Biodiversity & Conservation

SS.IMU.Ang.A12

Explanation of sensitivity and recoverability


Physical Factors

Substratum Loss
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Removal of the substratum, e.g. due to dredging, would remove the Potamogeton pectinatus beds, its rhizomes and tubes, associated species and infauna. Therefore an intolerance of high has been recorded. Recoverability is probably high (see additional information below).
Smothering
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Haslam (1978) suggested that Potamogeton pectinatus was the most tolerant species of conditions in rivers affected by turbidity or sediment deposition due to coal mining effluent. Potamogeton pectinatus grows through additional layers of deposited sediment (Haslam, 1978). Rhizomes can be buried up to 15cm in the substratum while tubers can be found at 47cm below the surface of the substratum. Tubers planted at 20cm produced plants with reduced growth rates, so that growth is probably dependant on depth (Kantrud, 1990). Smothering during winter may result in reduced growth the following spring, although winter months are generally associated with increased scour due to high water flow rates and turbulence.

Smothering by flora and fauna may be of greater importance. Smothering and hence shading by epiphytes and filamentous algae reduces growth rates in fennel pondweed in eutrophic conditions (Kantrud, 1990). The hydroid Cordylophora spp. was reported to grow on Potamogeton pectinatus, forming a gelatinous coating inhabited by harmful organisms, which suffocated and injured the pondweed (Kantrud, 1990). The accumulation of fine silt on the leaves was reported to harbour epiphytic diatoms and shade the plant, resulting in reduced growth (Kantrud, 1990). Smothering of the sediment surface by deposition of sediment is unlikely to adversely affect burrowing infauna such as polychaetes, oligochaetes and deposit feeding amphipods e.g. Corophium spp. But suspension feeders such as Mya arenaria or Cerastoderma edule, if present, are probably intolerant of smothering, especially as juveniles (see MarLIN reviews). Although the biotope would probably not be adversely affected, loss of intolerant suspension feeders will result in a loss of species richness.

Therefore, smothering by 5cm of substratum is unlikely to significantly harm the plants, although the build up of sediment may reduce growth rates in the following growth season, if it remains over winter. Smothering by algal mats and epiphytes and fauna may be more harmful but the pondweed beds will probably survive. Therefore, an intolerance of low has been recorded.

Increase in suspended sediment
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Potamogeton pectinatus is considered to be tolerant of turbid waters (see below) (Haslam, 1978). The physical effects of suspended sediment (scour, clogging) are unlikely to adversely affect the pondweed. Many of the associated organisms, such as gastropods (e.g. Hydrobia spp.), hydroids (e.g. Cordylophora caspia), bryozoans (e.g. Electra crustulenta or Conopeum seurati), and crustaceans (e.g. Gammarus salinus) are typical of estuaries, salt marshes and lagoons that are characterized by high suspended sediment levels, and therefore likely to tolerate increased suspended sediment levels. The pondweed is likely to reduce water flow and increase siltation, so that increased suspended sediment is likely to increase the overall rate of accretion and raise the level of the substratum, potentially allowing emergent species to colonize in time. But increased accretion is likely to be minimal in a month (see benchmark). Therefore, an intolerance of low has been recorded. The major effect of increased suspended sediment levels is the change in turbidity (see below).
Decrease in suspended sediment
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A decrease in suspended sediment levels, and the resultant decrease in turbidity may allow other species to compete with Potamogeton pectinatus, e.g. Myriophyllum alterniflorum. But Potamogeton pectinatus is the dominating pondweed in brackish water conditions exemplified by this biotope, so that competition is likely to be minimal. A decrease in suspended sediment levels may reduce the food availability for suspension feeding invertebrates such as hydroids, bryozoans and mysids. Therefore, a biotope intolerance of low has been recorded.
Desiccation
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Potamogeton pectinatus survives in submergent and emergent communities, tolerates fluctuating water levels and can survive aerial exposure, although it forms short plants in emergent communities (Kantrud, 1990). Kantrud (1990) reported that tubers were not dependant of surface water cover to germinate but were intolerant of desiccation as 60% failed to germinate in exposed to sediment moisture less that 23% for two weeks, while Preston (1995) reported that most Potamogeton pectinatus tubers were killed by 2 months desiccation. Fruit can survive emersion for over year, and will germinate in a few days once wetted (Kantrud, 1990). Fruit production was only important for the long term population survival in areas subject to desiccation and/or drastic changes in salinity (van Wijk, 1988; 1989a). However, van Vierssen & Verhoeven (1983) reported that in pools in which the outer edges dry out in summer, Potamogeton pectinatus was restricted to deeper parts of the pools by competition from Zannichellia pedunculata and Ranunculus baudotii. Zannichellia pedunculata was able to reproduce and fruit quickly before the pools dried and Ranunculus baudoti survived as a land-form, while Potamogeton pectinatus did not form tubers until late summer and autumn, and was therefore excluded form areas of pools that dried in summer. Van Vierssen & Verhoeven (1983) therefore suggested that Potamogeton pectinatus was not tolerant of desiccation.

