Date: 05/07/17

Time: 8:00 - 12:00 BST

Vessels: Xplorer

Low Tide: 08:48 UTC (1.42m)

High Tide: 14:51 UTC (4.07m)

Sea Surface Temperature: 15.5 degrees Celsius

The Falmouth estuary is well-known for its diverse range of habitats, where many different kinds of organisms can be found. Due to its high diversity in habitat types, the estuary has been classified as a Special Area of Conservation, which protects some extensive seagrass and maerl beds, that are important habitats for other organisms to live in. Maerl beds are also known to be particularly sensitive to climate change, and can act as a carbon sink. The SAC designation of the Fal estuary area also restricts the sampling and tampering of the seabed, in order to perserve the wildlife found in the area.

Group 3 conducted a sidescan survey on July 5th 2017 onboard the MTS Xplorer. The site survey was Site 2, the site further offshore on the western side of the Falmouth Bay, from Swanpool towards the headland. Three videos were also collected as a method for groundtruthing of the substrate.

The aim of this survey is to survey the seabed and identify the substrate type in the area that was sampled, as well as characterising the flora and fauna associated with that substrate type.

Site 2 was surveyed using the sidescan sonar, which sends out acoustic waves that reflect off the seabed and maps the habitat. Four parallel lines in total were recorded by the sidescan sonar alongside each other, 100 m apart. This identifies the type the substrate that is found in the area. The map projection was OSGB36, and the SONAR was at 100kHz. The equipment had a range of about 75 m.

Groundtruthing methods were also employed during the site survey of Site 2. A video camera mounted on a frame was deployed three times, and recorded video images across the four transect lines used for the sidescan sonar in order to identify the substrate type directly. No scale was available for any of the recordings. The camera was orientated in different directions for each video; at an angle, vertically, and horizontally.

A van Veen grab was not deployed as an alternative method of groundtruthing due to time constraints.

Species List:

- Brittle stars (Class Ophiuroidea)

- Red algae (Ceramium rubrum(?))

- Live and dead maerl (5% coverage)

- Sponges

- Hermit crab (video 1)

- Spiny starfish (Marthasterias glacialis)

- Unidentified fish (4)

- Unidentified algae

Discussion

Using the sidescan sonar, the site was found to contain mostly sand, with a small section of rockier substrate associated with kelp. The ripple width was 1.43 m, and as there is an accumulation of organisms like Ophiuroidea and algae in the trough, it is likely that these are relic ripples, and have not moved recently. The video recordings showed that the seabed was made of live and dead maerl, and maerl sands. The Fal and Helford Management Scheme has previously classified the area as 'mixed sublittoral sediments with dead maerl and maerl gravels'. The site is maintained under the conservation objectives in the management scheme, and is a monitoring priority. The JNCC Report in 2010 also classified the Falmouth Bay as coarse sediment, which includes sandy substrate.

The videos recorded high abundance of Ophiuroidea throughout, with more algae observed in video 1 and more sponges in video 2. A spiny starfish (Marthasterias glacialis) and three hermit crabs were also found in the first video. The abundance of Ophuiroidea may be attributed to reduced predation intensity or increased food supply due to high nutrients input from the Fal estuary nearby. (Metaxas and Giffin, 2004)

Fal & Helford Special Area of Conservation Management Scheme 2000, Ch. 2 'Reasons for designation as a special area of conservation (SAC)', 2.3 'Current condition of interest features', 19-22, Ch. 4 'Conservation objectives', 4.3 'Zoning map', 31.

McBreen F., Askew N., Cameron A., Connor D., Ellwood H. and Carter A. 2011. UKSeaMap 2010: Predictive mapping of seabed habitats in UK waters. JNCC Report, No. 446.