For the past couple of decades, late summer brings a bloom of algae in the Gulf of Mexico, just off the mouth of the Mississippi River. Last year, the bloom covered 17000 km2 –somewhat larger than the State of Connecticut. When the bloom dies, the decomposition of algal biomass consumes all the oxygen dissolved in the water—hypoxia. Without oxygen, fish and shellfish die. Hence, the bloom is known as the Dead Zone.
Similar blooms are found at the mouth of estuaries in other regions of the world, especially when their watersheds harbor a large amount of agricultural activity. Chesapeake Bay and the Baltic Sea are good examples, and a similar phenomenon is found in central Lake Erie. There is evidence that hypoxia began to occur in large rivers and lakes more than 100 years ago. What causes hypoxic zones to occur?
The answer lies in the large amount of nitrogen fertilizer used in modern agriculture, and also incidentally on home gardens and golf courses. When nitrogen fertilizer contains nitrate, it is easily lost to runoff waters, inasmuch as nitrate is highly soluble in water. Applications in excess of immediate plant demand are lost. Even when nitrogen fertilizer is applied in other forms, such as ammonium or urea, these are easily converted to nitrate by soil microbes and lost in runoff. By one account nearly 8-12% of the nitrogen fertilizer applied worldwide is lost from fertilized fields and transported to the sea. In some individual fields, the value can be as high as 50%.
Still more nitrogen is lost during the disposal of animal wastes from modern industrialized production of pork and chickens. Here nitrogen is lost during inadvertent overflow of waste lagoons, and nitrogen is transported to groundwater, which makes its way to stream channels. In North Carolina, the nitrogen isotopic composition of animal waste closely matches that in groundwater, establishing a close link between the two.
When the nitrate arrives at the coastal waters, it stimulates a bloom of algae. (It is fertilizer, after all). This may be aided by the simultaneous transport of phosphorus from agricultural lands, although phosphorus is by no means as mobile as nitrogen when it comes to runoff waters. The hypoxic zone in Lake Erie appears closely related to both nitrogen and phosphorus inputs.
Hypoxia is one of the side-effects of modern agronomic systems which strive to feed 7 billion of us with a nutritious diet. Unfortunately, hypoxia saps the ocean’s ability to supply protein in fish and shellfish, just at the moment that these resources are most needed by the human population. Once again, if there were fewer of us, the problem would be easier.
The solution will stem from a more judicious use of fertilizer, so that the largest percentage of it is assimilated by the crop plant of interest. We also need to treat nitrogen and phosphorus in human and animal wastes as a resource to be recycled, not an unfortunately byproduct to be disposed.
References
Blesh, J. and L.E. Drinkwater. 2013. The impact of nitrogen source and crop rotation on nitrogen mass balances in the Mississippi River Basin. Ecological Applications 23(5):1017-1035.
David, M.B., L.E. Gentry, A.D. Kovacic and K.M. Smith. 1997. Nitrogen balance in and export from an agricultural watershed. Journal of Environmental Quality 26: 1038-1048.
Gao, S. P. Xu, F. Zhou, H. Yang, C. Zheng, W. Cao, S. Tao, S. Piao, Y. Zhao, X. Ji. Z. Shang, and M. Chen. 2016. Quantifying nitrogen leaching response to fertilizer additions in China’s cropland. Environmental Pollution 211: 241-251.
Gardiner, J.B. and L.E. Drinkwater. 2009. The fate of nitrogen in grain cropping systems: a meta-analysis of 15N field experiments. Ecological Applications 19: 2167-2184.
Jenny, J.-P., P. Francus, A. Normandeau, F. Lapointe, M.-E. Perga, A. Ojala, A. Schimmeimann, and B. Zolitschka. 2016. Global spread of hypoxia in freshwater ecosystems during the last three centuries is caused by rising local human pressure. Global Change Biology 22: 1481-1489.
Karr, J.D., W.J. Showers, J. W. Gilliam and A. S. Andres. 2001. Tracing nitrate transport and environmental impact from intensive swine farming using delta nitrogen-15. Journal of Environmental Quality 30: 1163-1175.
Chang, C.C.Y., C. Kendall, S.R. Silva, W.A. Battaglin, and D.H. Campbell. 2002. Nitrate stable isotopes: tools for determining nitrate sources among different land uses in the Mississippi River Basin. Canadian Journal of Fisheries and Aquatic Sciences 59: 1874-1885.
Rabalais, N.N., W.-J. Cai, J. Carstensen, D.J. Conley, B. Fry, X. Hu, Z. Quinones-Rivera, R. Rosenberg, C.P. Slomp, R.E. Turner, M. Voss, B. Wissel, and J. Zhang. 2014. Eutrophication-driven deoxygenation in the coastal ocean. Oceanography 27: 172-183.
Schlesinger, W.H. 2009. On the fate of anthropogenic nitrogen. Proceedings of the National Academy of Sciences. 106:203-208. [doi:10.1073/pnas.0810193105]
Sebilo, M., B. Mayer, B. Nicolardot, G. Pinay, and A. Maniotti. 2013. Long-term fate of nitrate fertilizer in agricultural soils. Proceedings of the National Academy of Sciences 110: 18185-18189
