In a recent publication, Stephen Glaholt of Indiana University and other researchers, including Dartmouth provost Carol Folt, developed a new method of evaluating stressors in aquatic environments: Adaptive Iterative Design (AID). AID simplifies the method of evaluating stressors by quantifying both their individual and interactive effects, minimizing the amount of prior knowledge required to do the analysis. AID has also been found to be a more effective way to assess multiple stressor interactions than current methods.
As of now, the most common experiment being used by eco-toxicologists to develop criteria for stressors in aquatic environments is the acute Daphnia toxicity test. The test examines toxicity by exposing Daphnia (a small organism commonly known as the water flea) to different concentrations of the stressor in aquatic environments and measuring the percent mortality rate.
The acute Daphnia test, however, measures only individual stressors, which is problematic because in most aquatic environments, organisms are subject to multiple stressors at once. To account for this, eco-toxicologists currently run many single-stressor tests for each of the stressors and then combine the results using a mixture model.
Glaholt, et al., have developed an “iterative, quadratic, multivariate, logistic model” called adaptive iterative design, or AID in order to improve water quality assessment. It is meant to be easier to use than previous models, and uses improved statistical methods that require less information to work.
AID’s validity was tested by exposing Daphnia pulex to cadmium, zinc, and arsenic while varying the pH level and hardness of the water. Such conditions are common in polluted waters. The mortality rate of the Daphnia was determined after 48 hours by observing if individual Daphnia were moving.
AID was developed using the information from unsuccessful experiments seeking to determine the impact of the above five variables on Daphnia pulex. Multiple single-stressor tests were unable to predict the effects on Daphnia of common water pollutants. A model was needed that could handle non-linear response variables, and measure interactive effects.
The AID process starts by using a minimal amount of data about the stressors to estimate the response curve of the Daphnia. Next, the design model is used to determine the stressor concentration that best fits the response curve. This continues until the next set of concentration values does not improve the model’s approximation.
Once a suitable model is produced, the effects of stressors on organisms can be predicted. The above model analyzes the effect of each of the stressors, but can also be expanded to include “interactions for all possible combinations of the mixture variables.”
Experimentation found that two iterations of AID were sufficient to produce a stable response model. This model was used to identify all possible combinations of the five variables and their effects on Daphnia morality rates. The results indicate that Daphnia mortality is proportional to pH, arsenic, cadmium, and zinc values in the water.
Furthermore, some combinations of elements also had synergistic effects on Daphnia. For example, when arsenic and cadmium are in a system at the same time, there is a higher rate of mortality than the sum of the effects of arsenic and cadmium on mortality individually. In contrast, when combined with zinc, arsenic and cadmium had a less than additive effect.
Although the AID method was intended to test the effects of multiple stressors on aquatic organisms’ mortality rates, it can also be used to test for reproduction and growth rate, making it an effective and promising tool in analyzing the well being of aquatic organisms.
Further reading:Stephen P. Glaholt, Celia Y. Chen, Eugene Demidenko, Deenie M. Bugge, Carol L. Folt, Joseph R. Shaw. “Adaptive Iterative Design (AID): A novel approach for evaluating the interactive effects of multiple stressors on aquatic organisms.” Science of the Total Environment. Volume 432, 15 August 2012, pp. 57-64. Available at http://www.sciencedirect.com/science/article/pii/S0048969712007899 (5 July 2012).
“Daphnia.” Available at http://www.nih.gov/science/models/daphnia/ (5 July 2012).