The importance of carbon dioxide and dinitrogen fixation for seasonal coral reef metabolism: from organism to ecosystem functioning

Coral reef functioning is based on fixation and cycling of essential C and N. Here, research into these processes is presented from a northern Red Sea coral reef. Water column photosynthetic primary production was relatively low compared to other seas and lowest in the oligotrophic summer. Water column N2 fixation was stable throughout the year and may have contributed substantially to the N demand of planktonic primary producers during oligotrophic summer. Benthic primary production was dominated by hard and soft corals, while highest benthic N2 fixation was performed by microbial mats, the coral rock reef structure, the carbonate sediment community, and turf algae. The reef community as a whole acted net autotrophic in winter and spring, balanced in summer, and net heterotrophic in fall. Uptake of dissolved organic C via sponges was comparable to reef primary production. This thesis presents new insights into the major C cycles of a coral reef under variable seasonal conditions.