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Publication - Consultation Paper

Wild seaweed harvesting: strategic environmental assessment - environmental report

Published: 22 Nov 2016
Part of:
Marine and fisheries
ISBN:
9781786526229

Investigates the sustainability and potential environmental impacts of wild seaweed and seagrass harvesting, maerl extraction and removal of beach-cast seaweed.

263 page PDF

10.2MB

263 page PDF

10.2MB

Contents
Wild seaweed harvesting: strategic environmental assessment - environmental report
7. Climatic Factors

263 page PDF

10.2MB

7. Climatic Factors

7.1. Climate Change

7.1.1. Over the last 50 years, it has become increasingly apparent that the world's climate is changing at an unprecedented rate. Evidence of an increase in average global temperatures, along with an increase in the concentration of greenhouse gases ( GHG) in the atmosphere, has led to the conclusion that human activities such as the use of carbon based-fuels is the main reason for this increase [25] . Three of the major GHGs are carbon dioxide (CO 2), methane (CH 4), and nitrous oxide (N 2O). In 2011, concentrations of these GHGs exceeded pre-industrial atmospheric concentrations by approximately 40%, 150%, and 20% respectively ( IPCC, 2013).

7.1.2. The effects of this climate change include temperature increases (both air and water), sea level rise, and increases in extreme weather events, such as storms and flooding. Climate change is now considered to be one of the most serious environmental threats to sustainable development, with adverse consequences expected for human health, food security, economic activity, natural resources and physical infrastructure [26] . Adaptation to the effects of climate change is now acknowledged as necessary for responding effectively and equitably to the impacts of climate change.

7.1.3. Since 1961, average temperatures in all parts of Scotland have risen for every season (Sniffer, 2006), and over the past three decades sea-surface temperatures around the UK coast have risen by an average of 0.7°C ( UKCIP, 2011).

7.1.4. In the 20th century, average UK sea levels increased by around 1 mm/year ( UKCIP, 2011). Sea level will continue to rise with average global temperatures. Predictions suggest that by 2095 relative sea level will have risen by 23 to 39 cm ( UKCIP, 2010).

7.1.5. Combined with the expected increase in the occurrence of extreme weather events (such as storms and flooding) these effects have the potential to cause a major threat to marine and coastal environments as well as to the human activities that they support (Defra, 2012). Coastlines characterised by soft sediment are likely to be more vulnerable to these effects ( Figure 24) and coastal defences are already in place in parts of Scotland ( Figure 25).

7.1.6. In addition, the ocean has taken up approximately 30% of anthropogenic carbon dioxide emissions, which is altering ocean chemistry by increasing acidity. This is a concern for marine ecosystems, with calcareous organisms being most at risk, as more acidic water increases the rates of calcium carbonate dissolution (Scottish Government (2012).

Figure 24: Classification of the Scottish coastline (hard/mixed or soft)

Figure 24: Classification of the Scottish coastline (hard/mixed or soft)

Figure 25: Location of coastal defences

Figure 25: Location of coastal defences

7.2. Ecosystem Services

7.2.1. Seaweeds and seagrasses contribute to the following ecosystem services:

  • Natural hazard protection - provision of a natural coastal defence through their wave dampening effect and in preventing and/or alleviating coastal erosion.
  • Climate regulation -through their important role in the carbon cycle in terms of capturing, storing and exporting carbon.

Coastal Protection

7.2.2. Seaweeds, in particular kelp forests, and seagrasses are known for their capacity to attenuate waves and reduce current velocities (Fonseca & Cahalan, 1992; Mork, 1996; Bradley & Houser, 2009), which provides coastal protection. These species are able to provide such protection primarily due to their biogenic structures, as these protrude into the water column (Gambi et al., 1990; Bouma et al., 2005; Lowe et al., 2005; Luhar et al., 2010; Paul et al., 2012). The level of protection they provide varies seasonally, particularly during winter months, when they shed their blades or leaves or suffer storm damage and physical disturbance, which reduces the amount of biomass in the water column (Christianen et al., 2013).

