A recent news headline proclaimed the “dead zone” – an aquatic area devoid of oxygen and therefore life – in the Gulf of Mexico to be the largest ever observed spanning an area roughly the size of the state of New Jersey (1). Dead zones are an extreme case of a process called eutrophication that begins when excess nutrients from human activities including agriculture, industry and waste water treatment enter a water body causing harmful algal blooms that turn pristine waters from blue to soupy green. The problem of eutrophication is not isolated to the gulf, it’s also a concern in the Chesapeake Bay watershed (e.g. 2, 3), the Great Lakes (e.g. 4, 5), and Lake Champlain (e.g. 6) among many other bodies of water. Worse, research published last week in the journal Science suggests the problem is likely going to get worse in the future, not just in the United States, but in waterbodies around the globe (7).
Article: Sinha, E., A. M. Michalak, and V. Balaji. (2017) “Eutrophication will increase during the 21st century as a result of precipitation changes.” Science. 357, 405-408. DOI: 10.1126/science.aan2409
The Problem: Too much of a good thing?
In balanced ecosystems like that shown on the left side of Figure 1, algal growth is limited by the availability of certain nutrients. Typically, these are phosphorus (P) in freshwater systems and nitrogen (N) in estuaries and marine systems. In the balanced aquatic system phytoplankton (algae being a particular type) are limited in their growth by what is available of those limiting nutrients (N & P). Algae that does grow is then food for larger organisms in the ecosystem, like fish, which feed even larger organisms, and so on. The water in this balanced system is typically clear allowing sunlight to reach thriving bottom dwelling plants that in turn cycle oxygen into the system.
In unbalanced ecosystems like that depicted on the right side of Figure 1, much more of those critical nutrients are available allowing the algae to grow out of control creating algal blooms in a process called eutrophication. Algal blooms do a number of disruptive things to offset the balance of the system. For example, in a classic double whammy, the algae first use up the available nutrients and that excessive growth blocks sunlight from penetrating deep into the water cutting off energy to bottom dwelling plant communities. The algae then die and start a decay process that depletes the already stressed oxygen supply of the environment. This lack of oxygen is what leads to the so called “dead zone.” With no oxygen, fish and other mobile organisms are forced to evacuate the area in search of oxygenated water. Organisms that cannot get away or cannot move, like bottom dwelling plants, often die.
So where do these excessive nutrients that start the cascade come from? The short answer is: people. Human activities including agriculture, waste water treatment, industrial activity and increased storm water runoff all feed into the problem. In the case of agriculture, nitrogen and phosphorus limit the production of crops in a way similar to that of algae. When more of those critical nutrients are available, crops are able to grow and thus phosphorus and nitrogen are introduced across the landscape in the form of fertilizer. Knowing exactly how much fertilizer needs to be applied can be a tricky problem. The cost of fertilizer notwithstanding, a farmer, wanting the best crop yield, is thus incentivized to add more rather than less. The crops themselves are often unaffected or even positively affected by the availability of excess nutrients on the farm field just like excess nutrients isn’t a problem for the algae who happily grow and grow and grow!
The Forecast: More rain, more fertilizer, more eutrophication
Previous research has shown that the changing climate will impact regions of the U.S. and the globe differently. For example, some regions, like the Northeast of the U.S., are forecast to experience increases in precipitation amount, frequency and intensity (8). In the study by Sinha et al., the authors focus on nitrogen, which is the primary cause of eutrophication in estuaries and coastal waters. The authors used 21 climate simulations and applied them to the observed land use and related fertilizer application rates these areas to predict the magnitude of the eutrophication problem for three time periods: historical (1975-2005), near future (2031-2060), and far future (2071-2100). The historical time period simulation served as a way to ensure the results corresponded to already observed patterns that then allowed the authors to predict which factors will continue to influence the eutrophication problem at the regional and continental scale United States in the future. In the US, they find that areas with three conditions are most at risk: those with a) high historical nitrogen input from human sources, b) existing high precipitation patterns, and c) a high likely increase in precipitation under future climate scenarios. The authors conclude these same conditions can lead to increased eutrophication in water bodies around the globe.
How much worse is the changing climate likely to make the problem of eutrophication? And, are we on the right track to addressing the challenge? Using the example of the Gulf of Mexico dead zone, the authors suggest that current efforts to reduce nutrient input in the upper Mississippi watershed are either inadequate or will require massive management changes in light of the confounding impacts of future precipitation increases. To specifically clean up the dead zone in the of the Gulf under future conditions, the authors estimate that a 62% reduction in nitrogen input will be required, far higher than the current target of 20%. Global regions including India and eastern China are similarly burdened with a high risk profile. And those regions are also home to half the world’s population who are heavily reliant on surface water.
We know eutrophication is a problem, and the highlighted work by Sinha et al. tells us it is likely to get worse under future climate conditions. How this and similar findings are incorporated into the decision making process of farmers, regulators and others, and how that can contribute to restoring our environment is research to be shared in future posts. Stay tuned!
- NOAA News & Features. (2017) “Gulf of Mexico ‘dead zone’ is the largest ever measured: June outlook foretold New Jersey-sized area of low oxygen. August 2, 2017. http://www.noaa.gov/media-release/gulf-of-mexico-dead-zone-is-largest-ever-measured
- US EPA. Addressing Nutrient Pollution in the Chesapeake Bay. Webstie https://www.epa.gov/nutrient-policy-data/addressing-nutrient-pollution-chesapeake-bay. Accessed August 11, 2017.
- Boesch, D.F., R. B. Brinsfield, and R. E. Magnien. (2001) Chesapeake Bay eutrophication: scientific understanding, ecosystem restoration, and challenges for agriculture. J Environmental Quality. Mar-Apr; 30(2):303-20.
- Chapra, S. C. and A. Robertson. (1977) Great Lakes Eutrophication: The Effect of Point Source Control of Total Phosphorus. Science. 196(4297):1448-1450.
- Michalak, A. M., Anderson, E. J., et al. (2013) Record-setting algal bloom in Lake Erie caused by agricultural and meterological trends consistent with expected future conditions. PNAS. 110(16):6448-6452. DOI: 10.1073/pnas.1216006110.
- Levine, S. N., A. Lini, M. L. Ostrofsky, et al. (2012) The eutrophication of Lake Champlain’s northeastern arm: Insights from paleolimnological analyses. Journal of Great Lake Research. 38(S1):35-48.
- Sinha, E., A. M. Michalak, and V. Balaji. (2017) “Eutrophication will increase during the 21st century as a result of precipitation changes.” Science. 357, 405-408. DOI: 10.1126/science.aan2409
- Melillo, J. M., T. C. Richmond, and G. W. Yohe, Eds. (2014) Highlights of Climate Change Impacts in the United States: The Third National Climate Assessment. U. S. Global Change Research Program. Online at: nca2014.globalchange.org. Accessed August 11, 2017.