| Summary: | The deep sea covers more than 99% of available habitat on earth and is one of the most understudied ecosystems. Below 200-metre depth, we enter the deep sea where there is not enough light for photosynthesis. Animals living on the seafloor in the deep sea therefore depend mostly on organic matter produced at the ocean’s surface as food source. Only a small part of this ocean surface produced organic matter reaches the seafloor, and so large parts of the ocean are food limited ecosystems and sometimes seen as a marine desert. However, in specific areas in the deep sea, lively cold-water coral reefs and sponge grounds are found, which support many other animal species (for example fish and lobsters) and are hotspots of biological activity. This thesis investigates how these hotspots can exist in the otherwise food limited deep sea. We focused on two study areas: first, a cold-water coral region at 700-metre depth in the Northeast Atlantic Ocean, and second, a deep-sea sponge ground at 450-metre depth on the northern continental shelf of the Labrador Sea, Northeast Canada. We show that cold-water coral reefs are hotspots of organic matter mineralization and require a high food supply. Internal waves and tidal currents transport organic matter towards these reefs, and our model predictions show cold-water corals grow where food is replenished at a sufficient rate and bottom currents are strong. High sponge biomass is likely related to strong bottom currents and increased nutrient availability. This thesis implies that mixing in the water column is vital for cold-water coral reefs and sponge grounds, which accelerates food supply to the seafloor. These findings could be used to locate new and conserve existing cold-water coral reefs and sponge grounds in the deep sea, and aid in understanding their fate in a changing ocean.
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