Shiller Lab Summary

Estuarine and coastal systems play important roles in society, serving as port facilities, productive fisheries and rookeries, and scenic recreational areas. However, these same values to society mean that these areas can be significantly affected by human activities. Inputs of nutrients and trace metals are among these impacts. Our work is oriented towards the understanding of transport and cycling of nutrients and trace elements in river-dominated coastal systems. It is only by understanding the fluxes of these elements in coastal/estuarine systems that management models/scenarios can be developed to predict the effect of our activities on the chemical (and ultimately biological) health of these systems.

More specifically, we are interested in the waters of Louisiana Shelf, a coastal area which receives substantial fresh water input from the Mississippi River. The Mississippi River carries high concentrations of plant nutrients derived from fertilizer use on farms in the heartland of the US. These excess nutrients stimulate plant growth in the surface waters of the Louisiana Shelf. These plants, in turn, sink to the bottom waters of the shelf where they serve as food for respiring organisms. The input of this excess food then stimulates an excess of respiration thereby depleting the shelf bottom waters of oxygen during the summer. These oxygen-depleted (or “hypoxic”) waters then become a “dead zone” avoided by animals.

In my lab, we examine the distribution of trace metals dissolved in the water. By “trace metals” we are referring to elements such as iron (Fe), copper (Cu), and vanadium (V) that occur in parts-per-billion or lower concentrations. Despite their low concentrations, these elements can be important nutrients, toxics, or indicators of important environmental processes such as oxygen depletion. Thus, their study is an important aspect of our efforts to understand the functioning of the Louisiana Shelf ecosystem. Furthermore, processes occurring in coastal and estuarine waters can affect the transport of trace metals from the continent to the open ocean. So, our work has bearing on open ocean systems, too.

The overall goal of this research project is to better understand the mixing processes which occur as the waters of the Mississippi River and the Atchafalaya River enter the Gulf of Mexico, along the Louisiana coastline. The researchers will be taking water samples and making physical measurements at the mouths of the two rivers and across the continental shelf to look at how different chemicals react as they flow from the rivers into the coastal ocean.

      
Image courtesy of U.S. Geological Survey (left) and NASA (right).

The first part of understanding how the waters mix is to figure out where all the water comes from. Jim Krest, USF St. Petersburg, will be measuring radium isotopes in the water (the surface water and groundwater) and sediments. Previous studies have shown that these naturally-occurring, radioactive isotopes have higher concentrations in the coastal waters of the Gulf of Mexico water than can be accounted for by the flow of the two rivers. Currently, it is not clear what the source of the “extra” radium is, so figuring this out will be a be a big part of figuring out the sources of all the water. This group will be doing experiments to determine how much radium is currently in the Gulf of Mexico water, how much is coming out of the rivers, and how much can be coming from other sources. Some likely suspects for the extra radium include:

1. Natural movement of radium out of bottom sediments (mud) and into the water. By a process called “diffusion”, the radium will come out at a steady rate because it has a higher concentration in the sediments than in the water. The radium will move from higher concentration to lower concentration, just like water flowing downhill. http://hyperphysics.phy-astr.gsu.edu/hbase/kinetic/diffus.html

2. Radium being “pushed” out of the sediments and into the water. By a process called “advection”, any water moving through the mud will pick up radium from the sediments as it flows. If this moving groundwater then leaves the mud and enters the open water of the Gulf of Mexico, it will bring radium with it. http://ww2010.atmos.uiuc.edu/(Gh)/guides/mtr/af/adv/adv.rxml

3. Another possible contributor of radium is something called “produced water”. This is water that has been trapped in the ground for centuries, and it comes up when we bring up oil or natural gas. The produced water can have very high concentrations of radium, and just a little bit could make up a lot of the “extra” radium we see in the Gulf of Mexico. Produced water is usually pumped back down into the ground so that the surface water is not contaminated, but we don’t know if any of it leaks back out. http://web.ead.anl.gov/pwmis/intropw/index.cfm

One possible way that our results may be useful is for better understanding of some of the biological processes that occur in the Gulf of Mexico. For example, many researchers are interested in a problem called “hypoxia” which occurs nearly every summer in the coastal waters of Louisiana and Texas. Hypoxia means “low oxygen”. When the oxygen level in the water drops, fish and other organisms start to get stressed. And, if the oxygen level gets too low, the fish may die if they can’t get out of the area.

One of the causes of hypoxia is too much nitrogen and phosphorous, which act as food or nutrient sources for tiny plants called phytoplankton. By a process called “eutrophication” (literally, “too much food”), the phytoplankton are able to grow very quickly, and the number of phytoplankton in the surface water increases. Then, zooplankton (tiny animals) increase in population because they now have a good food source (the phytoplankton). All this growing, eating and dying produces lots of waste, and as the waste gets broken down by bacteria, the oxygen in the water gets used up.

It's difficult to track the nutrients involved in the hypoxia process. The nitrogen and phosphorus gets taken up by the phytoplankton very quickly, and the bacteria recycle it almost as fast, making it very difficult to understand. This is where the trace elements, the radium isotopes, and the modeling are important. There are radium isotopes and trace elements coming out of the rivers, sediments, and Gulf of Mexico water, and many of these "tracers" do not get used in the biological processes. We can determine how much of each tracer comes from each source (rivers, sediment, Gulf of Mexico, etc), and we can determine the ratio between the tracers and the nutrients from each source. From this information, we can create models of the sytem to determine how much of the nutrients are coming from each source, and how those nutrients are getting used and recycled in the coastal water.

For more information on hypoxia in the Gulf of Mexico:

• http://toxics.usgs.gov/hypoxia/other_info.html
• http://www.gulfhypoxia.net/