Sunday, August 9, 2015

Do coral reef ecosystems need fresh water?

Frank Kovalchek's photo of water flowing to the ocean at Oheo
The answer is obvious: No.  Nevertheless, it is a fairly commonly held belief on Maui. (I don't know about elsewhere.)

It came up, briefly, in the First Friday in Wailuku webcast that Jason Schwartz and I do (, usually from about 5-7 p.m.). We interview passers-by, tell jokes (Jason does blonde jokes, I do dog-walks-into-a-bar jokes) and talk about issues that interest us: food poisoning in China, affordable housing on Maui (Jason's big idea) and municipal water (my big idea).

People interested in restoring normal stream flows in east Maui rivers (that would be Steve, our technical master, who offers an unseen commentary as we talk) sometimes assert that fresh runoff is necessary to maintain the health of the offshore ecosystem.

Where this idea came from, I don't know, but for sure it doesn't argue for restoring stream flows. (There are other, better arguments for that.)

The reasons that it doesn't arise from natural phenomena that everybody on Maui experiences but few understand quantitatively. An immense amount of rain falls on east Maui, but about 94% falls on just 6 days out of  typical year.

These would be a couple of 3-day tropical storms, which drop several feet of water. Some soaks into the enormous fresh aquifer but most runs into the ocean.

Nahiku Stream ordinarily runs around 10 million gallons a day, or it would if East Maui Irrigation didn't divert most of it. Nahiku is one of the biggest rivers in east Maui but there are three dozen other rivers.

In a storm, the flow at Nahiku has been measured as high as 9 billion gallons per day.

EMI's ditches can carry off, at most, 200 million gallons per day, so Nahiku all by itself drops 50 times as much water into the ocean as can be diverted to central Maui.

Fresh water is poisonous to coral; that's why entrances through the reefs are at stream mouths.

But the dose makes the poison. It's a big ocean. 9 billion gallons are dispersed quickly.

If it were true that offshore ecosystems required fresh water, then we wouldn't have thriving systems on the leeward side, where most streams are dry nearly 99% of the time and it is usual for 10 months to pass between rainfalls.

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