Mobile species such as gammarids, mysids and fish will probably avoid drying conditions and move to deeper water, while hydrobids are probably tolerant of desiccation, e.g. Hydrobia ulvae can survive emersed in sediment at the high strandline for over a week. But, bryozoans and hydroids are restricted to damp habitats on the shore, so that colonies on emergent plants are likely to be adversely affected.

Overall, an increase in desiccation at the benchmark level is likely to increase competition from desiccation tolerant emergent macrophytes, and decrease the upper extent of the fennel pondweed bed, especially where desiccation occurred early in the season, before reproductive propagules are formed. Therefore, an intolerance of intermediate has been recorded. Recovery is likely to be rapid, aided by remaining vegetative propagules and the surviving plants (see additional information below).
Increase in emergence regime
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Potamogeton pectinatus is tolerant of water fluctuation (at least 0.5 -1.75m in brackish waters) and can survive periodic exposure in tidal conditions (Kantrud, 1990). A decrease in water level in turbid conditions may increase growth of the fennel pondweed by increasing light penetration. But an increase in emergence will expose the beds to increased risk of desiccation (see above) and competition from emergent macrophytes.

Mobile species (e.g. fish, gammarids and mysids) will probably avoid the factor and filamentous green algae (e.g. Ulva spp.) are probably tolerant of emersion, while emersed bryozoans and hydroids may be adversely affected due to the increased desiccation risk, potentially reducing species richness.

Therefore, an intolerance of intermediate has been recorded to represent the potential loss of the upper extent of the population. Recovery is likely to be rapid (see additional information below).
Decrease in emergence regime
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Potamogeton pectinatus is tolerant of water fluctuation (at least 0.5 -1.75m in brackish waters) and can survive periodic exposure in tidal conditions (Kantrud, 1990). The effects of a decrease in emergence and hence increased immersion time and depth will depend on turbidity. In highly turbid waters, the resultant reduction in light levels is likely to reduce growth and biomass of shoots, rhizomes and reproductive propagules. For example, in clear brackish waters fennel pondweed survives changes of 2m in water level while a 10cm increase in highly turbid waters greatly reduced production (Kantrud, 1990). Van Vierssen & Verhoeven (1983) demonstrated a positive correlation between macrophytes cover and insect species diversity, so that a decrease in pondweed biomass is likely to reduce the species richness. Alternatively, an increase in immersion may allow the pondweed to out-compete emergent macrophytes at lower turbidities and colonize a larger area.

Overall, a decrease in emergence may allow Potamogeton pectinatus and its associated community to increase in extent. Alternatively, increased average depth may result in a decrease in biomass and the species richness of the associated community, where additional habitat is not available for colonization. Therefore, an intolerance of intermediate has been recorded. Recoverability is likely to be rapid (see additional information below).

Increase in water flow rate
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Although a deep rooted species, Potamogeton pectinatus was considered to have a low anchoring strength when compared to other macrophytes, depending on substratum, anchoring more firmly in coarse sediments than fine (Haslam, 1978; Kantrud, 1990). In summer, its rhizome system is difficult to erode (see wave action) but is easily damaged. The pondweed produces small plants in fast flow but large plants in slow flow (Haslam, 1978). Kantrud (1990) reported that currents >1m/s limited growth in the pondweed and one study, while the pondweed grew in currents up to 2m/s in another and concluded that Potamogeton pectinatus was tolerant of currents. However, Haslam (1978) suggested that the pondweed was intolerant of fast flow or storm flow.

This biotope has only been recorded from saline lagoonal habitats with very weak tidal streams on muddy substrata. An increase in water flow from very weak to moderately strong may reduce growth but would probably not damage the pondweed bed in most circumstances. The intolerant of the bed to damage is partly dependant on the substratum, with soft fine muds substrata being more susceptible to increased water flow. Strong to very strong currents, however would probably remove vegetation and some rhizome material, although a proportion of the plant will probably remain. Strong to very strong water flow may remove more plant material than can be compensated for by growth resulting in loss of the pondweed beds in the long term. In addition, increased water flow will favour coarser substrata although the pondweed can colonize a variety of substrata. The hydroid Cordylophora caspia and bryozoan Conopeum reticulum occur in a wide range of water flow regimes and are unlikely to be affected directly but a proportion may be lost if vegetation was removed. The crustacean fauna is found in strong water flow and will be probably unaffected by the increased water directly. Any loss of vegetation, and loosely attached filamentous algal mats will reduce their food supply.

