Biodiversity & Conservation

IR.SIR.EstFa.HarCon

Explanation of sensitivity and recoverability


Physical Factors

Substratum Loss
(View Benchmark)
Removal of the available substratum will inevitably result in loss of species attached or embedded in it and hence the biotope. Therefore, an intolerance of high has been recorded. In the case of widespread substratum removal, the nationally rare Hartlaubella gelatinosa may take several years to recruit from apparently isolated populations. Therefore, a recoverability of moderate has been recorded (see additional information below).
Smothering
(View Benchmark)
Smothering by 5cm of sediment (see benchmark) is likely to prevent feeding and hence reduce growth and reproduction in encrusting bryozoans. The hydroid hydranths are relatively large and some parts of the colony are likely to protrude above 5cm of sediment. However reduced feeding, together with local hypoxic conditions under the sediment layer will probably reduce growth and reproduction rates. In addition, associated sediment abrasion may remove the bryozoan colonies. Balanus crenatus was also recorded as highly intolerant of smothering (see review). Brault & Bourget (1985) noted that loss of Balanus crenatus resulted in loss of several members of the community. Therefore, a biotope intolerance of high has been recorded. Hydroids will probably survive and recover abundance quickly, and the community recover rapidly. Therefore, a recoverability of high has been recorded (see additional information below).
Increase in suspended sediment
(View Benchmark)
This biotope was recorded from the upper and central regions of the Tamar estuary Plymouth exposed to relatively high suspended sediment loads, characteristic of estuarine habitats. This biotope is likely to experience variation in suspended sediment loads, being high in winter floods but lower in summer. The moderately strong currents experienced by the biotope probably reduce the likelihood of significant siltation, although some sediment probably accumulates at the base of hydroid stems. Therefore, this biotope is tolerant of high suspended sediment loads and an increase in suspended sediment at the benchmark levels is unlikely to have adverse effects but may reduce the feeding efficiency of the encrusting bryozoans and interfere with larval settlement. Therefore, an intolerance of low has been recorded.
Decrease in suspended sediment
(View Benchmark)
A decrease in suspended sediment may reduce the availability of organic particulates and hence reduce food availability. Reduced siltation may improve larval settlement for all species in the community. Encrusting bryozoans and hydroids require an adequate food supply to maintain their rapid growth rates and reproduction. Therefore, an intolerance of low has been recorded.
Desiccation
(View Benchmark)
This biotope is subtidal occurring from 0-5m. However, its upper extent may be exposed during periods of low water level. Intertidal populations of hydroids, Conopeum reticulum, and Electra sp. are restricted to damp habitats such as underboulders, overhangs or the interstices between macroalgae. The branched growth form of hydroids is likely to retain water on emersion (e.g. see Cordylophora caspia). Balanus crenatus has more permeable shell plates than littoral barnacles and therefore loses water quicker and dies sooner when exposed to air e.g. Foster (1971) recorded that Balanus crenatus adults of 6 mm and 11 mm diameter can withstand 17 hours and 40 hours of aerial exposure respectively. Similarly, Barnes et al. (1963) recorded that Balanus crenatus had a mean survival time of 14.4 hours in dry air. Hence, hydroids and Balanus crenatus are likely to be particularly intolerant of desiccation at the benchmark level and the upper portion of the population may be removed. Therefore a biotope intolerance of intermediate has been recorded. Dormant stages, if present, may survive, so that hydroids may recover relatively quickly and a recoverability of very high has been recorded.
Increase in emergence regime
(View Benchmark)
An increase in emergence will result in a larger proportion of the population being exposed to intertidal conditions, increased extremes of temperature, reduced ability to feed and an increased risk of desiccation (see above). Therefore, the upper proportion of the population is likely to be adversely affected and an intolerance of intermediate has been recorded. Infaunal polychaetes will probably be relatively unaffected. Recoverability has been recorded as very high.
Decrease in emergence regime
(View Benchmark)
A decrease in emergence, at the benchmark level, is likely to provide additional habitat for colonization by encrusting bryozoans, hydroids and barnacles, and reduce the risk of desiccation to existing colonies. Therefore, not sensitive* has been recorded.
Increase in water flow rate
(View Benchmark)
Water movement is essential for hydroids and other suspension feeders such as encrusting bryozoans and barnacles, to supply adequate food, remove metabolic waste products, prevent accumulation of sediment and disperse larvae or medusae. Hydroids are expected to be abundant where water movement is sufficient to supply adequate food but not cause damage (Hiscock, 1983; Gili & Hughes, 1995). Balanus crenatus is found in a wide range of water flows and is likely to be intolerant of change in this factor. Conopeum reticulum has been reported in strong to weak tidal streams (JNCC, 1999). Therefore, it is probably tolerant of a wide range of water flow rates. Similarly, Hartlaubella gelatinosa has been recorded in very strong, moderately strong and weak tidal streams (JNCC, 1999). Most hydroids have a narrow range of water flow rates for effective feeding, and feeding efficiency decreasing a high water flow rates. Overall, an increase in water flow from e.g. moderately strong to very strong may interfere with feeding and larval settlement and remove shells and small stones, to which members of the community were attached. Therefore, a proportion of the population may be lost or damaged and an intolerance of intermediate has been recorded. Recoverability is likely to be very high.
Decrease in water flow rate
(View Benchmark)
Water movement is essential for hydroids and other suspension feeders such as encrusting bryozoans and barnacles, to supply adequate food, remove metabolic waste products, prevent accumulation of sediment and disperse larvae or medusae. Hydroids are expected to be abundant where water movement is sufficient to supply adequate food but not cause damage (Hiscock, 1983; Gili & Hughes, 1995). A reduction in water flow may adversely affect recruitment, since Judge & Craig (1997) reported that initial settlement and recruitment of Obelia longissima increased several fold with a doubling of current speed from <2 to >50 cm/s. However, Hartlaubella gelatinosa has been recorded from very strong to weak tidal streams, and Conopeum reticulum from strong to weak tidal streams. Therefore, they are probably tolerant of a wide range of water flow rates.

