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The UK Government has recognised the need for nature-based solutions to climate change as part of the UK’s target of reaching Net Zero emissions by 2050. Understanding the location and extent of subtidal seagrass beds (Zostera Marina) is crucial to determining their contribution to ecosystem services. Therefore, we need to accurately define the extent of the known seagrass beds and prospect for new seagrass beds in areas where they may occur, using methods that are accurate, efficient, and repeatable.

During sea trials of Solstice Multi-Aperture Sonar (MAS®), we found that Solstice was very good at mapping subtidal seagrass. To understand this capability better, we mapped the seagrass beds in Plymouth Sound in October 2023 and August 2024 to assess the suitability of this new sonar system for wide-area seagrass and ecosystem mapping. Solstice MAS was mounted on a small, shallow-draft survey vessel and run over the many seagrass beds in Plymouth Sound. Solstice generates wide-area, repeatable, and georeferenced seabed images and bathymetry, enabling the precise mapping of features such as seagrass beds and capturing areas of the seabed as small as 1.5cm x 3.75cm. Solstice can be used for mapping areas of 1m2 and above, but there is no limit to the largest area of seabed that can be mapped.

Figure 1: Solstice MAS displays showing bathymetry and texture over an area of rock and sand seabed.

During the surveys, we found that seagrass was visible on the sonar record as a well-defined texture different from other seabed types such as rock, sand waves, and kelp. Seagrass grows in depths less than 5m in Plymouth Sound, and in some places it grows between rock reefs and right up to the low water mark. These areas are difficult to map safely using drop cameras or echo sounders because the boat must be directly over the area being recorded. Using Solstice, we could safely map these difficult areas using the sonar’s 100m range capability. The boat could be in deeper water offshore and still image the seagrass in amongst the rocks and shallows 100m away.

Figure 2: A single 200m wide sonar image of the seagrass bed off Drake’s Island in Plymouth Sound.

This method is an efficient and accurate way of mapping seagrass beds. Solstice is designed for survey speeds of up to 6 knots, achieving a maximum seabed coverage rate of 1.6 km² or 160 ha per hour. For example, the largest seagrass bed in Plymouth Sound is 1300m long, but this can be mapped in just 9 minutes. Post-processing and analysis can be conducted back in the office using standard sonar processing software, further minimising expensive vessel time. Unlike conventional side scan sonar, the image quality does not depend on tow speed. The sonar is also unaffected by poor underwater visibility.

Figure 3: A detailed image of the seabed off Drake’s Island showing seagrass, kelp, sand and rock reef.

Solstice records details about the type and texture of the seabed around a seagrass bed, illustrating the seagrass in context. This information helps us to understand why the seagrass grows where it does and can provide invaluable information about a site before any restoration work begins. Solstice images provide detailed insights into the spatial variation of seagrass beds, which is essential information for planning detailed investigations using divers, drop cameras, or ROVs. The Solstice sonar images are georeferenced and spatially accurate, so relocating significant features on the seabed using divers is straightforward.

Figure 4: Solstice creates images of the seabed that show variations in seagrass coverage and density over large areas.

We also used Solstice to prospect for seagrass in areas where it may grow. Solstice detects small, sparsely distributed patches of seagrass that are difficult to locate and even harder to accurately map using other methods. We used it to find an area of potential new growth in Cawsand Bay that was beyond the extent of the known seagrass bed. We also identified several previously unknown seagrass beds in Jennycliff Bay on the east side of Plymouth Sound, the largest being 390m2 in area. The results of these surveys have revealed significant differences between the previously known extent of the seagrass beds in Plymouth Sound and those defined by Solstice MAS.

Measuring Change in Seagrass Beds

Understanding the health of seagrass beds is crucial to determining their contribution to ecosystem services.  Accurately measuring change is an important part of understanding and monitoring seagrass.  The actions we need to take depend on knowing if the seagrass is improving or getting worse, and if the changes are a trend or temporary.  The efficiency, wide area coverage and inherent spatial accuracy of Solstice MAS® makes it ideal for assessing changes in seagrass beds over time.

The state of a seagrass bed may change for many reasons.  Seagrass is affected by annual cycles of growth, die-off, and senescence, which are currently not well understood.  We know that seagrass beds can also change due to storm events and other impacts such as water quality incidents and agricultural run-off.  Variations in the plants themselves tell us something about the health of the seagrass at a small scale, but larger scale measurements provide different information: is the seagrass bed spreading or receding, has the seabed sediment changed, is the seagrass being damaged by boat anchors or moorings?