However, at the benchmark level, although growth may be impaired the pondweed bed will probably survive. Therefore, an intolerance of low has been recorded. Recovery will probably be rapid (see additional information below). Communities in fine sediments may be more intolerant, and exposure to greater increases in water flow are likely to damage the bed resulting in lower biomass, cover and probably shorter plants, and probably reduced species richness.
Decrease in water flow rate
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Potamogeton pectinatus may dominate on soft sediments and slow water flow (Haslam, 1978) and its luxuriant growth may clog canals and streams, significantly reducing flow. It requires a modicum of water flow as an increase in water flow from 0.2 to equal to or >0.4mm/s increased its photosynthetic rate 1.5 fold (Kantrud, 1991). However, the above flow rate is negligible compared to coastal waters. This biotope was recorded from very weak tidal streams or negligible water flow so that a further decrease in water flow is unlikely and not relevant has been recorded.
Increase in temperature
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Kantrud (1990) concluded that Potamogeton pectinatus had a wide temperature tolerance, commensurate with its cosmopolitan distribution, and was adapted to temperature fluctuation. However, temperature affects growth and reproduction. For example:
  • van Wijk (1983) reported that tubers sprouted when water temperatures reached 5.5 °C in the field but that 25 °C was optimum for tuber germination in culture (Kantrud, 1990);
  • fruit were reported to germinate at 8 °C and flowering began at 15 °C in Canadian lakes (Kantrud, 1990);
  • optimum growth was observed at 23-30 °C,while little growth occurred at 37 °C and growth was slow at 10 °C;
  • in experimental ponds fennel pondweed and other pondweeds died at 38 °C;
  • in brackish waters growth was suppressed at 25 °C and the plants were covered in epiphytes, and
  • maximum net photosynthesis occurred at 25-28 °C (Kantrud, 1990).
Thermal effluent from a Canadian power station, averaging 7 °C above ambient, resulted in earlier and heavier flowering, a higher standing crop and increased vegetative growth of Potamogeton pectinatus, replacing Myriophyllum spicatum (Haag & Gorhan, 1977). Similarly, fennel pondweed, increased in areas affected by thermal effluent on the Finnish coast, although it was replaced by Myriophyllum spicatum and Cladophora glomerata in area subject to the highest temperature increases and highest water flow (Kantrud, 1990).

The majority of the characterizing species have broad temperatures tolerances or are widely distributed to the north or south of Britain and Ireland, and unlikely to be affected by changes in temperature at the benchmark level. But an acute increase in temperature may adversely affect spring populations of Neomysis integer (see species review).

Overall, an increase in temperature may result in increased growth of Potamogeton pectinatus and ephemeral green algae, providing additional food and cover for the invertebrate fauna. However, excessive growth of ephemeral algae may smother the pondweed in brackish water conditions. Otherwise the pondweed and its associated community may benefit.
Decrease in temperature
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Kantrud (1990) reported that Potamogeton pectinatus was distributed circumboreally, north to about 70° N and concluded that Potamogeton pectinatus had a wide temperature tolerance, commensurate with its cosmopolitan distribution. Kantrud (1990) reported that 5 °C was the lower limit of fennel pondweed growth, although tubers began to sprout at 5.5 °C. A cold snap was also reported to enhance tuber germination. In shallow water, fennel pondweed is likely to be damaged by frost if exposed but shoots may be found under ice in deeper waters (Kantrud, 1990).

The majority of the characterizing species have broad temperatures tolerances or are widely distributed to the north or south of Britain and Ireland, and unlikely to be affected by changes in temperature at the benchmark level.

