The Marine Life Information Network

Temperature increase (local)

Benchmark. A 5°C increase in temperature for one month, or 2°C for one year. Further detail

Evidence

No evidence on the effect of temperature on the formation and retention of fluid mud features was found. Localised thermal effluents (see benchmark) may increase mixing of the water column as warmer or colder water rise or fall respectively.  However, the main driver for the formation of the fluid mud features is tidal flow in highly turbid conditions. In addition, the mass transport of water in the large estuary systems in which these features form (e.g the Severn and Ouse) suggests that localised thermal plumes would be insignificant.

Temperature decrease (local)

Benchmark. A 5°C decrease in temperature for one month, or 2°C for one year. Further detail

Evidence

No evidence on the effect of temperature on the formation and retention of fluid mud features was found. Localised thermal effluents (see benchmark) may increase mixing of the water column as warmer or colder water rise or fall respectively.  However, the main driver for the formation of the fluid mud features is tidal flow in highly turbid conditions. In addition, the mass transport of water in the large estuary systems in which these features form (e.g the Severn and Ouse) suggests that localised thermal plumes would have an insignificant effect on the feature.

Salinity increase (local)

Benchmark. A increase in one MNCR salinity category above the usual range of the biotope or habitat. Further detail

Evidence

Estuarine systems are transitional habitats between riverine freshwater and fully saline seawater. The position of the 'salt wedge' in the estuary, where low salinity water of riverine origin flows under higher saline seawater, varies with the time of year, rainfall, high and low water, neap or spring tides and the degree of mixing due to wind waves, storms or tidal flow. The transition between high and low salinity waters is associated with 'flocculation' and a resultant increase in turbidity. A change in salinity at the benchmark level (i.e. from full to reduced or reduced to low) occurs along the length of the estuary. 

However, Kirby (2010) concluded that the main factor controlling the formation of fluid mud features was the semi-diurnal and semi-lunar tidal cycle. For example, he noted that heavy rain on exposed mudflats (and its associated localised decrease in salinity) perturbed the semi-diurnal and semi-lunar tidal mixing regime but was never large enough to disturb the cycle of the sediment behaviour observed (Kirby, 2010). In the Humber, the turbidity maximum occurred in the lower salinity reaches of the estuary but was displaced down the estuary by ca 12 km between spring and neap tides. Salinity was generally well mixed but large salinity inversions occurred in the presence of stationary suspended sediments (fluid muds) (ca 90 g/l). (Uncles et al., 2006).

Sensitivity assessment. An increase in salinity to over 40 is only likely in areas subject to hypersaline effluents or brines. Roberts et al. (2010b) suggested that hypersaline effluent dispersed quickly but was a concern at the seabed and in areas of low energy where widespread alternations in the community of soft sediments were observed. Therefore, given the strong tidal streams, mass water transport and riverine input typical of the estuarine systems in which fluid mud features form, a hypersaline effluent is unlikely to have any significant effect. Therefore, resistance is assessed as 'High' with 'Low' confidence. Therefore, resilience is probably 'High' and sensitivity assessed as 'Not sensitive' at the benchmark level.

Salinity decrease (local)

Benchmark. A decrease in one MNCR salinity category above the usual range of the biotope or habitat. Further detail

Evidence

Estuarine systems are transitional habitats between riverine freshwater and fully saline seawater. The position of the 'salt wedge' in the estuary, where low salinity water of riverine origin flows under higher saline seawater, varies with the time of year, rainfall, high and low water, neap or spring tides and the degree of mixing due to wind waves, storms or tidal flow. The transition between high and low salinity waters is associated with 'flocculation' and a resultant increase in turbidity. A change in salinity at the benchmark level (i.e. from full to reduced or reduced to low) occurs along the length of the estuary. 

However, Kirby (2010) concluded that the main factor controlling the formation of fluid mud features was the semi-diurnal and semi-lunar tidal cycle. For example, he noted that heavy rain on exposed mudflats (and its associated localised decrease in salinity) perturbed the semi-diurnal and semi-lunar tidal mixing regime but was never large enough to disturb the cycle of the sediment behaviour observed (Kirby, 2010). In the Humber, the turbidity maximum occurred in the lower salinity reaches of the estuary but was displaced down the estuary by ca 12 km between spring and neap tides. Salinity was generally well mixed but large salinity inversions occurred in the presence of stationary suspended sediments (fluid muds) (ca 90 g/l). (Uncles et al., 2006). Uncles et al. (2006) noted that salinity was insignificant to the dynamics of the estuarine turbidity maximum movement in the nose and upper core regions of highest suspended sediment concentrations.

