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Flushing Out the Truth About Sewage and Coral Reefs

April 15, 2013

Unfortunately, there is plenty of untreated sewage making its way into tropical seas. Photo: Flickr user Abizern

I never expected to be so intrigued and excited about poop, until a paper in PloS ONE came out in 2011 that demonstrated that a common human pathogen found in human wastewater, Serratia marcescens strain PDR60, caused white pox disease in elkhorn coral (Acropora palmata), the foundation species in Caribbean coral reefs.

Caribbean reefs have been plagued by disease in recent years and figuring out the source of the pathogens has been a challenge. Human sewage has long been a suspect, but the science behind this suspicion was always tenuous. I think most people would assume that exposing reefs to partially treated or untreated sewage couldn’t be a good thing, but there were no clear data that made the connection of human sewage to the degradation of corals so clearly until this paper.

Unfortunately, there is plenty of untreated sewage making its way into tropical seas.

In the Caribbean, most sewage isn’t actually treated, rather it is put into containers that sit in the ground — the ground being comprised of porous calcium carbonate rock (limestone) that is characteristically leaky.

In many places in the Pacific, the ocean is the toilet.

Human fecal contamination in near-shore and off-shore coral reefs has been well documented and has also been linked to causing human disease. We knew that humans were getting sick from their own waste, but what is so remarkable about this suite of disease-host interactions is that not only were vertebrates (us) transferring their disease to invertebrates (coral), but that this transmission took place across distinct realms, from the terrestrial to the marine — crazy stuff!

The offender wasn’t the nutrients in the sewage as some have argued, in places like the Florida Keys, but the human pathogens in that sewage. To give you a little background perspective on these ideas, there have been assertions by some scientists that an increase in nutrients in coral reef systems (which by definition are nutrient poor) has been a key driver in the conversion of coral reefs to algal reefs (especially in the Caribbean).

However, the experimental science has shown us that in most cases the primary driving factor in the coral vs. algal dominance situation is the number and type of herbivores present.

Herbivores play an important role in suppressing algal growth and are thus allies of corals when it comes to the battle for space with algae. This idea that healthy grazer populations are the key for sustaining healthy coral reefs has confused and distracted many whose intuition and observations tell them that runoff from land is a primary threat facing sensitive reef habitats.

The linkage between human sewage and coral disease helps to validate such intuitions and better clarify the contribution that terrestrial activities make in coral reef degradation. But it doesn’t stop there — there is evidence that nutrient additions (from things like agricultural runoff) may facilitate coral disease and coral predator outbreaks (such as crown of thorns starfish on the Great Barrier Reef).

So nitrogen is likely to have an effect, but perhaps not the one originally thought. One can see why the situation can be confusing and identifying priorities for threat abatement can be a challenge.

The 2011 paper gives us a better understanding of the potential for human sewage to wreak havoc on a reef and the research continues to better understand the linkage.

The papers are trickling in and I suspect the relationship will become pretty solid and well accepted over time.

The question for me right now is: When do we do something about it?

In a recent conversation with a well-respected academic scientist, who believes that sewage is a problem for reef health, I was cautioned to not jump the gun and go out and start a crusade against poop. He argued that we needed more science. But isn’t that often the mantra of scientists?

What happens if we invest in solving this problem and then we learn that it isn’t such a big deal for reef health?

I would argue that we have still done some serious good for the communities we focus on because sewage treatment and good sanitation systems are good for everyone. The worst-case scenario is that we help address a public health problem (i.e. reducing exposure to fecal pathogens that cause life-threatening diseases). That sounds like a reasonable gamble to me. The best-case scenario is that we solve a problem for reef health and public health.

What bothers me about the environmental movement is we have failed (or perhaps never really tried) to connect environmental health to public health — when the two are intimately intertwined. Giving this sewage problem some attention has become more intriguing to me of late — sewage is something we can actually do something about.

We know how to manage it, treat it, and even turn it into useful things — yet wastewater seems to be an afterthought when it comes to reef conservation strategies. Working side-by-side with municipal planners and public health organizations to elevate sewage treatment and implementation of sanitation systems as a local or national priority could lead to some very productive partnerships and help us each achieve our respective goals (i.e., public health and coral reef health).

So many problems in marine conservation are so big that people can’t figure out how to possibly solve them — the systems are so complex, the contributing factors are diverse. Figuring out that point of influence in which we can change the impacts is extremely challenging and sometimes would take superhuman effort to make happen.

So what we do is focus on what we can do — well, most of the time. There are definitely things that have a shut-off valve — things that I don’t think we are paying enough attention to. And I would argue that the problem of untreated sewage literally has a shut-off valve and is something worth exploring as a focal strategy in our coral reef conservation.

What do you think? 


Bruno, J.F., L.E. Petes, C. Drew Harvell and A. Hettinger. 2003. Nutrient enrichment can increase the severity of coral diseases. Ecology Letters, 6: 1056–1061. doi: 10.1046/j 1461-0248.2003.00544.x

Sutherland K.P., S. Shaban, J.L. Joyner, J.W. Porter, and E.K. Lipp. 2011. Human pathogen shown to cause disease in the threatened eklhorn coral Acropora palmata. PLoS ONE 6(8): e23468. doi:10.1371/journal.pone.0023468

Photo credit: Flickr user Abizern under a Creative Commons license.

Opinions expressed on Cool Green Science and in any corresponding comments are the personal opinions of the original authors and do not necessarily reflect the views of The Nature Conservancy.

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