Threats to both types of coral reefs, due to the general fragility of coral with regards to the effects of indirect and direct human influence, can cause the precarious balance of stability within the reef to tip and the ecosystem to collapse.
Direct human influence can include, but not limited to, things such as pollution, sewage emissions, global warming via CO2 release, dredging, overfishing and coral removal, whilst indirect effects can include bleaching, ocean acidification and eutrophication.
Cold water corals tend to face less of an issue from pollution and coral removal, due to the reefs being within deep water and away from human influx to the ocean, but accidental or deliberate dredging, along with overfishing for commercial stocks and global warming, can cause huge damage to these reefs. Slightly more stable than tropical reefs (which can be seen as almost balanced on a knife’s edge with regards to the stability), cold water reefs can still be devastated by human or human-influenced activities. Dredging – due to both coral removal from the reef and the destruction of the area of the sea bed, is especially devastating. Commercial fishing can remove a large portion of the food web from these reefs, leaving them in a state of disarray.
Tropical reefs suffer more from changes in the ocean pH, temperature and depths, due to their shallow locations and mild temperature bands for coral growth, whilst coral ‘poaching’ (due to the attractiveness of the coral species, and a much easier job to obtain it) is much more prevalent. It is due to this also that pollution can have a huge effect on the coral and the surrounding organisms, poisoning the areas and bleaching/killing the coral in the surrounding areas of the initial influx of polluted water, or sewage. Recently, population explosions on the coasts, especially within areas without top class sanitation and general natural protective laws, have led to a large increase in this example of damage to reefs.