Small Underwater Changes May Be Big Deal for Life Everywhere

by Bill Chameides | November 12th, 2009
posted by Erica Rowell (Editor)

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Green plants such as phytoplankton are essential to life on Earth. Is air pollution changing this basic building block? And if so, how? (NOAA)

Air pollution is causing a fundamental change in the way lakes work.

Virtually all life on Earth depends on green plants. Through photosynthesis, those plants produce the organic matter we eat and burn for the energy we need to do everything we do, and make up the stuff of our bodies. In lakes and oceans, those green plants for the most part are microscopic, floating organisms called phytoplankton.

The rate of photosynthesis is typically limited by the availability of nutrients. That’s why farmers add fertilizers to the soil to enhance crop production. Most commonly, the limiting nutrient is either:

  • nitrogen (N), as is often the case in terrestrial ecosystems including lakes, or
  • phosphorus (P), as is often the case in the ocean (in some remote ocean areas it’s iron (Fe)).

However, a paper published in Science last week suggests that that P factor might be changing in lakes around the world.

N Cycle A-Changin’

The Earth and its life systems are supported by chemical cycles — technically, biogeochemical cycles — that maintain the balance of the key elements needed to sustain life in sufficient abundance. Probably the most familiar cycle is the carbon (C) cycle, which

  • transforms carbon dioxide (CO2) and water via photosynthesis into the oxygen and organic material we respiring organisms need, then
  • transforms the oxygen and organic material via respiration back into the CO2 that green plants need.

The element N has its own biogeochemical cycle. In nature, atmospheric, molecular nitrogen (N2) is converted (or fixed) into reactive nitrogen compounds such as ammonia and nitrate by lightning, fires, and nitrogen-fixing microorganisms like bacteria.

These reactive forms of nitrogen are used by us living creatures and plants to form the proteins that are one of the basic building blocks of our bodies. Eventually, the reactive nitrogen is converted back into N2 (as well as the greenhouse gas nitrous oxide, N2O) by denitrifying bacteria, closing the cycle.

Oh, Those Cycle-Tinkering Humans

After the C cycle, the cycle that’s being most significantly upset by human activity is arguably the N cycle. By producing fertilizers and burning fossil fuels, we produce, or fix, more reactive nitrogen compounds than occurs naturally. Since 1970, we have more than doubled production of reactive nitrogen in the biosphere.

Thus, there’s a lot of reactive nitrogen kicking around, and the consequences have been significant. Some examples:

  • we can thank all that reactive nitrogen for the smog that plagues our atmospheric and bodily airways (nitrogen oxides, a precursor for smog);
  • high concentrations of reactive nitrogen in the form of nitric acid can lead to acid rain; and
  • enhanced production of nitrous oxide is fast becoming a leading contributor to ozone depletion — and is a significant contributor to global warming.

Air Pollution Affecting Our Diet?

Now, James Elser from Arizona State University and co-authors have identified another fundamental change caused by our perturbation of the N cycle.

On the basis of data from more than 2,000 remote lakes located in Colorado, Norway, and Sweden, the authors found lakes with high rates of atmospheric N deposition had significantly elevated surface-water concentrations of reactive nitrogen compared to lakes with low deposition rates. Additionally, their results showed that P rather than N became the dominant nutrient limiting photosynthesis in phytoplankton in the lakes with higher N deposition. Significantly, the switch was not limited to lakes in close proximity to sources of air pollution, but those in remote locations.

This is an interesting finding in a couple of ways that may have ramifications for the food web.

By favoring those phytoplankton that are more competitive under P-limiting conditions (compared to those dominant under N-limiting conditions), the basis of the food web is changed — and, quite possibly, not for the better. P-limited phytoplankton are believed to be less nutritious than N-limited species, so the abundance of the former could weaken the food web at higher levels.

It’s odd to think that our fossil-fuel intensive lifestyle, so influenced by a diet largely composed of highly processed, nutrient-light food, may be forcing a similar choice at the bottom of the food web. We can call that the “junk-food cycle.”

filed under: ecosystems, lakes, pollution
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