Biodiversity & Conservation

The rhizomes and roots of submerged macrophytes such as Potamogeton pectinatus help to stabilize and oxygenate the sediment surface, while the stems and leaves provide food and 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; Verhoeven, 1980a; van Vierssen & Verhoeven, 1983). Potamogeton pectinatus provides primary production and substratum within the biotope. Few organisms, except waterfowl, feed on Potamogeton pectinatus 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 the strandline (Verhoeven & van Vierssen, 1978; Verhoeven, 1980a; Byren & Davies, 1986; Kantrud, 1990).

Additional, primary productivity is provided by microbial (e.g. diatoms) and macroalgal epiphytes growing on the leaves of Potamogeton pectinatus, and a floating mat of filamentous algae (e.g. Ulva prolifera and Cladophora spp.) in more saline situations, and, when present, stoneworts (e.g. Chara aspera and Lamprothamnium papulosum).

Potamogeton pectinatus competes for light and space with other submerged macrophytes e.g. the stoneworts Chara aspera and Lamprothamnium papulosum, epiphytic microalgae and macroalgae (as above) or phytoplankton. With increasing or variable salinity Potamogeton pectinatus forms mixed stands with Ruppia species and may be replaced in the biotope £IMS.Rup£. In decreased salinity waters it competes with Myriophyllum spicatum, Ranunculus baudotii or Zannichellia pedunculata (van Vierssen & Verhoeven, 1983; Kantrud, 1990) forming mixed stands (NVC community A11; see Rodwell, 1995). Potamogeton pectinatus 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 Potamogeton pectinatus 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). Aquatic insects probably utilize any available aquatic macrophytes as substratum.

The epiphytes and algal mats may be grazed by gastropods (e.g. Hydrobia spp. or Potamopyrgus spp.), amphipods (e.g. Gammarus salinus and other Gammarus species) and isopods (e.g. Jaera spp. and Idotea spp.) and probably mysids (Mauchline, 1980)

Verhoeven & van Vierssen (1978) and Verhoeven (1980b) suggested that isopods and amphipods may feed directly on Potamogeton pectinatus. 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 and microbes in the detrital food chain (Byren & Davies, 1986).

The young leaves of Potamogeton perfoliatus were show to be the preferred food of freshwater caddis-fly larvae (Trichoptera) (Jacobsen & Sand-Jensen, 1994). Littoral Trichoptera species such as Limnephilus lunatus probably feed on Potamogeton pectinatus directly (Chen, 1976; van Vierssen & Verhoeven, 1983). Larvae of the beetle Haemonia appendiculata were reported to feed on Potamogeton pectinatus (Kantrud, 1990).

Suspension feeders filter both phytoplankton and detritus (organic particulates), for example amphipods e.g. Corophium volutator, the mysid e.g. Neomysis integer, bivalves e.g. Cerastoderma glaucum and Mytilus spat, hydroids, bryozoans, and polychaetes (Hediste diversicolor).

Surface and infaunal deposit feeders include polychaetes (e.g. Manayunkia aestuarina and Pygospio elegans), oligochaetes (e.g. Limnodrilus hoffmeisteri and Tubifex costatus), amphipods (e.g. Corophium volutator), and chironomid larvae.

Small invertebrates are preyed on by small mobile predators that use the Potamogeton pectinatus beds for shelter. For example, insect larvae (especially dragonfly larvae e.g. Ischnura elegans), water boatmen (e.g. species of Sigara), mysids, shrimp and sticklebacks (e.g. Gasterosteus aculeatus and Spinachia spinachia) (Verhoeven, 1980a; van Vierssen & Verhoeven, 1983).

Generalist predators use, but are not closely associated with, the Potamogeton pectinatus beds, e.g. the eel Anguilla anguilla, and the goby Pomatoschistus microps.

Several species of wildfowl feed directly on Potamogeton pectinatus, although the exact species will vary with location, season and salinity, e.g. the coot Fulica atra, the wigeon Anas penelope, the mute swan Cygnus olor, whooper swan and gadwall. Other species are omnivorous feeding on the vegetation and invertebrates, e.g. garganey, mallard, pintail, pochard, scaup, shoveler, teal, and tufted duck (Jupp & Spence, 1977; Kantrud, 1990; Preston, 1995).