Overall, the pondweed bed and its associated community is unlikely to be affected by long term decreases in temperature and will probably survive acute temperature decreases at the benchmark level.
Increase in turbidity
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Potamogeton pectinatus was considered to be of intermediate tolerance to turbid waters, compared with other macrophytes (Haslam, 1978) but tends to dominate highly turbid waters, e.g. due to suspended sediments, sewage or coal mining effluents, that are unsuitable for other macrophytes and was considered to have a high tolerance to turbid conditions by authors cited by Kantrud (1990). Potamogeton pectinatus exhibits local adaptations to turbid conditions in eutrophic (see nutrients) or brackish waters (van Wijk et al., 1988; Kantrud, 1990) including increased tuber formation and increased shoot length allowing the plants to rapidly reach the water surface and develop a canopy. Potamogeton pectinatus is also shade tolerant, growing under overhanging trees and under emergent macrophytes (Kantrud, 1990). However, increased turbidity reduces growth, biomass and production in the pondweed. For example, 100ppm of suspended sediment reduced fennel pondweed production by 50% in culture and production was low in silted, carp infested waters with Secchi disks depths of <30cm. Increased water depth reduces the amount of light available to submergent plants and algae, so that increased water depth may be detrimental while a reduction in water depth may offset the effects of increased turbidity (see emergence) (Kantrud, 1990). Very high turbidity will exclude most pondweeds including Potamogeton pectinatus. For example, fennel pondweed was reported to be absent from a New Zealand lake in areas of 100-300ppm suspended sediment, and from lakes with Secchi depths of <20cm. Kantrud (1990) concluded that Secchi depths of <20cm usually indicated waters that would not support fennel pondweed growth. Kantrud (1990) also noted that growth of fennel pondweed improved water transparency by anchoring the substratum, reducing water turbulence, oxygenating the water column, and sequestering nutrients.

Increased turbidity will also probably reduce the growth of epiphytic or filamentous green algae and charophytes (e.g. Lamprothamnium papulosum) but most invertebrates are unlikely to be affected directly, although loss of macrophyte or algal productivity will reduce the food supply for grazers and ultimately decomposers and deposit feeders.

Overall, Potamogeton pectinatus is probably relatively tolerant of turbidity at the benchmark level, and is only likely to be excluded under extremely turbid conditions (see benchmark). However, high turbidity will probably reduce the productivity of community, in terms of both macrophyte and macroalgal primary productivity and hence secondary production. Therefore, the biotope would probably survive long term change to high turbidity and even to extreme turbidity in the short term (one month, see benchmark), depending on depth, although a proportion of the biomass will be lost and an intolerance of intermediate has been recorded. Nevertheless, while Potamogeton pectinatus is tolerant of high turbidity levels, increased turbidity has been implicated in its loss form some wetlands (Kantrud, 1990) and in moderate to highly turbid waters the pondweed is probably highly intolerant of any further increase in turbidity.
Decrease in turbidity
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In freshwater systems, a decrease in turbidity will probably allow other submergent macrophytes to invade the habitat, forming mixed stands and increasing competition with Potamogeton pectinatus. However, in brackish water exemplified by this biotope, few species other than Myriophyllum spicatum, Potamogeton filiformis, Ranunculus baudoti, charophytes are likely to compete with the pondweed (see NVC A6 and A11, Rodwell, 1995). Therefore, the increased light is likely to increase the biomass and cover of the pondweed, and hence potential species richness and 'not sensitive*' has been recorded.
Increase in wave exposure
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Haslam (1978) suggested that Potamogeton pectinatus was of intermediate tolerance to turbulence caused by wind generated wave action or boat wash in lakes. Submerged macrophytes are usually restricted to wave sheltered areas of lakes such as river inlets and protected bays (Kantrud, 1990; Preston, 1995), and Potamogeton pectinatus may be restricted to deeper waters in less sheltered sites. Fennel pondweed is likely to be torn or broken during storms but recover from underground rhizomes and tubers (Haslam, 1978). Haslam (1978) reported that in one example, rapid growth in early summer negated the effects of storm damage, while in autumn storm damage removed 80% of vegetation. Haslam (1978) suggested the recurrent storm damage may result in loss of a population. Increased wave action is likely to increase the turbidity due to resuspension of the sediments, or remove of suitable substrata. For example, Kantrud (1990) reported that fennel pondweed was still recovering 22 years after a storm, probably due to removal of substrata and increased turbidity due to plankton blooms.

Where present, Potamogeton pectinatus beds stabilize the sediment, and buffer wave action for other plants, e.g. Verhoeven (1980a) suggested that the upper zone of charophytes (Lamprothamnium papulosum and Chara spp.) in the brackish lake Swartvlei, the Netherlands, was dependant on the protection from wave action afforded by the deeper stands of Potamogeton pectinatus.