Sensitivity assessment. Changes in the position of the 'salt wedge' may affect the position of the turbidity maxima in the estuary but given the highly turbid nature of the estuaries in which the fluid mud features are recorded, the effect is probably minimal. Therefore, resistance is assessed as 'High' with 'Low' confidence. Therefore, resilience is probably 'High' and sensitivity assessed as 'Not sensitive' at the benchmark level.

Water flow (tidal current) changes (local)

Benchmark. A change in peak mean spring bed flow velocity of between 0.1 m/s to 0.2 m/s for more than one year. Further detail

Evidence

Kirby (2010) concluded that the main factor controlling the formation of fluid mud features was the semi-diurnal and semi-lunar tidal cycle, therefore, changes in the tidal regime and water flow may affect fluid mud features depending on their scale and magnitude. The Severn has a tidla range of 14.7 m, with a mean spring tidal range of 12.3 m and a mean neap range of 6.5 m that results in strong but variable tidal streams with a maximum spring flood of 2.3 m/s and a neap flood of 1.2 m/s at Avonmouth (Kirby & Parker, 1983; Kirby, 2010). Kirby & Parker (1983) noted that suspended sediment started to settle as the tide decelerated but while the suspension was still moving at 0.5-1 m/s and documented 'lutoclines' (mobile fluid muds) at ca 0.2 to 0.8 m/s near the seabed (Figure 8; Kirby & Parker, 1983; Kirby, 2010). In the Ouse / Humber estuary system, the highest concentration of suspended sediment in the fluid mud layer at 40 g/l moved at less than 0.1 m/s but was generally stationary at 90 g/l (Uncles et al., 2006).

Sensitivity assessment. A change in peak mean spring bed flow velocity of between 0.1 m/s to 0.2 m/s for more than 1 year (the benchmark) over the entire estuary may alter the dynamics of fluid mud within the estuary. An increase may reduce their retention time while a decrease may increase their retention time. However, it is difficult to estimate the extent of the change as most mobile fluid muds form and move at speeds in excess of the benchmark. In addition, the system would probably return to prior conditions within a single tidal cycle once the pressure is removed (after one year). Therefore, resistance is given a precautionary assessment of  'Medium' while resilience is probably 'High' and sensitivity assessed as 'Low' at the benchmark level.

However, obstacles in the water column or on the seabed may create larger effects at their locality depending on their size, position and duration. In particular, extensive permanent structures that affect the tidal regime are likely to adversely affect the fluid mud features. Kirby (1997; 2010) concluded that the Cardiff-Weston barrage proposal would result in loss of the physical processes that form the fluid mud features recorded in the Severn, and hence the features themselves, upstream of the barrage. Nevertheless, such a change is significantly greater than the benchmark and permanent.

Emergence regime changes

Benchmark.  1) A change in the time covered or not covered by the sea for a period of ≥1 year or 2) an increase in relative sea level or decrease in high water level for ≥1 year. Further detail

Evidence

The fluid mud features are subtidal but changes in emergence reflect changes in the tidal regime and the intertidal mudflats are an important source and sink for the suspended sediment in the estuarine system (see Kirby & Parker, 1983; Kirby, 2010).  Kirby (2010) reported that the average sea level rise at Hinkley Point tide gauge was 4 mm/yr whereas the mean mudflat degradation rate at Stert Point was 16 mm/yr and cited CHaMP reported predictions of rapid intertidal habitat loss of 10-20% due to sea level rise in 100 years. Kirby (2010) estimated a foreshore erosion rate of ca 7 Mt/yr but also noted that Bridgewater Bay mud patch is both a sediment source and sink, while Newport Deep and is a long-term sink. Long-term changes in sea level are likely to submerge parts of the intertidal and affect currents within the estuary.  Kirby (1997; 2010) also concluded that the construction of the proposed that the Cardiff-Weston barrage would result in loss of the physical processes that form the fluid mud features recorded in the Severn, and hence the features themselves, upstream of the barrage. Nevertheless, such a change is significantly greater than the benchmark and permanent.

