Science Articles

Breathless: The Nitrogen Story Continued

Fri, 25th Nov 2011

This time, the lid is lifted on the ocean dead zone...

Robinson Fulweiler

How long can you hold your breath - 30 seconds? 2 minutes? Enough to cross from one end of the pool to the other? The world record for breath holding is 19 minutes and 21 seconds... but don't try it at home - this was accomplished after breathing pure oxygen for ten minutes and doctors worry such feats might have serious health complications. For most of us, while we go about our land-based business, we think little about breathing. We notice that when we run too fast we breathe more heavily, that when we get too excited we breathe faster, or - if you are like me - when we get stressed sometimes we hold our breath. We take our breathing for granted and we don't typically worry that we might run out of oxygen. But if you lived in the ocean, this would be a real and, I am sad to say, ever increasing concern. There are two main reasons for this - one due to the physical properties of water, and the other due to human activities on land.Ocean

We don't think about running out of oxygen on land for good reasons. First, air holds more oxygen than water. In fact, a given volume of air contains approximately 40 times more oxygen than the same volume of seawater. The actual amount of oxygen (or any gas) dissolved in seawater also varies according to the temperature and salinity of the water. The warmer and saltier the water is, the less oxygen it can hold. In addition, water is harder to mix than air, so if winds or currents do not stir up the water then the supply of oxygen won't be rejuvenated from the overlying air. In turn, as organisms breathe the local oxygen concentration drops. This is exacerbated in summer when the sun warms the water surface, setting up a temperature-induced stratification (layering of the water column). In addition, in estuaries - the coastal zones where freshwater and seawater meet and mix - stratification develops as freshwater enters the sea. The freshwater is lighter and more buoyant, so it floats on top of the seawater like a lid. Together, freshwater and warming temperatures decrease mixing, so the oxygen concentration towards the bottom of the water column is rapidly depleted with no way to replenish it.

PhytoplanktonTypically, such low oxygen events caused by freshwater and warmer temperatures are transient and water column oxygen would be rejuvenated with the changing of the tide or the summer-afternoon southerly ocean winds. However, these days low oxygen conditions are found in many coastal waters and, alarmingly, are occurring at both an increased frequency and duration. If you were to look at a map of these low-oxygen hot spots (and there some good ones available, see the links below) you'd notice that most of these locations are adjacent to major population centres. This is because where there are humans there is nitrogen, which is the limiting nutrient in marine ecosystems. This means that the amount of nitrogen available in the environment will determine how much plant growth occurs. In the marine environment, nitrogen can stimulate the growth of various plants including phytoplankton (the microscopic grass of the sea), submerged aquatic vegetation (e.g. eel grass, turtle grass) and macroalgae. Like all things, too much nitrogen leads to excess plant growth most often seen as increased phytoplankton or macroalgae production. If you can remember those long-distant science lessons you might recall learning that plants photosynthesise and in doing so release oxygen, so you're probably wondering why it is that more plant growth could lead to lower oxygen conditions?

Well, at first, as the phytoplankton grow in the surface waters they transform sunlight and nutrients into minute green cells. Oxygen is produced and its concentration within the water column does initially increase. But, when they stop photosynthesising - either temporarily at night or permanently when they die - they stop creating oxygen. In time, their small, delicate bodies fall through the water column to the depths of the sea floor or the estuary bottom. Here, these nutrient-laden carbon-rich morsels are decomposed by hungry bacteria. As the bacteria decompose the dead phytoplankton they also consume oxygen. This ultimately leads to hypoxic (low oxygen) or anoxic (no oxygen) conditions. In some cases these hypoxic conditions last just the night; in other cases these hypoxic and even anoxic conditions persist in both space and time earning the name of "dead zones."

Perhaps the most infamous dead zone is the one located in the Gulf of Mexico. Gulf of MexicoThis dead zone has been linked to nitrogen fertiliser runoff from the agricultural basin in the United States. Owing to heavy flooding this year, the Gulf of Mexico dead zone is predicted to be the largest yet recorded - somewhere between 22,000 and 24,000 square kilometers. That's almost the size of the Island of Sicily or the State of New Hampshire. Unfortunately, the prevalence of known dead zones is increasing, and since the 1960s their total number has doubled each decade (Diaz and Rosenberg 2008).

