositing eggs on Ruppia spp. and other macrophytes, e.g. insects, hydrobids, and some fish;
• species living in tubes attached to plants, e.g. the polychaetes Polydora ligni and Spirorbis spirorbis, and the amphipod Corophium volutator;
• species creeping over plants and other hard substrata but not the sediment, e.g. amphipods, isopods, gastropods, and insect larvae;
• species creeping over plants and the sediment bottom, e.g. Hydrobia spp. and Potamopyrgus spp.;
• benthic infauna, e.g. the oligochaete Tubifex spp., polychaetes Hediste diversicolor, Arenicola marina and Manayunkia aestuarina, the amphipod Corophium volutator, bivalves Cerastoderma glaucum, Macoma baltica and Mya arenaria and chironomids;
• mobile species in the vegetation canopy, e.g. sticklebacks and pipefish, and
• mobile species occurring within the vegetation and the surrounding area, e.g. shrimps, crabs, mysids, gobies, eels and flatfish.

Where the Ruppia beds accumulate sediment and/or lie adjacent to areas that dry out, the Ruppia beds may be associated with a succession of terrestrial saltmarsh or marsh plants, e.g. reeds and sedges, forming a hydrosere. The reader is directed to Rodwell (2000) for further information on saltmarsh communities and Rodwell (1995) for further information on aquatic plant communities.

Primary productivity
Verhoeven (1980b) suggested that under ideal conditions the largest possible standing crop of Ruppia spp. in European waters was about 300 g dry weight /m², which was low to moderately productive when compared to marine seagrass or freshwater aquatic plant communities. Verhoeven (1980b) reported values of productivity between 9 -290 g ash weight /m² in terms of biomass in European sites. Verhoeven (1980b) estimated a minimum annual productivity of 6-15 g C /m² for Ruppia cirrhosa beds in the Camargue lagoons, France and 15-20 g C/m² for Ruppia spp. beds in the Netherlands. In both cases the Ruppia spp. productivity was lower than the local phytoplankton productivity.
Ruppia spp. primary productivity is reduced by excessive turbidity, competition (probably for light) with other aquatic plants, algae and phytoplankton, Excessive wave action or water depth (Kantrud, 1991). Filamentous algae and epiphytes inhibit Ruppia productivity by shading and by entanglement; increasing the plants sensitivity to wave action. However, algal mats may also shade and reduce epiphytic microflora on the Ruppia leaves. Epiphytes reduce Ruppia productivity by shading, competing for nutrients and by interfering with exchange of gases and nutrients across the leaves of Ruppia spp., although Verhoeven (1980b) concluded that under eutrophic conditions inhibition by epiphytes was minor compared to the effects of shading and increased turbidity caused by phytoplankton blooms.

Secondary productivity
Fredette et al., (1990) estimated that Zostera spp. and Ruppia spp. seagrass beds supported about 200 g dry weight /m² /yr. of invertebrate (primarily isopods, amphipods and crabs) secondary productivity, roughly equivalent to 55.9 tonnes of invertebrate production over a year in a 140 ha bed, although they considered their value to be an underestimate. Verhoeven (1980a) reported up to 43,800 invertebrates /m² (biomass up to 22.9 g ash-free weight /m²) in Ruppia dominated communities, although only 15 of 75 species were closely associated and two species dominated (Verhoeven, 1980a; Kantrup, 1991). Further secondary production is generated through the detrital food chain. About 44% of the autumn decrease in Ruppia cirrhosa biomass was due to leaching and decomposition, while the remainder was taken by wildfowl and invertebrates (Verhoeven, 1978; Kantrup, 1991). In experiments, grazing by macro-invertebrates (Gammarus spp. and Sphaeroma spp.) reduced leaves and shoots of Ruppia cirrhosa to particles less than 1mm in 180days (Kantrup, 1991). Verhoeven (1980b) suggested that 90% of the plant material produced in Ruppia beds was decomposed and most mineralised (converted to available inorganic nutrient) within the following year.

Ruppia maritima is thought to be an annual while Ruppia cirrhosa is perennial (Verhoeven & van Vierssen, 1978), however, Kantrup (1991) reported that Ruppia maritima could also grow from over-wintering rhizomes. Ruppia species die back completely in winter, over-winter primarily as seed (druplets), that germinate in April (early spring) (Verhoeven & van Vierssen, 1978; Kantrup, 1991). Perennial species over-winter as leaf bearing rhizomes that bud in early spring with occasional development from seed (Verhoeven & van Vierssen, 1978; Kantrup, 1991). Annual species of Ruppia exhibit high fecundity, rapid development, early maturity and the production in a large amount of seed, and are able to survive in more ephemeral habitats. Ruppia maritima produces enormous numbers of seeds about two weeks after flowering (June -September), since the flowers are held underwater, where pollination is more efficient. Reproduction occurs in a temperature range of 15-19 °C but decreases above 30 °C. Seeds or duplets can remain viable in the sediment for up to 3 years (Verhoeven & van Vierssen, 1978; Verhoeven, 1979; Kantrup, 1991).

