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Geophysics

Geophysics poster (click to view PDF)

Date: 25/06/2015

Vessel: RV Explorer

Weather: Fair – no significant precipitation

Sea State: smooth

Cloud Cover: 3/8

Tides: High tide (4.10m) at 11:05 UTC.

Low tide (1.8m) at 17:35 UTC.

Departed: 11:45 UTC

Transect 1 (Ken 005)

Start time: 13:34:32 UTC

Start Location: 50°06.0N 005°07.0W

End Time: 13:38:07 UTC

End Location: 50°06.0N 005°06.6W

Transect 2 (Ken 004)

Start Time: 13:42:58 UTC

Start Location: 50°06.0N 005°06.6W

End Time: 13:46:37 UTC

End Location: 50°06.0N 005°07.0W

Video 1

Start Time: 14:03 UTC

Start Location: N 26374.0m, E 177265.8m

End Time: 14:20 UTC

End Location: N 26971.0m, E 177139.1m

Video 2

Start Time: 14:28 UTC

Start Location: N 271546m, E 177723.9m

End Time: 14:45 UTC

End Location: N 27115.5m. E 177686.2m

• Introduction
• Methods
• Results
• References

Contents

Information

Introduction

The Helford river estuary has been designated as a marine special area of consideration (SAC). This status was given due to its rich and varied ecosystems including maerl and eelgrass beds.

Aim:

To investigate the benthic seafloor within the Helford Estuary and map the geological and biological features of this area. Particular interest was given to the artificially planted eelgrass by Dr. Ken Collins.

Objective:

To use the side scan sonar to create a benthic habitat map and thus map the eelgrass and then use a drop camera to ground truth our findings.

A survey of the Helford lower estuary was conducted using a sidescan towfish and a live video feed to determine the bathymetric features.

Fal and Lower Helford is a Special Area of Consideration, it has numerous habitats that are rare and fragile (1). These include the Maerl beds, most abundant at the St. Mawes bank, which is in fact one of the UK’s only and largest maerl beds (2). The Helford Estuary, a part of the Falmouth Bay, is an idyllic location, with many boats moored along the edges and small beaches, this however is detrimental to the existence of the delicate eelgrass beds that lie near the banks of the Helford.

Figure 1 - The SAC area in Falmouth bay (highlighted in blue) (3)

Eelgrass

Eelgrass (Zostera spp), is one of the few angiosperms that permanently inhabits submerged marine environments. Human-induced disturbances, such as dredging, addition of docks, mooring of boats, harvesting of shellfish using rakes or trawls, and use of motorboats in shallow waters have created ‘scarred’ areas within eelgrass meadows (3). Seagrasses are also known to stabilise sediments and reduce wave and current energy thus creating a stable habitat for other organisms to grow within (4,5). If the eelgrass suffers, a whole community will be affected. Eelgrass is therefore designated as a Biodiversity Action Plan (BAP) species, as eelgrass beds constitute an important habitat under the EU Habitats Directive 1992 (6).

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Figure 2 - The Helford estuary, the yellow line represents the area we measured, it could not be plotted exactly since there were boats moored in the area so the tow was not straight.

Figure 3 - Recorded distribution of Zostera marina in UK as produced by The Marine Life Information Network (MarLIN) (7).

Methods

Sidescan Transect

Once the area of interest had been located-across the newly exposed moorings close to the implanted eelgrass using the survey vessel M.T.S Xplorer, a Subsurface Duel Frequency Analogue Side Scan Sonar was conducted using a towed fish (see image 9). The frequency of the sonar was 100kHz since a lower resolution ensures identification of eelgrass. The swath was 50m.

Video transects

Two transects were taken, one across the side scanned area, footage of the MCZ was also taken. This can be used to identify species present and percentage cover of the eelgrass.

Using the programme ImageJ a ratio was calculated that compared the two most dominant species in both transects algae and seagrass. This was done by selecting a number of frames throughout the video, in each of these frame using a free hand lasso tool a species was drawn around and the pixels in the area selected allow a percentage cover of the species to be generated.

Habitat Map

The habitat map was created by preliminarily aligning the two scan print outs and combining them to create one image of the survey area. Using time and position a skeleton was constructed on the programme Surfer, the time could cross-referenced with the side scan sonar and the video footage. This allowed a visualisation of the different benthic habitats.

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Results

Findings from the final profile were classified by the knowledge of sediment reflectivity and then comparing to the video feeds. The less reflective the sediment, the softer the benthos, this then indicates softer sediments. The sediments towards the bottom of the habitat in boundary 4 are hard since they produced greater backscatter. The eelgrass however produced more backscatter, this paired with the video footage showed the areas where these eelgrass communities were present.