The majority of the associated invertebrate species are probably adapted to wave sheltered conditions, or fine sediments associated with wave sheltered conditions. Mobile species such as mysids, gobies and sticklebacks will probably move to deeper water to avoid wave turbulence. More sedentary gammarids or hydroids may be washed away, while the benthic infauna may be changed due to changes in the substrata from fine to coarser sediment, and a proportion of the epifauna and epiflora will be lost on removed vegetation.

Overall, this biotope in characteristic of extremely wave sheltered conditions, so that an increase in wave exposure from e.g. extremely sheltered to sheltered is likely to result in loss of a proportion of the biotope and an intolerance of intermediate has been recorded. The biotope is likely to be highly intolerant of a further increase, e.g. to moderate exposure, resulting in prolonged and low recoverability, as cited by Kantrud (1990).

Decrease in wave exposure
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This biotope is characteristic of extremely to ultra wave sheltered conditions, so that any further decrease in wave exposure is unlikely.
Noise
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The majority of species in Potamogeton pectinatus dominated communities are unlikely to react to noise at the benchmark level. Wildfowl, however, are intolerant of disturbance from noise from e.g. shooting (Madsen, 1988) and from coastal recreation, industry and engineering works. For example, Percival & Evans (1997) reported that wigeon were very intolerant of human disturbance and, where wildfowling was popular, wigeon avoided Zostera noltii beds at the top of the shore.
Visual Presence
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The majority of species in Potamogeton pectinatus dominated communities have poor, if any, visual acuity, and are unlikely to react to visual disturbance. However, mobile fish may be disturbed by passing boats but probably with minimal effect. Wildfowl, however, are intolerant of disturbance from noise from e.g. shooting (Madsen, 1988) and from coastal recreation, industry and engineering works. For example, Percival & Evans (1997) reported that wigeon were very intolerant of human disturbance and, where wildfowling was popular, wigeon avoided Zostera noltii beds at the top of the shore.
Abrasion & physical disturbance
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The rhizome system of Potamogeton pectinatus is deep and extensive in summer and therefore difficult to erode, although easily damaged (Haslam, 1978). The pondweed bed will probably be damaged and torn by a passing anchor or mobile fishing gear (see benchmark) and a proportion of attached epifauna and epiphytes and filamentous algae will be lost. Recovery from remaining rhizomes and tubers will probably be rapid. Therefore an intolerance of intermediate has been recorded with a recoverability of high.
Displacement
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Potamogeton pectinatus would probably be severely damaged and fragmented by displacement, resulting in loss of filamentous algae, epiphytes and epifauna, and the associated fauna. The mobile invertebrates and fish, e.g. gammarids, mysids and fish will probably be unharmed and migrate to adjacent areas. However, the community would probably be lost and an intolerance of high has been recorded. Potamogeton pectinatus is able to root from fragments of rhizome and stem, so that recovery will probably be rapid (see additional information below).

Chemical Factors

Synthetic compound contamination
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Haslam (1978) suggested that Potamogeton pectinatus was very tolerant of sewage and industrial effluent pollution, often dominating affected waters where other plants can not survive. But herbicides and agricultural chemicals caused major damage, especially in still waters (Haslam, 1978). For example, Coyner et al.(2001) reported that the herbicide chlorsulfuron reduced growth rates, and vegetative production at 0.25 and 0.5ppb, while significant decrease in biomass and increased mortality occurred at greater than or equal to 1ppb chlorsulfuron. Several herbicides, including Atrazine and Diquat, have been used to control growth in Potamogeton pectinatus in irrigation ditches in North America (for details see Kantrud, 1990).

Similarly, most pesticides and herbicides were suggested to be very toxic for invertebrates, especially crustaceans (amphipods isopods, mysids, shrimp and crabs) and fish (Cole et al., 1999). For example, Lindane was shown to be very toxic to gobies (Gobius spp.: see Pomatoschistus minutus) (Ebere & Akintonwa, 1992) . The pesticide Ivermectin is very toxic to crustaceans, and has been found to be toxic towards some benthic infauna such as Arenicola marina (Cole et al., 1999).

