Posted by: reefrescue | June 30, 2012

What’s killing our corals?

A Summary of the Status Review Report of 82 Candidate Coral Species Petitioned Under the U.S. Endangered Species Act

The National Marine Fisheries Service (NMFS) has identified 82 corals species as candidates to be considered for protection under the U.S. Endangered Species Act due to a potential risk of extinction. Of the 82 corals, seven are found in Florida and the Greater Caribbean Basin. Of these seven, five topped the list of the 82 corals as most likely to become extinct by the end of the century.

The following are excerpts from the Biological Review Team’s “Status Review Report of 82 Candidate Coral Species”, that was presented at the NOAA/NMFS public meetings in Florida and Hawaii during June 2012. The full text is available online at: http://www.nmfs.noaa.gov/stories/2012/05/07_coral_documents_page.html

THREATS TO CORAL SPECIES

Human Population

The common root or driver of most, possibly all, of these threats is the number of humans populating the planet and the level of human consumption of natural resources. The combination of increasing numbers of humans and their persistently rising per capita resource demands are directly responsible for escalating atmospheric carbon dioxide (CO2) buildup and associated impacts, both direct (e.g., ocean warming, ocean acidification, and sea-level rise) and indirect (influential in the increased prevalence of many coral diseases, decreased ability of corals to deposit calcium carbonate skeletons, increased energy for storms, and the potential of increased input and re-suspension of coastal sediments by changing precipitation patterns or sea-level rise).

Increased human population and consumption of natural resources are also root causes for increases in fishing (particularly of herbivores) at many locations around the globe, for massive inputs of nutrients (eutrophication), toxic pollutants, and sediments into many coastal waters, and for the spread of invasive species.

Summary of global changes and their impacts

Corals have evolved during the last 240 million years under a naturally varying climate. Recent climate changes resulting primarily from anthropogenic greenhouse gas emissions likely are the most abrupt since the corals first evolved. The recent anthropogenic changes have been referred to as a new geological era, the Anthropocene (Crutzen, 2002; Zalasiewicz et al., 2008; Zalasiewicz et al., 2010), and the associated biodiversity changes have been predicted to be the sixth global mass extinction event (Thomas et al., 2004a).

Rising atmospheric CO2, and its concomitant impacts on the oceanic environment, has already contributed to the deterioration of coral reefs and coral species populations globally (Hoegh-Guldberg et al., 2007; Wilkinson, 2008). By the early 1980s, atmospheric CO2 levels had risen from preindustrial levels of about 280 ppm to in excess of 340 ppm. Thermal stress began causing mass coral bleaching events in the 1980s and became a global problem in the 1990s. By the 1990s, the return frequency of mass bleaching in parts of the Caribbean was exceeding the ability of many reefs and coral species to recover from bleaching and disease effects (Eakin et al., 2010), and the combination of stressors were decreasing coral reef architectural structure (Alvarez-Filip et al., 2009). Coral disease outbreaks first began in some locations in the Caribbean Sea in the 1970s (Bak and Criens, 1982; Gladfelter, 1982) and were followed by major outbreaks across the entire Caribbean Sea (Aronson and Precht, 2001). Presently, atmospheric CO2 levels exceed 390 ppm and this high concentration likely has contributed to the decline of many coral reefs through processes described herein. Human activities are releasing CO2 into the atmosphere rapidly and this rate is expected to increase, exceeding worst case scenarios used in modeling future climate change (IPCC, 2007b; WDCGG, 2010).

The atmospheric concentration of the dominant greenhouse gas, CO2, has steadily increased from ~ 280 ppm at the start of the Industrial Revolution to over 390 ppm by 2009 (WDCGG, 2010)—the highest concentration of the last 800,000 years (Luthi et al., 2008; Fig. 3.2.2; Petit et al., 1999) and probably the last 20 million years (Pearson and Palmer, 2000). Rates of human-induced CO2emissions are also accelerating, rising from 1.5 ppm per year during 1990–1999 to 2.0 ppm per year during 2000–2007 (Canadell et al., 2007; Raupach et al., 2007) exceeding rates seen during the past 720,000 years, including during glacial-interglacial transitions (Hoegh-Guldberg et al., 2007; Luthi et al., 2008). CO2 emissions to date have resulted in an acceleration of at or above the worst-case scenario used in the IPCC’s Third and Fourth Assessment Reports (Fig. 3.2.3).

