Estuaries are important players in the global cycle of elements. They are the meeting point for the land, atmosphere, and ocean, and are a significant point of exchange among these domains.
Wade McGillis and his doctoral student, Nadine Els, of Lamont Doherty Earth Observatory are particularly interested in the exchange of gases that occur at this interface.
On a global level, coastal waters are estimated to absorb carbon dioxide and thus mitigate the impacts of global warming. On a local level, animals and plants depend on sources of oxygen and carbon dioxide for survival. It is important to understand the specific mechanisms of gas exchange in order to better understand these local and global processes.
In August of 2009, McGillis and Els measured aqueous carbon dioxide concentrations in the Hudson River Estuary at the Piermont Pier HRECOS station using a dissolved gas equilibrator system. By collecting these measurements at the HRECOS station, they were provided a rich context of water quality and meteorological measurements to better understand the drivers of gas exchange in this system.
The data showed a striking relationship between dissolved oxygen (DO) and carbon dioxide (CO2) concentrations. Twice daily, CO2 levels spiked and DO levels plummeted in synchrony. By overlaying the tidal cycle on this data, they found that these events occurred in response to the high tide. Spiking CO2 and low DO concentrations occurred within 1.5 hours after each high tide.
The most likely explanation is that tidal waters were mixing with the Piermont Marsh, just to the south of the HRECOS station. The salt marsh is rich with decaying plant materials. The microbes breaking down these materials consume DO and release CO2 in large amounts. When tidal waters mix with the salt marsh, some of these gases are transferred into the estuary and we see an immediate impact on CO2 and DO concentrations in the main stem of the river.
Once in the river's main stem, the fate of the gases differ between night and day. During daylight hours, photosynthesizing plants and phytoplankton consume the excess CO2 and produce DO. As a result, DO and CO2 concentrations rapidly returned to prior levels. At night, however, high CO2 and low DO concentrations persisted much longer because photosynthesis is not occurring.
This work has both local and global implications. Locally, the spiking CO2 concentrations impact rates of photosynthesis off of Piermont Pier. Also, the dipping dissolved oxygen concentrations may have a negative impact on aquatic life.
Globally, the transfer of CO2 from the marsh to the estuary may represent a net sink for atmospheric carbon. The plants in Piermont Marsh fix atmospheric carbon through photosynthesis. The microbes breaking down dead plant material, release this carbon which is then transferred to the estuary by the tidal cycle. In this way, atmospheric carbon is transferred from the air to the estuary. If this carbon is transported to the open ocean, it would represent a net sink of atmospheric carbon. This mechanism is referred to as the plant CO2 pump and some scientists feel it could be an important mechanism to mitigate the impacts of anthropogenic CO2 emissions (Jahnke, 2008).
McGillis and Els travel the world in order to better assess global cycles of gas exchange. Currently, they are in the Gulf of Mexico examining the impacts of the oil spill on gas exchange in this system. We look forward to their return to learn more about the role the Hudson River plays in these global cycles.
How To Observe CO2 Exchange Using the HRECOS System:
HRECOS can observe the impact of the salt marsh even without measuring CO2. Carbon dioxide concentrations were tightly and inversely correlated with pH in this study(correlation = -0.92). This is because carbon dioxide forms carbonic acid when it is mixed with water, thus lowering the pH. When CO2 spikes, pH levels drop. We can use this relationship to identify peaks in CO2 concentrations. Compare water depth with pH for any date in July of 2010 at Piermont Pier on the HRECOS live data page and you'll see a drop in pH within 1.5 hours of the high tide.
Jahnke, RA. 2008. MAYBE IT'S NOT JUST ABOUT AIR-WATER GAS EXCHANGE. Oceanography 21(4) p.42-43