Therefore, synthetic chemicals found in agricultural, urban and industrial discharges are likely to adversely affect the biotope. Herbicides in particular are likely to reduce growth and productivity of the pondweed beds, and may result in its loss. In addition, loss of particularly intolerant crustaceans may result in unchecked growth of epiphytes, which would again reduce photosynthesis and productivity of the pondweed beds. Overall, synthetic chemical contamination will at least result in a reduction in productivity, seed set and ultimately the extent of the Potamogeton pectinatus bed. Therefore, fennel pondweed is probably highly intolerant of herbicide contamination. Recovery is probably dependant on recolonization of the habitat (once the contaminants have dispersed or depurated) but will probably take less than 5 years.
Heavy metal contamination
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Haslam (1978) suggested that macrophytes were little affected by heavy metals, since a countrywide survey had not been able to detect any correlation between plant distributions and heavy metal concentrations of Cr, Co, Cu, Fe, Pb, Mn, Ni, Sn and Zn. The chemical constitution of waters and sediments inhabited by Potamogeton pectinatus (including heavy metals concentrations) was given by Kantrud, 1990). But Greger & Kautsky (1991) reported that sediment concentrations of 4µg Pb, 13 µg Cu and 38µg Zn /g dry weight of sediment reduced the biomass of the pondweed.

Cole et al. (1999) suggested that Pb, Zn, Ni and As were very toxic to algae, while Cd was very toxic to Crustacea (amphipods, isopods, shrimp, mysids and crabs), and Hg, Cd, Pb, Cr, Zn, Cu, Ni, and As were very toxic to fish. Gobies were reported to be particularly intolerant of Hg (see Pomatoschistus minutus). Bryan (1984) reported sublethal effects of heavy metals in crustaceans at low (ppb) levels.

Bryan (1984) suggested that polychaetes are fairly resistant to heavy metals, based on the species studied. Short term toxicity in polychaetes was highest to Hg, Cu and Ag, declined with Al, Cr, Zn and Pb whereas Cd, Ni, Co and Se were the least toxic. He also suggested that gastropods were relatively tolerant of heavy metal pollution.
The intolerance of crustaceans to heavy metal contaminants suggests that amphipod and isopod grazers would be lost, allowing rapid growth of epiphytes, and reduced turnover of the detrital food chain. Additional growth by the epiphytes and algal mats, unless they are adversely affected themselves, could potentially compete with pondweed stands for light and nutrients reducing productivity. Overall, in the absence of other evidence, the Potamogeton pectinatus beds would probably survive, with reduced productivity, but several members of the community may be lost (e.g. fish and crustaceans) resulting in a reduced species richness. Therefore, an intolerance of intermediate has been recorded. Recoverability would probably be high (see additional information below).
Hydrocarbon contamination
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Little information on the effects of hydrocarbon contamination from, for example oil spills, on Potamogeton pectinatus beds was found. Where they occur, oil spills are likely to persist for some time in sheltered, soft sediment habitats.

Suchanek (1993) noted that gastropods, amphipods, infaunal polychaetes and bivalves were particularly sensitive to oil spills. For example substantial kills of Nereis, Cerastoderma, Macoma, Arenicola and Hydrobia were reported after the Sivand oil spill in the Humber (Hailey, 1995). Single oil spills were reported to cause a 25-50% reduction in abundance of Arenicola marina (Levell, 1976). The toxicity of oil and petrochemicals to fish ranges from moderate to high (Cole et al., 1999). The water soluble fraction of oils was shown to cause mortality in sand gobies and fish, especially their larvae, are thought to be intolerance of polyaromatic hydrocarbons (PAHs) (see Pomatoschistus minutus). PAHs are significantly more toxic when exposed to sunlight (Ankley et al., 1997) , and may have a greater effect in clear shallow waters inhabited by pondweed communities.

Therefore, while there no evidence was found to suggest that Potamogeton pectinatus spp. would be directly affected by hydrocarbon contamination, its associated community may be lost. Loss of grazers may increase epiphytic fouling resulting in lower growth and productivity. However, given the likely persistence of oils in sheltered, sedimentary habitats, an overall intolerance of high has been recorded.
Recovery will depend on recolonization by the associated invertebrate community, which is likely to be rapid (see additional information below).
Radionuclide contamination
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No information found
Changes in nutrient levels
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Potamogeton pectinatus is able to withstand a wide range of nutrient concentrations. It is capable of absorbing nutrients through its leaves and stems as well as its roots and influences nutrient cycling in natural waters (Kantrud, 1990). It requires Mg, Ca and P for active growth (Kantrud, 1990) but Ca is unlikely to be limiting in brackish water conditions. Kantrud (1990) suggested that Potamogeton pectinatus was seldom limited by nutrients.