Sensitivity assessment. An increase in sea level for ca 1 year (see benchmark) may result in loss of the intertidal area and may change the position of the salt wedge and turbidity maxima in the estuarine system. It may result in minor changes in the size, formation and retention rates of fluid mud features but recovery would be rapid once the system was returned to normal.  Therefore, resistance is given a precautionary assessment of  'Medium' while resilience is probably 'High' and sensitivity assessed as 'Low' at the benchmark level.

Wave exposure changes (local)

Benchmark. A change in near shore significant wave height of >3% but <5% for more than one year. Further detail

Evidence

Fluid mud features in the estuarine systems sheltered or extremely sheltered from wave action. However, Kirby (2010) reported that meteorological events such as wind waves perturb the semi-diurnal and semi-lunar mixing and settling of sediment responsible for the behaviour of fluid mud features in the Severn but were never large enough to destroy the observed cycle. Surface turbulence in the water column created by stormy days inhibited the process of suspended sediment settlement. But the largest amounts of fluid mud on neaps occurred after equinoctial spring tides that coincided with gales.  Therefore, a 3-5% change in significant wave height (the benchmark) is unlikely to have a significant effect on the fluid mud features. Hence, resistance is assessed as 'High', resilience as 'High' and the features are probably 'Not sensitive' at the benchmark level.

Transition elements & organo-metal contamination

Benchmark. Exposure of marine species or habitat to one or more relevant contaminants via uncontrolled releases or incidental spills. Further detail

Evidence

This pressure is 'Not assessed'.

Hydrocarbon & PAH contamination

Benchmark. Exposure of marine species or habitat to one or more relevant contaminants via uncontrolled releases or incidental spills. Further detail

Evidence

This pressure is 'Not assessed'.

Synthetic compound contamination

Benchmark. Exposure of marine species or habitat to one or more relevant contaminants via uncontrolled releases or incidental spills. Further detail

Evidence

This pressure is 'Not assessed'.

Radionuclide contamination

Benchmark. An increase in 10µGy/h above background levels. Further detail

Evidence

No evidence

Introduction of other substances

Benchmark. Exposure of marine species or habitat to one or more relevant contaminants via uncontrolled releases or incidental spills. Further detail

Evidence

This pressure is 'Not assessed'.

De-oxygenation

Benchmark. Exposure to dissolved oxygen concentration of less than or equal to 2 mg/l for one week (a change from WFD poor status to bad status). Further detail

Evidence

Kirby & Parker (1983) & Kirby (2010) note that fluid mud features are anaerobic, due to the presence of organic fines. The estuary bed and at times in summer the entire water body up-estuary of the Shoot Channel is deoxygenated by neap tide fluid mud or its resuspension in the neap-spring tidal cycle (Kirby, 2010).  Therefore, the presence of fluid muds probably increases deoxygenation in their locality. However, the presence or absence of oxygen in the system probably has little or no effect on the physical fluid mud features, especially as the faunal component is also transient. Therefore, this pressure is 'Not relevant' to these features.

Nutrient enrichment

Benchmark. Compliance with WFD criteria for good status. Further detail

Evidence

Fluid muds and suspended sediment are involved in the transport of nutrients and organic material within the estuary system. The flocculation of organic material, especially as the saltwater-freshwater interface increases turbidity and probably contributes to the turbidity maxima. However, an increase or decrease in nutrients or organic enrichment is unlikely to make a significant difference in the physical processes that determine the cycling of mobile or stationary fluid muds features within the estuary system. Therefore, this pressure is probably 'Not relevant'.

Organic enrichment

Benchmark. A deposit of 100 gC/m²/yr. Further detail

Evidence

Fluid muds and suspended sediment are involved in the transport of nutrients and organic material within the estuary system. The flocculation of organic material, especially as the saltwater-freshwater interface increases turbidity and probably contributes to the turbidity maxima. However, an increase or decrease in nutrients or organic enrichment is unlikely to make a significant difference in the physical processes that determine the cycling of mobile or stationary fluid muds features within the estuary system. Therefore, this pressure is probably 'Not relevant'.

Physical loss (to land or freshwater habitat)

Benchmark. A permanent loss of existing saline habitat within the site. Further detail

Evidence

Although unlikely in this instance, 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.

Physical change (to another seabed type)

Benchmark. Permanent change from sedimentary or soft rock substrata to hard rock or artificial substrata or vice-versa. Further detail

Evidence

Kirby (2010) suggests that the Severn estuary consists of a scoured seabed where most of the sediment is unconsolidated and mobile at any one place or time. The mobile and stationary fluid mud features consist entirely of fine sediments in various states of suspension and settlement.  Although unlikely, if the sediment was replaced by hard or soft rock to fluid mud features would cease to exist. Therefore, resistance is assessed as ‘None’ and resilience as ‘Very low’ (as the change at the pressure benchmark is permanent). Hence, sensitivity is assessed as ‘High’.

