When corals release eggs and sperm, impressive amounts of nutrients are released. Current reefs have means to deal with this natural pollution, but this may be compromised by algal overgrowth in the near future.

Coral spawning is an essential part of life on the reefs, which is a seasonal event. The Acroporids in the Caribbean spawn after the August full moon, and the corals in the Red Sea, Indian Ocean and the Pacific have their own rhythms. Broadcast spawning involves the release of buoyant, lipid-rich gametes (known as egg-sperm bundles) into the water. This massive broadcast spawning (next to broadcast spawners, brooding species exist as well, like Pocillopora damicornis, which release already developed planula larvae into the water column) is required to ensure cross-fertilization between colonies (similar to pollen from plants).

This release of gametes (sex cells; being either eggs or sperm) is a rich source of nutrients. Many eggs, full of yolk, are not fertilized and become a source of carbon, nitrogen and phosphorous due to bacterial breakdown. The nitrification processes which occur in the bacteria use up a lot of oxygen, and this can become a problem in relatively small water bodies like lagoons. Next to this, the gametes will cause undesired sedimentation on the reefs, which is damaging to corals (sedimentation reduces the water flow a coral receives,  suffocating the coral. Light is blocked as well).

Just like in the home aquarium, dissolved nutrient levels should be kept low, as reef species have adapted to this situation. High nutrient levels will cause algal overgrowth, which can be disastrous for reefs, especially ones which are recovering from stress (storm damage, bleaching events, overfishing, etc).

Fortunately, reefs have means to deal with this annual nutrient boost. Bacterial processes in the seabed are a key mechanism by which nutrients are kept low. Denitrification by bacteria causes conversion of nitrate and nitrite into nitrogen gas (N₂), and phosphates are converted into bacterial biomass. Furthermore, the huge amount of water will simply dilute the nutrients (in this case, the well-known slogan "the solution to pollution is dilution" applies here). 

However, such as with many other processes occurring on the reefs, this annual clean-up process has its disadvantages. In 2005, at Heron Island (Great Barrier Reef), a mass-spawning event caused a quick drop in oxygen concentration (similar to a drop in redox value in a home aquarium after overfeeding), and a rise in particulate organic matter (detritus) which caused a lot of sedimentation. This drop in oxygen, combined with high sedimentation, sometimes causes high mortality (death) amongst reef fish, corals and other invertebrates.

Left: Spawn slicks —aggregations of buoyant eggs, embryos, and larvae— at Scott Reef, Western Australia (James Guest, Science, 2008).

Particulate nitrogen concentrations peaked after spawning, and remained high for 17 days! The amount of sedimentation remained high as well, for 2 weeks. This shows that reef ecosystems recycle their own nutrients, and that they do so quickly.

Shortly after the spawning event, a bloom of phytoplankton occurred due to the elevated nutrient levels. This plankton quickly removed all the available nitrogen (ammonia, nitrate), but consumed only a small proportion of the inorganic phosphorous (phosphate). This showed that pelagic algae were nitrogen limited, and that there was an over-abundance of phosphorous. This is probably true as well for benthic algae  as phosphates efficiently bind to reef sediment and rock.

The problem which might arise in the near future is the disruption of the huge denitrification filter; the sand beds. The overgrowth of benthic algae - this has been reported from many reef locations worldwide - may affect the sanded, by preventing (in)organic matter to penetrate it. If this happens, the reefs will not be able to cope with the annual flooding of nutrients, causing even stronger growth of algae. This could become yet another problem for reefs, next to global warming and ocean acidification (see our Climate Change section for more detailed articles about this topic). What we have here, in essence, is a so-called positive feedback loop. More growth of algae leads to less nutrient breakdown, which leads to even more algae,  which leads to even less breakdown, etc.

Interestingly, small reef sections which are part of the Great Barrier Reef occur a various distances from the shore, which are sometimes sources of pollution. A good example is the occurrence of agriculture areas along the Australian coast, which cause nitrogen- and phosphorous-rich runoff. Studying these reefs - some of which receive a lot of nutrient input, some which do not - , might give crucial insights into how ecosystems respond to high nutrient input in combination with mass spawnings. It is expected that reefs which already are polluted, will have more trouble with removing the annual waste products caused by spawning.

Reference:

Guest J, How Reefs Respond to Mass Coral Spawning, Science, 2008, (320), pp 621-623