Oregon State University

College of Earth, Ocean, and Atmospheric Sciences

Shell Budgets: Oyster reefs buffer acidic inputs to Chesapeake Bay

April 23, 2013

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."

Figure 1

Figure. Conceptual depiction of oyster reef structure. An oyster reef generally is underwater at high tide, with the top visible only at low tide, if at all. Very limited measurements of the internal physical and geochemical structure of oyster reefs have been made. Figure adapted from Hargis and Haven (1999).

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.

Chesapeake Bay

A Landsat mosaic of the Chesapeake Bay, which borders the states of Maryland, Virginia, and Delaware. Source: NASA Goddard Space Flight Center, Scientific Visualization Studio.

Chesapeake Bay

An invading population of Pacific oysters, Crassostrea gigas, in the Oosterschelde estuary, Netherlands taken on November 2011. These reefs are growing vertically approximately 0.5" per year and were first observed in this location in 1965. Although nearly all of the Chesapeake Bay oyster reefs are currently subtidal, they were once extensive navigation hazards when settlers first arrived in Chesapeake Bay. These reefs appeared to be once an important component of the Bay's alkalinity cycle, taking up alkalinity through shell formation and slowly releasing as shells degrade to buffer metabolic acids. This ecosystem function, similar to an electrical capacitor, is estimated to be about 5% of that prior to extensive harvesting in the Bay. Photo by Roger Mann.

Links

Journal article on this research, Ecosystem effects of shell aggregations and cycling in coastal waters: An example of Chesapeake Bay oyster reefs (2013) George G. Waldbusser, Eric N. Powell, and Roger Mann.

Chesapeake Bay Watershed Tour (NASA Goddard Space Flight Center animation, Scientific Visualization Studio)


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