Land-based sources of pollution

A decade ago, it was estimated that 58% of the world’s coral reefs were potentially threatened by human activities such as coastal development, resource exploitation, and land-based and marine pollution (Bryant et al., 1998). A more recent assessment indicated that the situation has continued to deteriorate, as coastal human populations and their collective consumption of natural resources have continued to increase unabated (Burke et al., 2011). Human activities in coastal watersheds introduce sediment, nutrients, chemical contaminants, and other pollutants into the ocean by various mechanisms, including river discharge, surface runoff, groundwater seeps, and atmospheric deposition. Humans introduce sewage into coastal waters through direct discharge, treatment plants, and septic leakage, each bringing nutrients and microbial contamination. Agricultural runoff brings additional nutrients from fertilizers, as well as harmful chemicals such as pesticides. Elevated sediment levels are generated by poor land-use practices. Industry is a major source of chemical contaminants, especially heavy metals and hydrocarbons.

Several seminal review papers have described the effects of coastal pollution on coral reefs and provide a more detailed treatment of the topic than space allows here. These works include the effects of sewage (Pastorok and Bilyard, 1985), sedimentation (Rogers, 1990), nutrient enrichment (Dubinsky and Stambler, 1996; Szmant, 2002), terrestrial runoff (Fabricius, 2005), and contaminants (Peters et al., 1997). Many of these water quality parameters and their consequent biological effects co-occur in the field, making it difficult to definitively establish causative mechanisms (Fabricius, 2005). The situation is further confounded by the fact that some pollutants have both direct and indirect effects, while others may be beneficial in small amounts but are detrimental at elevated levels.

Sedimentation

Delivery of terrestrial sediment is likely to be the most pervasive sediment stress that corals experience, though dredging, beach re-nourishment, and winds and seas that remobilize in situ sediments can also result in important stresses to corals in some areas. Terrestrial sediments are also likely to have greater impacts than marine sediments because of their physical and chemical characteristics. Terrestrial sediments tend to be both finer (more easily re-suspended) and darker (more light-absorbing); consequently terrestrial sediments reduce light more effectively than marine sediments when suspended in the water column (Te, 1997). The high iron content of some terrestrial sediments may serve as fertilizers to certain components of some coral reef systems. Terrestrial sediments are also often associated with harmful organic compounds, heavy metals, nutrients or harmful bacteria (Bastidas et al., 1999; Hodgson, 1990; Jokiel et al., 2004). These associated constituents, combined with grain size and organic content, are primary factors in determining sedimentation stress in corals (Weber et al., 2006).

Toxins and contaminants

As is the case with the other pollutant stressors (with which they co-occur), toxins and bioactive contaminants may be delivered to coral reefs via either point or non-point sources. Several reviews have been conducted on contaminants, including heavy metals, synthetic organics, and petroleum products (Howard and Brown, 1984; Loya and Rinkevich, 1980; Pait et al., 2007; Peters et al., 1997). However, the analytical ability to detect contaminants sheds little insight on the ecological effects that contaminants might have on corals. A substantial body of literature documents bioaccumulation of contaminants, and over the previous decade scientists have developed sophisticated molecular techniques as biomarkers (Downs et al., 2005; Morgan et al., 2005). The presence or constituent changes in a biomarker under exposure to a toxicant stress may provide some mechanistic understanding of the organismal response, but only if these mechanisms are well established in basic physiology and traditional dose-response experiments. Instead, effects to corals to date have most often been inferred from environmental correlations.

Disease

Disease is broadly defined as “any impairment that interferes with or modifies the performance of normal functions, including responses to environmental factors such as nutrition, toxicants, and climate; infectious agents; inherent or congenital defects, or combinations of these factors” (Wobeser, 1981). A disease state results from a complex interplay of factors including the cause or agent (e.g., a pathogen, an environmental toxicant, a genetic defect), the host, and the environment. In this case, the host is a complex holobiont that includes the coral animal, dinoflagellate, and microbial symbionts. For the purposes of this Status Review Report for the 82 candidate coral species, the effects that the BRT incorporates and ranks as “coral disease” are those characterized as presumed infectious diseases or those attributable to poorly-described autogenous malfunctions (e.g., genetic defects) and often associated with acute tissue loss. Other manifestations of broad-sense disease, such as coral bleaching or toxicological effects, are incorporated in other threat sections (e.g., toxins, acidification, warming).

Disclaimer: This is a very brief edited summary of the NMFS BRT, 581 page report and cannot possibly reflect the full content and conclusions from the NMFS BRT document. It is only intended to provide the reader with an abbreviated overview. We recommend interested parties access the complete documents on line at: http://www.nmfs.noaa.gov/stories/2012/05/07_coral_documents_page.html

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