An article looks at the positive benefits of ash to the nutrient cycle in the ocean... There are a few good papers on this, including Frogner, et al 2001 in Geology.

"By now, you may know that airborne ash from the volcano that erupted through Iceland's Eyjafjallajökull glacier this past Wednesday is like floating Kryptonite for airplanes. The ash clogs engines and threatens aeronautic safety. And in the right concentrations, it can be bad for both people and animals when it lands.

The falling cinders—a mix of crushed rock, glass and some toxic chemicals—are considered generally bad for the body. So what happens when this particulate cloud of doom settles on the surface of the ocean? You might picture it creating a floating blanket of poison that would give the people at Ocean Conservancy cardiacs. But does it?

A plume of volcanic ash rises into the atmosphere from a crater under about 656 feet ice at the Eyjafjallajokull glacier in southern Iceland April 14, 2010. A huge ash cloud from the Icelandic volcano turned the skies of northern Europe into a no-fly zone on Thursday, stranding hundreds of thousands of passengers. Photo: Olafur Eggertsson/Reuters

Well, according to several scientific studies, the ash could actually be good for oceans.