Seagrasses are generally known as ecosystem engineers, as they reduce flow velocities in their canopies. In perennial subtidals they usually lead to increased net sedimentation rates and reduction of the grain size (4). This could explain why sediment seems appears to be a harder substrate on the benthic habitat map.

Left: Figure 6 - Laminaria digitata & eelgrass (Zostera marina)

Figure 5 - The constructed Sonograph of transect 1 taken from the MTS Xplorer.

Analysing and comparing the video footage of the two different areas allows a comparison between a well-established eelgrass community and a newly introduced eelgrass community.

Transect 1 had both a side scan sonar and video footage to record the bathymetry. This transect covered the area where the seagrass was implanted.

Transect 2 had just a video recording of the benthos. The bedform and biota was monitored in this area since it is a Marine Conservation Zone that acts as a well-established habitat (6)

Right: Figure 7 - eelgrass (Zostera marina) & common starfish (Asterias rubens)

These two images (figures 6 and 7) are from each of the video footage transects. A clear difference can be seen between transect 1, in image 3 and transect 2, in image 2. There is a much greater density of seagrass and other macroalgal species in image 2 in the well-established MCZ. The seagrass also appears to have healthier taller propagules. The seagrass has a patchy distribution in transect 1; as seen in the images below (figure 8 and 9). There were not bare spaces in the established Seagrass community, this means the bare sand could be residual effects from the removed moorings.

Figure 8 - Shot of Sargassum muticum, also seen at 4:13 in transect 1, there is sandy benthos not occupied by seagrass or macroalgae.

Figure 9 - 4:45 in transect 1 substrate becomes bare and hard (possible mooring scar/recent removal of mooring).

Table 1 - Ratios of image frames from the video footage in the Helford Estuary

The density of seagrass was greater in the natural habitat as seen in the video footage, however on analysis of the ratio between seagrass and algae Transect 1 had higher ratios of Seagrass to Algae whereas transect 2 has less seagrass to algae. There is a similar abundance of eelgrass seen in both transects however it is unknown how much is the implanted biota. The success can also be judged upon the health of the surrounding community. Hardly any Laminaria species were identified in transect 1 whereas in transect 2 much of the eelgrass was flowering and there were algal pockets as well as more shoals of juvenile fish can be seen. This could mean that the gradient-in-time model of succession can be applied where biological characteristics of species can be used to explain their distribution along temporal gradients (8)

References

1. Langston, W.J., Chesman, N.C., Burt, G.R., Hawkins, S.J., Readman, J. and Worsfeild, P. 2003, ‘Site Characterisation of the South West European Marine Sites- Fal and Helford cSAC’, Marine Biological Association Occasional Publication No. 8.
2. Birkett, D.A., C.A.Maggs, M.J.Dring. 1998. Maerl (volume V). An overview of dynamic and sensitivity characteristics for conservation management of marine SACs. Scottish Association for Marine Science. (UK Marine SACs Project). 116 page
3. Kemp WM, Twilley RR, Stevenson JC, Boynton WR, Means JC (1983) The decline of submerged vascular plants in upper Chesapeake Bay: Summary of results concerning possible causes. Mar Technol Soc J 17:78–89
4. Bos, A., Bouma, T., de Kort, G. and van Katwijk, M. (2007). Ecosystem engineering by annual intertidal seagrass beds: Sediment accretion and modification. Estuarine, Coastal and Shelf Science, 74(1-2), pp.344-348.
5. Bouma, T.J., et al., 2005. Trade-offs related to ecosystem engineering: a case study on stiffness of emerging macrophytes. Ecology, 86,2187-2199
6. JNCC, 2011.Intertidal sediments dominated by aquatic angiosperms. [Online]
   Available at: http://jncc.defra.gov.uk/page-5793
   [Accessed 30/06/15].
7. Tyler-Walters, Dr. H., 2008. Zostera marina. Common eelgrass. Marine Life Information Network: Biology and Sensitivity Key Information Sub-programme [Online].
   Available at:  http://www.marlin.ac.uk/generalbiology.php?speciesID=4600
   [Accessed 28/06/15].
8. Pickett, S. (1976). Succession: An Evolutionary Interpretation. The American Naturalist, 110(971), p.107.

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Figure 10 - Fish used off the MTS Xplorer to create a sidescan sonar.

Figure 4 - Benthic habitat map representing both the video data and the side scan sonar data constructed to identify boundaries between habitats.

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