When European settlers arrived on Chesapeake Bay, it was encrusted with a treasure trove of oysters and other bivalves. The living oyster reef and its stockpile of empty shells was voluminous enough to influence the water chemistry of the bay, says marine ecologist George Waldbusser and colleagues. Based on harvest records from the 17th century, he estimates that the oyster-impoverished bay of 2013 is running "at least 100 million bushels behind where it was before we started harvesting, in terms of shell budget."
Oysters eat microscopic phytoplankton, including algae, which the bay generally has overabundance of thanks to excess fertilizer runoff. Oysters are not just a tasty economic resource – they make Chesapeake Bay cleaner. The missing shells are a direct loss to oyster restoration, because oyster larvae are choosy about where they glue themselves down and start building their shells. They prefer other oyster shells as anchorage.
But in addition to providing habitat for future generations, oyster reefs appear to alter their local water chemistry. Like slow dissolving Tums in the belly of the estuary, disintegrating oyster shells are slow release capsules of calcium carbonate, an alkaline salt and a buffer against acidity. Seawater mixing in on the tide has a relatively high capacity to absorb acid inputs without a large change in pH. Fresh water flowing out to sea generally has a low buffering capacity, and is sensitive to acid sources, whether from human made point sources like coal plants or natural processes like the oysters' own respiration. Coastal estuaries, where the waters meet, are also where oysters tend to cluster.