Impact of climate change on the distribution of sponges and cold water corals in the Antarctic and Subantarctic

Climate change and ocean acidification, fuelled by the release of anthropogenic carbon dioxide in the atmosphere, impact the physico-chemical dynamics of the ocean. Current changes are undeniable and their rate is believed to be much faster than anywhere else in the geological records. Modifications of the seawater temperature and pH, amongst other parameters, impact marine organisms across all levels of biological organisation and extinctions or shifts in the distributional range of several species are expected. Unable to escape the alteration of their environment, sedentary benthic organisms are particularly exposed. The fate of corals and sponges, the main habitat-forming species in the benthos, is considered critical because of their aragonite skeleton (corals) and slow growth (sponges). In this thesis, I intended to estimate the impact of climate change and ocean acidification on the distribution of sponges and cold water corals in the Antarctic and the Subantarctic. For this purpose I focused on two main case studies: - Case study I: Climate-induced changes in megabenthic communities along the Antarctic Peninsula and their consequences on the distribution of glass sponges. - Case study II: Sensitivity of the cold water coral Desmophyllum dianthus to ocean acidification inferred from its distribution along a pH gradient in a Chilean fjord. In both cases, I chose to work mainly with non-destructive sampling devices, namely Remotely Operated Vehicles (ROVs) acquiring underwater videos for quantification of the benthos and simultaneously recording several physico-chemical parameters. For monitoring purposes a standardized method was needed to obtain comparable abundance data from videos recorded by different vehicles. Case study I (=> Manuscript I and Manuscript II) On the Antarctic continental shelf, glass sponges (Porifera, Hexactinellida) are a dominant component of rich megabenthic communities sustained by the seasonal melting of sea-ice and the resulting phytoplankton blooms. Under permanent ice shelves, however, a depauperate deep-sea like fauna survives in oligotrophic waters, relying on the advection of allochtonous food. The catastrophic collapse of ice shelves attributed to rapid regional warming along the Antarctic Peninsula is creating new conditions in the formerly permanently ice-covered water column. In the south of the Larsen A embayment, a ROV transect conducted in 2007 was repeated in 2011 to monitor the evolution of the benthic life after the collapse of the overlaying ice shelf in 1995. A slow response of the Antarctic benthos was expected but, in only four years, a rapid successional pattern was observed with pioneer ascidians quickly colonizing the area, becoming dominant and then collapsing completely to the advantage of a new population of glass sponges. The ophiuroids at the site shifted simultaneously from a population of abundant filter-feeders to a dominance of deposit-feeders. Sustained by a low predation pressure, the reduced competition for space and food and the new but irregular supply of pelagic organic material, hexactinellid sponges nearly doubled their biomass and abundance in Larsen A between both surveys. These unexpected results, together with findings from other studies, imply a paradigm shift in the consideration of Antarctic benthic dynamics, where glass sponges may undergo boom and bust cycles, allowing them to quickly colonize new habitats. If the current rate of ice shelf retreat continues, hexactinellids may spread along the Antarctic Peninsula and thus find themselves on the winners’ side of climate change. Case study II (=> Manuscript III) Cold water corals (CWC) occur primarily in the deep-sea, where they provide habitat for a rich reef-associated benthic fauna. In spite of gaps in our knowledge of their ecology and physiology, these invertebrates are considered extremely sensitive to ocean acidification. Due to the remoteness of their natural environment investigations into their response to natural variations of the seawater carbonate chemistry or to artificially elevated pH in experimental setups are rare. In Comau Fjord, in Chilean Patagonia, an exceptionally shallow population of the CWC Desmophyllum dianthus is known to dwell across a wide pH gradient. We deployed a ROV at seven sites across this fjord, in order to quantitatively assess the vertical and horizontal distribution of D. dianthus and determine the physico-chemical parameters driving its occurrence. None of the recorded parameters seems to be a major controller of the abundance, distribution or size of this coral. D. dianthus is ubiquitous in the fjord, occurring in high abundances down to 280 m depth at a pH < 7.8. Below this depth, this coral is still present in smaller and less dense patches in a suboxic water mass with low pH. Our findings show that D. dianthus can survive and even prosper in aragonite undersaturated waters. However, its sensitivity to other alterations linked to climate change remains unknown. Methodology (=> Manuscript IV) For the quantitative analyses of video transects, a major difficulty exists in the determination of the area surveyed. Depending on the substrate topography, camera orientation and the availability of reference scales, several methods can be employed for the scaling of underwater images. The task is particularly difficult when comparing data originating from several vehicles. Despite a decidedly time-consuming procedure, 3-D modelling of the seafloor based on video data and a reference scale emerged as a powerful tool for standardized analysis of images obtained by three different ROVs. To date, it also is the only scaling method offering the possibility of integrating small scale topography to the measurements.