by Matt McNamara, former SNR intern

Upstairs in the Rubenstein lab there is a bright room where florescent lights are lined up on the tables and a variety of glass beakers and plastic cups fight for space beneath them. The room speaks of some type of growth going on; a culture of sorts. At closer examination, the beakers and cups reveal tiny (and some not-so-tiny) creatures of freshwater lakes and streams. Common names for these animals are water fleas, scuds, midges, or even just fish food. To the water quality specialists at the Rubenstein lab, these are macrozooplankton, a primary consumer in the Lake Champlain ecosystem and a fundamental link in the varied and sometimes complex Lake Champlain food web. It is not uncommon for ecological "indicator species" to be found towards the bottom of the food web. In other words, the well-being of these prolific little creatures plays a large part in determining the structure of the lake's fish communities (Brown et al. 1993). This is the crux of current toxicity tests going on at the Rubenstein. The lives of these creatures tell of the inner workings of the aquatic ecosystems we love so much but often take for granted.

As a part of the Burlington Bay project, current testing is aimed at addressing the toxicity issues facing Burlington Bay as a high risk area for toxic contamination from urban storm drains. It is through the cooperation of the University of Vermont's School of Natural Resources and funding from Green Mountain Power that these tests can occur.

How is what we put into the lake affecting the health of the organisms that live there as well as our own health? As of 1994, there were 7 industrial effluents running directly into the lake, 34 hazardous waste sites in the basin, 73 sewage systems, 65 "active" landfills, and a plethora of private septic systems (Halpern 1993). This is a lot to take into consideration. The Burlington Bay project seeks to quantify the risks associated with toxins from storm drains in the bay specifically. This type of localized work facilitates well directed clean up measures and increased public awareness by essentially bringing the problem home.

Currently being worked with at the lab are macrozooplankton from the class Cladocera. Ceriodaphnia is the specific species. Ceriodaphnia mature enough to produce young in as little as 72 hours under ideal conditions. This makes them an ideal candidate for toxicity testing, because it is possible to monitor the life cycle of several generations in relatively short order. The EPA sets strict standards for how toxicity testing is performed on Ceriodaphnia that must be followed in order for the results to be considered acceptable. Some of these quality controls include monitoring the Ceriodaphnia in order to be certain that there are no reproductive disorders, mutations, or parasitic organisms in the culture before they are exposed to a toxin. With every test, a group of control organisms that are not exposed to any toxin are kept and monitored along with the organisms undergoing the test in order to be sure there are no outside influences affecting the health of the organisms.

Quantifying toxicity is a complex issue because a "toxic effect" will always depend on how you characterize it. Death is probably the most common endpoint for toxicity testing, as the large quantity of data on Lethal Concentrations (LCs) can attribute to. However, there are a wide variety of other endpoints used in toxicity tests such as mutations, reproductive problems, and behavioral changes, etc. When the organism exhibits a specified endpoint, the concentration of a toxin that caused it is called the Effective Concentration(EC). LCs and ECs are calculated for different proportions of the population tested on. The most common calculation is done for when 50% of the population exposed exhibits the endpoint characteristics. This is called the LC50 or EC50. The most common way of determining a LC50 or EC50 is to do several tests with various concentrations, and keep track of the proportion of organisms reaching the endpoint in each test. Then all the tests can be represented graphically with percentage reaching the endpoint on the X axis and concentration of the toxin on the Y axis. This is a very simplistic way of getting an idea of what the LC50 or EC50 is. There are a variety of statistical variations on this basic idea.

Toxicity testing is most widely known through short term "acute" tests, as opposed to longer exposure tests called "chronic" tests. Acute tests are often used in determining acceptable toxic concentrations in outflows for waste treatment plants and other possible toxin sources that require a permit. While these tests are conclusions in and of themselves in a permitting situation, they are only precursors for further research in a scientific setting.

In the case of testing stormwater, the organisms are first exposed to stormwater from each of the drains running into the bay in various concentrations. This initial "reference test" gives the analyst an idea of where the most pressing toxicity problems exist. The next step involves breaking down the different components of the stormwater to determine what exactly is causing the toxic effect. It is then that specific problem spots can be pin-pointed for clean up efforts.

Stepping back from the tests, we are forced to ask, "What do we consider to be an unacceptable outcome as a result of toxins entering the lake, and what is our risk of that occurring with conditions as they are and how they will be?" These are difficult questions. In the coming months, the Rubenstein lab will be shifting into high gear to help address this issue and many more of the pressing questions about how we are impacting the Lake Champlain ecosystem.

Halpern, S. 1993. Along the Eastern Shore: A Report on the Uncertain Times of Lake Champlain. pp. 57-66.

Toxicity Testing