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Thirty years of unique data reveal what’s really killing coral reefs

July 24, 2019

By Paul Homewood



Coral reefs are considered one of the most threatened ecosystems on the planet and are dying at alarming rates around the world. Scientists attribute coral bleaching and ultimately massive coral death to a number of environmental stressors, in particular, warming water temperatures due to climate change.

A study published in the international journal Marine Biology, reveals what’s really killing coral reefs. With 30 years of unique data from Looe Key Reef in the lower Florida Keys, researchers from Florida Atlantic University’s Harbor Branch Oceanographic Institute and collaborators have discovered that the problem of coral bleaching is not just due to a warming planet, but also a planet that is simultaneously being enriched with reactive nitrogen from multiple sources.

Improperly treated sewage, fertilizers and top soil are elevating nitrogen levels, which are causing phosphorus starvation in the corals, reducing their temperature threshold for "bleaching." These coral reefs were dying off long before they were impacted by rising water temperatures. This study represents the longest record of reactive nutrients and algae concentrations for coral reefs anywhere in the world.

"Our results provide compelling evidence that nitrogen loading from the Florida Keys and greater Everglades ecosystem caused by humans, rather than warming temperatures, is the primary driver of coral reef degradation at Looe Key Sanctuary Preservation Area during our long-term study," said Brian Lapointe, Ph.D., senior author and a research professor at FAU’s Harbor Branch.

A key finding from the study is that land-based nutrient runoff has increased the nitrogen:phosphorus ratio (N:P) in reef algae, which indicates an increasing degree of phosphorus limitation known to cause metabolic stress and eventually starvation in corals. Concentrations of reactive nitrogen are above critical ecosystem threshold levels previously established for the Florida Keys as are phytoplankton levels for offshore reefs as evidenced by the presence of macroalgae and other harmful algal blooms due to excessive levels of nutrients.

As can be seen by this bleached coral in Looe Key in the lower Florida Keys in 1987, these coral reefs were dying off long before they were impacted by rising water temperatures. Credit: Brian Lapointe, Ph.D., Florida Atlantic University’s Harbor Branch Oceanographic Institute

Researchers gathered data from 1984 to 2014 and collected seawater samples during wet and dry seasons. Lapointe and collaborators from the University of Georgia and the University of South Florida also monitored the living coral and collected abundant species of seaweed (macroalgae) for tissue nutrient analysis. They monitored seawater salinity, temperature and nutrient gradients between the Everglades and Looe Key. They wanted to better understand how nitrogen traveled from the Everglades downstream to the coral reefs of the Florida Keys National Marine Sanctuary, which now has the lowest amount of coral cover of any reefs in the wider Caribbean region.

Data revealed that living coral cover at Looe Key Sanctuary Preservation Area declined from nearly 33 percent in 1984 to less than 6 percent in 2008. The annual rate of coral loss varied during the study, but increased from 1985 to 1987 and 1996 to 1999 following periods of heavy rainfall and increased water deliveries from the Everglades. Between 1991 to 1995, significant increases in Everglades runoff and heavy rainfall resulted in increases of reactive nitrogen and phytoplankton levels at Looe Key above levels known to stress and cause die-off of coral reefs. Despite reduced Everglades flows, the water quality has not yet recovered to the levels of the 1980s.

Nitrogen loading to the coast is predicted to increase by 19 percent globally simply as a result of changes in rainfall due to climate change, which suggests the need for urgent management actions to prevent further degradation.

"The future success of the Comprehensive Everglades Restoration Plan will rely on recognizing the hydrological and nitrogen linkages between the Everglades, Florida Bay and the Florida Keys," said Lapointe. "The good news is that we can do something about the nitrogen problem such as better sewage treatment, reducing fertilizer inputs, and increasing storage and treatment of stormwater on the Florida mainland."


There is the usual armwaving about climate change, but no evidence is presented as to why a small increase in CO2 levels should have the slightest effect on water temperatures or coral development, when the climate has been much warmer for most of the period since the end of the ice age.

As is usually the case with environmental degradation and species decline, it is the obvious factors which are responsible.



I am reminded that cold also kills corals:


NOAA and partners from 12 organizations surveyed sites in the Florida Keys to determine the extent of coral bleaching, and death, in the wake of record low-water temperatures. Scientists assessed coral health at more than 78 sites from January 25 to February 12 to determine the severity of coral bleaching and reefs most affected.

During the first two weeks of January 2010, water temperatures in some parts of the Keys dropped into the upper 40s and lower 50s, which is about
20 degrees Fahrenheit lower than the typical temperatures of the upper 60s and lower 70s. The lethal lower limit for corals is 60 degrees Fahrenheit.

According to reef biologists, the influx of cold water from Florida and Biscayne bays appears to be responsible for the coral deaths in nearshore waters of the Florida Keys National Marine Sanctuary. The hardest hit areas were the inshore and mid-channel reefs from Biscayne Bay (southeast Florida) to Summerland Key (island in the Florida Keys). Fortunately, the offshore reefs most frequented by divers and sportfishers were buffered by warmer waters of the Florida Current and spared severe impact.

Bleached coral

Coral bleaching occurs when corals, stressed by changes in conditions such as temperature, light, or nutrients, expel the symbiotic algae living in their tissues.

A widespread cold-water coral die-off has not occurred in Florida since the late 1970s. While all coral species were affected, the impact of the cold water was very distinct from location to location. Corals within 100 meters (328 feet) of each other experienced 10- to 15-degree Fahrenheit temperature differences.

  1. July 24, 2019 10:31 am

    Reblogged this on Climate- Science.

  2. tim leeney permalink
    July 24, 2019 10:52 am

    See also Peter Ridd’s excellent chapter in “Climate Change the Facts 2017” available from Amazon.

  3. dearieme permalink
    July 24, 2019 12:18 pm

    The problem is now ascribed to agriculture, meaning that the solution is, as usual, population control.

  4. HotScot permalink
    July 24, 2019 12:23 pm

    What we also must always bear in mind is that the The Lancet discovered that 50% of scientific studies could not be replicated. Bayer put that number at 75%.

    The usual criticism of this is that ‘that’s medical science they are talking about’ to which the response is, why does anyone imagine that climate science is any better than medical science which, if it goes wrong (Thalidomide being a case in point) the personal tragedies and financial implications are horrendous.

    No one will be around when even the best climate predictions will supposedly manifest themselves, other than the loopy Matt McGrath who can be humiliated, once again on this blog, in 18 months.

  5. Jon permalink
    July 24, 2019 12:34 pm

    I’m sceptical of ANYTHING being claimed about the marine environment in the current era of heavily politicised ‘science’.

  6. Gerry, England permalink
    July 24, 2019 1:46 pm

    ‘….as a result of changes in rainfall due to climate change,’

    Why do I wonder that if this is investigated it will turn out to be a lie.

  7. swan101 permalink
    July 24, 2019 4:10 pm

    Reblogged this on ECO-ENERGY DATABASE.

  8. A C Osborn permalink
    July 24, 2019 5:31 pm

    During the period of this study –
    Cold kills Coral as well.

  9. M E permalink
    July 24, 2019 10:04 pm

    If the problem is run off from agricultural land where does it leave the vegans? Are they going to be causing global warming with their recommended diet.
    A tangled web.!

  10. July 25, 2019 5:16 am

    Reblogged this on Climate Collections.

  11. Paul Spensley permalink
    July 25, 2019 9:57 pm

    Anyone spot today ? It was the ” hottest day since 1947 ….then changed to hottest day since 2003 record. Nothing to do with co2 emissions in 1947 not fitting their agenda….

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