However, in the turbid waters of the upper estuary, a decrease in water flow may significantly increase siltation rates, potentially resulting in smothering (see above) and loss of suitable hard substratum. Therefore, an intolerance of intermediate has been recorded. Recoverability has been assessed as very high (see additional information below).

Increase in temperature
(View Benchmark)
Growth rates were reported to increase with temperature in several bryozoans species, however, zooid size decreased, which may be due to increased metabolic costs at higher temperature (Menon, 1972; Ryland, 1976; Hunter & Hughes, 1994). Temperature is also a critical factor stimulating or inhibiting reproduction in hydroids, most of which have an optimum temperature range for reproduction (Gili & Hughes, 1995). Most of the hydroid and bryozoan species within the biotope are recorded to the north or south of the British Isles and are unlikely to be adversely affected by long term increases in temperature at the benchmark level, e.g. Hartlaubella gelatinosa has been recorded from southeast Sweden to the Mediterranean (Cornelius, 1995b). Several members of the community may also survive acute temperature change, e.g. in Conopeum reticulum and Electra pilosa upper lethal temperatures of 30-32 °C and 25-29 °C respectively were reported (Menon, 1972, 1974). Therefore, an increase in temperature at the benchmark level is likely to affect growth and reproduction in the encrusting bryozoan and hydroid species but otherwise have few adverse effects.
However, Balanus crenatus is a boreal species, and likely to be intolerant of long term chronic or acute short term increases in temperature. Brault and Bourget (1985) reported that loss of the dominant colonizing species (e.g. Balanus crenatus) resulted in a destabilized and poorer community. Therefore, a biotope intolerance of intermediate has been recorded. Recoverability is likely to be very high (see additional information below).
Decrease in temperature
(View Benchmark)
Conopeum reticulum and Electra pilosa were reported to survive below freezing temperatures (Menon, 1972) although colonies are probably more tolerant of low temperatures in winter than summer (see review for details). Low temperatures may trigger regression or dormancy in hydroids (e.g. Cordylophora caspia). Brault & Bourget (1985) noted that recruitment was delayed until spring on settlement plates deployed in winter. However, all the dominant species within the biotope are boreal or recorded from north of the British Isles. Therefore, although growth and reproduction may be reduced, they are unlikely to be adversely affected by reductions in temperature in British waters and an intolerance of low has been recorded.
Increase in turbidity
(View Benchmark)
A decrease in light penetration may decrease competition for space between characterizing species and macroalgae. However, an increase in turbidity is likely to result in a decrease in phytoplankton and macroalgal primary production, which may reduce food available to the suspension feeders within the community. Therefore, an intolerance of low has been recorded.
Decrease in turbidity
(View Benchmark)
An decrease in turbidity may increase phytoplankton and hence zooplankton productivity and potentially increase food availability. It may also allow settlement of foliose algae that would compete with the otherwise dominant animal species. However, the biotope includes much bare hard substratum so that competition is likely to be minimal. Therefore, an intolerance of low has been recorded.
Increase in wave exposure
(View Benchmark)