7.2.3. Indirect coastal protection may be provided by beach-cast seaweeds that release nutrients to dune habitats, which in their turn stabilise local sediments and contribute to coastal protection (Orr, 2013). No scientific evidence has been found that suggests that drift deposits on beaches directly provide coastal protection. However, if present on a large enough scale, it is possible that they could dissipate wave energy, protecting sediments on a local scale from wave scour.

7.2.4. No evidence has been found to suggest that maerl habitats contribute significantly to coastal protection, apart from where they contribute to a large proportion of the sand in beaches ( e.g. Coral Beach in Skye). Similarly, very little information regarding the coastal protection properties of seaweeds other than kelps has been found. The only exception to this is understorey macroalgae associated with kelp forests reducing current velocities (Eckman et al., 1989).

7.2.5. The following sections focus on the mechanisms by which kelps and seagrasses provide coastal protection.

Wave Dampening

7.2.6. Laminaria hyperborea forests are known to provide a buffer against storm surges through wave dampening and by reducing the velocity of breaking waves (Lovas & Torum, 2001). The extent of wave dampening is strongly influenced by the morphology, drag co-efficient, and density of the dominant kelp species; thus the magnitude of protection provided varies with species, and therefore may also vary with location (Gaylord et al., 2007). Smale et al. (2016) also found wave exposure to be an important factor in structuring L. hyperborea populations, with kelp density, biomass and age being greater in more exposed sites.

7.2.7. L. hyperborea forests off the coast of Norway have been found to reduce wave heights by as much as 60%, resulting in wave energy losses of 70-80% (Mork, 1996). It is expected that L. hyperborea forests around Scotland provide a similar level of protection that is likely to be locally important to coastal communities (Smale et al., 2013). The rocky seabed off the west coast of Uist supports a vast L. hyperborea forest which extends approximately 6-8 km offshore. Waverider buoys deployed off the western coast of the Outer Hebrides suggest that gross wave energy reduces from 58.1 to 14.9 kW/m between depths of 100 m and 15 m (Mollison, 1983). The greatest energy loss has been found to occur between 23 and 15 m depth where kelp beds are abundant (Mollison, 1983; Orr, 2013). Wave heights of over 11 m have been recorded 30 to 40 km offshore. When these same waves broke on the coast they had been reduced to around 1.5 - 2 m in height. The combination of shallow gradient and roughness created by the kelp forest is considered to greatly dissipate wave energy (Comhairle Nan Eilean Siar, 2013). The extreme damage caused by the 2005 storm in was attributed to the elevated sea state raising the wave base sufficiently to disengage from the protective effects of the kelp forest (Angus & Rennie 2014). Although this particular conclusion may be speculative, it is supported by the other evidence presented above that suggests kelps (specifically L. hyperborea) do play an important role in local coastal protection.

7.2.8. Seagrass beds are also effective at attenuating wave energy; however, small above-surface biomass means that the potential for direct attenuation is smaller than that of kelps. Despite this, the presence of the seagrass canopy helps to protect the sediment by deflecting water flow, reducing shear stress experienced by the bed and therefore erosion (Le Hir et al., 2007). Seagrasses are also effective at reducing current velocities, an ability that is a function of leaf density (Gambi et al., 1990; van Keulen & Borowitzka, 2002; Koch & Gust, 1999; Jackson et al., 2012). Dense seagrass beds are capable of reducing current speeds up to ten times more than unvegetated areas (Jackson et al., 2012). Evidence suggests that the deposition and accumulation of sediment within the bed can ultimately lead to a reduction in water depth, increasing the wave attenuation potential of the local area (Madsen et al., 2001; Bos et al., 2007; Houser & Hill, 2010). However, such effects may be ephemeral, with sediment release occurring in winter (Bos et al., 2007). Several studies have provided evidence to suggest that sediment accumulation and subsequent bathymetric changes do not occur in all seagrass beds. This process appears to be reliant upon the physical conditions of the location as well as the seagrass species present (Mellors et al., 2002; van Katwijk et al., 2010).