Physical change (to another sediment type)

Benchmark. Permanent change in one Folk class (based on UK SeaMap simplified classification). Further detail

Evidence

The fluid mud features are composed of fine cohesive sediments or muds (Kirby, 2010; Manning et al., 2010).  Although unlikely, a change from muds to sands or mixed coarse sediments would prevent their resuspension on the scale required to form the fluid muds features and probably result in the loss of the features. Therefore, resistance is assessed as ‘None’ and resilience as ‘Very low’ (as the change at the pressure benchmark is permanent). Hence, sensitivity is assessed as ‘High’.

Habitat structure changes - removal of substratum (extraction)

Benchmark. The extraction of substratum to 30 cm (where substratum includes sediments and soft rock but excludes hard bedrock). Further detail

Evidence

Removal of the substratum to a depth of 30 cm would result in loss of a portion the fluid mud in the footprint of the activity. Therefore, resistance is assessed as 'Low'. However, the fluid mud would return and be replaced within days in the neap cycle or weeks in the spring cycle. Therefore, resilience is 'High' and sensitivity is assessed as 'Low'. 

Abrasion / disturbance of the surface of the substratum or seabed

Benchmark. Damage to surface features (e.g. species and physical structures within the habitat). Further detail

Evidence

Fluid mud features are by definition unconsolidated sediments in suspension. Any artificial structure or mechanism (e.g. trawl or dredge) passing through the suspension may result in very localised mixing of the suspension and disruption of the layering that results from the natural dewatering of the suspension. However, as the fluid muds are unconsolidated, and do not support a benthic community, little if any damage is envisaged. Therefore, resistance is assessed as 'High'. Hence, resilience is 'High' and the feature is probably 'Not sensitive' to this pressure.

Penetration or disturbance of the substratum subsurface

Benchmark. Damage to sub-surface features (e.g. species and physical structures within the habitat). Further detail

Evidence

Fluid mud features are by definition unconsolidated sediments in suspension. Any artificial structure or mechanism (e.g. trawl or dredge) passing through the suspension may result in very localised mixing of the suspension and disruption of the layering that results from the natural dewatering of the suspension. However, as the fluid muds are unconsolidated, and do not support a benthic community, little if any damage is envisaged. Therefore, resistance is assessed as 'High'. Hence, resilience is 'High' and the feature is probably 'Not sensitive' to this pressure.

Changes in suspended solids (water clarity)

Benchmark. A change in one rank on the WFD (Water Framework Directive) scale e.g. from clear to intermediate for one year. Further detail

Evidence

This pressure addresses the increases in suspended sediment due to changes in sediment & organic particulate matter and chemical concentrations or activities disturbing sediment and/or organic particulate matter and mobilizing it into the water column. However, the suspended sediment concentrations typical of fluid mud features (ca 0.1 - 200 g/l) are significantly higher than the minimum value given for 'very turbid' conditions in the benchmark. Therefore,  an increase in turbidity at the benchmark level is 'Not relevant'. However, a decrease in suspended sediment could represent a major change in the fluid mud features. Although unlikely, any activity that significantly decreased the suspended sediment load within the estuary system could result in a reduction in the extent, duration and retention of the fluid mud features. But such a change in pressure is probably outside the scope of the definition and benchmark. 

Smothering and siltation rate changes (light)

Benchmark. ‘Light’ deposition of up to 5 cm of fine material added to the seabed in a single discrete event. Further detail

Evidence

Fluid mud features are composed of high concentrations of suspended sediments in various phases of settlement, flow and resuspension.  In addition, the fluid mud features can vary in thickness from ca 0.5 m to up to 5 m (Kirby & Parker, 1983; Kirby, 2010). Therefore, the deposition of 5 or 30 cm of fine sediment is unlikely to have any noticeable effect. Therefore, resistance is assessed as 'High'. Hence, resilience is 'High' and the feature is probably 'Not sensitive' to this pressure.

