Dead Zones The Loss of Oxygen from Rivers, Lakes, Seas, and the Ocean

Rivers and oceans are easily neglected when it comes to pollution. Out of sight, out of mind and all that. Except that the oceans do not forget. Of all the water pollution problems, oxygen loss is probably one of the more abstract ones. Even the words used to describe it, hypoxia and anoxia, will be meaningless to those without a background in biology. In Dead Zones, marine scientist and microbiologist David L. Kirchman provides a general introduction to the problem of oxygen loss and why it matters.

I thought you could not tell me anything new on this topic. After all, for my PhD, I spent five years in Finland studying how fish behaviour in the Baltic Sea is affected by eutrophication, the pollution caused by excess nutrients. The Baltic is one of several badly affected water bodies in the world due to the large land area that drains into it and the narrow connection with the North Sea. However, my studies focused on decreased underwater visibility due to algal blooms rather than oxygen loss. Dead Zones was thus a welcome surprise and refresher, showing that even I had only an incomplete picture of the problem.

So what are dead zones? They are areas of fresh or marine water that have low (hypoxia) or virtually no (anoxia) dissolved oxygen. As Kirchman clarifies in his prologue, the term is a bit dramatic. It does not mean that the whole water column is affected – often it is limited to the bottom layer. And they are not always completely dead – some organisms actually thrive here. But many creatures are sensitive to oxygen loss, leading to slower growth, fewer offspring, and poorer health. In extreme cases, it leads to the death of fish and bottom-dwelling invertebrates, reduced biodiversity, and reshuffled food webs.

Dead zones vary in size through time, influenced by weather and currents. But since about the 1950s, they have been steadily growing both in size and number, and we know the cause. Fed by excess nutrients, algae bloom. When they die, the bacteria decomposing them use up oxygen quicker than it can be refreshed. But before we get to this point in our understanding, the first half of the book takes the circuitous route through the history of their discovery.

Some of the first dead zones that were widely noticed were the Great Stinks. Not that long ago, many rivers running through bustling cities in Europe and the US received large loads of untreated sewage. The resulting bacterial feast would rapidly consume all the oxygen and then switch to sulfate metabolism, releasing hydrogen sulfide with its characteristic smell of rotten eggs. Wastewater treatment facilities brought this problem under control, with rivers recovering and fish returning.

Much harder to notice were the dead zones developing in coastal areas, such as the Gulf of Mexico or Chesapeake Bay, or inland seas, such as the Black Sea or the Baltic Sea. Kirchman takes you through their discovery in the 1980s and the research by Nancy Rabalais and others to address the question whether they have always been there? The answer required the meticulous collection of long time series of oxygen measurements and the study of sediment cores that record past oxygen levels. The conclusion is that, yes, they were a natural occurrence in the past, but the frequency and severity have increased, and it all centres on roughly 1950. How so? More nutrients, specifically nitrogen and phosphorus. Where did they come from? Artificial fertiliser used in agriculture. As discussed here, there have been naysayers, both in scientific circles and amongst farmers and industry, but the evidence is pretty convincing by now.

Although Kirchman does not explicitly mention it, the 1950s marks a transition period dubbed the Great Acceleration during which our impact on the environment boomed. And the increase in nitrogen and phosphorus release is one of many indicators used by scientists to define the start of the Anthropocene. Two key figures Kirchman introduces here are Fritz Haber and Karl Bosch. The former developed a process to cheaply and efficiently fixate nitrogen, the latter scaled it up for the industrial production of explosives and artificial fertiliser. The Haber-Bosch process was one factor facilitating the human population explosion driving the Great Acceleration. As Kirchman mentions, "Depending on your diet, about half of the nitrogen in your body came from a factory using the Haber-Bosch process" (p. 96), and Vaclav Smil is quoted as writing that, without it, Earth would probably have only half the human population it has now.

Having arrived at the status quo, the second half of Dead Zones explains why these problems have been so difficult to tackle. One factor is the inherent complexity of the system. Which nutrient is the limiting one runs the full gamut from nitrogen in marine waters (mostly) to phosphorus in lakes – and sometimes neither. There is no one-size-fits-all solution. Similarly, as Kirchman explains, fish kills are not a problem suitable for a bumper sticker slogan to rally behind: "Dead Zones Kill Fish and Fishery Jobs" (p. 128). The reality is more nuanced. Oxygen depletion can, for example, boost fish landings when fish flee hypoxic zones and become easier to catch. And this leads to the second factor – all these complexities make it very easy for those who stand to lose from tougher environmental regulations to call for more research, and for legislators to hesitate to implement anything. However, as Kirchman points out: "[...] we cannot ignore dead zones just because the effect of hypoxia is complex" (p. 133). To that end, he offers a range of solutions, including things that you, the reader, can do.

Dissolved oxygen levels in water was never going to win the prize for the sexiest topic. Given this inherent handicap, Kirchman's conversational tone makes Dead Zones nicely accessible. However, I do feel that the overall structure and clarity suffer somewhat, although I cannot quite put my finger on the why. The narrative ricochets between different topics and some things are only touched upon (e.g. the rise of jellyfish, or the release of neurotoxins during certain algal blooms). I did enjoy the historical sketches, they are Kirchman's forte. At only 172 pages, this book is not a thorough overview but more a general introduction. Since these topics are normally discussed in scientific journals, reports, academic monographs, and edited collections, it is an introduction that is long overdue.