Of course, the ultimate question is does it matter? Why do we care that vast areas of the coastal ocean are experiencing low oxygen? And what does it mean for marine systems or for us? In Part III I will discuss the many negative consequences of dead zones. Meanwhile, please don't hold your breath...

References

Diaz, R. J., and R. Rosenberg. 2008. Spreading dead zones and consequences for marine ecosystems. Science 321: 926-929.

Maps of Dead Zones:

1. earthobservatory.nasa.gov/IOTD/view.php

2. www.cop.noaa.gov/stressors/extremeevents/hab/features/Diaz_08.aspx

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As the human population continues to grow, there is more and more pressure on farming production, and thus more fertilizer.  There are agricultural practices that can minimize crop runoff...  but, it can be expensive, and potentially not as productive as the current practices.  And, of course, some "better" practices might be more labor intensive.

Flooding?
Perhaps that is part of the problem, inundating farmland that ordinarily would not have nitrogen runoff.  This might be hard to prevent, except by choosing crops in flood zones that would require less added nitrogen, and perhaps stabilize the soil and trap sediments.

One of the problems with coastal "dead zones" is that the coastal areas are also a host for numerous types of aquatic life.  I.E.  the small fish, corals, and etc do better living in shallow water, than in the open ocean.  And, thus a dead zone can be devastating to the fisheries.

The waves naturally aerate the sea.  Is there a way to enhance this process?  Could it be as simple as adding rocky outcroppings to the beaches?  Or perhaps building aeration pumps driven by underwater currents.
CliffordK, Fri, 25th Nov 2011

It's serious.

Oceanic Dead Zones Continue to Spread (2008 Scientific American)

And "in the summer, northerly summer winds work together with the Earth's rotation to push oxygenated surface water offshore; this coastal water is replaced by low-oxygen but nutrient-rich waters from the depths of the continental shelf in a process known as upwelling. (See illustration.) Once this nutrient-rich water reaches the ocean's sunlit layers, it fertilizes blooms of phytoplankton. Resulting phytoplankton blooms feed the food chain and thereby help make the Pacific Northwest one of the nation's most productive fisheries. But the decomposition of unconsumed, sunken phytoplankton promotes the formation of deep pools of low-oxygen water.

Periods of upwelling-favorable northerly winds may be interrupted by relatively short periods of southerly winds during the summer and by longer periods during the fall. These southerly winds work together with the Earth's rotation to drive oxygenated surface waters back towards the shore and to drive low-oxygen bottom waters away from the shore in a process known as downwelling.  Periods of strong downwelling have traditionally occurred frequently enough to flush the low-oxygen pools from the continental shelf, and thereby prevent them from expanding all the way to the shore.

LEAVING NORMAL
But underwater surveys conducted by the research team of waters off the Pacific Northwest have identified the following new phenomenon:

    Pools of low-oxygen water have expanded from the continental shelf to near-shore waters off Oregon and Washington every summer since 2002; the close proximity of these dead zones to the shore had never been reported before that year.

    Coastal dead zones have been more hypoxic than the low-oxygen pools located on the continental shelf, with some coastal areas periodically completely stripped of their oxygen.

    Areas of hypoxia that have seasonally dotted the Pacific Northwest coast, "have been connected to one another by a ribbon of low-oxygen water that runs along the coastal sea floor," says Barth. So far, the most hypoxic year for the Pacific Northwest was 2006, when the research team discovered a dead zone off Newport, Oregon that sprawled over almost 1,200 square miles, and pressed so close to the shore that "a baseball hit from Highway 101 during the summer could land in it," says Barth. Covering up to 80 percent of the water column and lasting for an unusually long time (four months), "this dead zone transformed a teeming habitat into a fish-free zone that was carpeted with dead crabs, worms, severely stressed anemones and sea stars, and what looked like potentially noxious bacterial mats," says Barth." Is Climate Change Suffocating Our Seas?


yor_on, Sat, 26th Nov 2011

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