Are Oysters the Solution to Pollution?

New study shows restored oyster reef worth its weight in nutrients

“Our study showed that a successfully restored oyster reef can remove significant levels of nutrients from the water column,”...We found that annual denitrification rates at the restored site were enhanced by an order of magnitude and that rates in August were among the highest ever recorded for an aquatic system...

Denitrification—the conversion of reactive nitrogen compounds back into the inert nitrogen gas that makes up most of our atmosphere—is a holy grail of restoration science. In its pure, gaseous form, nitrogen contributes little or nothing to the over-fertilization that plagues coastal waters worldwide.

Denitrification is a bacterial process that provides energy for a certain class of bacteria, called, get this, denitrifiers.  It most often occurs in oxygen depleted environments, say the smelly muck that accumulates around oysters as a result of their filtering out plankton, and building mounds of feces and pseudofeces (don't ask) around them.

Uptake into the tissues and shells of living organisms provides another way to remove nitrogen compounds from the water. The team’s research shows that the restored reef provided habitat for almost 25,000 bottom-dwelling organisms per square meter compared to just over 2,000 organisms per square meter at the control site. Nitrogen in shells of live oysters and mussels accounted for 47% of all nitrogen incorporated into living organisms at the site.

Wow, that much! I'd truly surprised.  It doesn't happen that often.

Kellogg says incorporation into shell is particularly important because “the shells of oysters and mussels can persist long after the organisms die, removing nutrients from the water for years, decades, or even centuries if the shells become buried.” Nutrients in soft tissues, on the other hand, can be quickly recycled back into the water, once again contributing to over-fertilization.

This is a factor I hadn't thought about before, but which makes perfect sense in hindsight.  Oyster and mussel shells contain a small but distinctive amount of both phosphorus and nitrogen.  Phosphorus as hydroxy apatite, a mineral which helps strengthens the shell (it's also an important mineral in teeth), and nitrogen as proteins which are incorporated into the structure of the shell as it is laid down and help guide its form.

Putting their findings into a management context, the team used their results from the Choptank reef to estimate the scale of oyster restoration needed to meet the EPA’s new nitrogen restrictions for this Chesapeake Bay tributary. “We estimate that more than 475,000 pounds of nitrogen need to be removed from the Choptank to meet current restrictions,” says Kellogg. “According to our calculations, this could be done by restoring high-density oyster reefs to 23% of the river’s available bottom.”

While I doubt 23% of the bottom is suitable oyster habitat, that is impressive, and oysters certainly could make a big difference.  So why don't we stop harvesting them until we're sure they're really on the rebound?