Lamiae
Extrait du cours oberon bank seasearch
4 DISCUSSION
Of 17 dives conducted during the expedition, 10 were on Oberon Bank itself and, when bad weather precluded diving offshore, seven in the shelter of the Sound of Arisaig. Most of the survey team were experienced Seasearch divers completing 14 Surveyor forms between them, with nine from Oberon Bank. The Surveyor form divides the sites into different habitats, allowing more detailed species lists and habitat descriptions.
Since kelp species were identified to species level on all Seasearch Surveyor forms, tertiary infralittoral biotope codes (e.g. EIR.Lhyp.pk) could be assigned according to Connor (1997):
see APPENDIX 3. Tertiary, secondary and primary codes were assigned for circalittoral biotopes where possible (e.g. MCR.ErSSwi). Details from the Seasearch Observer forms were not in themselves sufficient for biotope ascription.
4.1 Oberon Bank
4.1.1 Rocky infralittoral biotopes
The shallowest part of Oberon Bank, rising to a minimum of 15.7m bcd (see APPENDIX 2), comprised a series of rounded bedrock ridges crowned by Laminaria hyperborea park with foliose red and brown seaweeds (EIR.LhypR.pk), thickening to forest density (EIR.LhypR.ft) in some places (dives 4, 7 and 10). Although the common urchin Echinus esculentus was recorded on 8 of 10 Oberon Bank dives, it was ‘Rare’ at 3 and only ‘Occasional’ at another 3.
Grazing was therefore minimal and surfaces beneath the kelp correspondingly rich in foliose and filamentous red algae (including Delesseria sanguinea, Phycodrys rubens and Heterosiphonia plumosa), brown algae (Dictyota dichotoma), encrusting coralline algae, hydroids, bryozoa and featherstars. Beneath the kelp park zone, Dictyota dichotoma densities were not great enough to merit classification as the EIR.FoR.Dic biotope.
4.1.2 Rocky circalittoral biotopes
Through and beneath the infralittoral zone, Oberon Bank descended to the north, west and south in a series of bedrock ledges, interspersed with boulders and cobbles. Mixed animal turf (MCR.XFa) comprised, hydroids (including Nemertesia and Kirchenpaueria species), bryozoa (such as Securiflustra securifrons and Alcyonidium species) and featherstars (Antedon bifida, Antedon petasus and Leptometra celtica), increased in density as foliose algae diminished.
Bedrock ledges descended to a mixed ground of coarse shell sand, pebbles, cobbles and small boulders. Barnacles, the colonial white anemone Parazoanthus anguicomus and, particularly on the verticals, Devonshire cup corals (Caryophyliia smithii) all increased in abundance with depth, particularly to the south and west. Here they were associated with deeper circalittoral boulders, cobbles and pebbles among mixed sediments. The colonial anemone Epizoanthus couchii was ‘Frequent’ on boulders, cobbles and pebbles to the west of the bank, with Swiftia pallida and the bryozoans Porella compressa and Alcyonidium diaphanum ‘Frequent’ or ‘Common’ to the south. With increasing depth the football seasquirt Diazona violacea was also found, albeit ‘Occasional’ or ‘Rare’, on rock and boulders surfaces to the north, west and south. It is also worth noting that poor cod (Trisopterus minutus) were recorded as ‘Common’ or ‘Abundant’ at deeper habitats on dives to the south and west of Oberon Bank.
Oberon Bank descended to the east in a series of steep, smooth rounded bedrock faces of varied filter-feeding animal turf cover (MCR.XFa): some patches were characterised by the branched bryozoans Porella compressa and Omalosecosa ramulosa; others by featherstars with the mobile sea-cucumber Holothuria forskali, whilst the Devonshire Cup Coral (Caryophyllia smithii) was ubiquitous throughout, increasing in abundance with depth.
Similarly, the northern sea fan Swiftia pallida, the erect cupped sponge Axinella infundibuliformes (and perhaps Phakellia ventilabrum: see 4.3), the crinoid Leptometra celtica and the ‘Occasional’ Diazona violacea increased in abundance with depth.
Although patchy, the prevalence of erect sponges, C. smithii and S. pallida characterised the deeper bedrock faces as a more exposed, more tideswept example of the MCR.ErSSwi biotope.
Although ledges of coarse circalittoral sand and small boulders were found at ~30m bcd in accordance with the description in 1.2, during the course of the present expedition no fan shells were found here (see 3.1.1 and 4.1.4).
……
Si le lien ne fonctionne pas correctement, veuillez nous contacter (mentionner le lien dans votre message)Un document sur Oberon bank seasearch (4,52 MO) (Cours PDF)
