GROUP 9 FALMOUTH 2017

Methodology

On 04/07/17 between 08:27 and 08:58 GMT, group 9 undertook a basline survey of the seafloor at St Mawes harbour in the Falmouth Estuary, UK on the MTS Xplorer.

The weather conditions were calm and cloud cover was 8/8. The aim of the survey was to produce a habitat map of the area which could be used in the monitoring of the area, as part of its Special Area of Conservation (SAC) status.  A subsurface dual frequency side scan sonar instrument was used to produce a carbon copy of the texture of the seafloor to give an impression of the types of substrate and consequently, the habitats present. In order to verify the types of substrate and habitats (“ground-truthing”), a high-definition video camera was attached to a plastic fin and lowered at a slight angle towards the seafloor and the vessel was allowed to drift over the sample area. The video was observed and recorded.

On the following day, the carbon copy of the side scan was scaled and aligned according to the position of the vessel. Four different habitat types were identified and superimposed onto the scaled side scan. These habitat types were found to be: gravel and sand; patchy eelgrass beds; dense eelgrass beds and sub-tidal mixed muddy sediment. The key species living in each habitat were identified using the video footage. Using this information, a habitat map was produced, as seen below.

HABITAT MAPPING   

Introduction

On 4th July 2017, we conducted a sidescan survey to create a comprehensive benthic habitat map of the sea floor in St Mawes harbour.

The results of this survey can be used as a baseline habitat map to compare change in the ecosystems in future years. We carried out the survey on the MTS Xplorer which is a 12m long catamaran (Limited, 2017), covering four transect lines between 8:27 and 8:59 GMT. The equipment used to carry out the survey was a dual frequency Sidescan Sonar and a drop-down video system. Results of the sidescan sonar produced a textural carbon copy of the seafloor; we then ground truthed using the video footage in the lab.

Results & Discussion

Analysing the side scan sonar and the video drifts, we were able to identify four different habitat types within the St Mawes harbour; gravel and sand communities, patchy eelgrass beds, dense eelgrass beds and subtidal mixed muddy sediments.

Each habitat provides a different niche with varying pros and cons for species, even though the habitats are in close proximity to each other there will still be a clear difference in fauna present.

Zostera marina responsible for the dense eelgrass beds create a habitat that is used by many species for protection. Juvenile cod and plaice rely on being able to hide and escape predators while they are at the most vulnerable stages of their lifecycle. It is vital to monitor the existing eelgrass beds as a reduction of the habitat size (by moorings or anchors) can have a cascading effect on adult fish populations, consequently causing fish stocks falling at an increasing rate.

Surveying the whole of St Mawes was not possible due to yachts and moorings obstructing transect lines 3 and 4. Consequently, it was not possible to calculate the complete size of eelgrass beds present. This could cause problems when monitoring changes in the habitat in future years because changes in the eelgrass could occur at areas that were unable to be sampled. Using side scan sonar and video drifts provide little information on the species present and diversity of the area. A grab sample could be used to access the infauna and epifaunal species, however this was not possible due to the eelgrass being a protected area.   

References

McCloskey, R. and Unsworth, R. (2015). Decreasing seagrass density negatively influences associated fauna. PeerJ, 3, p.e1053.

Introduction

Methodology

Poster

Results

Discussion

References

Side Scan SONAR

This is a technique used by scientists to map the seafloor. Sound is beamed down from the tow-fish which creates an image of the seafloor beneath the transect taken by the boat. The ‘loudness’ (strength) of the return echo is measured, with variable results.

Hard areas of the seafloor (like rock formations) reflect more sound and have a stronger/louder return signal than softer areas (i.e. sand). Once the image has been rendered, areas with louder echoes appear darker than those of quieter return signals.

Objects or features that rise above the seafloor cast shadows in the SONAR image, and represent areas where no sound hit (e.g. shipwrecks) – the size of the shadow can be used to interpret the size of the feature.

Meta Data

MTS Xplorer

Date: 04/07/17

Time: 08:27:22 until 08:59:29 GMT

Location: St Mawes Harbour Latitude: 50’09.0N

Longitude: 005’01.4W

Low Tide: 07:47 (1.54m)

High Tide: 14:00 GMT (4.19m)

Cloud Cover: 8/8

The views and opinions expressed re those of the individual and not neccesarily those of the university