Estuaries, oysters, and ocean acidification

What can a bottle of smelly marsh water and an oyster superglued to a plate tell us about ocean acidification?

As part of ongoing ocean acidification research focused on estuaries and near shore environments, Whitman Miller, Trace Element Lab scientist Fritz Riedel, and I are trying to find out the answer to that very question. In the last two centuries, atmospheric concentrations of carbon dioxide (CO2) have risen from 280 to 380 microatmospheres (µatm), a unit essentially equivalent to parts per million by volume. One third of this anthropogenic CO2 is absorbed by the earth’s oceans, which significantly lowers the pH of the oceans and alters carbonate chemistry in their surface waters, a phenomenon called ocean acidification: as the partial pressure of CO2 (pCO2) increases, pH decreases. This shift in pH lowers the availability of carbonate ions in the water column, ions used by calcifying creatures like corals, pteropods, and oysters to build their shells, making it more difficult for them to maintain the integrity of their structures. Most acidification research has been focused on open oceans, with little attention paid to lower salinity estuaries and temperate near shore environments. But estuaries and coastal ecosystems are incredibly complex, both biologically productive and economically important; yet their shallow waters, and lower salinity and alkalinity make them more susceptible to pH changes than open ocean environments. How do we attempt to study the effects of ocean acidification on biota in such a complex and diverse environment? That’s where water chemistry and oysters on plates come into play.

Complementing our ocean acidification research in the lab on the Eastern Oyster and Asian Oyster larvae (Crassostrea virginica and Crassostrea ariakensis respectively) in estuarine waters, we are now conducting pilot field studies in the Rhode River with Eastern Oyster spat. We have chosen three study sites: the Smithsonian Environmental Research Center’s (SERC) boat dock, the Kirkpatrick Marsh weir, and the river basin just outside Kirkpatrick Marsh. Our preliminary carbonate chemistry data show that our sites are very different from one another despite their close relative proximity, with the weir experiencing the most dramatic shifts in pH and pCO2 levels; pCO2 levels above what occur at the SERC dock and well above what occurs in the atmosphere and most surface waters of the open ocean. The diurnal and tidal cycles heavily influence the water chemistry of our study sites, as phytoplankton and marsh plants alternatively photosynthesize and respire, benthic microbes breathe, and as higher salinity waters from the ocean meet the freshwater rivers draining into the Bay. For organisms to survive in these waters, they must be able to withstand dramatic changes in carbonate chemistry and thus pH on a daily basis, as well as extreme chemical variation between habitats.

In September 2010, several volunteers helped us superglue tiny oyster spat onto pvc (Poly-vinyl chloride plastic) plates, which we then attached to floating cages. We aim to track the growth and survivorship of these spat through time in order to learn what levels and how much variation in pCO2 our Eastern Oyster can endure. Along with the chemical data we have collected so far, we are also periodically taking snapshots of all the oyster plates in order to analyze growth, and we will take subsamples of our oysters to do stable isotope analyses of their shells. So far the results have been visually impressive; even without doing formal growth analyses it is clear that the dock oysters have grown much larger than their oyster brethren at the weir oysters, with the basin oysters falling somewhere in between.

We ultimately hope to understand how pCO2 affects population dynamics and ecology of oysters and other species, and to establish a baseline to compare the effects of rising levels of atmospheric pCO2 on our estuaries and coastal ecosystems. How will elevated levels of pCO2 affect recruitment of native species and biological invasions? How will it affect fisheries and restoration efforts of oyster reefs? The beginnings of an answer may just lie with a bottle of smelly marsh water, and oysters superglued to plates.

Amanda Reynolds, Smithsonian Environmental Research Center, March 2011. Article.

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