Flow Reduction

7.2.9. Kelp habitats are also able to attenuate current flow, an effect which can range from the seabed up to twice the height of the kelp (Lovas & Torum, 2001). As with wave dampening, this ability is also a function of the morphology of the kelp canopy, but also relies upon density and the underlying assemblage of other red, green and brown seaweeds (Gaylord et al., 2007; Eckman et al., 1989). These underlying species help to further reduce flow in close proximity to the seabed, allowing for the deposition of sediment and larvae (Gaylord et al., 2007).

7.2.10. The reduction in flow speeds and the subsequent deposition of sediment tends to be a more important component of coastal protection in habitats where flora possess a relatively small above-ground biomass. This applies to seagrass beds composed of relatively short and highly flexible leaves that possess a smaller potential for direct wave-attenuation compared to stiffer, larger vegetation such as kelps (Bouma et al., 2005; Christianen et al., 2013).

7.2.11. A reduction in the blade density of seagrass habitats would have a knock-on effect on coastal protection by redistributing accumulated sediment, increasing water depth and subsequently reducing wave attenuation. Historical anecdotal evidence from the Isles of Scilly exists suggesting that dieback of seagrasses resulted in large fractions of mud being transported to the adjacent area (Jackson et al., 2012).

7.2.12. The predicted effects of climate change may place greater importance on the coastal protection ecosystem services provided by seaweed and seagrass communities, particularly in relation to wave dissipation and the protection of coastal areas from erosion. Predicted increases in sea level, increased frequency and magnitude of storm events and larger waves have the potential to significantly alter the coastline shape and the depth of near-shore areas, which could have associated impacts on the distribution and abundance of seaweeds and seagrasses in these areas (Hoegh-Guldberg et al., 2007). These features are at particular risk of being lost from more exposed locations.

7.2.13. Relative sea level rise over recent decades has been recorded as almost 6 mm per year in the Outer Hebrides (Rennie & Hansom, 2011). The soft sandy low-lying coasts of the Uists and Barra are particularly vulnerable and erosion may be expected to accelerate over coming years. As the wave base rises above the seabed and kelps in line with relative sea level rise, larger waves and therefore more energy will reach the shore than over past years thereby increasing vulnerability of the coast to flooding and in turn erosion.

Shoreline Stabilisation

7.2.14. In addition to hydrodynamic dampening, seaweeds and seagrasses are also able to contribute to coastal protection through shoreline stabilisation (Fonseca & Cahalan, 1992). Although shoreline stabilisation does not alter energy inputs, it allows the shoreline to be more robust and resilient to received wave energy, by binding sediments and reducing the amount of resuspension. Seagrasses possess roots and rhizomes that extend beneath the sediment surface. These structures can form dense mats that have an anchoring effect, stabilising sediments and increasing the critical bed shear stress required for bed erosion (Le Hir et al., 2007). Recent experimental evidence demonstrated that short, grazed seagrass beds with low above-surface biomass can still be effective at providing sediment stabilisation (Christianen et al., 2013). Seasonal changes in above-surface biomass resulting from the shedding of leaves or degradation due to high turbidity therefore do not mean that seagrass beds have lost their coastal protection value. Thus, seemingly insignificant low-biomass seagrass meadows may still offer significant coastal protection services, and should be valued as such (Christianen et al., 2013).

Formation of Dunes

7.2.15. The formation of dunes and the protection of the coastal zone from erosion is possibly one of the most important services supplied by beach-cast seaweed in the Uists, which is considered to be threatened by accelerated sea level rise (Angus, 2012) and the loss to human lives and property associated with an increase in flooding and storm surge (Orr, 2013). Beach-cast seaweed provides nutrients to dune plants, and promotes their growth, reproduction and survival, and thereby reduces the windblown transport of sand. This facilitates the retention of sediment and dune formation, thus buffering the coast against risks of erosion from flooding (Dugan & Hubbard, 2010; Orr, 2013).

