Coral reef

Background:

Coral reefs occupy a narrow ecological niche. Corals live in limited temperature ranges, require high sunlight, strong current flow, and oligotrophic waters. Since the time reef building corals first emerged in the fossil record, they have faced two mass extinctions and took millions of years to recover to their former population levels and diversity. Ironically, though a growing number of people are dependant on a healthy, functioning reef system, anthropogenic stressors inhibit the overall health of these ecosystems (1). Approximately 20% of coral reefs have already been lost, and over one third of the remaining reefs are threatened (2).

Contents: • Fishing Practices o Overfishing o Destructive Fishing Practices • Warming Oceans • Ocean Acidification • Eutrophication

Fishing Practices:

Overfishing

Currently, the status over 75% of the world’s fisheries range from overexploited to fully depleted (3). Overfishing of predators can cause trophic cascades. A classic trophic cascade is the over-hunting of sea otters off of California’s coast, which resulted in a boom of herbivorous sea urchins. The overconsumption of kelp by the urchins resulted in ecosystem collapse. In coral reefs, similar overfishing of the Triton Snail (Charonia tritonis) and the Humphead Wrasse (Cheilinus undulatus) has resulted in the reduced predation of the coralivor, Acanthaster planci. The overconsumption of coral by the sea stars can result in ecosystem degradation analogous to the kelp forests. Fishing down the food web has resulted in shifted baselines of what are considered healthy ecological systems, and a collapse of higher taxa within the fisheries (4). A decrease in biodiversity and biomass negatively impacts ecosystem functioning, the resilience of an ecosystem to recover from a disturbance, and productivity.

Destructive Fishing Practices

Blast fishing is the practice of throwing a lit stick of dynamite in a shoal of fish and collecting them as they float to the surface. Depending on the proximity, the blast can leave a crater of fractured and shattered corals as large as 2 m in the surrounding reef. The substrate left behind is not suitable for coral larval recruitment, and so recovery is often slow. This practice can instantly reverse decades of coral growth. Cyanide fishing is designed to stun a certain fish and capture it alive, typically for the aquarium trade. In excessive concentration, cyanide will kill the targeted fish and cause severe damage to surrounding organisms. Corals exposed to sublethal concentrations of the chemicals have been shown to undergo partial bleaching, which reduces corals’ growth, fecundity, and ability to photosynthesize. Corals may take up to a year to recover from the bleaching effects due to cyanide (5).

Warming Oceans

As corals reach their thermal limit, they begin to exhibit impaired functioning. The breakdown of coral functioning begins with compromised carbon fixation of the coral symbiont (6). Prolonged exposure to high temperatures can cause photosynthesis to decline and can eventually cause bleaching. Coral reefs that have experienced a mass-bleaching event tend to have a survival rate around just 40% (7). Temperature stress has also been connected with an increase in coral disease, such as white band disease, black band disease, and skeletal eroding band disease.

Ocean Acidification

About 50% of CO2 emitted remains in the atmosphere, terrestrial life takes up about 20% and the world’s oceans take up the remaining 30% (8). This makes the oceans the largest sink of atmospheric CO2. This sink can alter the oceanic chemistry and disturb the carbon cycle (link: carbon) (9). Small deviations from equilibrium can shift the balance and remove crucial minerals necessary for several marine life forms. As a result of this shift in equilibrium, the mineral aragonite is not formed at the necessary rate for growth (28). Acidification not only affects corals, but also crustose coralline algae, which is essential to coral recruitment, Diadema antillarum, which keep macroalgae in check, and any other calcifying organisms. When carbon dioxide concentrations approach 450 ppm, calcification might not be able to occur at all (10).

Eutrophication

Urban and agricultural development, along with deforestation and demolition of mangroves, are strongly linked to diminished water quality and eutrophication. Sedimentation and eutrophication increase turbidity, which restricts available light for photosynthesis and aggravates coral tissue. Excess nutrients, such as the nitrogen inputs from development, also promote macroalgal growth. When there is not enough herbivory to control the growth sustained by eutrophication, such as the case in the Caribbean, coral dominated ecosystems tend to undergo a phase shift towards a macroalgae dominated ecosystem (11). Additionally, the larvae of Acanthaster planci, a corallivorous sea star, can obtain a major energy input from runoff (12). Eutrophication may provide enough nutrients to the larvae to accelerate their growth and development (13). While predation can often increase space and diversity in ecosystems, a reef subjected to an Acanthaster boom can be completely decimated and left barren.