22 Jul

Tracking acidification in the coastal zone


One of our goals is to understand an important aspect of climate change called “coastal acidification” and to demonstrate that this process is a bit different than “ocean acidification” occurring in the open oceans.

The process of “ocean acidification” occurs when excess carbon dioxide from the atmosphere enters the oceans to lower the pH and dramatically alter the carbonate chemistry.  This has been termed the “other CO2 problem” and, together with a recent two degree increase in ocean temperatures, is threatening ocean life.  For example, acidification prevents some shellfish from constructing calcium carbonate shells, and is devastating the west coast shellfish industry.  Calcification is also disrupted in many species of coral.  We’ve shown that it alters the metabolism of marine plants and fish grazing rates.  Acidification even hinders the ability of fish to smell and hear, and therefore to find prey, hide from predators, and communicate.  This is the natural consequence of having too much CO2 in the atmosphere.  Carbon dioxide levels are now over 400 ppm, up from 260 ppm at the dawn of the industrial revolution.  The oceans have taken up ~ 1/3 of this excess carbon, slowing climate change, but at a price.  The price is the acidification of the open oceans.

Here’s a description from one of our recent works: “Since the industrial revolution atmospheric carbon dioxide levels have increased from 280 to 400 ppm (parts per million), the highest levels occurring on our planet in 800,000 years (Doney et al. 2009; Sabine et al. 2004). Approximately one-third of the CO2 emitted from human activities has been absorbed by the oceans, slowing the rate of global climate change. However, this generates carbonic acid (H2CO3), resulting in an increase in the total H+ concentration (i.e., increasing seawater acidity and lower pH) and conversion of carbonate ions (CO32-) to bicarbonate ions (HCO3) in seawater. The establishment of high CO2/low pH conditions in blue waters is termed ‘ocean acidification’ (OA).  In the open oceans this process is relatively well understood. In the past 150 years, the oceans have become net CO2 sinks and the average ocean pH has dropped from 8.21 to 8.10 (Royal Society 2005). By the end of the century it is expected to fall another 0.3 to 0.4 units (Doney et al. 2009; Orr et al. 2005). This shift in ocean chemistry represents a 150% increase in hydrogen ions and a 50% decrease in levels of carbonate ions (CO32-) (Doney et al. 2009; Orr et al. 2005). OA lowers the availability of CO32-, and therefore the seawater saturation states with respect to several carbonate minerals (Ω), so that the formation and deposition of new CaCO3 minerals is reduced, and the dissolution of existing minerals is enhanced. This disrupts the growth of many calcifying organisms, including important species of shellfish, plankton, and corals, which struggle to form CaCO3 shells, skeletons, and tests.”

Here’s a good description of the problem, from the Pacific Marine Ecology Lab: http://www.pmel.noaa.gov/co2/story/Ocean+Acidification

And from National Geographic: http://ocean.nationalgeographic.com/ocean/critical-issues-ocean-acidification/

Acidification is also occurring along our coastlines, but the process is somewhat different.  We call it “coastal acidification”.  Here the excess carbon comes mostly from the land as run-off, via rivers, bays, and marshes.  Much of this results from land use practices related to agriculture, nutrient over-enrichment, development, deforestation, etc.  The amount of carbon input is much higher, and much more variable.  For example, the daily fluctuations in carbonate chemistry near coastal marshes can be a thousand times greater than the fluctuations recorded in the open ocean over a period of many years!

Here’s how we described in recently: “Within the Chesapeake Bay, and other estuaries, the process is more complex. The sources of carbon are different: for example, estuarine waters do not generally absorb CO2 from the atmosphere but rather as dissolved organic carbon (DOC) from the land. Powerful biological processes, e.g. respiration and decomposition (i.e., heterotrophy), convert DOC to inorganic carbon.  High levels of CO2 can also be generated from DOC by microbial respiration in anoxic bottom waters as a consequence of eutrophication (Wallace et al 2014; Melzner et al 2013). Other processes can be influential as well. For instance, in some areas acid sulfate soils or nitrogen deposition may contribute to acidification. In other areas, ocean mixing or coastal upwelling may deliver high CO2/low pH waters from the open ocean.  The Chesapeake, like most estuaries, is surrounded by terrestrial and intertidal environments which export massive amounts of organic carbon to the oceans via the “land–ocean continuum” (Cia, 2010). Historically, this terrestrial organic carbon flowed to the oceans where heterotrophy converted it CO2, which was in turn emitted to the atmosphere. However, due to human activities, this has changed in several ways: (1) the mass flow of organic carbon along this land–ocean continuum has increased dramatically and (2) the oceans act as carbon sinks, absorbing excess anthropogenic carbon from the atmosphere. In short, today’s estuaries import terrestrial organic carbon, convert it to inorganic carbon, release a fraction to the atmosphere as CO2, store some, and export the rest as organic carbon to the oceans.”

Both processes are driven by excess carbon.  But to study them, we must employ different strategies.  To work on open ocean systems one must be very precise and patient, to work in coastal systems one must become comfortable with constant variation in large data sets, and design experiments appropriately.

It can be a challenge to obtain the measurements needed to accurately characterize a coastal community – a tidal marsh or seagrass meadow, for example.  Modern technologies are helping us to monitor these systems at multiple locations at rates as high as one observation per second.

In the past we’ve been using an in situ real-time pCO2 monitor, developed by Dr. Whitman Miller (from the Smithsonian Environmental Research Center), to measure carbon dioxide concentrations in estuarine waters.

IMG_0439We have recently expanded our capabilities to monitor atmospheric carbon concentrations in the coastal zone using airborne sensor packages.  This means that, together with our collaborators, we can now monitor carbon above and below the waterline simultaneously, in real-time in coastal waterways, marshes creeks, and in the air above them – all without getting wet!

To see how it works check out this video: IMG_0415

By coordinating many sensor packages, and taking frequent water samples for later analyses, we can track carbon into and out of coastal ecosystems.  By analyzing plant tissues and conducting tracer experiments with stable (non-radioactive) isotopes we can follow the flow of carbon through plants and into metabolic pools of carbohydrates, proteins, and phenolic substances – above and below ground.  We can even track the carbon among trophic levels as plants are consumed by herbivores, and so forth.

Questions we hope to answer:

How to salt marshes process carbon?  How do they take it in and release it?  What are the local conditions that estuarine organisms experience near marshes?

Can seagrass meadows take up and safely store large amounts of excess carbon – acting as a helpful “carbon sponge” to help slow climate change?

Does the carbonate system change in predictable ways within seagrass beds?  And what are the consequences for organisms, such as bay scallops, that live there?

We thank Dickinson College’s Research and Development Committee for funding, and the folks at Valarm for technical assistance.