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 Scandinavia and western Europe (Finland, the Netherlands, France and Portugal) are given by Verhoeven and his co-author (Verhoeven & van Vierssen, 1978; Verhoeven, 1980a; van Vierssen & Verhoeven, 1983), Jacobsen & Sand-Jensen (1994) and Cunha & Moreira (1995).

Most Potamogeton species, including Potamogeton pectinatus, are rhizomatous perennials, the majority of the plant dying back to tubers and either rhizomes with short leafy shoots or just rhizomes (Preston, 1995). Potamogeton pectinatus populations may act as perennials in some environments or annuals in others. For example, in a sheltered brackish pool in Yerseke, the Netherlands, Potamogeton pectinatus was perennial, dying back to rhizomes with leafy shoots and tubers, as well as producing a persistent seed bank. But, in a wave exposed lake, the population was annual, dying back to tubers only in winter (van Wijk, 1988, 1989a, Preston, 1995). Other populations may fall between the above two extremes (Preston, 1995).

In temperate areas, Potamogeton pectinatus is one of the first species to grow in spring. Over-wintering propagules begin to shoot in late March to June when water temperatures reach about 10 °C (Kantrud, 1990). Healthy stands can cover the water surface two weeks later. The plant dies back in late August to October, and most decomposes or is washed ashore before the winter freeze in north temperate zones (Kantrud, 1990). In meso-haline lagoons in the Netherlands, Potamogeton pectinatus flowers in mid May to mid July (Kantrud, 1990).

Growth of filamentous algae and algal mats is greatest in the summer months, potentially smothering and shading Potamogeton pectinatus (Kantrud, 1990). Van Vierssen & Verhoeven (1983) noted that the abundance and diversity of the coleopteran (beetle) and heteropteran (true-bugs) fauna was positively correlated with the available pondweed cover. Cunha & Moreira (1995) reported seasonal changes in the macrofauna of Potamogeton and Myriophyllum beds in Portugal. They reported that polychaetes showed little seasonal changes in abundance while molluscs and leeches showed high densities in spring to summer but low numbers or even absence in autumn to winter. Crustaceans (e.g. gammarids) were most abundant in autumn, while insects were rare but abundant in winter and summer. Oligochaetes were most abundant in winter, although some species of oligochaete were also abundant in spring. Seasonal changes in the macrofauna was related to seasonal changes in temperature, dissolved oxygen, tidal regime and low or high rainfall and hence freshwater runoff and salinity (Cunha & Moreira, 1995). Grazing bird species probably vary seasonally, with resident species feeding all year round and migrant birds grazing on rhizomes and tubers in the winter months.

The leaves and stems of Potamogeton pectinatus 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. Verhoeven and his co-author recognized the following elements of the submerged macrophyte Ruppia spp. communities, which are probably equally representative of Potamogeton pectinatus communities:

• the Potamogeton pectinatus and other associated aquatic macrophytes or macroalgae;
• mats of filamentous algae, e.g. Cladophora spp., and Ulva spp., that harbour high densities of invertebrates e.g. aquatic insects, chironomid larvae, amphipods, copepods and juvenile bivalves (Verhoeven & van Vierssen, 1978; Verhoeven 1980a; van Vierssen & Verhoeven, 1983);
• epiphytic species attached to the plants e.g. diatoms, filamentous diatoms, blue green algae, bacteria, fungi, hydroids, and bryozoans;
• temporary epiphytic species, e.g. larval or juvenile anemone, bivalves, and aquatic insects;
• species depositing eggs on Potamogeton pectinatus and other macrophytes, e.g. insects, hydrobids, and some fish;
• species living in tubes attached to plants, e.g. 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
• mobile species occurring within the vegetation and the surrounding area, e.g. shrimps, crabs, mysids, gobies, and eels (Verhoeven & van Vierssen, 1978; Verhoeven 1980a; Howard-Williams & Liptrot, 1980; van Vierssen & Verhoeven, 1983).

