Disclaimer: the views and opinions expressed are those of Group 9 members and not necessarily those of the University of Southampton, National Oceanography Centre or Falmouth Marine School

Figure 1 - A photo of the varying depths within the Fal estuary

On Tuesday 30th June, Group 9 surveyed the seabed in order to ascertain the range of available marine habitats in the Fal Estuary. Working onboard the MTS Xplorer, a sidescan sonar towfish was used to carry out the survey. The survey duration was 08:45-12:45UTC. 3 parallel lines were surveyed in total: between St Anthony Head Lighthouse and the Northern extent of the entrance to St Mawes harbour. To verify and further examine the habitats observed on the sidescan image (and identify the abundant seabed fauna/flora) a seabed video survey was performed. Snapshots of the footage obtained are presented in the below gallery. Utilising all of the above, a detailed habitat map was created.

The next day was spent in a lab doing detailed analysis of the sidescan image. Notable features were highlighted and cross-referenced with the video survey footage and grab samples (when taken).

AIM - To produce a habitat map of the area surveyed.

Sampling Difficulties;-

• A number of sampling difficulties arose due to strong winds and low tide. Data collected from the side scan was effected by strong winds generating relatively large waves that resulted in the vessel to become unstable.
• The low tide meant that the area of interest, specifically transect 3, was inaccessible.
• The combination of strong winds and tidal state meant that the boat travel at various speeds. This causes a number of sampling variation within each transect, creating analysis complications within the lab.
• As the area is protected under SAC, grabs were not permitted and thus benthos analysis was unobtainable.

HABITAT DISCUSSION

The majority of the survey area was sandy gravel, highlighted in yellow, consisting of sands and bivalve shells.

However, extensive and densely populated Maerl beds, highlighted in pink, of up to 100% coverage were also found at the northern extent of the entrance to St Mawes harbour. Video6 had between 75-100% Maerl cover at points compared with Video4 which had 25% as a maximum. . Seasearch last surveyed the extent of Maerl in 2012 therefore this year’s transects, 2015, may have found extensions of the Maerl covered zone. The accumulation rate for Maerl in Cornwall is 0.5m ky-1 (Bosence, 2003).

Some points within the gravel dominated areas, highlighted in purple, were found to have dense algal growth. One patch of rocky shore with kelp was found on the boundary of the third transect, highlighted in green. On the map this is shown by streaky shadows indicating the presents of weeds. Lines upon the graph show path of the video tracks using the start and end coordinates. The cameras upon release were able to flow using the current so the track shows this drift. Due to the drifting pattern Video6 was able to flow over Video1 allowing them to be merged.

There were high abundances of echinoderms and cnidarians throughout all video recordings, particularly when compared to previous years. As these groups are predators, eg. Asterias rubens predates cockles (Wolff, 1977), within the ecosystem it implies a rich food source is available to exploit.

The environment may have had all of the necessities needed, and at optimum levels, for these species to reach their carrying capacity.

Figure 2 - A habitat map obtained from the data collected on the survey

From the side scan data, 6 areas of interest were chosen to video for classification clarification. A number of species were present in all video footage e.g. Jellyfish (Aurelia Aurita (common jellyfish) Rhizostoma pulmo) and Starfish (Asterias rubens (common starfish), Henricia sp.). A thorn back ray (Raja clava) was sighted in V5. The sediment in the area was identified as being coarse sand/gravel with Maerl was found in V5, located near St Mawes harbour. Red, green and brown algae was present in a relatively patchy distribution. Zonation of algal types was noted with change in colour prominent with variations in depth. An abundance of dead myidae shells, inhabited by tube worms were view from video track 2.   

Figure 5 - Deploying the camera

At the start of the first transect, wave action on the towfish caused interference bands to occur on the trace. Additionally, after the line was started, the sensitivity of the sonar was adjusted by John Davies to optimise the image quality. As a result, the first 2 minutes of line 1’s trace is distorted. Once the sidescan was operating efficiently, several interesting features were observed. Long-wavelength bedforms were seen mainly on the first line. Rocks were identified from objects with a strong reflectance and shadows behind them. What was later confirmed by a video survey to be seagrass was imaged as a selection of clumped strong reflectors with short shadows.  

Figure 4 - The towfish used to survey the transect area

Figure 3 (Above)- A map from google earth showing the area transected, and presented in the habitat map. (Left)

Figure 6 - Zoning the geophysical map

Figure 8 - Spotting for life in the water for later calibration with towfish

Figure 7 - Deploying the towfish off the stern

Bosence, D. and Wilson, J., 2003, ‘Maerl growth, carbonate production and accumulation rates in Atlantic’, Aquatic conservation in marine and freshwater ecosystems, 13, 21 – 31.

Wilson, C.,  Berjes. J. A. and Maggs, C. A., 2004, ‘Environmental tolerances of freeliving coralline algae (maerl) implication for European marine conservation’, Biological conservation, 120, 283 – 293.

Wolff, W. J., Sandee, A. J. J. and De Wolff, L.., 1977, ‘ The development of a benthic` ecosystem’, Hydrobiologica, 52, 107 – 115.

Figure 9 - This is a poster created to display the finding of the area surveyed on the 30/06/2015. Descriptions of flora and fauna findings from video footage is displayed as well as a habitat map, created from sidescan data and video findings. There is a link to a better reolution version HERE