Habitat

The aim of this survey was to create a benthic habitat map for the area between Carricknath Point and Shag Rock. This was done by using sidescan sonar in conjunction with ground-truthing by using underwater camera transects.

Methods

Sidescan model: Kongsberg dual-frequency

Operating frequency (kHz): 100 and 410

Paper trace software: GeoAcoustics

Navigation software: HYDROpro navigation

Trackplot software: Surfer

Co-ordinate system: OSGB13

Sidescan swath (m): 150

In total, four transects were taken and numbered Lines 4-7. A preliminary line without the sidescan was first taken between Lat: 50⁰08'435N Long: 005⁰01'083W (N31945.845m, E184449.979m) and from that, the transects were plotted. Each sidescan was spaced 100m apart to give some overlap for each, allowing us to produce a map by stitching the traces together.

Three camera transects were taken by selecting a position within the sidescan transects and allowing the boat to drift for an average of 12 minutes. This gave a curved transect which can be seen on the benthic habitat map. The camera used was a boat tech camera. Some issues were encountered on camera transect 1 due to a fuzzy picture, making identification difficult but not impossible. No grab was taken as we were located in an SAC.

Date: 23.06.2015

Time: 12:50 (UTC)

Location:

Line 4: Start of line: 50° 08.4’ N 5°01.3’ W

End of line: 50° 08.9’ N, 5° 01.3’ W

Line 5: Start of line: 50° 08.8’ N 5°01.2’ W

End of line: 50° 08.4’ N, 5° 01.2’ W

Line 6: Start of line: 50° 08.4’ N 5°01.1’ W

End of line: 50° 08.9’ N, 5° 01.1’ W

Line 7: Start of line: 50° 08.8’ N 5°01.0’ W

End of line: 50° 08.4’ N, 5° 01.0’ W

High tide: 9:20 4.3m

Low tide: 15:50 1.5m

Cloud cover: 5/8

Sea state: 2/10

Disclaimer- The views shown here are solely of group 4 and do not necessarily reflect the views or opinions of the University of Southampton.

Falmouth 2015 Group

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Benthic Habitat Map – Results and Discussion

Click Here to view Habitat Mapping Poster

Click Here to view Map as PDF

Rock – Unknown Flora

This habitat type comprises of the areas enclosed by boundaries 5, 6 and 10. We have no ground-truthing for these areas, so this assumption is made due to the sidescan trace texture being similar to that in areas 8 and 9. Camera transect 3 passed through these boundaries and showed rocky areas with kelp and other algae. From this, we deduced that these areas were rock outcrops, but we have left the title as “unknown flora” due to lack of ground-truthing. Boundary 10 is also close to the coast, where other confirmed rocky areas were located.

Muddy Substrate

We have inferred that boundaries 1,2 and 3 enclose sections of finer substrate. However, we are unable to verify this as we have no camera footage or grab samples for ground-truthing, but these have a similar texture on the sidescan to the area covered by camera transect 1, which shows a sandy/muddy substrate.

Depression

Boundary 4 encloses depression and shadow. Again this is an educated guess due to having no ground-truthing but the shadow is very clear. This could be due to currents scouring the seabed, which can occur when water flow is disrupted by a feature such as an anchor chain.

Sandy Substrate – Denser Seagrass

Boundary 7 contained a muddy substrate with varying densities of seagrass as can be seen on camera transect 2. This could be seen on the sidescan trace as spotty areas, which we can assume are patchy seagrass.

Sandy Substrate – Patchy Seagrass

The majority of the substrate around the features outlined is a sandy substrate with very patchy seagrass, as seen in camera transect 1. This is because the brightness around these areas are similar to those seen in the area of camera transect 1.

Rock – Kelp Forest

Boundaries 8 and 9 contain kelp and other marine flora and rocky substrate. This is based on camera transect 3, which shows these features.

Note

While there is a slight brightness change between EO184074m, NOO31612m and EO184073m, NOO31479m, this line is not clearly defined and we have no ground-truthing data. As such, we have not included this as a boundary on our final habitat map.