The biotope occurs in very wave sheltered situations, although storms may create significant oscillatory water movement in shallow depths. The oscillatory water flow caused by wave action is potentially more damaging to delicate marine organisms than unidirectional flow. The encrusting bryozoans and Balanus crenatus are tolerant of a wide range of wave exposures (e.g. see Conopeum reticulum review). Wave exposed conditions tend to favour small, less branched species of hydroid than are found in this biotope (Boero, 1984; Gili & Hughes, 1995). Hartlaubella gelatinosa has only been recorded from wave sheltered conditions. Therefore, it is likely than an increase in wave exposure at the benchmark level (e.g. from 'very sheltered' to 'moderately exposed') is likely to result in loss or damage of the hydroid colonies. The most likely effect adverse effect of wave action is the displacement of hard substrata (e.g. small rocks, cobbles or pebbles) and attached organisms. The resultant movement of the substratum and sediment scour may remove attached hydrorhizae and even resting stages of hydroids but many are likely to survive. Therefore, an intolerance of intermediate has been recorded. Recovery will depend in part on recruitment from other areas. A recoverability of very high has been suggested.
Decrease in wave exposure
(View Benchmark)
A decrease in wave exposure to ultra sheltered is unlikely to have any adverse effects since there is adequate water movement caused by strong tidal streams. Therefore, not sensitive has been recorded.
Noise
(View Benchmark)
Hydroids, bryozoans or barnacles are unlikely to be sensitive to noise or vibration at the benchmark level.
Visual Presence
(View Benchmark)
Hydroid and bryozoan polyps or barnacle cirri may retract when shaded by potential predators, however the community is unlikely to be affected by visual presence.
Abrasion & physical disturbance
(View Benchmark)
Abrasion by an anchor or fishing gear is likely to remove relatively delicate uprights of hydroids, damage bryozoan colonies and crush barnacles. However, in hydroids, the surface covering of hydrorhizae may remain largely intact, from which new uprights are likely to grow. For example, Standing (1976) noted that clipped stems of Obelia dichotoma took only 8 days to grow back. In addition, the resultant fragments of hydroids may be able to develop into new colonies (see displacement). Populations on small hard substrata (e.g. cobbles, pebbles or stones) may be removed by fishing gear, constituting substratum loss (see above). Overall, a proportion of the community is likely to be destroyed and an intolerance of intermediate has been recorded. However, recovery is likely to be rapid (see additional information below).
Displacement
(View Benchmark)
Colonies of hydroids and bryozoans attached to mobile substrata may suffer damage during displacement but are likely to survive. However, removal of a bryozoan of hydroid colony from its substratum would probably be fatal, and encrusting bryozoans are not known to be able to reattach. Similarly, Balanus crenatus is permanently attached to the substratum and could not survive if it was removed. Fragmentation is thought to be a possible mode of asexual reproduction in hydroids (Gili & Hughes, 1995) and it is possible that a proportion of displaced hydroid fragments may attach to new substrata, enhancing recovery. However, the encrusting bryozoans, hydroids and barnacles are likely to be lost and an intolerance of high has been recorded. The recovery of Hartlaubella gelatinosa is likely to be aided by the presence of fragments (see additional information below). Therefore, a recoverability of high has been recorded.