Potamogeton pectinatus is characteristic of and often dominant in naturally eutrophic waters and polluted, oxygen-poor, waters high in nutrients due to agricultural runoff, sewage or municipal wastes (Haslam, 1978; Kantrud, 1990; Preston, 1995). However, hyper-eutrophicated conditions the biomass of the pondweed may decrease. Kantrud (1990) reported that extremely high nutrient concentrations injure or destroy the plant resulting in its replacement by algae, probably partly due to increased turbidity caused by increased phytoplankton blooms. Kantrud (1990) suggested that submerged macrophytes out -compete and hence inhibit the growth of algal epiphytes and phytoplankton in nutrient limited waters. However, phytoplankton blooms in eutrophic waters, either due to natural fluctuations in macrophyte abundance, phytoplankton predators, or eutrophic conditions, increases the turbidity by removing photosynthetically active light wavelengths, and hence, greatly reduce the biomass of the pondweed, often restricting it to shallow waters.

Gammarus salinus and Cordylophora caspia have been associated with polluted waters (see reviews), while most epiphytic and epistatic grazers would probably benefit from the increased algal growth stimulated by eutrophic conditions. But it was suggested that the nationally rare foxtail stonewort Lamprothamnium papulosum was intolerant of nutrient enrichment being absent from water with >20 µg/l, and preferring nutrient poor sites (Bamber et al., 2001). Therefore, if present the foxtail stonewort will probably be lost due to nutrient enrichment.

Overall, Potamogeton pectinatus beds are probably tolerant of increase in nutrients at the benchmark level, and would possibly even benefit from enrichment, detrimental effects only manifesting at extreme high nutrient levels. Loss of a few intolerant species, e.g. some macrophytes and the foxtail stonewort will probably reduce species richness. Nevertheless, a not sensitive* has been recorded.
Increase in salinity
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Potamogeton pectinatus and Potamogeton filiformis are the only Potamogeton species to penetrate brackish water. Potamogeton pectinatus grows optimally between 5-14g/l in brackish waters with a maximum salinity tolerance of 8ppt Cl¯ (ca 15psu). It grows well below 4 ppt Cl¯ (ca 7.25psu) but is replaced by Ruppia dominated communities above 9 ppt Cl¯ (ca 16.25 psu), forming mixed stands at intermediate salinities (see £IMS.Rup£).

The nationally rare foxtail stonewort Lamprothamnium papulosum was reported to prefer 8-28psu but tolerate up to 32psu. Most brackish water species are adapted to a wide range or variable salinities, e.g. Hydrobia ulvae, Gammarus salinus and Gammarus insensibilis, however the mysid Neomysis integer is predominantly brackish water and has an upper tolerance limit of 20 - 25psu (see review).

Overall, a short term increase in salinity e.g. from low to variable for a week would probably stress the pondweeds and a few members of the invertebrate community but otherwise have limited effects. However, a long term change from e.g. reduced to variable salinity would probably result in loss of the Potamogeton pectinatus bed, and a change in the invertebrate community to more marine species, probably resulting in its replacement by Ruppia dominated communities in the long term (see £IMS.Rup£). Although the invertebrate fauna of brackish water Potamogeton pectinatus dominated communities and Ruppia dominated communities are similar, the biotope would effectively be lost and an intolerance of high has been recorded.
Decrease in salinity
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A further decrease in salinity e.g. from reduced to low or to freshwater, will probably exclude the marine or estuarine components of the invertebrate fauna, e.g. Gammarus salinus and bryozoans, while allowing more freshwater species to colonize, e.g. insects. Potamogeton pectinatus would probably experience greater competition form other submerged macrophytes, such as Myriophyllum spicatum, forming mixed stands similar to the more species rich NVC A11. Therefore, although many members of the faunal community will probably remain, and Potamogeton pectinatus would probably still be a dominant macrophyte, NVC A12 (the biotope) would probably become NVC A11 and be lost, therefore an intolerance of high has been recorded.
Changes in oxygenation
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Potamogeton pectinatus, like many submergent macrophytes, is adapted to grow in hypoxic sediments. Air channels within the leaves and stem supply the roots with oxygen. Fennel pondweed is characteristic of polluted and oxygen-poor waters. In some cases its night-time respiration was reported to reduce dissolved oxygen to unacceptable levels in wetlands. Spencer & Ksander, (1997) reported that anoxic conditions caused propagules to sprout earlier than in aerobic conditions and noted no difference in the proportion of propagules that sprouted under either oxygen regime.

Most of the species identified as characterizing can probably tolerate low oxygen concentrations (e.g. Cordylophora caspia, benthic infauna, the mud snails Hydrobia spp. and the bryozoan Conopeum spp.) as they are characteristic of wave sheltered and low water flow environments subject to low oxygen conditions. Mobile gammarids and fish are probably able to avoid low oxygen conditions.

Overall, an intolerance of low has been recorded at the benchmark level. Recovery will probably be rapid.

