Liebig’s Law and Haber’s Tragedy

The fertilizers commonly used by gardeners have many ingredients, but the biggest two are nitrogen and phosphorus, either of which can limit plant and algal growth. The idea that only one nutrient limits growth is encapsulated by Liebig’s Law of the Minimum, named after Justus von Liebig, a 19th-century German chemist. Liebig is also called the “father of fertilizer” because of his work on formulating and promulgating commercial fertilizers. However, he wasn’t the first to discover the Law, and he was wrong about the most important ingredient of fertilizers. This chapter outlines the arguments among limnologists, oceanographers, and geochemists about whether nitrogen or phosphorus sets the rate of algal growth and thus production of the organic material that drives oxygen depletion. The chapter discusses that the limiting nutrient varies with the type of aquatic habitat. In dead zones like the Gulf of Mexico, parts of the Baltic Sea, and Chesapeake Bay, bioassay experiments have shown that nitrogen is usually limiting. The nitrogen necessary for fertilizer and ammunitions comes from the Haber-Bosch process. The chapter reviews the life of one of the two German inventors, Fritz Haber, and how it was full of contradictions if not tragedy.

The Case for Phosphorus

As this chapter explains, one approach to evaluate nutrient limitation is to compare nutrient concentrations with the Redfield ratio. Alfred Redfield had no formal background in oceanography, yet he made one of the most fundamental discoveries in the field. He found that the ratio of nitrogen to phosphorus in marine microorganisms is the same as the ratio of the two elements in nutrients dissolved in the oceans. Because of work with the ratio, the current Hypoxia Action Plan for the Gulf of Mexico mentions phosphorus as well as nitrogen. In the Baltic Sea, it was argued that the focus should be solely on phosphorus to limit toxic cyanobacterial blooms, but other work demonstrates the importance of limiting nitrogen for minimizing eutrophication. Once considered to be a dead lake, Lake Erie improved after the construction of wastewater-treatment plants and the banning of phosphorus-rich detergents, as the chapter shows. But the lake continues to have problems with hypoxia and harmful algal blooms, because of continuing inputs of phosphate and organic nitrogen. The chapter ends by arguing that both nitrogen and phosphorus must be considered in efforts to solve the dead-zone problem.

Giving the Land a Kick

When it became clear that nutrients cause the rise of dead zones, scientists next examined the possible sources of the nutrients. This chapter argues the biggest source today is agriculture. The expansion of the Gulf of Mexico dead zone directly follows the huge increase in agricultural productivity, especially for corn. Yields increased over six times since 1930 in part because farmers used more fertilizer, “to give the land a kick.” As the chapter explains, Nancy Rabalais and Gene Turner found a direct link between fertilizer use and nutrient levels in the Mississippi River. In spite of opposition from agribusinesses, their work led to the formation of a White House committee and passage of legislation to support work on the hypoxia problem. Agriculture is also the main source of nutrients feeding dead zones in other regions of the world. The chapter later points out that the biggest user of fertilizer is now China, where excessive nutrients have caused massive harmful algal blooms and other environmental problems.

What Happened in 1950?

This chapter discusses what happened around 1950 that led to the expansion of dead zones. For the Gulf of Mexico, there are many reasons to think the flow of the Mississippi River has changed since the days of Mark Twain, considering the construction of so many levees, dikes, floodways, spillways, weirs, and revetments. Rain-absorbing grasslands and forests have been replaced by asphalt, roof shingles, and other hydrophobic material that hasten rainwater to the Gulf. But the flow of the Mississippi has not changed enough to explain why the Gulf dead zone grew around 1950. As the chapter discusses, what did change was nutrients. It shows that concentrations doubled in the Mississippi River from the 1930s to the 1990s, which stimulated algal growth and production of organic material that eventually led to depletion of dissolved oxygen. In addition to creating dead zones, the increase in nutrients has stimulated harmful algal blooms, leading to fish kills and beach closings.

Dead Zones Discovered in Coastal Waters

This chapter describes the discovery of coastal dead zones, such as the Gulf of Mexico and Chesapeake Bay in North America and the Baltic and Black Seas in Europe. Gene Turner sailed out of Pascagoula, Mississippi, in the spring of 1975, on the first of seven cruises that led to the discovery of the Gulf of Mexico dead zone. In the Chesapeake Bay, an unlikely environmentalist, Charles Officer, sounded the alarm in 1984. The biggest dead zone, however, is the Baltic Sea. Even as early as 1969, ecologists feared hypoxia was turning the Baltic into a “biological desert.” The northwest shelf of the Black Sea turned hypoxic in the 1970s, which killed bottom-dwelling fish like goby and flounder. Many coastal regions around the world have low oxygen waters that devastate marine life and habitats. The early studies emphasized one or two of three ingredients—sewage, fresh water, and plant nutrients—thought to be essential in creating a dead zone. This chapter and Chapter 3 discuss these ingredients before revealing which is most important.

