365 Days of Climate Awareness 49 - Eutrophication

This is somewhat of a side glance, as this is not an issue of major climatic importance, but it is of great ecological importance. In the last post we looked briefly at plankton, an extremely large grouping of organisms which spans many different biological classifications ("taxa", from Greek (of course!) taxis = arrangement) including algae. A little more Greek for today: the term eutrophication comes from the roots "eu" = good, and "troph" = food.

The term refers to how surface water--rivers, ponds, lakes, the ocean--comes to have a large amount of dissolved artificial fertilizers in it, which leads to an algal bloom. These algae use up the fertilizer in their photosynthesis, and add oxygen to the water. However, after the fertilizer is exhausted, the algae die, and decompose. The bacteria which break down the algal tissues consume all of the oxygen, leaving the water hypoxic (low oxygen) or anoxic (no oxygen).

The two principal fertilizer components are phosphate (one phosphorus atom and four oxygen atoms, PO4) and nitrate (one nitrogen, three oxygen: NO₃), both very important for photosynthesis. When fertilizers are applied to farmland, lawns and gardens, some is carried away in runoff, and not absorbed by the ground. The nutrients in this runoff gather in rivers, ponds, lakes, and even the ocean, creating hyper-fertile environments (the goal in putting them down in the first place). Another large nutrient source is raw sewage.

These large algal blooms can lead to dead zones, where the large concentration of photosynthesizing algae at the surface block sunlight and sea-air gas exchange. The oxygen from the algae's photosynthesis is not all given to the water. Much of it is released into the atmosphere, so after the algae decompose, the water suffers a net loss in oxygen content. This occurs in a large zone, roughly the size of New Jersey, in the Gulf of Mexico. Every summer O₂ levels in the surface water drop below 2 mg/l. The pattern is repeated elsewhere around the world, notably in the North and Baltic seas, and the East China Sea.

On a widespread scale it is called "cultural eutrophication", as the process (and its opposite, meiotrophification, from Greek, mei = less) can also occur naturally. But the repeated, anthropogenic circumstance of overabundant nutrients in water is labeled cultural. There is currently no legal effort underway to limit the causes of this global problem which impacts fisheries and many other aspects of the ecosystems where it occurs.

Tomorrow: phytoplankton.

Be well!