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Camera Transects – Results and Discussion

Camera Transect 1

Camera transect 1 showed fine sand and muddy sediments with sparse algae and seagrass (Zostera marina). No fauna are seen at the surface, but burrows and pits indicate the presence of infauna, such as burrowing polychaete worms.

Camera Transect 2

Camera transect 2 shows sand and muddy substrate with more abundant tufts of seagrass (Zostera marina) and other algae. The presence of seagrass adds to information which suggests that the sediment is sandy, as Z. marina grows in sandy sediment in subtidal areas of 4 meters or less (Tyler-Walters, 2008). The seabed along this transect is at a depth of 6m, explaining the patchiness of Z. marina. There is occasional presence of epifaunal organisms, such as the common seastar (Asterias rubens) which thrives on areas of fine sand covered with an algal carpet (Budd, 2008). This can also be clearly seen in the second still of camera transect 3. Small sized fish can also be seen, which may include Couch’s Goby (Gobius couchi), a protected species only found in south Cornwall and Ireland (JNCC, No DateC).

Camera Transect 3

Camera transect 3 showed a shallow (4m depth) rocky habitat colonised by mixed vegetation including kelp (Laminaria species) and seaweed species, such as Saccorhiza polyschides and Heterosiphonia plumosa. H. pulmosa is a red seaweed which grows as an epiphyte on kelp species (Rowley, 2008a). The kelp from boundary sites 8 & 9 indicates several features associated with kelp growth. While kelp grows in most coastal areas with a temperate climate, most kelps require a hard substrate on which they can anchor and a high level of current. Currents cause sweeping which prevents sediment build up, and pushes away herbivorous organisms. The movement of canopies allows light to reach fronds which would otherwise remain in the shade. High current levels also allow for higher nutrients, particularly nitrate, as this is a very important requirement for kelp growth. (Dayton, 1985).

Camera transect 3 also shows ripple marks in the areas of sandy sediment covered by an algal carpet. These are characteristically formed in areas of constant high current, however these ripples are not big enough to be picked up by the sidescan sonar. Indications of infaunal organisms were also present.

Juvenile pollack (Pollachius pollachius) use estuarine algae as a nursery ground during their first year of life (Rowley, 2008b) and can be observed in the last still.

References

Budd, G., 2008. Asterias rubens. Common starfish. Marine Life Information Network [online] Available at: <www.marlin.ac.uk/speciesfullreview.php?speciesID=2657> [Accessed 28/06/2015]

Dayton, P. K., 1985. Ecology of Kelp communities. Annual Review of Ecology and Systematics. 16. pp. 215-245.

Deeble, M., Stone, V., 1985. A port that could threaten marine life in England’s Fal Estuary. Oryx. 19:02 pp. 74-78.

JNCC, No Date A. Fal and Helford. [online] Available at: <jncc.defra.gov.uk/protectedsites/sacselection/sac.asp?EUcode=UK0013112> [Accessed 27/06/2015]

JNCC, No Date B. Special Areas of Conservation. [online] Available at: <jncc.defra.gov.uk/page-23> [Accessed 27/06/2015]

JNCC, No Date C. Couch’s goby. [online] Available at: <jncc.defra.gov.uk/page-5633> [Accessed 28/06/2015]

Rowley, S., 2008a. Heterosiphonia plumosa. A red seaweed. [online] Available at: <www.marlin.ac.uk/speciesinformation.php?speciesID=3485> [Accessed 28/06/2015].

Rowley, S., 2008b. Pollachius pollachius. Pollack. [online] Available at: <www.marlin.ac.uk/speciesinformation.php?speciesID=4155> [Accessed 28/06/2015].

Somerfield, P. J., Gee, J. M., Warwick, R.M., 1994. Soft sediment meiofaunal community structure in relation to long term heavy metal gradient. Marine Ecology Progress Series. 105. pp. 79-88.

Tyler-Walters, H., 2008. Zostera marina. Common eelgrass. Marine Life Information Network [online] Available at: <www.marlin.ac.uk/speciesfullreview.php?speciesID=4600> [Accessed 28/06/2015]