Reef paradox
The reef paradox (in the broader sense also Darwin's paradox ) describes the apparent contradiction between the high biomass production of coral reefs and the relative nutrient poverty ( oligotrophy ) in their environment. It was formulated by Darwin as early as 1842 and is still not fully understood today. In this context, the reef is also viewed as an oasis in a nutrient desert. The gross primary production can have values of up to 1000 kg carbon fixationper square meter and year. However, the existing wealth is only preserved if the nutrient loss does not exceed the nutrient intake.
Nutrient intake
Various factors can help a reef gain nutrients. The input of nitrates , known as nitrification , can play a role here, but is difficult to quantify. The influx of plankton plays a role in all reefs. The importance of groundwater inflow (e.g. Kaneohe Bay, Hawaii ) and nutrient-rich, upflowing seawater ( Great Barrier Reef ) is controversial, as both factors only play a role near the coast, but reefs away from the coast are equally productive.
Nutrient losses
The reef inhabitants excrete substances containing carbon , nitrogen and phosphorus , which are displaced by the current. 100 g fish / m² results in an estimated release of 0.03 g nitrogen, 0.004 g phosphorus and 4 g carbon per day, which corresponds to about 4% of the gross primary production. Nutrients are also bound in the sediment via the calcareous skeleton of the corals. Assuming 4.2 kg of calcium carbonate are formed per year, 1.3 g of phosphate are bound in it. This corresponds to the release by the fish. Then there is the drifting of reef particles and denitrification by degrading microorganisms.
Nutrient cycles within the reef
There are short-circuited nutrient cycles between zooxanthellae (algae) that live as symbionts in a variety of organisms, such as B. corals, snails or tunicata . Many of these interactions are still unknown. The algae can carry out photosynthesis in a highly productive manner in a nutrient-enriched environment .
The corals release mucus (6% lipid, 34% protein, 60% carbohydrates such as arabinose ), which protects them from drying out, UV radiation, overgrowth and sedimentation at low tide, among other things . 80% of the mucus is water-soluble and serves as a nutrient for the pelagial fauna and flora. Corals invest about 50% of their carbon assimilate in slime. As it ascends through the water column, the slime traps particles and organisms growing in them. It finally gathers on the surface of the water to form a stinking slime carpet several meters wide and about 3 cm thick, which is carried by the wind towards the beach. It decreases with increasing particle content. It then serves as food or fertilizer for numerous organisms. The nutrients come back to the corals in increased numbers via the sediment. Your mucus thus serves as a nutrient trap. These processes have been demonstrated on the Great Barrier Reef and need not apply to all reefs.
The sieve structure of the reef sediment is finer than long thought. This has been shown with endoscopic examinations of fringing reefs in the Gulf of Aqaba . The surface of the discovered cavities was 80% populated by sponges and the rest in equal parts by mussels, sea squirts, polychaeta and bryophytes. Furthermore, an upward flow of water could be detected through low-lying entrances and high-lying exits. The organisms take up nitrogen, which exceeds other processes such as fish migration (2.4–7.2 mmol N / m²d) with the production of 22.3 mmol nitrogen per day and m².
See also
swell
- Christian Wild, Markus Huettel, Anke Klueter, Stephan G. Kremb, Mohammed YM Rasheed, and Bo B. Jørgensen: Coral mucus functions as an energy carrier and particle trap in the reef ecosystem . 2004, Nature 428, 66-70