Benthic foraminifera associated to cold-water coral ecosystems

Cold-water coral reef ecosystems occur worldwide and are especially developed along the European margin, from northern Norway to the Gulf of Cadiz and into the Western Mediterranean Sea. The dominant reef builder in these areas is the scleractinian coral Lophelia pertusa, often associated with the scleractinian coral Madrepora oculata. These species settle on suitable hard substrates, in environments characterized by elevated currents and high food availability. Along the European margin cold-water coral reefs developed during different times and with different morphologies. In particular, on the Norwegian shelf and upper slope, extended active/living reefs have developed on elevated hard substrata. Along the Irish margin on the Rockall Bank, on the Porcupine Bank, and in the Porcupine Seabight, L. pertusa has built large fossil and/or active carbonate mounds. In the Gulf of Cadiz and in the Alboran Sea buried reefs and patch reefs with often strongly fragmented coral rubble are generally found in association with mud volcanoes below a (hemi-) pelagic sediment cover. Cold-water corals have been known since the eighteenth century but the development of new technologies has resulted in the discovery of huge cold-water coral ecosystems and carbonate mounds during the last few decades. Their widespread occurrence presents a challenge to understand their development, preservation and possible importance in the geologic record. In modern oceans, they provide important ecological niches for the marine benthic fauna in the deep-sea. In comparison to the macrofauna the microfauna, particularly the foraminifera associated with these systems, are poorly known. The present study focuses on the foraminiferal assemblages associated with cold-water coral ecosystems from the European continental margin. Samples were collected in three key regions: surface sediments along the Norwegian margin and in the Porcupine-Rockall region, gravity cores were taken on the top of two mud volcanoes in the Alboran Sea. Planktonic and benthic foraminiferal assemblages were quantitatively analyzed. Since patterns of community structures are often not readily apparent, hierarchical cluster analysis, multidimensional scaling (nMDS), and diversity analysis were applied to emphasize differences in the foraminiferal assemblages. Planktonic foraminiferal assemblages provide information about the conditions in surface waters. The assemblage of planktonic foraminifera in the coral-rich layer of two sediment cores in the Alboran Sea and in surface sediments from the Norwegian margin indicate similar conditions in the uppermost waters of both regions: nutrient-rich with enhanced phytoplankton blooms. At the transition from the coral-rich layer to the overlying (hemi-)pelagic sediments in the Alboran Sea a main shift in the planktonic foraminiferal assemblage indicate a reorganization of water masses towards more oligotrophic conditions. Low food availability could be the main reason for the decline of the cold-water coral reefs in this area. The benthic foraminiferal assemblages in the Porcupine/ Rockall region on the Irish margin are strictly related to the distribution of different sedimentary facies. On the Norwegian margin, benthic foraminiferal assemblages show a clear separation of cold-water coral reef associated fauna and off-reef associated fauna. However, different assemblages for each facies are only weakly defined and grade one into the other preventing a strict facies attribution as that observed along the Irish margin. The benthic assemblages from coral-rich layers in the Alboran Sea and those from cold-water coral reefs associated to carbonate mounds in the Porcupine/Rockall and cold-water coral reefs along the Norwegian margin also show remarkable similarities. In particular, the assemblages are dominated by epifaunal-attached species such as Discanomalina coronata, Cibicides refulgens, and Lobatula lobatula but also infaunal foraminifera such as Globocassidulina spp., Epistominella spp., Cassidulina spp. are highly abundant. The benthic fauna provide information on currents, oxygenation and organic matter content. In particular, the benthic fauna associated with cold-water coral ecosystems indicates an environment characterized by high energy, well oxygenated waters and high organic matter supply derived from phytoplankton blooms and reaching the sea floor. In the investigated areas D. coronata is restricted to living cold-water coral reefs facies only and/or in co-occurrence with coral fragments. Based on these observations D. coronata is interpreted to require similar ecological conditions to cold-water corals and, therefore, proposed as an indicator species for healthy cold-water coral ecosystems on the European continental margin. In conclusion, our data suggest that although cold-water coral ecosystems occur at different latitudes, the associated foraminiferal assemblages are consistent from Norway to the Western Mediterranean. Thus they can be used to identify these ecosystems even in the geologic record, when the corals are often strongly dissolved like in the Alboran Sea.