Editing Coral reef (section)

===Algae===
Reefs are chronically at risk of algal encroachment. Overfishing and excess nutrient supply from onshore can enable algae to outcompete and kill the coral.<ref>{{cite web |title=Coral Reef Biology |url=http://www.coral.noaa.gov/component/content/article/138.html |access-date=April 6, 2011 |archive-url=https://web.archive.org/web/20110927181624/http://www.coral.noaa.gov/component/content/article/138.html |archive-date=September 27, 2011 |url-status=dead |publisher=[[NOAA]]}}</ref><ref>{{cite book |last=Glynn |first=P.W. |year=1990 |title=Ecosystems of the World v. 25-Coral Reefs |editor-first=Z. |editor-last=Dubinsky |publisher=Elsevier Science |location=New York |isbn=978-0-444-87392-7}}</ref> Increased nutrient levels can be a result of sewage or chemical fertilizer runoff. Runoff can carry nitrogen and phosphorus which promote excess algae growth. Algae can sometimes out-compete the coral for space. The algae can then smother the coral by decreasing the oxygen supply available to the reef.<ref>{{Cite journal|last1=Murphy|first1=James W.A. |last2=Richmond|first2=Robert H.|date=2016-04-19 |title=Changes to coral health and metabolic activity under oxygen deprivation|journal=PeerJ|volume=4 |pages=e1956|doi=10.7717/peerj.1956|pmid=27114888|pmc=4841221|issn=2167-8359 |doi-access=free }}</ref> Decreased oxygen levels can slow down calcification rates, weakening the coral and leaving it more susceptible to disease and degradation.<ref>{{cite web|title=THE EFFECTS OF TERRESTRIAL RUNOFF OF SEDIMENTS, NUTRIENTS AND OTHER POLLUTANTS ON CORAL REEFS|url=http://coralreefs.org/wp-content/uploads/2014/05/ISRS-Briefing-Paper-3-Water-Quality.pdf|access-date=2015-12-05|url-status=dead|archive-date=March 4, 2016|archive-url=https://web.archive.org/web/20160304091903/http://coralreefs.org/wp-content/uploads/2014/05/ISRS-Briefing-Paper-3-Water-Quality.pdf}}</ref> Algae inhabit a large percentage of surveyed coral locations.<ref>{{Cite journal |title=Algae-Dominated Reefs |first1=Peter S. |last1=Vroom| first2=Kimberly N. |last2=Page |first3=Jean C. |last3=Kenyon |first4=Russell E. |last4=Brainard |journal=American Scientist |year=2006 |volume=94 |issue=5 |pages=430–437|doi=10.1511/2006.61.1004}}</ref> The algal population consists of [[turf algae]], [[coralline algae]] and [[macroalgae|macro algae]]. Some sea urchins (such as ''[[Diadema antillarum]]'') eat these algae and could thus decrease the risk of algal encroachment.