The physical niche of the bathyal Lophelia pertusa in a non-bathyal setting: environmental controls and palaeoecological implications

Abstract

The habitat-forming scleractinian coral Lophelia pertusa supports an ecosystem that is widely known to occur in the bathyal marine ecologic realm along deep shelves, oceanic banks, seamounts and continental margins. Therefore, L. pertusa is generally considered a ‘deep-water’ or ‘deep-sea’ coral. In contrast, this study analyses the environmental controls of this bathyal ecosystem where it is thriving well in the non-bathyal shallow-water setting of the Swedish Kosterfjord area (NE Skagerrak). This is one of several shallow-water L. pertusa occurrences in Scandinavian waters where saline and temperature stable oceanic waters intrude as topographically-guided underflows onto the inner shelf and adjacent fjords, driven by an estuarine circulation.

The L. pertusa occurrence of the Säcken site in the northern Kosterfjord exists at 80–90 m water depth and only a few tens of metres beneath a permanently brackish surface water layer. At the depth of the coral patches, however, the hydrographic data reveal fully marine conditions, which are ensured by a deeper inflow of Atlantic water through the Norwegian Trench into the Skagerrak. SEM analyses of resin casts taken from dead L. pertusa skeletons yield an endolith assemblage dominated by boring sponges such as Cliona spp. (trace: Entobia ispp.), the boring bryozoan Spathipora, the fungus Dodgella priscus (trace Saccomorpha clava) and an unknown fungus (trace: Orthogonum lineare). Such a composition exclusively of heterotroph organisms resembles the Saccomorpha clava / Orthogonum lineare ichnocoenosis which is regarded as indicative for fossil and Recent, open marine, aphotic environments. This interpretation is supported by direct light measurements at the Säcken site, which indicate aphotic conditions for at least most of the year.

The finding of bathyal communities in comparatively shallow waters is linked to factors that force deeper oceanic water masses to surface. Such situations are likely to be expected where an estuarine circulation prevails, or in deep-sea basins bordered by narrow shelves and with local upwelling cells driven by the wind regime, facilitating the intrusion of eutrophic deeper waters to shallow depths — including the benthic communities.

This circumstance reveals a major potential pitfall in the palaeobathymetric interpretation of fossil L. pertusa occurrences, which tend to be interpreted as bathyal palaeoenvironments. Strikingly, almost all known exposed ancient L. pertusa locations (e.g. Rhodes and Messina Strait in the Mediterranean Sea or the Cook Strait, New Zealand) derive from tectonically active regions with steep bathymetric gradients and a specific confined topography which could have forced deep water to the near surface.