Coral Reefs in the 21st Century: Is the Past the Key to the Future?
Pamela Hallock Muller (University of South Florida, St. Petersburg, Florida, USA)
Dept. of Geology & Assoc. for Women in Science, NE Ohio Chapter Colloquium,
Kent State University, Kent, Ohio, USA
19 October 2000
What’s a reef? Historically, a reef has been any hazard to navigation. Any hazard to navigation is still called a reef. Example: the Exxon Valdez oil spill was caused by the ship running into a “reef”, but it wasn’t a biologic reef. Biogenic reefs include things like oyster reefs at the mouth of the Crystal River in Florida, and coral reefs.
Ships hitting the Florida Keys reefs was a major impetus for the establishment of a marine sanctuary down there. Coral reefs are wave-resistant carbonate structures that develop in place and are composed of skeletons of reef-building corals, as well as shells and skeletons of other organisms. Reefs have to be accreting.
Corals are cnidarians, and are usually colonial. Zooxanthellate corals (non-deep water corals) are dependent on symbiotic dinoflagellates (zooxanthellae), require sunlight, and are mixotrophic (both photosynthetic and capable of feeding). The best environments for the mixotrophic lifestyle are low-nutrient environments where the principal source of nutrients is in particulate organic form. Corals bleach by sloughing off the entire endodermal layer, the layer which contains the zooxanthellae.
Azooxanthellate corals are heterotrophic and don’t require dinoflagellate zooxanthellae.
Ahermatypic corals don’t build reefs, and can be zooxanthellate or azooxanthellate.
Hermatypic corals are reef-builders, and can be zooxanthellate or azooxanthellate.
There may be ahermatypic corals in marginal environmental conditions.
Hermatypic vs. ahermatypic is more a function of environment, not something inherent in the organism.
Coral reef occurrences - warm water reefs in Caribbean, Gulf of Mexico, a few off the eastern and western coasts of South America, lots in the western Pacific basin and several in the Indian Ocean. There are few in the eastern tropical Atlantic. Why aren’t there more reefs in the Pacific Ocean? Mainly because there is little shallow water substrate available (only a couple of exceptions, such as Galapagos, etc.). The occurrence of coral reefs is controlled by occurrence of shallow water habitats, availability of low-nutrient areas, and availability of warm areas. Upwelling zones are usually where you don’t find coral reefs.
Conditions favouring coral reef growth - 18-31˚ C, 32-40‰ salinity, water movement, clear water, limited terrigenous sediment, limited nutrient input (a key condition).
Coral reefs are threatened worldwide. Bryant et al. (January 1998) - “Reefs at Risk”, published during a worldwide coral bleaching event. This study found that 58% of coral reefs on Earth are at risk. The Caribbean and western Pacific regions are areas of maximum coral reef threat. The factors behind the risk include human overpopulation, deforestation, high erosion rates, etc.
Acropora (3 spp. total) is considered a major component of most western Atlantic and Caribbean coral reefs. Acropora palmata and Acropora cervicornis have been considered to be endangered species by some workers.
Coral reefs are threatened by several things:
1) Nuisance algal blooms. Most of these are not really algal, but cyanobacterial (often Lymbia [sp.?]). They are often seen covering sea fans, for example.
2) Increased rates of bioerosion - occurs when black band disease and bleaching weakens corals. Critical bioeroders include clionid sponges and siphonodictyid sponges, both of which are boring sponges. Sponge borings results in loss of reef structure. Hallock Muller has been seeing large meadows of clionid sponges more and more commonly recently in coral reef settings.
3) Loss of habitat - many fish species (such as grunts) are losing habitats from coral reef structure loss.
Local impacts on coral reefs - ship groundings, recreation, construction, spills. Example: cutting of mangrove swamps and forests, resulting in brown, turbid water nearby from erosion & runoff.
Example: lots of tourist recreation nowadays in coral reefs areas, globally - lots of people & lots of sunscreen chemicals, etc.
Local impacts on a global scale - coastal pollution, sediments, overfishing.
Regional impacts - new diseases, major rivers, dust.
Global change - increasing CO2 levels, ozone depletion.
Some claim that there are few to no reefs left in the world that aren’t affected by overfishing. Basic problem here with all these problems is human overpopulation (we’ve exceeded the carrying capacity of the Earth).
Causes of regional reef decline:
- Diadema die-off (Echinodermata, Echinoidea, “hatpin urchin”) is a good example of how new diseases can decimate coral reefs on a regional scale (this die-off started in 1983). This event was devastating to Caribbean reefs, because Diadema was a very important grazer; now, little grazing of pest algae & cyanobacteria is occurring, and they are overgrowing the corals. The Diadema kill started off the Panama Canal, so it seems likely that the pathogen causing the die-off came from the Pacific through the Panama Canal.
- Nutrification by major rivers due to deforestation and agricultural & urban pollutants.
- Nutrification via atmosphere (atmospheric dust & air pollution).
White-band disease has been considered a good reason to put Acropora on the endangered species list. White-band disease has only been prevalent in the last 20 years. Geological and sedimentological studies show that coral destruction from white-band disease is unprecedented since the last glacial retreat [= end of Pleistocene]. We don’t know the ultimate cause for white-band disease’s prevalence right now. Head corals have been hit by black band disease and other pathogens in the 1990s.
