This month I’m departing from my usual practice of zeroing in on a specific emerging technology. Rather, I’m looking at a little-discussed consequence of several contemporary technologies as disparate as agricultural practices, sewerage treatment, and our reliance on fossil fuels.
     Hypoxic (low oxygen) dead zones occur in both fresh and saltwater bodies when nutrients such as nitrates and phosphates are introduced. Under normal conditions, these are present in minute (dilute) quantities. They are critical for plant(algae)growth. But because of their scarcity, algal abundance is usually held in check. Increasing the availability of dissolved nitrates and phosphates always increases the growth of these waterborne plants.
     Lots of members of the food chain feed on algae. From zooplankton to filter feeders like oysters, mussels, and clams. Even certain vertebrate fish rely on it as a food source. So, what’s the problem?
     Algal growth is so rapid, herbivores can’t keep up. All that excess algae sinks to the bottom of the water body where it inevitably dies, becoming lunch for sediment-dwelling microbes. Dissolved oxygen is used up, replaced by the byproduct of respiration, CO2, and eventually methane. All those zooplankton, filter feeders, and vertebrates in the ensuing dead zone suffocate, adding their biomass to the microbial feeding frenzy occurring on the seafloor or lakebed. This vicious cycle continues as long as the unnatural nutrient levels persist.
     There are two primary sources of excess nutrients: agriculture and human sewerage—both raw AND treated. As a child growing up in Seattle, I witnessed the eutrophication (the formal name of the process of explosive nutrient-fed algal growth and resultant decline of free oxygen) of Lake Washington. Fish kills were routine. As a result, a regional entity was created—METRO—that built sewerage treatment plants and a vast network of sanitary sewer lines to carry sewerage from homes and apartments to those plants. Treated effluent was discharged into Puget Sound, where strong currents  diluted the nutrient load, avoiding dead zones.
     As an adult, fish kills became commonplace in a remote, long and narrow arm of Puget Sound called Hood Canal. The south half of Hood Canal was shallow and experienced little of the tidal currents seen farther north. The surrounding communities, though rural, grew and were numerous. All relied on septic systems. While well-maintained septic systems prevented bacterial contamination, the dissolved nitrates and phosphates leached into Hood Canal. The dead zone became an annual summer event. Fish kills were common. The famous oyster beds were affected.
     Most Americans are familiar with the summer dead zones that annually develop around the perimeter of the northern Gulf of Mexico sea floor. The largest nutrient contributor is the Mississippi River. Its muddy waters carry far more than eroded sediment. Roughly 350 thousand square miles of the Mississippi River’s 1.245 million square mile basin is agricultural land. Not to single out agriculture, the effluent of some 20 million people finds its way into the river as well. The result? The Mississippi transports some 1.6 million metric tons of nitrogen and 145 thousand metric tons of phosphorus into the Gulf of Mexico per year.
     OK, if hypoxic dead zones are common knowledge, what’s the “little-discussed consequence?” I’m glad you asked. In addition to higher CO2 and methane levels in hypoxic dead zones another gas is also generated by anaerobic seafloor microbes—H2S—hydrogen sulfide.
     Those who have visited mud flats at low tide are familiar with the ubiquitous rotten egg smell given off by exposed tidal sediments. In the aquatic environments of dead zones, the lower layers of water can become saturated with H2S.
     Hydrogen sulfide is quite toxic to oxygen-breathing organisms. Prolonged human exposure results in severe headaches followed by confusion, unconsciousness—and if the victim is not removed to fresh air—death.
     If nutrient loads are constant, rising water temperatures decrease free oxygen in dead zones, increasing hydrogen sulfide concentrations. Under certain conditions, H2S will migrate to the surface, where it is released into the atmosphere. These events are called hydrogen sulfide eruptions. Extensive H2S eruptions occur in the southeast Atlantic Ocean off the coast of Namibia. As water in the Gulf of Mexico continues to warm, H2S eruptions may arise there as well.
     Scientists have implicated high marine and atmospheric H2S levels for the Great Dying extinction event at the end of the Permian Era 250 million years ago. 96% of all marine species and 70% of land species went extinct. Runaway climate warming is understood to be the proximate cause of the high atmospheric H2S concentrations. Many scientists today are concerned that if global temperatures increase five degrees Celsius a similar H2S mass extinction event could be triggered.
     I don’t say this to be alarmist. In fact, I believe human-caused climate change will be arrested and reversed before humanity faces a similar catastrophe. The climate at the end of the Permian was ten degrees Celsius warmer than today—hellishly hot, even at the poles. But I am convinced of an increase in H2S eruptions within the Gulf of Mexico, and I’m willing to include the phenomenon in my fifth book.
     In the meantime, federal and state authorities are regulating Mississippi River nutrient levels with the ultimate goal of ending the seasonal dead zones. But while that effort proceeds, I’m feverishly writing books 4 and 5. With a little luck and persistence, book 4 should be published late this summer. Till then, keep cool—and happy reading.

For further Reading

https://oceanservice.noaa.gov/facts/deadzone.html#:~:text=%22Dead%20zone%22%20is%20a%20more,of%20oxygen%20in%20the%20water.&text=Less%20oxygen%20dissolved%20in%20the,as%20fish%2C%20leave%20the%20area
https://education.nationalgeographic.org/resource/dead-zone
https://www.epa.gov/sites/default/files/2015-10/documents/htf_report_to_congress_final_-_10.1.15.pdf
https://earthobservatory.nasa.gov/images/13155/hydrogen-sulfide-eruptions-along-the-coast-of-namibia#:~:text=These%20bacteria%20emit%20hydrogen%20sulfide,to%20precipitate%20into%20the%20ocean
https://www.nationalgeographic.com/science/article/permian-extinction#:~:text=About%20250%20million%20years%20ago,species%20in%20the%20seas%20survived
https://www.space.com/permian-extinction-microbes-hydrogen-sulfide.html