Dead Zones: The True Oceanic Horror Story
Scientific American has an article about dead zones, those regions of ocean bottom that become too depleted of oxygen to support many common forms of living organisms. Worldwide, the article reports that over 400 such regions have been identified. A handy graphic depicts the Caribbean, Gulf of Mexico, and US Eastern seaboard, and what it shows comes as no surprise; the US coastline is dotted with dead zones, while they are far less common in the waters of countries with less agribusiness.
The basic issue concerns nitrogen, an element needed for robust plant growth and the basis of fertilizers used for agriculture. Nitrogen provided through fertilizers, though, does not stay put in the fields where it would work as intended. Nitrogen-rich runoff makes its way into streams and rivers, and thence to the ocean. There, it provides a boost in available resources to algae, encouraging blooms. Other microbes feast on the abundant algae and deplete the available oxygen as they do so. Mobile animals may be able to escape an incipient dead zone, but many will not, and those animals unable to travel far — or at all, as various molluscs, barnacles, and other sessile organisms cannot leave their points of attachment — will die in place when the oxygen drops below the concentration they need for basic metabolism. This triggers another round of growth, this time of anaerobic bacteria that feast on organic matter and produce toxic byproducts like hydrogen sulfide gas. The combination of low to non-existent oxygen concentration and toxic chemicals yields the dead zones that are the subject of the linked article.
Some of these dead zones are fluctuating, coming and going with seasonal variations in nitrogen inflow, currents, and temperature. But others appear to be (ironically) long-lived, as in the year-round dead zone that is the Baltic Sea. The article notes that even when nitrogen influx is terminated, enough may already be sequestered in a region that recovery may take a long time to happen, and may require large-scale alterations in currents, such as those accompanying powerful hurricanes, in order to convert a dead zone back into an area that is productive for oxygen-requiring life.
I’m saddened by the number of dead zones surrounding the state of Florida in particular. While growing up there, I became interested in science largely through appreciation of the natural history of the marine environment. While many people got their sunburns and frolicked on Bradenton Beach, I would be a mile to the south across the Longboat Key inlet snorkeling in the shallow bay waters of Beer Can Island. There were several diverse ecological regimes to be seen within a few acres of shallow-water habitat, with characteristic flora and fauna for each. High current flow and somewhat deeper water next the rocks and concrete of an old bridge piling gave a distinctly reef-like habitat with small serranid fish, mantis shrimp, and highhats. There were solitary corals and soft corals as well there. A hundred yards to the south, though, water depths of three to five feet and modest tidal fluxes yielded abundant algae (“seaweed”) with populations of blue crab, horse conch, sunray venus, pipefish, seahorses, marginella, bubble shells, and many tiny gastropod species living on the algae. Two hundred yards to the east, and the very shallow water and high temperature gave a different environment, with pink algal films over mud flats with lugworms, ceriths, and nassa snails. I’m going by memory of times long gone by, and I’m sure things have changed a lot there since I was a young man getting acquainted with a world of wonders. I’d hate to see the region devoid of multicellular animals and plants, coated in noxious slime from anaerobic microbes. But if not the particular place of my acquaintance, the map shows that many other formerly vital and productive places now must be choked off and unavailable as nurseries for the young of many familiar species. Worse than that, these places are now death traps for planktonic juveniles who happen to drop too low in the water column where the oxygen has been sucked out of the system and toxic by-products of anaerobic life have accumulated.
There’s an obvious tie-in to discussion of alternative fuels here. Biofuel initiatives will come with costs that don’t have neat entries in accounting ledgers. If we continue with using a grain crop like corn for biofuel production, we certainly cannot expect to reduce fertilizer use dramatically. Instead, we are likely to see a total increase in fertilizer use despite measures noted in the article, such as greater efficiency of crop production via genetically modified varieties. That would be because a dependence on biofuel production will require greater land utilization in order to meet fuel consumption demands. Besides that, agricultural land used for biofuel crops is not available for food crops, and we are still going to want to eat and to have surplus food for export. These issues indicate that the alternative energy discussion has to be broadened to encompass the impacts that, quite literally, lie downstream.