7.3. Enhanced Ecosystem Resilience

7.3.1. Beaches which accumulate seaweed function as an interface for the processing and exchange of organic matter with other environments, rather than existing as enclosed ecosystems. Accumulations of beach-cast seaweed also increase the resilience of sandy beach food webs to perturbations, mainly through diversifying food resources available to higher trophic level fauna (Orr, 2013). Such perturbations may include erosion of sediments during and after storms.

7.4. Climate Regulation - Carbon Cycling, Storage and Sequestration

7.4.1. The term "carbon cycle" includes the exchange of carbon between the ocean and the atmosphere and has both a physical and biological component. The physical component is ultimately affected by the chemistry of seawater and is highly influenced by uptake of carbon dioxide by the oceans. The fixation of carbon by autotrophic (photosynthetic) organisms living within the oceans and its subsequent respiration forms the basis of the biological carbon cycle.

7.4.2. The proportion of carbon incorporated into biomass is said to be 'stored'; thus coastal ecosystems such as kelp forests, maerl beds and seagrass beds are able to store carbon. The stored carbon is removed from the environment; however, respiratory processes following predation, physical disturbance or mortality of the seaweeds/seagrasses release the stored carbon back into the environment. Should carbon sequestration processes be in place on a large enough scale, seaweed and seagrass ecosystems possess the potential to have important climate implications and are termed 'Blue Carbon Sinks' (Nellemann et al., 2009). Their effectiveness as a carbon sink is highly dependent upon their long term capacity to store carbon.

Kelp

7.4.3. The most recent estimates of kelp cover (defined as areas where kelps exceed 20% total cover of the habitat) suggest that this comprises 2,155 km 2 of the seabed around the coast of Scotland (Burrows et al., 2014a). The average standing stock of kelps in Scotland has been estimated at two different values, 94 g C/m 2 (Walker and Richardson, 1955) and 187 g C/m 2 (Kain, 1979). Burrows et al. (2014b) suggests that the value provided by Walker and Richardson (1955) is lower than Kain (1979) because it is based on estimates of standing crop at the shallowest depths rather than the entire depth range. Using a cover value of 2,155 km 2, total estimates of kelp standing stock around Scotland are 202,000 t and 404,000 t C respectively which corresponds to fresh weight equivalents of 4.5 and 9.0 Mt (Burrows, et al., 2014b). Using averaged production rates (685 g C/m 2 /yr), the estimated total production of Scottish kelps is 1,732,000 t C/yr. These values must be treated with caution due to the variability of production estimation methods, differing habitat- and depth-specific rates, varying biomass and the availability of light, nutrients and temperature. Furthermore, Smale et al. (2016) found that the range and maximal values of estimated standing stock of carbon contained within kelp forests may be greater than in historical studies, suggesting that this ecosystem property may have previously been undervalued. Despite these considerations, the estimated total production value gives a clear indication that kelps in Scottish waters represent a significant store of carbon (Smale et al., 2013). This comprises approximately 2% of the global kelp standing stock of carbon that was estimated by Laffoley & Grimsditch (2009).

7.4.4. While a small proportion of kelp-derived material is directly consumed by grazers and therefore transferred to higher trophic levels in situ (Sjøtun et al., 2006, Norderhaug & Christie, 2009), the vast majority of kelp-derived matter (>80%) is exported as detritus or dissolved organic matter into adjacent habitats (Krumhansl & Scheibling, 2012). These exports form an important food source for coastal food webs (see Section 4.3), but may also be incorporated into adjacent coastal sediments (Burrows et al., 2014b). As the majority of kelp beds grow on rocky substrates where burial is not possible, they do not possess the ability to directly sequester carbon. The only pathway available for kelp habitats to sequester carbon is by acting as carbon donors to other habitats capable of long term storage (Hill et al., 2015). Although significant amounts of carbon are exported out of kelp habitats, little evidence exists to quantify the rate of short or long term incorporation of kelp detritus into coastal sediments. Consequently, kelp beds are considered to have very little ability to sequester carbon.

