Treating wastewater is expensive: to get rid of organic matter,
phosphates, and nitrogen compounds that farms and cities release
into rivers, you need to pump the water through costly complexes
of pumps, filters, and tanks. A tidal marsh, on the other hand,
does the same job free of charge. The polluted water fertilizes
plants and microbes, which in turn support a huge food web, and
by the time the water makes its way out of the marsh, it’s
scrubbed clean. One acre of tidal marsh performs about $2,800
worth of water purification every year. Multiply that by the 165
million acres of coastal wetlands on the planet, and you get a
tidy annual bill of $462 billion. That sort of figure doesn’t
often make its way into environmental decisions, but Robert
Costanza thinks it should. Every time we do something that
affects these ecosystems, we’re making a trade-off, but we don’t
realize it, says Costanza, an ecological economist from the
University of Maryland. Costanza is part of a growing movement
of researchers who a