The Marine Life Information Network

The distribution of fossilised foraminifera is used to track changes in bottom water temperatures, as each species occurs in a particular temperature range (Archer & Martin, 2001) suggesting that some species are intolerant of temperature changes.

Thyasira flexuosa does not occur in the southernmost part of the North Sea but is distributed from Norway to the Azores, and extends into the Mediterranean (Tillin & Tyler-Walters, 2014). 

In the northern North Sea, foraminiferan dominated biotopes containing Thyasira equalis, Saccammina sp., Psammosphaera sp. and Astrohiza arenaria is present in ‘constant Boreal water’ where the bottom temperature among foraminifera communities has been noted to have a low temperature range (Stephen, 1923), with temperatures oscillating between 6 and 8 °C (McIntyre, 1961).The foraminiferan and Thyasira biotopes of the Forties  Further south, a greater range of bottom temperatures occurs and the biotope is not present. But this could also be due to an increase in sediment particle size with decreasing depth in this area. However, the Atlantic community which contains Thyasira flexuosa and Crithionina granum lives in ‘varying Boreal water’, with temperatures varying between 7 and 13 °C (McIntyre, 1961).

Sensitivity assessment. Available information suggests that the community is highly dependent on a relatively constant temperature and that different species of Thyasira sp. and foraminifera thrive in different temperature ranges. This pressure is not assessed due to lack of evidence.

There is no information on the response of the biotope to a decrease in temperature. Thyasira flexuosa does not occur in the southernmost part of the North Sea but is distributed from Norway to the Azores, and extends into the Mediterranean (Tillin & Tyler-Walters, 2014). However, Thyasira populations in the British Isles are restricted to areas where the bottom waters remain cool all year round (Jackson, 2007). No specific information on temperature tolerances of Thyasira spp. was found.

The distribution of fossilised foraminifera is used to track changes in bottom water temperatures, as each species occurs in a particular temperature range (Archer & Martin, 2001). This suggests that they are intolerant of temperature changes.

In the northern North Sea, foraminiferan domianted biotopes containing Thyasira equalis, Saccammina sp., Psammosphaera sp. and Astrohiza arenaria is present in ‘constant Boreal water’ where the bottom temperature among foraminifera communities has been noted to have a low temperature range (Stephen, 1923), with temperatures oscillating between 6 and 8 °C (McIntyre, 1961). Further south, a greater range of bottom temperatures occurs and the biotope is not present. But this could also be due to an increase in sediment particle size with decreasing depth in this area. However, the Atlantic community which contains Thyasira flexuosa and Crithionina granum lives in ‘varying Boreal water’, with temperatures varying between 7 and 13 °C (McIntyre, 1961).

Sensitivity assessment. Available information suggests that the community is highly dependent on a relatively constant temperature and that different species of Thyasira sp. and foraminifera thrive in different temperature ranges. This pressure is not assessed due to lack of evidence.

No evidence.

This biotope is found in full salinity (30-35 ppt). Thyasira spp. can inhabit waters of reduced salinity with 25-30 psu being optimal. However, adults exposed to lower than optimal salinities produced non-viable or slow developing eggs (Jackson, 2007).

This circalittoral biotope that has not been recorded from locations with brackish waters and so is probably highly intolerant of a decrease in salinity. In the northern North Sea, foraminiferan dominated  biotopes containing Thyasira equalis, Saccammina sp., Psammosphaera sp. and Astrohiza arenaria, is present where salinities remain fairly constant, between 35.20 and 35.26 ppm (McIntyre, 1961). However, the Atlantic community which contains Thyasira flexuosa and Crithionina granum (foraminifera) occurs in waters where salinity varies between 33.86 and 34.33 (McIntyre, 1961). This suggests that the community is highly dependent on a relatively constant salinity. Mobile species would be able to avoid the change in salinity by moving away, but localised densities would decline.

Sensitivity assessment. Resistance has been assessed as ‘Low’. Recoverability may be ‘Medium’ and sensitivity is assessed as ‘Medium’.

This biotope occurs in deep waters where tidal flows are weak (<0.5 m/s) (JNCC, 2015). The community occurs in fine soft mud that only develops in areas of weak tidal streams. Following an increase in water flow rate at the pressure benchmark the surface sediments and epifaunal foraminifera may be subject to some winnowing but the cohesive nature of subtidal muds will limit the impact. The lower substratum inhabited by mature specimens of Thyasira sp., infaunal foraminifera, polychaetes and  other species are  likely to remain unchanged.

Sensitivity assessment.  Since the majority of characterizing species are likely to persist, resistance has been assessed as ‘Medium’ as some removal of epifaunal foraminifera may occur. On return to normal water flow rates, resilience is assessed as ‘High’ as water transport from periodic storm events may restore populations of adult foraminifera. Biotope sensitivity is, therefore, assessed as 'Low.

Not relevant to sublittoral biotopes.