Smothering and siltation rate changes (heavy)

Benchmark. ‘Heavy’ deposition of up to 30 cm of fine material added to the seabed in a single discrete event. Further detail

Evidence

Fluid mud features are composed of high concentrations of suspended sediments in various phases of settlement, flow and resuspension.  In addition, the fluid mud features can vary in thickness from ca 0.5 m to up to 5 m (Kirby & Parker, 1983; Kirby, 2010). Therefore, the deposition of 5 or 30 cm of fine sediment is unlikely to have any noticeable effect. Therefore, resistance is assessed as 'High'. Hence, resilience is 'High' and the feature is probably 'Not sensitive' to this pressure.

Litter

Benchmark. The introduction of man-made objects able to cause physical harm (surface, water column, seafloor or strandline). Further detail

Evidence

Not assessed.

Electromagnetic changes

Benchmark. A local electric field of 1 V/m or a local magnetic field of 10 µT. Further detail

Evidence

This is a physical habitat with little or no faunal component. Therefore, this pressure is Not relevant.

Underwater noise changes

Benchmark. MSFD indicator levels (SEL or peak SPL) exceeded for 20% of days in a calendar year. Further detail

Evidence

This is a physical habitat with little or no faunal component. Therefore, this pressure is Not relevant.

Introduction of light or shading

Benchmark. A change in incident light via anthropogenic means. Further detail

Evidence

This is a physical habitat with little or no faunal component. In addition, the high turbidity prevents light penetrating into the water column. Therefore, this pressure is Not relevant.

Barrier to species movement

Benchmark. A permanent or temporary barrier to species movement over ≥50% of water body width or a 10% change in tidal excursion. Further detail

Evidence

This is a physical habitat with little or no faunal component. The definition of this pressure addresses the effect of obstructions to the movement of species rather than physical features. Technically, therefore this pressure is Not relevant.  However, the construction of a tidal barrage in the Severn estuary has received considerable attention in the literature. Kirby (1997, 2010) concluded that the proposed Cardiff-Weston barrage would result in the loss of the physical processes that created the fluid mud features in the Severn estuary.  Therefore, a precautionary resistance of 'None' is recorded. In the case of a barrage, a permanent structure, recovery would not be possible and resilience is assessed as 'Very low' and sensitivity as 'High'.

Death or injury by collision

Benchmark. Injury or mortality from collisions of biota with both static or moving structures due to 0.1% of tidal volume on an average tide, passing through an artificial structure. Further detail

Evidence

This is a physical habitat with little or no faunal component. Therefore, this pressure is Not relevant.

Visual disturbance

Benchmark. The daily duration of transient visual cues exceeds 10% of the period of site occupancy by the feature. Further detail

Evidence

This is a physical habitat with little or no faunal component. Therefore, this pressure is Not relevant.

Genetic modification & translocation of indigenous species

Benchmark. Translocation of indigenous species or the introduction of genetically modified or genetically different populations of indigenous species that may result in changes in the genetic structure of local populations, hybridization, or change in community structure. Further detail

Evidence

This is a physical habitat with little or no faunal component. Therefore, this pressure is Not relevant.

Introduction or spread of invasive non-indigenous species

Benchmark. The introduction of one or more invasive non-indigenous species (INIS). Further detail

Evidence

This is a physical habitat with little or no faunal component. Kirby & Parker (1983) and Kirby (2010) argue that the unstable, transient and anaerobic nature of the fluid mud features means that the habitat is largely uncolonized by species and depaurate. It follows that the habitat is unlikely to be suitable for invasive non-indigenous species. Therefore, this pressure is assessed as Not relevant.

Introduction of microbial pathogens

Benchmark. The introduction of relevant microbial pathogens or metazoan disease vectors to an area where they are currently not present (e.g. Martelia refringens and Bonamia, Avian influenza virus, viral Haemorrhagic Septicaemia virus). Further detail

Evidence

Kirby (2010) noted that contaminants and microbial pathogens can be carried by mobile fluid mud suspensions. But fluid muds are physical rather than biological features. Therefore, this pressure is Not relevant.

Removal of target species

Benchmark. Removal of species targeted by fishery, shellfishery or harvesting at a commercial or recreational scale. Further detail

Evidence

This is a physical habitat with little or no faunal component. Therefore, this pressure is not relevant.

Removal of non-target species

Benchmark. Removal of features or incidental non-targeted catch (by-catch) through targeted fishery, shellfishery or harvesting at a commercial or recreational scale. Further detail

Evidence

This is a physical habitat with little or no faunal component. Therefore, this pressure is Not relevant.