Where the Potamogeton pectinatus beds accumulate sediment and/or lie adjacent to areas that dry out, the 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
Potamogeton pectinatus, other aquatic macrophytes, macroalgae and microalgae provide primary productivity in the community. Potamogeton pectinatus alone may be extremely productive, depending on location and conditions. For example, 840 individual plants/m² or shoot densities of 1000 shoots per m² were reported by Howard-Williams (1978; cited in Howard-Williams & Liptrot, 1980 and Kantrud, 1990). Growth rates and hence productivity is greatest early in the growth season. Values of 668 mg C/m²/day were reported in Loch Leven (Jupp & Spence, 1977), and may range between 548 -1400 C/m²/day depending on location (Kantrud, 1990). Potamogeton pectinatus biomass can be high, e.g. 72 g organic dry weight/m² in Loch Leven (Jupp & Spence, 1977), and 60 -210 g/m² in Canal de Mira , Portugal (Cunha & Moreira, 1995), while Kantrud (1990) suggested that a maximum standing crop of <200 g/m² might indicate limited growth. The below ground biomass varies between 4-78% of the total depending on grazing, substratum type (fine sediment or gravels and sand) and the allocation to vegetative production. The production of vegetative tubers and turions or seed can also be high, e.g. Kantrud (1990) reported that in culture 36,000 tubers, 800 turions, 6,000 seeds were produced from a single plant in single growing season, while over 4000 seeds /m² were observed deposited on substrata in the vicinity of Potamogeton pectinatus beds.

Secondary productivity
The macrophyte primary productivity is only directly available to grazing water fowl and a few grazing invertebrates (e.g. trichopterans) (Jacobsen & Sand-Jensen, 1986; Kantrud, 1990). Microalgal and macroalgal primary productivity probably support a large number of grazing species such as molluscs and isopods. Fennel pondweed dies back rapidly shortly after flowering, with stems becoming washed ashore or decomposing on the bottom (Kantrud, 1990). Decomposition is accelerated by shredding and grazing invertebrates (e.g. amphipods) that increase the surface area for microbial decomposition, while other species feed on the microbes (Byren & Davies, 1986; Kantrud, 1990). Decomposed pondweed provides a food source for benthic filter-feeding and deposit feeding organisms (Kantrud, 1990). For example, Bryen & Davies (1986) reported 9 invertebrate taxa in bags of decomposing Potamogeton pectinatus in South Africa, with a maximum biomass of 64 mg of invertebrates per g dry weight of the pondweed, dominated by grazing amphipods and predatory dragonfly larvae.

The Potamogeton pectinatus beds themselves also support a high biomass of invertebrates, providing secondary production further up the food chain. For example Howard-Williams & Liptrot (1980) reported that the submerged macrophyte beds in the Swartvlei region of South Africa supported 410g/m² of the bivalve Musculus spp., 4000 individuals/m² of an amphipod (ca 0.27 g/m²), large numbers of juvenile marine fish, and a resident population of 2000 -3000 coot. Cunha & Moreira (1995) reported average annual invertebrate densities of 40,318 - 225,806 individuals/m² (of all species) in Canal de Mira, Portugal.

Potamogeton pectinatus
Potamogeton pectinatus is a rhizomatous perennial, dying back in winter to leafy shoot bearing rhizomes and/or tubers in the winter months but may behave as an annual in some environments (see seasonal change). Fennel pondweed flowers in mid May to mid July, shortly after peak biomass of shoots is reached. Flowers are borne on long stalks (peduncles) to the water surface, where pollination by buoyant pollen occurs. But the peduncle is often not rigid enough to hold flowers at the surface and pollination can occur underwater between adjacent flowers by bubble pollination, although submerged pollination is not as efficient as at the water surface (Preston, 1995). Fruit (drupelets or achenes) begin to form about 3 weeks after flowering (Kantrud, 1990). Mature fruit sink to the bottom or temporarily float and are deposited on shore and germinate from late March to early summer. Most fruit are recovered close to shore (Kantrud, 1990). Seedling mortality is high in shallow waters (<2m) due to physical damage and smothering by litter and stranded vegetation and in deeper water due to lack of light (Kantrud, 1990).

There is little evidence of the importance of reproduction by seed in Potamogeton species, seedlings are rarely observed in nature, and van Wijk (1988, 1989a) concluded that seed was probably only important for dispersal and survival to exposure to long-term desiccation or drastic variations in salinity (in effectively annual populations), and that the maintenance of populations was due to vegetative persistence or reproduction (Kantrud, 1990; Preston, 1995).