Seed germinate in a wide variety of temperatures and salinity. For example, in Europe Ruppia maritima seeds began to germinate when the water temperature exceeded a minimum/maximum interval of 10/15 °C, and mainly between 15-30 °C. Prior desiccation may stimulate germination. Seeds will germinate in as little as 5-10cm of water in culture, although seed production is reduced in shallow waters (Kantrup, 1991). The effect of salinity on germination is temperature dependant. For example, Ruppia maritima seeds germinate well at 43.4 psu at 28°C but germination rates is lower at high temperatures and low salinities (<3.5psu) than at low temperatures and salinities up to 26 psu (Kantrup, 1991). Germination may also be affected by oxygen levels and seeds in poorly oxygenated sediments lie dormant until the next year (Kantrup, 1991).

Ruppia maritima can also colonize by rhizomes. Over-wintering rhizomes bud in early spring, at about the same time as germination, probably in response to temperature (Kantrup, 1991). Over-wintering rhizomes is of greater importance than seed set in perennials such as Ruppia cirrhosa. In perennials the pollination occurs at the water interface, which is less efficient than underwater, and their allocation to reproductive shoots is less than to vegetative production. Orth & Moore (1982) reported that recolonization of sediment denuded of Ruppia spp. by a boat propeller occurred at about 0.25 m/yr.

Ruppia species distribution is affected by the isolated nature of their habitats (e.g. lagoons) and their ability to disperse. Seeds and rhizomes can be transported by currents attached to floating detached plant material. After desiccation, dried plants and attached seed can be transported considerable distances by the wind. A proportion of the seed consumed by wildfowl pass through the birds unharmed, therefore, wildfowl could potentially transport seed considerable distances. For example, 30% of the freshwater eelgrass Naja marina seeds fed to ducks in Japan survived and successfully germinated after passage through their alimentary canals and could be potentially transported 100-200km (Fisherman & Orth 1996). Verhoeven (1979) noted that Ruppia maritima produces large amount of seed and was the most cosmopolitan Ruppia species, suggesting the potential for wide dispersal.

However, competition with infauna such as Hediste diversicolor or Arenicola marina have been suggested to hamper potential recruitment in Zostera noltii (see review) (Hughes et al., 2000; Philippart, 1994a). Similarly, Corophium volutator has been reported to inhibit colonization of mud by Salicornia sp. (Hughes et al., 2000). Therefore, the above infaunal species could potentially inhibit recruitment in Ruppia spp.

The microalgae and filamentous macroalgae found within the biotope are wide-spread and ubiquitous, producing numerous spores, and can colonize rapidly. Similarly, bryozoans and hydroids probably produce numerous but short lived pelagic larvae, so that local recruitment from adjacent populations is probably rapid.

Boström & Bonsdorff (2000) examined the colonization of artificial seagrass and Ruppia maritima beds by invertebrates. They reported colonization by abundant nematodes, oligochaetes, chironomids, copepods, juvenile Macoma baltica and the polychaete Pygospio elegans within 33-43 days. Disturbance by strong winds after 43 days resulted in a marked increase in the abundance of species by day 57, except for Pygospio elegans. They noted that settlement of pelagic larvae was less important than bedload transport, resuspension and passive rafting of juveniles from the surrounding area in colonization of their artificial habitats. Other polychaetes, such as Arenicola marina do not possess a pelagic larvae, but migrate as juveniles and can swim as adults. Recolonization in Arenicola marina is thought to be rapid where adjacent populations are present, although recolonization may take longer in isolated populations.

The sticklebacks Gasterosteus aculeatus and Spinachia spinachia are associated with Ruppia beds. In both species the males set up a territory and build nests, in which the female lays eggs that are subsequently fertilized and guarded by the males (Fishbase, 2000). The abundance of vegetation provided by the Ruppia bed and its associated algal mats probably provides nesting material for the males and a refuge for developing juveniles. While associated with this biotope, sticklebacks are mobile species capable of colonizing the habitat from adjacent areas or the open sea.

Ruppia vegetation dies back in autumn and winter, and over-winters either as seed or rhizome, only to germinate or bud in early spring. Therefore, the Ruppia bed and its associated community (except the infauna) develops annually. In subtropical climates wintering waterfowl were reported to consume entire stands of Ruppia spp., which re-established within weeks in optimal conditions (Kantrup, 1991). However, if the rhizomes and seed bank is removed community developed may be prolonged.

Ruppia spp. seed and rhizomes can be transported considerable distances by wildfowl or by water currents and wind (when dry). Floating fragments of Ruppia spp. grow roots freely, sink and attach to the bottom. For example, Orth & Moore (1982; cited in Kantrup, 1991) reported that sediments denuded by a boat propeller were recolonized at about 0.25m /year. However, little other evidence of colonization rates was found.

Community development will depend on the time taken for Ruppia propagules to reach the available habitat. Once rhizomes or seed arrive in the habitat recovery may take several years. In areas connected by water flow or regularly frequented by wildfowl recovery will take many years, but in isolated area habitat recovery may be prolonged, possibly taking up to 5-10 years.

The benthic infauna probably colonizes the associated sediment more slowly but still relatively rapidly. For example, Broström & Bonsdorff (2000) found that abundant infauna colonized artificial seagrass and Ruppia maritima habitats within 33 - 57 days (1-2 months). Few species found in Ruppia dominated communities are associated with Ruppia spp. alone (Verhoeven & van Vierssen, 1978; Verhoeven, 1980a) and most probably colonize the vegetation from the surrounding habitats.

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