We used Solstice MAS to generate detailed, accurate and repeatable measurements required to determine changes in seagrass beds.  For example, we found a sonar image from Solstice tests in April 2018 that showed the southern end of the seagrass bed in Cawsand Bay in Plymouth Sound, and we then mapped the same area in October 2023.  The picture below shows Solstice sonar images of the same area of seabed taken five years apart.  The brown stripe across the pictures is the area imaged by the sonar, and the same large rock is visible at the bottom of both pictures that we can use as a reference when comparing the images.

Figure 5: Comparison between Solstice sonar images from 2018 and 2023 for the southern end of the seagrass bed in Cawsand Bay, showing significant changes to the coverage

The seabed around the rock is flat sand, and the white ‘fluff’ on the sand is how seagrass appears on the Solstice sonar.  The upper picture from 2018 shows the southern end of the large seagrass meadow in Cawsand in the upper left corner, and there are a few patches of seagrass between that and the rock.  By 2023, the seagrass had spread, covering much of what was once bare seabed and covering ground further west beyond the rock.

Notice how the seagrass forms clumps and patches with bare or sparse areas in between.  Measuring seagrass coverage in this area using diver transects would give very different results depending on where the transect was laid.  Solstice provides a much wider view of the seagrass from which we can calculate more accurate and representative measurements of coverage.

A change was also noticed in the seagrass bed at Ramscliff on the east side of Plymouth Sound, this area was mapped in detail using Solstice MAS sonar in October 2023 and again in June 2024.

Figure 6: Variation in seagrass coverage at Ramscliff between October 2023 and June 2024

The change in eight months is noticeable.  The overall extent of the seagrass bed was similar, but the coverage was much less and had become very patchy.  The coverage was 1.33 hectares in 2023, but eight months later it was just 37% of the 2023 area.

Figure 7: Solstice sonar images showing a variation in sediment depth at Ramscliff between 2023 and 2024 in an area of seabed approximately 50m x 55m

The picture above shows one possible cause for this significant loss in coverage.  A close inspection of seabed rock features near the seagrass shown on the Solstice sonar image suggests that there were changes to the sediment levels at the site.  The picture on the left shows the seabed in 2023; note the small rock in the lower left corner, marked with a white arrow.  The same rock is shown in the picture on the right when seen in the summer of 2024, and now far more of the same small rock is visible.  More of the rock would be visible if the sediment level around it dropped, which suggests that the level of seabed sediment reduced at the Ramscliff site between 2023 and 2024, perhaps taking some of the seagrass with it.

Habitat Suitability Models

The UK Government has recognised the need for nature-based solutions to climate change as part of the UK’s target of reaching Net Zero emissions by 2050.  Seagrass is one of the key species that contribute to a wide range of ecosystem services, so restoration of seagrass beds is a priority.  Subtidal seagrass needs a particular environment in which to grow, and we can predict where reseeding may be successful using a habitat suitability model.

The ability for subtidal seagrass to grow depends on several factors.  There needs to be sufficient light reaching the seabed, which is defined by the water clarity and water depth.  Water clarity varies according to location; in some places where the water is clear seagrass can be found down to 10m depth, but the maximum depth range is usually 5m in Plymouth Sound where we are doing this research.  Seagrass grows on sand and mud sediment and in places where there is not too much water current and not too many waves to scour the seabed.  There are other factors, such as water quality, but this must be good enough because seagrass already grows in Plymouth Sound.

We used Solstice MAS® to help create habitat suitability models in Plymouth Sound.  We used its bathymetry capability to make a 3D model of the seabed and from that identified the flatter areas that are less than 5m deep.  The seabed texture information from Solstice is good for identifying different types of seabed, like sand and rock, and it can often be used to identify marine life such as seagrass and kelp. Published models exist that show the type of seabed in each port of the UK coast, but in some areas, such as Plymouth Sound, the models are too coarse to be used for accurately planning rewilding projects.  Solstice can provide the detailed and accurate spatial information needed before planning restoration work.  In some cases, it can also say if restoration work is feasible.  Solstice was used to help create a suitability model for seagrass off Drake’s Island at the northern end of Plymouth Sound.  The sonar records were interpreted to identify areas of sand/mud, rock, and kelp that were shallower than 5m depth.  The model showed that the existing seagrass bed is bounded on both sides by kelp-covered reef and that there was little room for expansion to the sides.

Figure 8: Habitat suitability model for Drake’s Island in Plymouth Sound with unoccupied suitable areas shown in red. The existing seagrass bed does not cover the predicted area, suggesting that some other factor is limiting growth.

