DOC or: How I learned to start worrying about carbon in water

Post by: Peter Regier

My research with the FCE-LTER works to better understand where organic carbon comes from, how it changes in the environment and where it ends up.  Organic carbon is the stuff that makes up all living things, and when plants and critters die, the organic carbon they are made of can be sequestered in soils or mobilize into the water or the air.  Since the Everglades is a subtropical system that usually doesn’t freeze and gets lots of sunlight (Florida is called the sunshine state for a reason…), plants can grow year-round.  This means we end up with ton of organic carbon moving in and out of environments like sawgrass marshes, mangroves and seagrass beds (all of which produce and store organic carbon).    

I’m interested in understanding how environmental drivers like hydrology and climate impact organic carbon dissolved in natural waters, aptly named dissolved organic carbon (DOC).  Since we can easily quantify DOC in the lab, we should be able to collect some water samples, measure DOC, explain the Everglades organic carbon cycle and call it a day right?  WRONG, because not all DOC is equal.  For instance, DOC that comes from freshwater marshes has different molecules than DOC from mangroves.  Some DOC looks delicious to bacteria and microbes, and they will eat it and poop out DOC that looks chemically different.  Some DOC has molecular structures which can be changed by sunlight.  Other DOC looks like garbage to microbes and could care less about sunlight, so it just floats downstream.  Because biological and photochemical processes are simultaneously interacting with DOC as it moves through the Everglades, any given sample may represent a wide variety of DOC sources and can be highly transformed during transit.  This means the current composition of DOC may look nothing like the parent material.   To help visualize what processes might affect the molecular composition of DOC (labeled as DOM), here is a fun map of potential pathways (Troxler et al., 2013):

If that doesn’t scare you, here’s a mass spectrum of DOC (Tolic et al. 2016), where each line in the graph below represents a different molecular formula.  That’s all from one DOC sample folks...

So DOC composition is way too complex and understanding how it reacts and transforms in the environment is just impossible and we should all go home and rethink our lives, right?  ALSO WRONG.  DOC is a master variable (meaning it’s critically important in the environment) that influences how nutrients, pollutants and metals get distributed throughout the Everglades.  These changes can alter drinking water quality, impact underwater plants, alter food-webs, lead to algae blooms, influence where specific species of plants and animals live, the list goes on and on…  Thus, it is incredibly important that we understand how DOC is produced and moves through the ecosystem, but this is a daunting challenge since DOC is so dynamic.  

Fortunately for us, some smart cookies have figured out simple ways to look at the complex mixture of molecules comprising DOC.  For instance, there are certain molecules that absorb sunlight in very specific ways, and really cool molecules that both absorb and re-emit sunlight as fluorescence.  Since pictures are better than words, here are two examples of fluorescence signatures of DOC components from the Everglades (courtesy of Chen et al., 2010): the first component is commonly associated with freshwater DOC and the second is primarily found in saltwater environments.    

Super different, right?  Using tools like fluorescence, mass spectrometry and stable isotopes (and many, many more), we are able to trace and separate sources of DOC and even decipher how molecules have changed as they travel through aquatic systems.  Recently, the scientific community studying DOC has been developing and using water quality sondes equipped with sensors that measure fluorescence directly in the water column (it’s the skinny blue thing in the picture):

These sensors collect valuable information about DOC at very short time-scales which wouldn’t be feasible with our current sampling and laboratory methods.  This has opened up a whole new world of DOC dynamics where we can study rapid changes over short temporal and spatial scales with high accuracy.  As these technologies continue to develop, we can integrate continuous monitoring using sondes with more traditional laboratory analyses to get unprecedented access to DOC composition and cycling dynamics.  In turn, this new data will help us better predict how the Everglades, and other regions around the world, will respond to environmental changes like rising sea levels, ecosystem restoration or shifting climate regimes.

References

Chen, M., Price, R.M., Yamashita, Y., Jaffé, R., 2010. Comparative study of dissolved organic matter from groundwater and surface water in the Florida coastal Everglades using multi-dimensional spectrofluorometry combined with multivariate statistics. Applied Geochemistry 25, 872–880.

Tolic, N., Tfaily, M.M., Shen, Y., Chu, R., Fillmore, T., Zhao, R., Bloodsworth, K., Robinson, E., Paa-Toli, L., Hess, N.J. (2016). Soil Organic Matter (SOM): Mass Spectrometry Data Analysis, in: Encyclopedia of Soil Science, Third Edition. CRC Press, pp. 2154–2158.

Troxler, T.G., E. Gaiser, J. Barr, J.D. Fuentes, R. Jaffé, D.L. Childers, L. Collado-Vides, V.H. Rivera-Monroy, E. Castañeda-Moya, W. Anderson, and others (2013). Integrated carbon budget models for the Everglades terrestrial-coastal-oceanic gradient: Current status and needs for inter-site Comparisons. Oceanography 26(3):98–107.