Chemical Factors

Synthetic compound contamination
(View Benchmark)
Chemical contaminants are likely to affect different species I the biotope to varying degrees, depending on the nature of the contaminant and its concentration.
  • Stebbing (1981) reported that Cu, Cd, and tributyl tin fluoride affected growth regulators in Laomedea (as Campanularia) flexuosa resulting in increased growth.
  • Bryan & Gibbs (1991) reported that virtually no hydroids were present on hard bottom communities in TBT contaminated sites and suggested that some hydroids were intolerant of TBT levels between 100 and 500 ng/l.
  • Rees et al. (2001) reported that the abundance of epifauna had increased in the Crouch estuary in the five years since TBT was banned from use on small vessels. This last report suggests that several species of epifauna may be at least inhibited by the presence of TBT.
  • However, Hartlaubella gelatinosa has been recorded in the Crouch estuary where TBT were very high, and was recorded under boat moorings at Cargreen in the river Tamar, and may be relatively tolerant of TBT (Keith Hiscock pers. comm.).
  • Bryozoans are common members of the fouling community, and amongst those organisms most resistant to antifouling measures, such as copper containing anti-fouling paints (Soule & Soule, 1977; Holt et al., 1995). Bryan & Gibbs (1991) reported that there was little evidence regarding TBT toxicity in bryozoans with the exception of the encrusting Schizoporella errata, which suffered 50% mortality when exposed for 63 days to 100ng/l TBT.
  • Hoare & Hiscock (1974) suggested that Polyzoa (Bryozoa) were amongst the most intolerant species to acidified halogenated effluents in Amlwch Bay, Anglesey.
  • Hoare & Hiscock (1974) found that Balanus crenatus survived near to an acidified halogenated effluent discharge where many other species were killed, suggesting a high tolerance to chemical contamination. However, barnacles have a low resilience to chemicals such as dispersants, dependant on the concentration and type of chemical involved and Holt et al. (1995) concluded that concluded that barnacles are fairly sensitive to chemical pollution.
The species richness of hydroid communities decreases with increasing pollution but hydroid species adapted to a wide variation in environmental factors and with cosmopolitan distributions tend to be more tolerant of polluted waters (Boero, 1984; Gili & Hughes, 1995). Bryozoans and barnacles are probably intolerant of chemical pollution. Overall, the extent or abundance of the species in the biotope may be decreased, although they may survive and an intolerance of intermediate has been recorded, albeit at low confidence. A recoverability of high has been recorded (see additional information below).
Heavy metal contamination
(View Benchmark)
Various heavy metals have been show to have sublethal effects on growth in the few hydroids studied experimentally (Stebbing, 1981; Bryan, 1984; Ringelband, 2001). Bryozoans are common members of the fouling community and amongst those organisms most resistant to antifouling measures, such as copper containing anti-fouling paints. Bryozoans were also shown to bioaccumulate heavy metals to a certain extent (Soule & Soule, 1977; Holt et al., 1995). Barnacles may tolerate fairly high levels of heavy metals in nature, accumulate heavy metals and store them as insoluble granules, for example, they are found in Dulas Bay, Anglesey, where copper reaches concentrations of 24.5 µg/l due to acid mine waste (Foster et al., 1978; Rainbow, 1987).
Therefore, the community may tolerate a degree of heavy metal contamination and an intolerance of low has been recorded.
Hydrocarbon contamination
(View Benchmark)
Although subtidal, this biotope is relatively shallow and may be exposed to oils and hydrocarbons adsorbed onto particulates and ingested or through the water soluble fractions of oils and hydrocarbons.
Species of the encrusting bryozoan Membranipora and the erect bryozoan Bugula were reported to be lost or excluded from areas subject to oil spills. (Mohammad, 1974; Soule & Soule, 1979). Houghton et al. (1996) reported a reduction in the abundance of intertidal encrusting bryozoans (no species given) at oiled sites after the Exxon Valdez oil spill. Encrusting bryozoans are also probably intolerant of the smothering effects of oil pollution, resulting in suffocation of colonies.
Littoral barnacles generally have a high tolerance to oil (Holt et al., 1995) and were little impacted by the Torrey Canyon oil spill (Smith, 1968) so Balanus crenatus is probably also fairly resistant to oil.