Satellite photos show regular plumes of wind-blown sediment from the Saharan Desert of northern Africa, and also extensive oceanic sediment plumes from the Orinoco River runoff (northern South America). Some have pointed out that there has always been wind-blown dust from the Saharan Desert. But, modern desertification trends (mostly anthropogenic, likely), especially during ENSO years (El Niño Southern Oscillation), magnifies the effect. Some terrestrial organisms (pathogens) that affect sea fans in coral reefs are known to have arrived in the Caribbean from Saharan wind-blown sediment.
Global threats to coral reefs - alteration of global nutrient cycles, ozone depletion & resulting increase in UVB radiation, increased CO2 levels (global warming & decreased CaCO3 saturation in oceans - something not considered much by people with respect to coral reef growth).
Nutrification - increased nutrient influx (by natural or anthropogenic processes) that promotes a visible shift in community structure. Implications of nutrification are increased levels of phytoplankton and higher benthic algae abundances and growth.
Eutrophication - elevated nutrient flux resulting in episodic anoxic conditions. This occurs in canals of the Florida Keys, but it has not been observed to occur in coral reef environments.
Source of nutrients in coral reef systems - 1) Upwelling; 2) atmospheric input (dust, aerosols, volcanic ash); 3) runoff; 4) groundwater discharge (including sewage from so-called “deep-injection wells” in urban areas of southern Florida); 5) nitrogen fixation (a natural process, but humans have doubled nitrogen fixation in terrestrial environments); 6) advection.
Ozone depletion - results in bleaching, especially in hot & still weather. Bleaching causes corals to have reduced immunity & increased susceptibility to disease. We’re seeing lots of new diseases globally in coral reefs (Are the diseases new or are the corals now more susceptible? Or both?). Where are these diseases coming from? There has been observed lots of cyanobacterial growth globally as well (Note: cyanobacteria were around on Earth before there was even an ozone layer! They are tolerant of UV radiation!). Ozone depletion & resulting increased UVB radiation also causes damage to coral larvae and causes increased mutation rates. UV radiation is not a problem in aquatic systems, some say. But, this isn’t quite true. UVB intensities from April to August of every year now are the same as seen for just the 1960 summer solstice.
Large shelled forams are bleached too, many times. This was seen after the Mt. Pinatubo eruption. Forams have been observed to be bleached every year since 1991 in the Florida Keys during summer solstice, actually before & after summer solstice - summer solstice is just the peak time interval for foram bleaching.
Bleaching affects scleractinian corals and Millepora and hydrocorals. The causes of coral bleaching include temperature stress, UV stress, salinity stress, and combinations of stresses, especially high temperatures & UV radiation. Bleaching is an oxidative stress response by corals - high temperatures put corals in a photo-inhibitive condition.
UV water penetration - doesn’t occur in coastal waters that are nutrient-rich (lots of suspended sediment). But, UV does penetrate clear water, which is where coral reefs like to grow. In less clear waters, forams have been observed to not bleach as much during maximum UV radiation intensity times of the year. Bleaching often doesn’t kill corals, but weakens them - they may miss a whole year of reproduction or they won’t grow as much.
Increased CO2 implications - results in global climate change, changes in sea level (higher), changes in storm intensity (more intense & more frequent), etc.
Bleaching was a rare event in corals prior to 1980 in Jamaica. Since 1990, bleaching has occurred somewhere in the Jamaica area every year.
Kleypas et al. (1999) - CO2 will be double that of pre-industrial levels by the year 2065. Aragonite saturation state in the tropics will drop 30% due to this. Biogenic aragonite precipitation will also drop 14-30% due to this. Another prediction is that by 2100, the oceans will go below critical saturation levels of calcium carbonate for reef-building to occur.
The last time CO2 levels were this high was the Eocene (double pre-industrial levels of CO2). In the Eocene, had CO2 levels increase, resulting in falling pH, falling CaCO3 saturation, increased carbonate solubility (especially for aragonite), etc.
Future? Eventually, we’ll get equilibrium again, and pH will return to ~8, carbonate saturation will increase again, etc. But, this will only happen after 1000s of years.
Future predictions - photosynthetic aragonite producers are losers. Eocene-like tropical biotas will benefit.
Higher levels of UVB radiation - losers will be shallow-water photosynthetic symbiotic organisms. Winners will be cyanobacteria. This pattern has been seen after major extinctions in the geologic record - lots of cyanobacteria around after extinctions.
Corals will continue to persist in deeper water. Coralline algae will persist in making some sort of reef-like structures.
Polar systems & coral reef systems are the 2 key ecosystems on Earth that do show the reality of global warming.
Livebottom communities should persist.
Some stony corals species are more vulnerable to disease & bleaching than other species (Example: Acropora is more susceptible than Porites).
Some sclerochronology studies show low growth years within corals in the long-distant past. Whether this was due to bleaching events or due to too-cold years (which also causes bleaching), we can’t tell.
We seem to be approaching Eocene conditions.