Biological hotspots in the deep sea: Environmental controls and interactions in deep-sea sponge and coral assemblages

The deep sea is one of the least studied environments of our planet. It is often considered an unfavorable habitat due to its immense pressure, darkness and low temperature. Life there relies mostly on sinking organic material from the surface ocean as a food source. However, only an extremely small amount of this material reaches the sea bottom. Surprisingly, in some places we do find thriving communities, like cold-water coral reefs and sponge grounds, similar to oases in the desert. Both sponges and cold-water corals are so-called ecosystem engineers, increasing the local habitat heterogeneity and complexity with their skeletons protruding from the otherwise muddy seafloor. Thereby, they provide a habitat, nursery, shelter and foraging area for many benthic and mobile species and create hotspots of biodiversity and biomass. In order to find out why we can find these hotspots in certain places, we investigated where they establish and how they sustain themselves. Environmental conditions in different sponge grounds and cold-water coral reefs within the Atlantic were recorded by long-term deployments of ocean bottom observatories with a multitude of biochemical and hydrodynamic sensors. Underwater robots additionally provided us with the opportunity to collect discrete samples and carry out in-situ experiments. These measurements showed that cold-water corals as well as sponges were occurring in a broad range of environmental conditions, from ice-cold waters in the Arctic to nearly anoxic waters in the oxygen minimum zones of the western African margin. A consistent finding was that these deep-sea hotspots occur in areas with special hydrodynamical conditions, which lead to a constant water exchange and delivery of otherwise limited resources, enhancing their food availability. A food web study showed that sponges are not only relying on sinking particulate matter from the surface, but are also able to take up the abundant dissolved resources, which are normally inaccessible for animals. Further studies revealed that sponges transfer particulate matter as detritus to the wider food web, which would otherwise be highly limited (the so-called sponge contribution). We were also able to quantify how much dissolved nutrients and particulates are taken up from the water column by the sponge ground and how much dissolved metabolites are released back to the water column, which can be taken up by other (micro-)organisms. By this, sponges drive carbon and nitrogen cycling as well as the benthic-pelagic coupling in the deep sea. This thesis provides for the first-time insights into the environmental variability within and around deep-sea hotspots and shows that sponges play a pivotal role in the sustenance of deep-sea ecosystems. We were able to show that these ecosystems thrive over a large range of environmental conditions, however, they often depend on a fragile equilibrium of these conditions. Hence, it is important to extend our knowledge about critical thresholds in order to be able to predict how climate change or anthropogenic activities, like fisheries, deep-sea mining or ocean acidification, might have negative effects on these ecosystems. This is vital for marine management and policymaking and ultimately human well-being.