The Drake’s Island seagrass bed does not extend northward as far as the expected 5m depth limit, which suggests some other factor is limiting growth.  We can test the assumptions used for the habitat model using the detailed seabed models created using Solstice.  We can compare the theoretical coverage of seagrass with the actual cover as seen on the sonar, and any difference will highlight some other factor not yet considered (Fig. 1).  The north side of the island faces the Tamar River channel and is subject to strong tidal currents, so these may be limiting the spread of the Drake’s Island seagrass bed.  Further work to test this idea will be done shortly using a Sonardyne Origin ADCP to measure the actual water current inside and outside of the seagrass bed.

The habitat model for Cawsand Bay on the west side of Plymouth Sound shows that an area to the north of the existing seagrass meadow is suitable for seagrass.  Solstice sonar surveys and observations by divers suggest that the seagrass is expanding northwards beyond Sandway Point (Fig. 2).  Within a few years, the seagrass in Cawsand may have naturally expanded to cover all the suitable real estate in that area.

Figure 9: Habitat suitability model for subtidal seagrass in Cawsand Bay, Plymouth. The unoccupied areas suitable for seagrass are shown in red.

Detecting Seagrass Damage

The UK Government has recognised the need for nature-based solutions to climate change as part of the UK’s target to reach Net Zero emissions by 2050.  Seagrass is one of the key species that contribute to a wide range of ecosystem services, so the restoration and maintenance of seagrass beds is a priority.  Subtidal seagrass is easily damaged, so accurate monitoring is required to ensure that the seagrass is in good condition. This requires detailed records of the seabed at high spatial accuracy so subsequent records can be directly compared.  Damage can come from several sources; seagrass beds can be physically damaged by storms, small boat moorings and anchoring, as well as large marine litter.

Seagrass usually grows in sheltered environments, but these areas may occasionally be affected by severe storms that disturb the seabed over large areas.  The edges of seagrass beds can also be eroded by variable riverine currents, such as the bed on the north side of Drake’s Island in Plymouth Sound.  Here, the spread of the seagrass appears to be limited by the outflow of the Tamar River, and the northern extent of the seagrass expands and retreats over time.

Figure 10: Solstice waterfall image of an area off Drake’s Island in Plymouth Sound. The edge of the seagrass affected by riverine currents is on the left. Inset is a bare patch in the seagrass caused by the presence of large marine litter.

Damage to the seagrass will occur if a small boat deploys its anchor in a seagrass bed.  Once on the seabed, the anchor digs into the sediment and displaces the seagrass by its roots, while the anchor chain drags across the seagrass, damaging leaves over a wide area.  This continues while the boat swings on its mooring as the chain drags across a swathe of seagrass.  When the anchor is later recovered, more seagrass roots are disturbed as the anchor breaks out of the seabed.

Small boat chain moorings in tidal areas can damage seagrass.  Simple moorings have a metal chain attached to a heavy mooring block on the seabed at one end and a floating buoy at the other.  Here, the chain can drag over the seabed around the mooring block when the tide goes out.  This is a problem in areas where the depth of water is small, but the range of tide is large, such as Cawsand Bay in Plymouth, where the depth and the tide range are the same at approximately 5m.  The action of the chain leaves a bare patch around the mooring where seagrass cannot grow.  Alternative mooring designs replace the chain or use floats to lift it clear of the seabed so the seagrass can grow around the mooring block.

Figure 11: An image of the seabed created by Solstice MAS showing damaged areas in a seagrass bed caused by small boat moorings.

During recent surveys of seagrass beds in Plymouth Sound using Solstice MAS, we found that large marine litter can also damage seagrass beds by creating a bare patch of seabed around the object.  The reason for this is not clear, but it may be due to localised seabed scouring around the object caused by tidal currents.

We are using Wavefront Solstice MAS® to record and monitor damage to seagrass beds in Plymouth Sound:

Solstice can locate features in the seabed, such as mooring blocks, bare patches and anchor scours.

• Solstice produces high-resolution, scaled images that can be used to accurately measure damaged areas of seagrass.
• The efficiency and spatial accuracy of Solstice data make it ideal for assessments of change in seagrass beds over time, improvements after damage is mitigated.
• Solstice is efficient and can cover up to 160 ha per hour, so it can be used for rapid assessments of large seagrass beds.

We are using Solstice to accurately record the position, size and shape of any damage to the seagrass in Plymouth.  Repeat surveys will highlight changes in the damaged areas to see if they are improving, particularly where yacht moorings have been upgraded or marine litter has been removed.

Figure 12: Marine litter can also damage seagrass. This image from Solstice MAS shows bare patches in the seagrass caused by large marine litter on the seabed.

A comprehensive report has been published on this work, please click here to download a copy.