FALMOUTH FIELDCOURSE

Group 5

The views expressed are those of the individuals concerned and do not express the views of the University of Southampton or those of the National Oceanography Centre Southampton.

The Fal estuary is under constant anthropogenic influences, giving reason for the formation of the Fal and Helford Marine special area of conservation (SAC) in 2006 (CIFCA, 2017).  This was to limit the impacts of activities such as a £20m dredging project in 2013, that could have damaged the protected seaweed beds (BBC, 2011). Remote sense images from a side scan sonar were coupled with discrete bed observations through a mounted camera to survey the seafloor topography and species diversity at site 11 shown by the blue pin on the image below.

The aim of this project was generate a seabed habitat map for this particular location. This site was chosen due to its unique location which is adjacent to the SAC boundary within the protected area.

Introduction and Aims

Methodology

Side scan sonar and videography were used to assess habitat type at site 11. Side scan sonar measures the amount of sonic backscatter reflected from the substrate and produces a sidescan trace of the surveyed area. On the trace darker colours represent harder substrates and lighter colours represent softer substrates. Shadows created by vertical objects can be used to determine their height, e.g a shadow from a prominent rock. The side scan system being used in this case worked at 100KHz with a swath width of 150m and the survey speed was ~4Kn (~2ms-1).

Once the four side scan transects were complete a video system was used to ground truth the area. Three sites were selected using the side scan read out; a rocky site, a sandy site, and a site that transitioned from rocky to sandy. Videography is a useful tool for ground truthing as it enables observation of the seafloor which can be used to verify substrate type; in this case, rocky substrates with kelp beds were most common and there were a number of sandy areas with small pebbles.

A sediment grab was not conducted due to time constraints and the relative unreliability of grabs on rocks and pebbles.

Habitat Types Found

There were two main types of habitat found at site 11. The first being kelp beds anchored to rocky substrates (see below left), and the second, coarse grained sand and pebbles (see below right).

The kelp have strong holdfasts which attach to the rocky substrates, allowing them to withstand high wave action. The kelp bed provides protection to the understory and shoreline by reducing turbulence and wave action. The blades of the kelp provide an ideal habitat for small encrusting organisms, principally Bryozoans (Førde, H. 2014). Although kelp beds are robust, certain environmental conditions such as a lack of nutrients, or storms can cause their destruction (Smale and Vance, 2015). Most common kelp; Laminaria sp.

The sandy substrates found off the coast of Falmouth have a large amount of biogenous sediment. The high wave action in coastal regions causes the lose grains to travel forming ripples in the sediment (Leckie, 1988). The kelp and Rhodophyta in these sandy regions are unable to grow to their full size as the pebbles they are attached to are small and less capable of anchoring them to the  substrate.

References

Førde, H. 2014, ‘Development of bryozoan fouling on cultivated kelp (Saccharina latissima) in Norway’, Institutt for biologi.

Smale, D.A. and Vance, T. 2015, ‘Climate-driven shift in species’ distributions may exacerbate the impacts of storm disturbances on North-east Atlantic kelp forests’, Marine and Freshwater Research, 67, 65-74.

Leckie, D. 1988, ‘Wave-Formed, Coarse-Grained Ripple and their Relationship to Hummocky Cross-Stratification’, Journal of Sedimentary Petrology, 58, 607-622

Cornwall Inshore Fisheries and Conservation Authority (CIFCA). (2017).Fal and Helford SAC : Cornwall Inshore Fisheries and Conservation Authority (CIFCA). [online] Available at: http://www.cornwall-ifca.gov.uk/UK0013112 [Accessed 8 Jul. 2017]

BBC. (2011) Falmouth harbour dredging 'may start 2013' chancellor says. BBC news. Available from: http://www.bbc.co.uk/news/uk-england-cornwall-15963791. [Accessed: 08/ 07/ 2017]

Click to expand

Click to expand

Following the Sidescan Sonar Trace Produced onboard Xplorer, the boundaries between substrate need to be transferred onto an accurate Track plot produced using Surfer 8 in order to map the habitats in the surveyed area. The trace was cut and rearranged to show each of the four linear transects adjacent to each other as a sonar ‘map’. From this, the boundaries can be identified and points along each boundary can be plotted to calculate both the real world location of the boundary and its location relative to the 4 measured transects.

Using these calculations, the boundaries can be plotted on the Surfer 8 plot, to show the shape of the seafloor. The Surfer 8 plot will be more accurate than the Sidescan trace, because the trace view is only relative to the ships track. For example, the Towfish does not travel in a perfectly straight line in the water, will be towed at a position behind the ship (and the ships navigation instruments), and will be elongated or truncated if the ship does not keep uniform speed – all of which will contribute to distorting the view on the Trace. In our case, the fish was 12 – 14m behind the ship. On a larger ship, this distance would be larger and would need to be taken into account. The difference we observed in shape were due to the change in speed of the ship – the speed decided was 4 knots, however this fluctuated along each transect.

We encountered problems transferring the boundaries between the Trace and Surfer plot. The boundaries plotted on the on the Trace did not match when plotted on the Surfer plot, to an extent beyond fluctuating ship speed, so that boundaries overlapped. Despite this, the boundaries were plotted and the discrepancies were ‘manipulated’ to a sensible outcome that we predict to be the correct result.

Once the map was successfully plotted, the two substrates (sediment and rock) could be colour coded to show habitats. Furthermore, our three video drift transects could be plotted. Again, the problems we had during plotting were overcome by matching the timestamps from points along the video track to the timestamps from the ships track, rather than the inaccurate Eastings and Northings.

Creating the Habitat Map

Click here to access a PDF of our habitat mapping poster