Detailed visual seabed survey at drilling site 7218/11-1

3 About sponges and challenges for assessment
3.1 OSPAR habitat classification of sponges
Offshore sponges in the south-western Barents Sea are classified under the OSPAR habitat
descriptionas"DeepSeaspongeaggregations". Thefollowingtext is modifiedfrom OSPAR(2005):
Deepseaspongeaggregationsare principallycomposedof spongesfrom two classes:Hexactinellida
andDemospongia.Glasssponges(Hexactinellidae)tend to be the dominantgroupof spongesin the
deep sea although demospongidssuch as Cladorhizaand Asbestoplumaare also present. The
massivespongesthat dominate some areas include Geodia barretti, G. macandrewi,and Isops
phlegraei.Theycan occurat very high densities,particularlyon the slopein areaswhere substrate
and hydrographicconditionsare favourable.Surveymaterial from a spongefield in the northern
North Seaand other locationshad a comparablediversityanddensityof spongeswith tropical reefs
(Konnecker,2002). The spongesalso influence the density and occurrenceof other speciesby
providingshelterto smallepifauna,within the osculaand canalsystem,and an elevated perch,e.g.
for brittlestars(Konnecker,2002).Deep-seaspongeshavesimilarhabitat preferencesto cold-water
corals,and henceare often found at the samelocation.Researchhasshownthat the densemats of
spiculespresent around spongefields may inhibit colonisationby infaunal animals,resulting in a
dominanceof epifaunalelements(Gubbay,2002).Information indicatesthat dominant speciesare
slow growingtaking severaldecadesto reachlargesize(Klitgaard& Tendal,2001).Thehabitat and
the rich diverse associatedfauna is therefore likely to take many years to recover if adversely
affected(Konnecker,2002).
Theyoccurbetweenwater depthsof 250-1300m(Bett & Rice,1992),where the water temperature
rangesfrom 4-10°Candthere is moderate currentvelocity(0.5knots).Deep-seaspongeaggregations
may be found on soft substrataor hard substrata,suchas bouldersand cobbleswhich may lie on
sediment.Icebergplough-mark zonesprovidean ideal habitat for spongesbecausestableboulders
and cobbles,exposedon the seabed,providenumerousattachment/settlementpoints(B.Bett, pers
comm.). However,with 3.5kg of pure siliceousspiculematerial per m 2 reported from some sites
(Gubbay,2002),the occurrenceof spongefields canalter the characteristicsof surroundingmuddy
sediments.Densitiesof occurrenceare hardto quantify,but spongesin the classHexactinellidahave
been reported at densitiesof 4-5 per m², whilst ‘massive’growth forms of spongesfrom class
Demospongiahavebeenreported at densitiesof 0.5-1 per m 2 (B.Bett,pers. comm.).
3.2 Sponges in the south -western Barents Sea
Spongesare found in most sea-floor (benthic)faunalassemblages,both on hard andsoft substrates.
In the south-westernpart of the BarentsSea(precisearea not specifiedhere),spongescompriseup
to 57 % of the benthic biomass(Ereskovsky,1995).Slightlyfarther east,along the Murman coast,
almost80%of the recordedbiomasswasdue to sponges(Propp,1971).Extremelyhigh densitiesof
spongesalso have been recorded in the Tromsøflaketarea west of the Troms/Finnmarkarea in
northern Norway(Figure7).
Detailed site survey at PL531 Darwin
Akvaplan-niva AS Rapport 6051 - 1 13