Ocean Dead Zones
How they form and why they are concerning
There are a lot of issues facing the oceans today, from climate change to plastic pollution to overfishing, but one issue that doesn’t get as much press are ocean “dead zones ”. Dead zones are basically aquatic environments (oceans, seas, lakes, or estuaries) that are devoid of oxygen — which results in the death of anything living in them. One would think that something this terrible would generate a lot of press coverage, but this is often not the case. Part of the reason for this is that dead zones are a local phenomenon, so if you do not live in near the mouth of a river where dead zones typically occur, you probably have never heard of them. It also doesn’t mean it is small, as the dead zone near the mouth of the Mississippi River can spread over hundreds of square miles and continuing to grow in size (Dybas 2005).
This obviously begs the question of what causes these ocean dead zones, and why are they getting worse? To start with, dead zones are closely related to a more well-known phenomenon — “red tides”. Red tides, or harmful algal blooms, are enormous overgrowths of toxin producing microalgae (single celled species of algae). Microalgae is always present in the water and forms the basis for all food chains in the ocean (similar to how plants form the basis for all food webs on land). The issue is that when conditions are right, microalgae can multiply out of control and form enormous blooms that cover miles of the ocean.
As previously mentioned, when these blooms of algae produce toxins that can directly harm plants and animals, we call them red tides. However, even when algae do not produce toxins they can still cause serious harm because they multiply quickly and once they have consumed all the nutrients in the water, they quickly die. Once dead, they fall to the sea floor, where bacteria feed on them. Bacteria breaking down dead organisms is a normal part of the cycle of life, and like nearly all metabolic processes, it requires oxygen. We tend to think of oxygen in the environment as constant and limitless, but this is not actually the case. In aquatic environments oxygen can run out quickly if the water is not being constantly mixed up , if the body of water is small, if there are not a lot of plants and algae producing oxygen in the area (such as during the night), or if the oxygen is being consumed at a fast enough rate.
For several reasons, the mouth of rivers during the warm months of the year are the perfect place for oxygen to be depleted. First, river mouths are an ideal place to cause algal blooms. Rivers naturally collect the nutrient runoff from land, and even in developed countries our storm drains generally go straight into the local river, carrying everything from fertilizer to motor oils. Lawn fertilizers are a major issue, since unlike large farms where fertilizer runoff is often monitored and regulated, there are few rules governing how much fertilizer we use to keep our lawns green. This is not to say that large agricultural operations do not contribute to the fertilizer runoff problem, but it is important to recognize that everyone with a lawn carries some of the blame here. Unfortunately, the nitrogen and phosphorous in fertilizer that makes plants grow is equally good at fertilizing algal blooms. Each individual lawn may not release much fertilizer after a rain, but collectively our fertilizer use is a major problem The Chesapeake Bay watershed alone covers parts of five states and has over 18 million people living on it. When you consider the cumulative effect of these millions of people washing their excess fertilizer into a small geographic area, it is unsurprising that issues may arise.
The second reason the mouths of rivers are the perfect place to deplete oxygen is due to fluid dynamics. Freshwater is less dense than saltwater, so when rivers run into the sea the light freshwater tends to sit above the denser salt water. This is even more likely to occur in the summer when the river is often much warmer than the ocean, since differences in water temperature further encourage stratification. As seen in the image below, the lack of mixing between the sediment rich river water and clearer ocean water is often visible to the naked eye.
The lack of water mixing is a problem because oxygen can only enter an aquatic ecosystem in two ways: (1) it can be absorbed from the air at the water surface, or (2) plants or algae in the water photosynthesize and release oxygen directly into the water column (you can see this if you visit a place with a healthy seagrass bed during the day, as oxygen bubbles are constantly being released from the seagrass). As mentioned earlier, the first process is often interrupted during the summer at the mouths of rivers since the warm river water and cooler ocean water often stratify. When algal blooms occur, the second process is also interrupted since the algae, combined with the silt from the river water, can block light from penetrating to plants or macroalgae (kelp) on the seafloor. Once the algal bloom consumes itself, the algae will sink to the seafloor and decompose, depleting the oxygen in the deeper saltwater. Given the size of some algal blooms, this can completely deplete oxygen across miles and miles of ocean within hours. When the oxygen runs out, anything that lives in the water that cannot breathe air — i.e. fish, shrimp, crabs, clams, and anything else with gills — will quickly suffocate and die.
Algal blooms are a natural phenomenon, and therefore so are the dead zones they produce. However, just because they are natural does not mean that our actions and decisions as humans are not making dead zones worse. The fertilizers we liberally add to our yards and gardens often get washed into the ocean, where they make large algal blooms more likely. We need to make a change. Part of this involves the esteem which we impart to a “perfect lawn”. Grass is not only a massive consumer of fertilizer, but also a massive consumer of water. If we could make the cultural change of replacing our grass lawns with native trees, shrubs, and other plants, it would not only mean less fertilizer running into our oceans but also less demand for water — particularly in places like the American Rockies and west coast of the United States where drought is now a constant issue.
We also need to do a better job with how we choose to develop land. Forests naturally do a great job holding soil and water, while condominium complexes, paved parking lots, and ploughed fields do exactly the opposite. When we cut down forests to develop the land for human use, we start a process of soil erosion and runoff. Sometimes the problem is obvious, such as in Ellicott City, Maryland, where developing the hills around the historic downtown resulted in an inability of the land to absorb water when it rains. The result was historic flooding that killed three people and destroyed the town twice in three years. The process can also be seen in our river shipping channels, which are being filled with eroded soil that comes from the land. As these previously deep rivers fill in with silt, they require multimillion dollar dredging projects to combat the issue and allow ships to continue to pass. This same silt is eventually carried out to the ocean, where it contributes to the problem of dead zones. In addition to helping create dead zones, this silt can bury sea grasses, coral, and mollusks that live on the sea floor– strangling and killing these foundational species in the aquatic food chain. If we do not address the ways in which our actions contribute to algal blooms and dead zones, the only result will be larger and larger swaths of ocean devoid of life. We cannot allow this to happen, since we as a species are far more dependent on the ocean than it is on us.
References:
Dybas CL. (2005) Dead Zones Spreading in World Oceans. BioScience 55(7): 552–557.
Karstensen J, Fiedler B, Schutte F, Brandt P, Kortzinger A, Fisher G, Zantopp R, Hahn J, Visbeck M, Wallace D. (2015). Open ocean dead zones in the tropical North Atlantic Ocean. Biogeosciences 12:2597–2605.