The water soluble fractions of Monterey crude oil and drilling muds were reported to cause polyp shedding and other sublethal effects in the athecate Tubularia crocea in laboratory tests (Michel & Case, 1984; Michel et al., 1986; Holt et al., 1995). However, hydroid species adapted to a wide variation in environmental factors and with cosmopolitan distributions tend to be more tolerant of polluted waters (Boero, 1984; Gili & Hughes, 1995). Calder (1976) suggested that hydroids found in the low salinity areas of south Carolina, such as Cordylophora caspia, were also present in relatively polluted waters, such as Charleston Harbour. Therefore, while the Balanus crenatus is probably tolerant of hydrocarbon contamination and hydroids are probably tolerant to a degree, the above evidence suggests that encrusting bryozoans are highly intolerant. Loss of the dominant, abundant species Conopeum reticulum and Electra spp. from the biotope would result in significant change in the community and perhaps loss of the biotope as described. Therefore, an intolerance of high has been recorded. Recovery would depend on recolonization by encrusting bryozoans, which is likely to be rapid (see additional information below).
Radionuclide contamination
(View Benchmark)
Insufficient information
Changes in nutrient levels
(View Benchmark)
Estuarine habitats are generally higher in nutrient levels than coastal waters. A moderate increase in nutrients may increase food availability for suspension feeders, in the form of organic particulates. Eutrophication may result in local hypoxic conditions (see below) and /or blooms of ephemeral algae. However, in this turbid environment, ephemeral algae are likely to be limited to the very shallow water near the top of the shore, and unlikely to adversely affect the biotope. Therefore, not sensitive has been recorded.
Increase in salinity
(View Benchmark)
The hydroids and bryozoans present in the biotope are characteristic of brackish waters. The dominance of hydroid and bryozoans species in this biotope is probably due to the exclusion of predators and competitors (con-specifics and ascidians) by reduced and variable salinity. An increase in salinity at the benchmark level is likely to allow more marine species to colonize the biotope and potentially out-compete the hydroid and bryozoan members of the community. The biotope will probably survive a short term, acute increase in salinity. However, a long term increase in salinity will probably result in loss of the community at the marine limit of its range. It may be able to colonize new space at the upper estuarine limit of its range providing suitable hard substrata are present. Therefore, an intolerance of intermediate has been recorded to represent a loss of the extent of the biotope. Recoverability has been recorded as very high (see additional information below).
Decrease in salinity
(View Benchmark)
The hydroids and bryozoans present in the biotope are characteristic of brackish waters. Lower limits of tolerance include, for example, 6.2 psu for Hartlaubella gelatinosa, 1psu for Obelia geniculata, 12psu for Obelia dichotoma, 21.5psu for Conopeum reticulum and 13.7psu in Electra crustulenta (Ryland, 1970; Cornelius, 1995b; Hayward & Ryland, 1998). [Please note: Conopeum seurati is considered a truly brackish water species surviving down to 1psu, and often confused with Conopeum reticulum (Ryland, 1970; Hayward & Ryland, 1998)]. Therefore, they are tolerant of the low and variable salinities experienced within the habitat and probably not sensitive to short term changes in the salinity for a week (see benchmark). However, a long term change in salinity from e.g. low to <5psu is likely to adversely affect the community. In the Tamar estuary, Plymouth, Devon, this biotope (SIR.HarCon) is replaced by the biotope £SIR.CorEle£ at the riverine/estuarine transition where the salinity is always below 20psu and can drop to 0psu (Hiscock & Moore, 1986). Therefore, a long term decrease in salinity is likely to result in loss of the biotope in the upper most parts of the estuary although it may extend its range into reduced salinity waters further down the estuary. Therefore, an intolerance of intermediate has been recorded to represent a loss of extent. Recoverability is likely to be very high (see additional information below).
Changes in oxygenation
(View Benchmark)
Sagasti et al. (2000) reported that epifauna communities, including dominant species such as the bryozoans Conopeum tenuissimum and Membranipora tenuis, and the hydroid Obelia bicuspidata were unaffected by periods of moderate hypoxia (ca 0.35 -1.4 ml/l) and short periods of hypoxia (<0.35 ml/l) in the York River, Chesapeake Bay. Their study suggests that estuarine epifaunal communities are relatively tolerant of hypoxia. However, Balanus crenatus was reported to survive an average of 3.2 days in the absence of oxygen (Barnes et al., 1963), and it is probable that a proportion of the Balanus crenatus population would be lost. Balanus crenatus has been shown to significantly affect community structure in early settlement communities (Brault & Bourget, 1985). Overall, although the majority of the species within the biotope are likely to survive, a proportion may be lost and an intolerance of intermediate has been recorded. Recoverability is likely to be very high (see additional information below).