As this biotope occurs in circalittoral habitats it is not directly exposed to the action of breaking waves.  The characterizing Thyasira spp. and associated polychaete species that burrow are protected within the sediment but the characterizing foraminiferans would be exposed to oscillatory water flows at the seabed. No specific evidence was found to assess this pressure.

Sensitivity assessment.  The biotope is considered to be ‘Not sensitive’ to changes in wave height, at the pressure benchmark, based on the depth of the habitat.

Not Sensitive at the pressure benchmark that assumes compliance with all relevant environmental protection standards.

This pressure is Not assessed but evidence is presented where available.

This pressure is Not assessed but evidence is presented where available.

No evidence.

This pressure is Not assessed.

Dando & Spiro (1993) found that numbers of Thyasira equalis and Thyasira sarsi decreased rapidly following the de-oxygenation of bottom water in the deep basin of Gullmar fjord in 1979-80.

Sensitivity assessment: A proportion of the characterizing species in these biotopes is likely to be lost, a resistance of ‘Low’ (>75% loss) is suggested based on Thyasira spp. Resilience is probably ‘High’, and therefore the biotopes’ sensitivity to this pressure if likely to be ‘Low’.

This pressure relates to increased levels of nitrogen, phosphorus and silicon in the marine environment compared to background concentrations.  The pressure benchmark is set at compliance with Water Framework Directive (WFD) criteria for good status, based on nitrogen concentration (UKTAG, 2014).  

Sensitivity assessment. This biotope is considered to have ‘High’ resistance to this pressure and ‘High’ resilience, (by default) and is assessed as ‘Not sensitive’.

Many of the species present are deposit feeders characteristic of organically enriched areas. An input of organic matter at the pressure benchmark is likely to provide a food subsidy to these species.

Thyasira spp. are characteristic of organically enriched offshore sediments and have been identified as a ‘progressive’ species, i.e. one that shows increased abundance under slight organic enrichment (Leppakoski, 1975, cited in Gray, 1979). In the development of the AMBI index to assess disturbance (including organic enrichment), both Borja et al. (2000) and Gittenberger & van Loon (2011) assigned Thyasira flexuosa to their Ecological Group III (defined as ‘species tolerant to excess organic matter enrichment). These species may occur under normal conditions, but their populations are stimulated by organic enrichment (slight unbalance situations)’. 

Sensitivity assessment The evidence presented suggests that the majority of the characterizing and associated species in the biotopes are likely to be able to utilize additional organic load as food and are present in enriched habitats. Biotope resistance is therefore assessed as ‘High’ and resilience as ‘high’ so that the biotope is assessed as ‘Not sensitive’.

All marine habitats and benthic species are considered to have a resistance of ‘None’ to this pressure and to be unable to recover from a permanent loss of habitat (resilience is ‘Very Low’).  Sensitivity within the direct spatial footprint of this pressure is therefore ‘High’.  Although no specific evidence is described confidence in this assessment is ‘High’, due to the incontrovertible nature of this pressure.

The biotope is characterized by the sedimentary habitat (JNCC, 2015), a change to an artificial or rock substratum would alter the character of the biotope leading to reclassification and the loss of the sedimentary community including the characterizing bivalves, polychaetes and echinoderms that live buried within the sediment.

Sensitivity assessment. Based on the loss of the biotope, resistance is assessed as ‘None’, recovery is assessed as ‘Very low’ (as the change at the pressure benchmark is permanent and sensitivity is assessed as ‘High’.

The change referred to at the pressure benchmark is a change in sediment classification (based on Long, 2006) rather than a change in the finer-scale original Folk categories (Folk, 1954).  For muddy sediments, resistance is assessed based on a change to either mixed sediments or sand and muddy sands . The characterizing Thyasira spp. prefer fine sediments including mud, muddy sand and sandy mud (Jackson, 2007).

Sensitivity assessment:  A change in Folk class from mud  to sand or muddy sand would probably not eliminate the characterizing Thyasira spp and the habitat may still be suitable for foraminiferans, however, some grades of mixed sediments with low fractions of fine sediments would probably be unsuitable. Resistance is therefore assessed as ‘Low’ (loss of 25-75%) and resilience is considered 'Very low’ given the permanent nature of this pressure. Sensitivity is therefore assessed as ‘High’.

Resistance is assessed as ‘None’ as extraction of the sediment swill remove the characterizing and associated species present. Resilience is assessed as ‘Medium’ as foraminiferans may require longer than two years to re-establish (see resilience section) and sediments may need to recover (where exposed layers are different). Biotope sensitivity is therefore assessed as ‘Medium’.

No evidence was found to assess this pressure. Abrasion may result in some damage and mortality, resistance is therefore assessed as 'Medium' and resilience as 'High', biotope sensitivity is therefore assessed as 'Low'.

Activities that disturb the surface and penetrate below the surface would remove /damage epifauna and infaunal species such as the characterizing species within the direct area of impact.