Seagrass

7.4.5. Although the exact extent of seagrass beds ( Zostera spp) is currently unknown, it is estimated that seagrass habitat covers an area of approximately 15.9 km 2 (Burrows et al., 2014b). Production rates of seagrass beds are generally high but vary between species. Annual primary productivity of Zostera marina can range from 69 to 814 g C/m 2 (Borum & Wium-Andersen, 1980; Wium-Andersen & Borum, 1984. In contrast, Mediterranean species such as Posidonia oceanica can fix 550-1000 g C/m 2 /yr, a value comparable to kelp habitats (Borum & Wium-Andersen, 1980).

7.4.6. The ability of seagrasses to slow current flows provides the potential to trap both seagrass detritus and detritus of allochthonous origin (terrestrial and planktonic) (Kennedy et al., 2010). Thus seagrass beds have the potential to act as carbon receivers, potentially storing carbon from external ecosystems as well as their own. An average net sequestration rate for seagrass beds has been estimated at 83 g C/m 2 /yr (Laffoley & Grimsditch, 2009). Combining this average with the estimated extent of seagrass beds in Scottish waters gives a national sequestration capacity of 1321 t C/yr (Burrows et al., 2014b).

7.4.7. However, this estimate must be treated with caution as the average net sequestration rate of 83 g C/m 2 /yr is based on beds populated by Cymodocea nodosa and Posidonia oceanica, whereas seagrass species in Scotland comprise Zostera marina, Z. noltii, and Ruppia maritima. There is little knowledge of the carbon burial rates within beds made up of these species, thus the role of Scottish seagrass beds as carbon sequesters is mostly unknown (Jackson et al., 2012). One of the few pieces of research into the sequestration rates of Zostera spp. indicated that a Spanish Z. marina bed sequestered carbon at a rate of 0.52 g C/ha/yr (Cebrián et al., 1997) [27] . This value is undoubtedly variable on a temporal and spatial scale due to changes in the physical environment (such as differing rates of accretion) and vegetative traits of the seagrasses (Jackson et al., 2012; Kennedy et al., 2010). However rates of this scale are so small they are negligible, therefore there is the potential that Scottish seagrass beds are not able to significantly sequester carbon.

Maerl

7.4.8. Much like seagrasses, given the correct environmental conditions, maerl can form extensive beds. Unlike other seaweed, the calcium carbonate skeleton of maerl does not break down quickly. Consequently maerl beds in Scottish waters represent a continuous standing stock of organic and inorganic carbon that has likely been accreted since the Holocene deglaciation period (Burrows et al., 2014b).

7.4.9. Primary productivity of maerl can reach 407 g C/m 2 /yr which is then trapped in the skeleton, resulting in maerl beds representing a long term carbon store irrespective of whether the algae within the skeleton is living (Burrows et al., 2014b). Relative to seagrasses and kelps, growth rates of maerl are slow at approximately 0.25 mm/yr. Despite this, beds can be extensive and deep, resulting in accretion rates varying from 420 to 1,432 g CaCO 3/m 2 /yr depending upon species composition of the bed (Freiwald & Henrich, 1994). Based on these values, Burrows et al. (2014b) estimate that 440,561 tC are locked within maerl deposits in Scottish waters. Again this is expected to be an underestimate due to the volume of dead maerl in other sediments not identified as maerl beds and the likely presence of a number of undiscovered beds. While their slow growth rates provide a small annual sequestration capacity, their longevity (centuries) means that sequestered carbon is locked away at geological timescales.

Effects of Climate Change on Seaweeds and Seagrasses

7.4.10. Climate change has the potential to affect the carbon sequestration capacity of kelp, seagrass and maerl habitats. The effects of climate change are not well understood, but are mostly predicted to be detrimental. Kelps and seagrasses are likely to be vulnerable to the increases in the occurrence of severe storms which may cause physical damage to and reduce carbon stored in the standing stock. For seagrasses, reduction in canopy density resulting from physical damage may also decrease the habitat's ability to trap sediment and deflect wave energy away from the bed. Sediments storing carbon are therefore likely to be more vulnerable to wave scour and subsequent re-suspension in severe storms. Such storm events are also likely to increase the turbidity of the water, through increased sediment input, which could detrimentally affect growth rates and therefore the carbon sequestration capacity of kelp, seagrass and maerl beds.

