The effects of human-induced pollution on the replenishment of coral reefs

Coral reefs are suffering severe declines world-wide caused by multiple anthropogenic disturbances combined with natural events such as storms. Their future depends on the resilience of replenishment processes. However, few studies exist on the effects of multiple stresses on early life history of corals. This review describes coral early life history processes including reproduction, settlement and post-settlement survival. Relevant literature is then reviewed, focusing on the effects of four major human-induced threats, sedimentation, eutrophication, fishing and rising sea water temperature on reproduction, settlement and post-settlement survival. Research shows that in many cases different stresses affect the same replenishment process. Using two simple models results show that additive and synergistic stress can harm not only the energy budget of a single coral colony, but could also drive a species to extinction. A coral confronted by a disturbance may show a decrease in its energy budget due to either a change in environmental conditions such as reduced light penetration, or increased energy use for defensive mechanisms such as increased sediment rejecting activities. Adding other disturbances may reduce growth and/or reproductive output. If the energy budget decreases even further, processes involved in basal metabolism may suffer and the coral might finally die. Multiple stresses decrease maximum per capita growth of coral populations due to, for example, lower larval survival and decreased settlement success. Coral population extinctions may occur when interacting stresses are combined with Allee effects. Reduced coral densities and reduced reproductive output can lead to decreased fertilisation success. Hence, multiple stresses threaten not only adult corals, but also impact upon replenishment of coral reefs. This is a severe concern and highlights the importance of long-term studies of coral reef recovery and stress mitigation, particularly at a time where threats to reefs are expected to increase further in number, frequency and severity