The shells of Thyasira spp. are thin and fragile and penetration and disturbance of the sediment is likely to lead to damage and mortality within the population. Sparks-McConkey & Watling (2001) found that trawler disturbance resulted in a decline of Thyasira flexuosa in Penobscot Bay, Maine. However, the population recovered after 3.5 months.  The direct mortality (percentage of initial density) of Thyasira flexuosa from trawling was estimated as 0-28%, based on samples taken with a Day grab before and 24 hours after trawling (Ball et al., 2000a). 

Gilkinson et al. (1998)  found that otter trawling that disturbed the sediment displaced small bivalves but that these were unharmed. Sediment penetration may therefore, disturb and displace foraminiferans but these may survive due to small size and robust tests.

Sensitivity assessment: A proportion of the characterizing species in these biotopes is likely to be lost or severely damaged, depending on the scale of the activity (see abrasion pressure). Therefore, a resistance of ‘Low’ (>75% loss) is suggested based on Thyasira spp. Resilience is probably ‘High’ (where foraminiferan populations undergo only small declines), and therefore biotope sensitivity to this pressure is likely to be ‘Low’.

No evidence. Changes in suspended solids are considered unlikely to affect the characterizing species and resistance is assessed as 'High' and resilience as 'High' so that the biotope is considered to be 'Not sensitive'.

Bijkerk (1988, results cited from Essink, 1999) indicated that the maximal overburden through which small bivalves could migrate was 20 cm in sand for Donax and approximately 40 cm in mud for Tellina sp. and approximately 50 cm in sand. No further information was available on the rates of survivorship or the time taken to reach the surface. This suggests that the characterizing species Thyasira spp. may be able to re-borrow through similar overburdens. Thyasira flexuosa have ‘highly extensible feet’ (Dando & Southward, 1986) allowing them to construct channels within the sediment and to burrow to 8 cm depth.

Epifaunal foraminifera may not be able to burrow to the surface and at least a proportion of the population may be lost. However, little information on foraminiferan biology was found, and so, in the absence of information tolerance has been assessed as 'None', albeit with very low confidence. Loss of the characterizing species of foraminifera would mean that the biotope is no longer SS.SMu.OMu.ForThy and so resistance is assessed as 'None'. Resilience is assessed as 'Medium' and sensitivity is assessed as 'Medium'.

Bijkerk (1988, results cited from Essink, 1999) indicated that the maximal overburden through which small bivalves could migrate was 20 cm in sand for Donax and approximately 40 cm in mud for Tellina sp. and approximately 50 cm in sand. No further information was available on the rates of survivorship or the time taken to reach the surface. This suggests that the characterizing species Thyasira spp. may be able to re-borrow through similar overburdens. Thyasira flexuosa have ‘highly extensible feet’ (Dando & Southward, 1986) allowing them to construct channels within the sediment and to burrow to 8 cm depth.

Epifaunal foraminifera may not be able to burrow to the surface and at least a proportion of the population may be lost. However, little information on foraminiferan biology was found, and so, in the absence of information, tolerance has been assessed as 'None', albeit with very low confidence. Loss of the characterizing species of foraminifera would mean that the biotope is no longer SS.SMu.OMu.ForThy and so resistance is assessed as 'None'. Resilience is assessed as 'Medium' and sensitivity is assessed as 'Medium'.

Not assessed.

No evidence.

Not relevant.

Not relevant.

Not relevant to off-shore biotopes.

Not relevant’ to seabed habitats.  NB. Collision by grounding vessels is addressed under ‘surface abrasion.

Not relevant.

Key characterizing species within this biotope are not cultivated or translocated. This pressure is therefore considered ‘Not relevant’ to this biotope group.

No evidence.

More than 20 viruses have been described for marine bivalves (Sinderman, 1990). Bacterial diseases are more significant in the larval stages and protozoans are the most common cause of epizootic outbreaks that may result in mass mortalities of bivalve populations. Parasitic worms, trematodes, cestodes and nematodes can reduce growth and fecundity within bivalves and may in some instances cause death (Dame, 1996).

Little information specifically concerning the effects of microbial pathogens and parasites on the viability of the characterizing species was found. A viral infection of the mutualist bacterium living on the gills of Thyasira gouldii has been suggested as the reason for a major decline in the Loch Etive population (Jackson, 2007, references therein),

Sensitivity assessment. No direct evidence of the biotopes being affected by the introduction of microbial pathogens was found to assess this pressure.

No species within the biotope are targeted by commercial or recreational fishers or harvesters. This pressure is therefore considered ‘Not relevant’.

Removal of the characterizing species would reduce the ecological services provided by these species such as secondary production and nutrient cycling.

Sensitivity assessment. Species within the biotope are relatively sedentary or slow moving although the infaunal position of Thyasira spp. may protect this genus from removal. Biotope resistance is therefore assessed as ‘Low’ and resilience as ‘High’ as the habitat is likely to be directly affected by removal but Thyasira spp.  will recolonize rapidly.  Some variability in species recruitment, abundance and composition is natural and therefore a return to a recognisable biotope should occur within 2 years. Repeated chronic removal would, however, impact recovery.