Biological Factors

Introduction of microbial pathogens/parasites
(View Benchmark)
No information was found.
Introduction of non-native species
(View Benchmark)
No information was found.
Extraction
(View Benchmark)
It is extremely unlikely that any of the species indicative of sensitivity would be targeted for extraction and we have no evidence for the indirect effects of extraction of other species on this biotope.

Additional information icon Additional information

Recoverability
The dominant members of this biotope, the encrusting bryozoans Electra spp., Conopeum reticulum, hydroids Hartlaubella gelatinosa and Obelia spp., and the barnacle Balanus crenatus are rapid colonizers (see 'additional ecology). These species have been reported to colonize new substrata within 1-4 months in spring and summer (perhaps longer in the winter months). Hydroids and encrusting bryozoans reproduce asexually and grow rapidly so that they are effective colonizers of space, mature and reproduce rapidly. In addition, the ability to form resistant resting stages in hydroids together with their potential ability to regenerate from fragments provides rapid recovery from damage. Overall, the community is likely to recovery very rapidly from disturbance or damage, probably less than 6 months in most cases. If the community is destroyed it is likely to recolonize and reach its original composition and abundance within a year at most.

However, the potentially short lived planula larva in Hartlaubella gelatinosa probably results in good local recruitment but poor long range dispersal. Long distance dispersal is probably limited in hydroids (Gili & Hughes, 1995). Long distance dispersal is probably dependant on passive dispersal by currents on floating debris or shipping and is likely to be un-predictable, potentially taking many years in areas isolated from other populations. Hartlaubella gelatinosa has a limited distribution (nationally rare) and it may take many years to recruit from disparate populations.


This review can be cited as follows:

Tyler-Walters, H. 2002. Hartlaubella gelatinosa and Conopeum reticulum on low salinity infralittoral mixed substrata. Marine Life Information Network: Biology and Sensitivity Key Information Sub-programme [on-line]. Plymouth: Marine Biological Association of the United Kingdom. [cited 22/08/2014]. Available from: <http://www.marlin.ac.uk/habitatbenchmarks.php?habitatid=86&code=1997>