7.4.11. Shelf seas around the UK are predicted to be 1.5 to 4°C warmer by the end of the 21st Century ( UK Climate Projections, 2009). The direct effect of temperature increase on kelp, seagrass and maerl communities is likely to vary between each species. However, species present in Scotland are temperate and generally become stressed by high temperatures. Consequently, increased water temperatures are likely to reduce growth rates (Steneck et al., 2002; Short & Neckles, 1999; Hiscock et al., 2004). Such an effect may be offset in kelp and seagrass species that are able to utilize increased CO 2 concentrations associated with ocean acidification (Koch et al., 2013). In contrast, ocean acidification will make it more difficult for maerl to deposit calcium carbonate and increase the dissolution rate of deposits. Ocean acidification therefore has the potential to slow the sequestration of carbon in maerl beds and release some of the carbon laid down by these deposits, including those incorporated in sediments not classed as maerl beds.

7.4.12. The exact effects of climate change are unclear; however, Connell & Russell (2010) suggest that turf-forming algae may become more competitive than large macroalgae under the effect of elevated CO 2. This successional change from larger to smaller plants might see a reduction in the potential carbon sequestration offered by seaweeds around Scotland.

7.5. Environmental Effects of Harvesting

Natural Hazard Protection - Coastal Protection

Kelps

7.5.1. Harvesting live kelps will reduce their density and height, attributes which are crucial to the coastal defence capabilities of this habitat. Dune erosion along the Norwegian coast, for example, has been attributed to the extraction of kelps (Løvas & Tørum, 2001). Density of the kelp canopy has been positively linked with the ecosystem's ability to attenuate wave and current velocities (see Section 4.4 ). Other research has highlighted the importance of water level in coastal erosion and the likelihood of marine flooding (Angus & Rennie, 2014; Løvas & Tørum, 2001), suggesting that raising water levels disengages the wave base from the protective effects of the kelps, increasing the energy reaching the shoreline, and therefore increasing coastal erosion. Using this information, it can be inferred that reducing the height of the kelp canopy through harvesting is likely to affect the habitat's ability to attenuate wave energy and therefore provide coastal protection on a local scale.

7.5.2. The effects of seaweed harvesting have been found to result in changes that are comparable to those caused by natural disturbance ( e.g. storms), as both remove either a proportion or all of the targeted species (Foster and Barilotti, 1990). Such removal provides space for other species to colonise the harvested area. As the potential to offer coastal protection is closely linked with the morphological traits of seaweed species, the successional species moving into the area post-harvest may not offer the same level of protection as the original species.

7.5.3. The extent to which a reduction in kelp density, height, and/or the level of succession will affect coastal protection is likely to depend upon the intensity and method of harvesting. Modern trawling/dredging is used as a sustainable harvesting practice for kelps in Norway. The Norwegian kelp dredge involves removing the entire mature adult plant (including the holdfast) but leaves small immature plants less than 20 cm length. Mechanical cutting of kelps is less widely used, having been phased out in the late 60s (Vea & Ask, 2011). This method involved the removal of kelps irrespective of size and is therefore deemed to be a less sustainable method of harvesting that would have a larger impact upon coastal defences as recovery times are far longer. Hand harvesting of live kelps is assumed to have a negligible effect on coastal protection as its potential scale and magnitude is likely to be much less than dredging and cutting practices.

Beach-casts

7.5.4. Drift kelp deposits on beaches are unlikely to provide significant direct coastal protection, however it is an important source of nutrients to dune plants that stabilise coastal sediments (see Section 4.4 ). The removal of drift kelps may therefore have an indirect effect upon coastal protection by having a negative effect on the growth of coastal plants and therefore on dune formation (Dugan & Hubbard, 2010).