The Great Stinks

This chapter discusses one of the first dead zones, the River Thames near London in the 19th century. London used the river as a sewer to dispose of untreated human waste and garbage, causing oxygen to disappear and gut-wrenching odors to well up, shutting down the city in the summer of 1858, aka the Great Stink. The sewage also carried pathogens that contaminated drinking water. The chapter also points out that dead zones were common in other rivers near large cities, including the Delaware River south of Philadelphia. Wastewater treatment solved the problem, and oxygen has returned to the River Thames, the Delaware River, and many other urban rivers in rich countries. Also discussed is the fact that fish and other aquatic life have also returned, but not completely. Adequate dissolved oxygen is essential, but more is needed to make a habitat livable and to ensure the complete recovery of aquatic life.

Reviving Dead Zones

Oxygen has returned to some dead zones, but many problems remain. As this chapter explains, nutrient input from agriculture in some regions has decreased because farmers use buffer zones, cover crops, and precision agriculture. But voluntary efforts to minimize nutrient pollution aren’t enough. In Iowa, the Des Moines Water Works, led by a charismatic CEO, Bill Stowe, unsuccessfully sued to reduce nitrate leaching from local farms. The value of government action has been demonstrated in Denmark, whereas its absence has led to many environmental problems in China. The chapter argues that one solution is tied to human health and climate change: our diet. Eating less, especially eating less red meat, would be better for our health, and it would reduce nutrient pollution and abate climate change. Agriculture accounts for nearly a third of all greenhouse gas emissions. The chapter ends by suggesting that the successful bans against DDT and phosphorus detergents are among the reasons to be optimistic about solving the dead-zone problem.

Fish and Fisheries

As the cause of dead zones became understood, research was devoted to figuring out the impact of hypoxia on aquatic life. The Gulf of Mexico dead zone overlaps with the Fertile Fisheries Crescent that stretches from Alabama to Texas, home to a multibillion dollar seafood industry. The chapter argues that the effect of hypoxic waters on benthic invertebrates is clear, while the story for mobile species like fish is complicated. Sessile invertebrates on the bottom, food for many fish and other animals, are wiped out when dissolved oxygen disappears. This chapter explains that even when mobile organisms are able to swim away to oxygen-rich waters, they are concentrated into a smaller habitat where they are more easily caught by predators and fishers. In the Gulf, the effects of hypoxia on fisheries are difficult to separate from the response of the fishing industry and overfishing, but effects especially on shrimp fisheries have been documented. As the chapter summarizes, hypoxia has many other impacts on aquatic biota, including rearranging food webs and contributing to the rise of jellyfish in coastal waters. Even when fishing yields are not affected, dead zones can devastate aquatic life and habitats.

Dead Zones in the Oceans

As this chapter shows, the open oceans are also running out of dissolved oxygen as seen at Station Papa in the subarctic Pacific Ocean, thanks to work done on Canadian weather ships starting in the 1950s. Not only are areas of severe hypoxia, or oxygen minimum zones, expanding, but the level of dissolved oxygen in all oceans is decreasing. The open oceans are losing oxygen because of climate change. The warming of the oceans reduces the solubility of oxygen in water and stimulates oxygen use by respiring organisms. This chapter explores how climate change is also altering circulation and the mixing of oxygen into oxygen-poor waters. Even where oxygen remains above dead-zone levels, its depletion is another sign of how climate change is reshaping the biosphere. The expansion of low-oxygen water has shifted the habitats of fish and invertebrates, such as the giant squid, over thousands of miles, and has disrupted the nitrogen cycle of the entire biosphere. The chapter explains that because of oxygen depletion, biological production of the oceans may decline due to the loss of nitrogen, while release of a potent greenhouse gas (nitrous oxide) may increase.

Coastal Dead Zones in the Past

This chapter discusses the dead zones of coastal areas: specifically the Gulf of Mexico and Europe’s Adriatic and Baltic Seas. Scientists were able to eliminate sewage from the list of ingredients that make a dead zone in the Gulf of Mexico and other coastal regions, but there was still the possibility that the loss of oxygen was natural. It wasn’t clear when the dead zone rose in the Gulf and the Baltic Sea. Systematic monitoring of dissolved oxygen in the Gulf, led by Nancy Rabalais, once called “Queen of the Dead Zone,” started only in 1986, so scientists have had to use indirect ways to deduce oxygen levels in the past. Studies using foraminifera (“forams”) and other oxygen-sensitive indices found that the proliferation of the Gulf dead zone and others started around 1950. Both the Gulf and the Baltic experienced low oxygen levels in the 19th century or earlier, but hypoxia became more common and more extensive in the middle of the 20th century.