Wreck Site 3: Marine Biology
Introduction
A survey of the marine life living on and around the A7 submarine was undertaken at the start of the diving fieldwork, the data collection was completed by Allen Murray and the identification by Dr Keith Hiscock. The biology survey was done before the top of the conning tower was disturbed by removing the abandoned trawl net and rope.
The species complement is typical of a steel wreck and quite similar to the nearby Rosehill wreck, although with fewer species, but also unusual in that no dead man’s fingers Alcyonium digitatum were seen. No sea fan anemones, Amphianthus dohrnii were seen; these are a nationally scarce and Biodiversity Action Plan Priority Species.
The description of the marine life is separated into four areas on the site; periscope, conning tower, hull and seabed.
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The conning tower and periscope before cleaning, showing the full complement of anemones
Periscope
The copper alloy periscope provides a smooth vertical surface well above seabed level where tidal currents are strongest.
Species noted on the periscope include:
- Plumose anemones, Metridium senile
Conning Tower
The steel conning tower itself provides a smooth vertical surface, the copper alloy top and hatch a horizontal surface, the ventilator tubes with their tangle of rope and net offer many hiding places for marine life (Fig. 68). Species noted on the conning tower include:
- Spiny starfish, Marthasterias glacialis
- Encrusting bryozoan (seamat), unidentified
- Solitary sea squirt, Ascidia mentula
- Hydroid, Ectopleura larynx
- Coral barnacles, Megalomma anglicum
- Devonshire cup coral, Caryophyllia smithii
- Pumice stone bryozoan, Cellepora pumicosa
- A silty bryozoan/hydrozoan turf of unidentified species
- Erect bryozoan, cf Bugula flabellata
- Encrusting bryozoan, possibly a Schizomavella sp.
- Barnacles, Balanus crenatus
- Unidentified sea squirts Ascidiacea indet.
Base of the conning tower and hull, port side looking aft, with many pink sea fans
White dead man’s fingers, Alcyonium digitatum, were expected to be present but were not seen.
Hull
The outer corroded surface of the steel hull was quite soft and very loose in places (Fig. 69). Species noted on the hull include:
- Common starfish, Asterias rubens
- Pink sea fans, Eunicella verrucosa
- Hydroid, Nemertesia ramosa
- Hydroids Obelia dichotoma, Bugula flabellata
- Erect branching bryozoan, Crisiidae indet.
- Jewel anemones, Corynactis viridis
- Female conger eels, Conger conger
- Swimming crab, Necora puber
- Edible crab, Cancer pagurus
- Common lobster, Homarus gammarus
- Topknot, Zeugopterus punctatus
- Poor cod, Trisopterus minutes
Pink sea fans, Eunicella verrucosa were in good condition and had a consistent orientation at right angles to the prevailing water movement which was fore and aft along the hull.
Seabed
Plan photograph of the flat, silty seabed
The seabed around the submarine is flat and featureless, consisting of very light silt that was easily disturbed then remained suspended in the water column. Burrows in the seabed are possibly from callianasid crustaceans (Fig. 70).
The depth of the silt could not be established but a 1m long probe pushed in to it did not encounter any resistance along its entire length. The 25kg sinker weight from the mooring buried itself approximately 500mm into the seabed in just two weeks.
During salvage operations on the submarine in 1914 the seabed was described as being ‘a bed of sand and mud’; at one point the A7 was ‘more covered with sand than before’ and later the ‘divers found a clean sandy bottom as the mud had been washed away by storms’. In 1981 when the submarine was first found by sports divers the seabed was reported as being ‘hard silt’ and the bed was firm enough for the compass binnacle to be found lying on the seabed beside the wreck.
The seabed is modern and appears to be the result of recent dumping activity nearby. The dumping ground lies just 1400m to the east of the site so the wreck site is likely to be affected by dredge spoil dumped in the area. A seabed sample was collected from the port side amidships.
Conclusion
Areas of the site provided a different habitat for marine life and this was largely differentiated by height above seabed, with the flat seabed, main hull, conning tower and periscope colonised differently. The flat seabed around the hull is comprised of light, easily disturbed sediment that provides little foothold for any colonising life but can provide a home for burrowing animals. The hull of the submarine does provide a horizontal and firm substrate but perhaps a temporary one as the surface of the steel corrodes and flakes of surface rust fall away. The steel conning tower offers a solid, vertical surface but also suffers from the effects of surface corrosion. The top of the conning tower and the periscope are furthest from the seabed so may be washed by stronger tidal currents than the areas below. The copper alloy material from which they are made may also offer a more secure footing for colonising life as it remains solid and does not flake off.