7.5.5. Overall, based on the evidence it is possible that harvesting living or beach-cast kelps in areas where the inshore coastline is soft ( e.g. beaches) could increase their potential for coastal erosion. Areas that are particularly vulnerable are the beaches on the west coast of the Outer Hebrides, Tiree, Orkney, Shetland and the west coast of the Scottish mainland ( Figures 21 and 22). Conversely, areas of the coastline that are already protected by coastal defences or comprise hard rocky substrate are unlikely to be affected by a reduction in coastal protection associated with wild harvesting ( Figures 21 and 22).

Other Groups of Seaweed

7.5.6. The potential coastal protection offered by other seaweeds (namely wracks, red seaweeds and green seaweeds) is generally limited to their contribution to beach-cast seaweed and where they comprise understorey macroalgae associated with kelp beds ( Section 4.4 ). Although no evidence of this was found in the literature, living intertidal seaweeds are also likely to absorb and reduce wave energy. However, given that they are primarily found on exposed rocky shores they are unlikely to contribute significantly to coastal protection.

Seagrasses

7.5.7. Were commercial harvesting to take place, this assessment assumes that the effects would be similar to those caused by better documented pressures such as shellfish trawling and grazing. Grazing is used as a proxy for harvesting using cutting methods (both hand and mechanical cutting), while shellfish trawling is used as a proxy for harvesting by trawling/ sledging/ dredging.

7.5.8. The extent to which harvesting will affect coastal protection provided by seagrasses is likely to depend upon the harvesting method used. Harvesting seagrasses through cutting the leaves from live plants is likely to result in similar effects to those caused by grazing (Christianen et al., 2013) as below ground biomass is left in place while above ground biomass can be significantly reduced (Vonk et al., 2010). Although these beds are likely to have lost some of their ability to attenuate and deflect wave energy directly, the below ground biomass of grazed seagrass beds is still likely to provide coastal protection through sediment stabilisation (see Section 4.4 ). Thus, although more wave energy is able to propagate to the seabed relative to a dense seagrass bed, the anchoring effect of the below ground biomass would still help protect against wave scour.

7.5.9. Trawling is a more destructive practice, with a single pass capable of removing 65% of seagrass biomass (Peterson et al., 1987). As dredges penetrate the sediment surface above and below ground biomass is removed. This removes the sediment stability afforded by roots and rhizomes, ultimately making the local sediments more vulnerable to wave scour and subsequent erosion. This would lead to increases in suspended sediment concentrations and a lowering of the seabed, reducing the wave attenuating potential of the local area, increase energy propagating to the coast and therefore increasing the potential for coastal erosion.

7.5.10. Seagrass beds may also be disturbed by trampling during harvesting activities which could lead to habitat loss/fragmentation (Reed and Hovel, 2006) and in turn result in an increase in the potential for coastal erosion. The level of disturbance (and in turn coastal erosion) is related to the intensity and duration of trampling, as well as the firmness of the substrate, with firmer substrates being less susceptible to damage compared to softer substrates (Eckrich and Holmquist, 2000).

7.5.11. Regrowth rates of grazed seagrasses ( Zostera spp.) tend to be fast, with even intensively grazed beds returning to a pre-grazed state within a year (Peterken & Conacher, 1997; Ganter, 2000). This relatively fast recovery rate is likely to be a result of regrowth from the unaffected below ground biomass. The duration of effect from the cutting of seagrasses is therefore likely to be in the order of one year. The more destructive effects of trawling have been found to result in longer recovery times that range from two years (Preen et al., 1995) to no recovery occurring at all (Giesen et al., 1990). The recovery of seagrass, as well as adjacent seagrass not directly affected by dredging, is likely to be hindered by the increased levels of turbidity associated with the dredging activity. The scale of harvesting that would maintain adequate levels of coastal defence are currently unknown, however cutting appears to be the least detrimental harvesting option.

7.5.12. It is unlikely that the amount of seagrass detritus washed up on beaches has a significant effect on coastal protection. It is therefore not considered in this assessment.