Potamogeton pectinatus may over-winter as rhizomes and/or tubers in the sediment. For example, in Swedish brackish waters, 100% of the biomass in wave exposed sands was reported to be tubers while in sheltered muds 75% of the biomass was over-wintering shoots (Kautsky, 1987 cited in Kantrud, 1990). Tubers are produced by all populations of Potamogeton pectinatus and begin to develop as early as May. Germination of tubers begins in March and is stimulated by a cold snap or prior low temperatures (Kantrud, 1990; Preston, 1995).

Potamogeton species have considerable powers of dispersal (Preston, 1995). Pondweeds can disperse via specialized asexual propagules such as turions (see glossary of scientific terms), fragments of stems or rhizome or fruits that float (aided by their buoyancy) and can be carried long distances by currents or flood waters. For example, the fruit of Potamogeton pectinatus was reported to be able to float for 48-60 hours (Preston, 1995). Plant fragments may be transported on the bodies (e.g. feet) of water fowl, while fruits may be transported in their digestive tracts. A proportion of ingested fruit survive in the gut of birds. Viable fennel pondweed fruit were reported to take an average of 44 hours to pass through mallard ducks, potentially providing long-range dispersal. The abrasion received on passing through the gut may enhance germination (see Kantrud, 1990; Preston, 1995). The potential fecundity can be extremely high. For example, Yeo (1965 cited in Kantrud, 1990) grew 36,000 tubers, 800 turions, and 6,000 fruit from a single tuber and 63,300 fruit and 15,000 tubers from a single seed. Overall, Potamogeton pectinatus is considered to be a pioneering species, able to quickly colonize newly flooded areas or areas reclaimed from the sea, and often becomes dominant is areas that become temporarily unsuitable for other species, e.g. due to pollution (Kantrud, 1990).

Other species
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. For example, Electra crustulenta is probably adapted to rapid growth and reproduction (r-selected), capable of colonizing ephemeral habitats, but may also be long lived in ideal conditions (Hayward & Ryland, 1998). In settlement studies, Electra crustulenta recruited to plates within 5 -6months of deployment (Sandrock et al., 1991). Hydroids are often initial colonizing organisms in settlement experiments and fouling communities (Jensen et al., 1994; Gili & Hughes, 1995; Hatcher, 1998). In settlement experiments in the Warnow estuary, Cordylophora caspia was found to colonize artificial substrata within ca one month of deployment, its abundance increasing from June to the end of September with a peak in July (Sandrock et al., 1991). Similarly, Roos (1979) reported that Cordylophora caspia recruited to and grew luxuriantly on water lily stalks in summer after early reproduction of nearby colonies in early spring. Cordylophora caspia releases a planula larva, although planula may occasionally develop in the parent gonophores being released as juvenile polyps. Planula larvae swim or crawl for short periods (e.g. <24hrs) so that while local recruitment may be good, dispersal away from the parent colony is probably very limited (Gili & Hughes, 1995). Fragmentation and rafting on floating debris may also provide other routes for short distance dispersal.

Boström & Bonsdorff (2000) examined the colonization of artificial seagrass 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. The above observation suggests that most macrobenthic species in macrophyte beds may recruit rapidly.

Mobile species, such as the gammarids, small gastropods and mysids are probably able to recruit and colonize available habitats from the surrounding area. Hydrobid molluscs produce pelagic larvae capable of considerable dispersal and may also colonize new habitats by rafting. Coleoptera (beetles), Odonata (dragonflies) and Heteroptera (true-bugs), with adults capable of flight, will probably be able to colonize available habitats relatively quickly once established, although the ability to fly varies between species (van Vierssen & Verhoeven, 1983).

The sticklebacks Gasterosteus aculeatus and Spinachia spinachia may be associated with Potamogeton pectinatus beds. 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 pondweed 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.

Potamogeton pectinatus 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 Potamogeton pectinatus bed and its associated community (except the infauna) develops annually. Growth rates are high in spring and Potamogeton pectinatus can colonize space rapidly. Colonization by mobile species is probably rapid. Cunha & Moreira (1995, Figure 9) noted that peak abundance of molluscs, leeches and insects occurred in spring and summer, probably coincident with the peak of macrophyte biomass, while oligochaete and crustacean abundance peaked during late autumn and winter probably coincident with decomposition of senescent macrophytes. Therefore, the species richness and density of invertebrates fluctuates seasonally with macrophyte abundance or decomposition, suggesting that different invertebrate groups can colonize the pondweed beds readily, depending on season.

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