Biological Factors

Introduction of microbial pathogens/parasites
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The loss of large areas of fennel pondweed and other macrophytes occurred in North American wetlands between 1918 and 1926. The decline was thought to be caused by fungal infection by Rhizoctonia solani and possibly other fungi. Fennel pondweed was particularly susceptible at 3-7psu. But Kantrud (1990) suggested that the evidence for direct role of pathogens in the decline was inconclusive. The aphid Rhopalsiphum nymhaeae uses several Potamogeton species as a secondary host, causing in-rolling of the leaf margin Preston, 1995). The smut fungus Doassansia martianoffiana forms pustules of the underside of leaves of several Potamogeton species (Preston, 1995). Many species of invertebrates, including crustaceans and gastropods are secondary hosts for fish or bird parasites (see individual species reviews for examples).

Any form of infestation or disease is likely to reduce the viability of the infected population. Gastropod molluscs may also be castrated by heavy trematode infestation. Therefore, in the absence of other evidence an intolerance of low has been recorded.

Introduction of non-native species
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No information found.
Extraction
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Potamogeton pectinatus beds are a significant food plant for water birds. Water birds were reported to excavate holes in the bed 10 cm wide by 0.3 m deep in search of tubers. Water birds were estimated to remove 21% of the biomass, or 40% of the standing crop and 43% of tubers in separate studies (Jupp & Spence, 1977; Kantrud, 1990). For example, in Loch Leven losses from grazing were less than those caused by wave action, and tuber density was not affected by grazing (Jupp & Spence, 1977; Preston, 1995). Van Wijk (1988) reported that above and below ground biomass were reduced by grazing by coot, mallard and mute swan (Preston, 1995). But beds of fennel pondweed were reported to have remained for 20 years under heavy water bird grazing (Kantrud, 1990). Overall, a proportion of the standing crop and biomass would be removed by water bird predation and an intolerance of intermediate has been recorded by definition. But the bed would probably recover rapidly, possibly within the growing season depending on the time of year, and hence community will probably survive.

Additional information icon Additional information

Recoverability
Zieman et al. (1984) noted that the recovery of seagrass ecosystems depended primarily of the extent or magnitude of damage to the sediments, i.e. the rhizome and root system. This is probably also true of aquatic macrophyte dominated communities.

Potamogeton pectinatus dies back in winter and grows back from over-wintering tubers and/or rhizomes, or from seed in annual populations. A single tuber or seed may be highly productive (see recruitment) and growth rates are high, especially in early spring (see productivity). Control of Potamogeton pectinatus may involve cutting back of the leaves and shoots (Rodwell, 1995). Fennel pondweed will not grow back immediately if cut late in the growing season, but in summer two or even three cuts may be required (Rodwell, 1995). Therefore, where tubers and/or rhizomes remain or a seed bank is present a well developed fennel pondweed bed is likely to develop within one growing season. Haslam (1978) reported that aquatic macrophytes could recover within a single growing season after shallow dredging, where a proportion of plant propagules remained. For example, after shallow dredging in the Great Ouse, Potamogeton pectinatus developed stable populations with 2-3 growing seasons (Haslam, 1978). But Haslam (1978) suggested that if plants are completely removed, recovery would probably take several years.

Potamogeton species have considerable powers of dispersal via specialized asexual propagules such as turions, fragments of stems or rhizome or fruits that float (aided by their buoyancy) and can be carried long distances by currents, flood waters, or by birds (Kantrud, 1990; Preston, 1995). 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).

The epiphytic and epifaunal species will probably recruit to the available habitats quickly, as will mobile species such as crustaceans, insects and fish (see recruitment).

Overall, recovery is likely to be rapid, probably within 1 year or at most 2-3 years where vegetative or sexual propagules remain in the sediment, or where neighbouring or upstream population exist. Where, the plants and propagules are completely removed or destroyed, recovery will take longer, but due to its potentially high dispersal potential , probably no longer than 5 years. Isolated habitats, e.g. lagoons, will depend on dispersal by water birds or flood waters from upstream populations, which could occur within a year or be protracted, although a stable population will probably establish quickly after recruitment.

This review can be cited as follows:

Tyler-Walters, H. 2002. Potamogeton pectinatus community. Marine Life Information Network: Biology and Sensitivity Key Information Sub-programme [on-line]. Plymouth: Marine Biological Association of the United Kingdom. [cited 29/08/2014]. Available from: <http://www.marlin.ac.uk/habitatbenchmarks.php?habitatid=320&code=1997>