Then there were two. Three days after the Waterfront Partnership towed its patchwork floating wetland to a mooring off the World Trade Center (seen below), the National Aquarium put a second one in the Inner Harbor between Piers 3 & 4.

This one looks a bit different.  The aquarium's is almost kidney shaped and solid, compared with the checkerboard array of the earlier floating wetland. Grasses and plants poke out of a floating bed of plastic mesh that looks a bit like a Brillo pad.

But the intent is the same - to see how these small patches of vegetation might soak up some of the nutrients fouling the water, and provide shelter and living space for fish and other aquatic critters.

The platform of the 200-square-foot wetland was made in Utah, of all places, then shipped in pieces across country. 

Aquarium staff assembled it Wednesday morning and then, with the help of local student volunteers, planted a mix of native salt marsh plants: smooth cordgrass, softstem bulrush, common three-square, seaside goldenrod and hibiscus. 

The porous plastic base will allow water to reach the roots, and conversely let the roots grow down and out to the water, explained David Nemerson, a conservation biologist with the aquarium.

Scientists with the University of Maryland plan to monitor the two wetlands to see what impact they may have on water quality and what kind of aquatic life they support.

"We're already learning things we didn't expect," said Dan Terlizzi, a UM water quality specialist who's based at the Columbus Center nearby.  He said tests of the wetland medium have found it quickly "colonizes" with bacteria, algae, worms and other tiny aquatic creatures.  That unseen life below the surface can soak up as much or more nutrients than the wetland vegetation poking out of the top.

These two wetlands are among the first steps of a campaign launched earlier this year by the Waterfront Partnership to make the harbor swimmable and fishable by 2020. 

That's a tall order, especially for these tiny patches of green. Dissolved oxygen readings in the vicinity have been quite low this summer, making it a stressful environment for fish (though aquarium staff say they still see plenty in the water around their facility.) 

No one thinks these tiny swatches of floating wetlands can make much of a dent.  But advocates say the aim of thes wetlands is to try them out, then seek to expand them assuming they've proven themselves. 

And as we noted here earelier, their greater value may lie in getting visitors to the Inner Harbor to look more closely at the water, to look beyond the trash floating in it and maybe think about the harbor in a different way.

Click here to see more photos of the Aquarium's wetland coming together. 

(Top photo courtesy National Aquarium)