7.6. Climate Regulation - Carbon Cycling, Storage and Sequestration

Kelps

7.6.1. The accumulation of detritus within kelp habitats is very small. Kelp-derived matter is respired, consumed or exported to adjacent habitats. Consequently, kelp habitats are not effective in acting as long term carbon stores. The majority of carbon stored within kelp habitats is contained within the living kelps and is therefore a function of the standing stock (Laffoley & Grimsditch, 2009). Harvesting wild kelps will remove some of the standing stock, reducing the amount of stored carbon in kelp beds throughout Scotland.

7.6.2. The large biomass turnover of kelp habitats results in large amounts of kelp-derived detritus being produced. Approximately 80% of this detritus is exported to adjacent habitats (Krumhansl & Scheibling, 2012; Burrows et al., 2014b). If the correct processes are in place, kelp habitats are able to donate significant amounts of carbon to adjacent carbon stores (Hill et al., 2015), however the proportion of exported material incorporated into carbon stores is unknown but likely to be small (Burrows et al., 2014b). Furthermore, a proportion of the carbon stored in this kelp detritus is released back into the atmosphere through bacterial breakdown. Reducing standing stocks of kelps through wild harvesting is likely to reduce the amount of detritus produced and subsequently exported and stored in adjacent habitats.

7.6.3. The magnitude of the harvesting effect is likely to depend upon the scale and type of harvesting undertaken, as well as the recovery rate of kelps. Kelps harvested by trawling/dredging/sledging leaves small kelps in situ, thus the habitat is able to return to pre-harvest size and density within a few years of the disturbance if harvesting levels are sustainable (Christie et al., 1998). As a result, any reduction in carbon storage is only likely to last a few years if appropriate management is in place. Harvesting via hedge cutting removes all kelps irrespective of size and is therefore more likely to be less sustainable. Effects may therefore be similar to those caused by overgrazing (Mann, 1977) and are likely to result in longer recovery times relative to those of habitats where juvenile kelps have been left intact. The collection of beach-cast seaweeds and seagrasses could potentially result in a reduction in the release of stored carbon back into the atmosphere through a reduction in microbial decomposition.

Maerl

7.6.4. The calcium carbonate skeletons produced by maerl form long lasting carbon deposits, and although the production rates are small relative to seagrasses and kelps, the extensive and deep beds form a significant carbon sink in Scottish waters (see Section 4.4 ). Harvesting practices are likely to impact upon the amount of live maerl, directly (as a result of removal) and indirectly, because of reduced survival resulting from increased levels of suspended sediment and physical disturbance (Burrows et al., 2014b). This may not greatly affect the overall carbon standing stock held in maerl deposits, but is likely to reduce the potential for future carbon sequestration, slowing the growth of the carbon reservoir (Burrows et al., 2014b). The magnitude of effect would therefore be dependent on the scale of commercial harvest and the ratio of live:dead material extracted.

Other Groups of Seaweed

7.6.5. There is no evidence that other seaweeds (namely wracks, red seaweeds and green seaweeds) are significant carbon stores and potential sinks of carbon. The impacts of harvesting these other types of seaweed on the carbon cycle, storage capacity and sequestration rates are therefore unknown but unlikely to be significant.

Seagrasses

7.6.6. The extent of any reduction in carbon storage potential due to harvesting is likely to be a function of the harvesting method. Cutting live seagrasses is likely to have an effect similar to grazing, decreasing the density of above surface biomass and therefore reducing the ability of the bed to slow current flow and trap sediment. The reduction in density of above surface biomass is also likely to increase the exposure of the bed to wave scour. However, this method leaves the stabilizing roots and rhizomes in place. Grazed seagrass beds may recover to a pre-grazed state within a year (Peterken & Conacher, 1997; Ganter, 2000), hence the effect of cutting on the ability of seagrasses to act as a carbon sink is minimal. Dredging is likely to have a more detrimental, long-lasting impact on carbon sequestration. Recovery times from this disturbance tend to be longer (see section above on Natural Hazard Regulation - Coastal Protection) and the removal of below ground biomass make sediments more susceptible to erosion, ultimately resulting in previously stored carbon being reintroduced into the marine environment.


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