Bet-settlin'-filter: Exponential pond scum growth?

July 25, 2006 6:36 PM

Mrs. Everichon is convinced that there is a law or principle (as in "Foo's Law") which describes the phenomenon wherein a pond can take weeks to become 50% covered in algae, but then the remaining 50% takes only a day.

This isn't about algae or ponds, just the phenomenon.

The only thing I know to describe this is geometric (or exponential) growth. Anyone?

posted by everichon to Science & Nature (9 answers total)

Sounds to me like a power law. Tons of power laws in action in biology linked on that wikipedia page.
posted by ChasFile at 6:38 PM on July 25, 2006

Best answer: The idea is called "The 29th Day" and it is summed up fairly well in here. It's basically a doubling problem.

It's better understood if you state it backwards. Imagine there is a lily pond, and it will fill with plants in 30 days. You also know that the number of plants doubles every day. On what day is the pond half-full?

The answer: the 29th day.
posted by adipocere at 6:41 PM on July 25, 2006 [1 favorite]

Sounds to me like a power law

Actually, the original poster is correct: this would be considered an example of an exponential growth phenomenon. Power law growth is slower than exponential.
posted by mhum at 7:17 PM on July 25, 2006

Pond (aka "Turn Over") Inversion:

"During late spring and summer your pond develops layers as a result of temperature and density differences. Rapid warming in the upper level of the pond results in less dense water, which has a lower oxygen holding capacity as well. Warmer water holds less oxygen than cooler water. However, with photosynthesis and oxygen exchange at the water surface - atmosphere interface - enough oxygen is provided for the minimum needs for the fish. The cooler layer of water in the bottom of the pond has the potential to hold more oxygen, but due to the lack of photosynthesis and the decomposition of organic matter there is actually less dissolved oxygen in the lower level compared to the top layer. When a turnover of the layers occurs due to strong winds, a rapid temperature change, or a inflow of a large amount of water, the cooler (less oxygenated) layer goes to the top and the warmer (more oxygenated) layer goes to the bottom."

posted by ericb at 7:27 PM on July 25, 2006

After turn-over/inversion occurs many ponds/lakes are treated with Bluestone (Copper Sulphate) or with Cutrine-Plus to rid it of the algae.
posted by ericb at 7:32 PM on July 25, 2006

To restate -- over time the water in a pond develops layers of differing temperatures. An outside force (i.e. "strong winds, a rapid temperature change, or a inflow of a large amount of water") can cause turnover/inversion to occur in an instant/in a flash, bringing algae to the surface "over night."
posted by ericb at 7:56 PM on July 25, 2006

The scenario is used as an analogy for the possible consequences should the risk of environmental collapse follow some sort of power law.

ie. There may come a point where we are faced with an exceedingly short time between recognizing the results of our actions (two days before the pond goes belly-up, it had only 25% duckweed and actually looked reasonably healthy and happy!) and trying to remedy our inattention (oops! we didn't recognize it was growing exponentially!)

We hope the power laws for our global problems are small ones...
posted by five fresh fish at 1:20 AM on July 26, 2006

See also: Tipping point.
posted by klangklangston at 9:05 AM on July 26, 2006

Response by poster: I marked "29th day" as best, on the grounds of it being a nice, succinct handle, and slightly more intriguing than "geometric growth". Thanks everybody!
posted by everichon at 9:22 AM on July 26, 2006