Conceptual Model Developed to Understand Contaminant Pathways between Aquatic and Terrestrial Ecosystems Active

Adjacent terrestrial and aquatic ecosystems are linked by organisms that move across ecosystem boundaries. For example, in freshwater ecosystems, aquatic insect larvae emerge and as adults move to terrestrial systems. These insects play a key role in nutrient and energy transfer from aquatic to terrestrial food webs.

These linkages can also be affected by contamination in freshwater ecosystems.  Contaminants are one of the multiple stressors that can interrupt the flow of energy and nutrients between aquatic and terrestrial food webs. Depending on the contaminant, the overall biomass of aquatic insects that emerge, the timing of emergence, and the dietary value of emerging adults can all be affected.  Contaminants can also be transferred to terrestrial systems as adult insects emerge from contaminated aquatic ecosystems.  There is little information of these linkages, which prevents a full understanding of wildlife exposure and limits mitigation actions designed to reduce contaminant exposure.

U.S. Geological Survey (USGS) scientists on the Ecologically-Driven Exposure Pathways Science Team developed a conceptual model based on known contaminant properties and ecotoxicological principles to predict the effects of various classes of contaminants on aquatic insects and discuss implications for terrestrial insectivores living near contaminated freshwaters. They also developed a conceptual model to understand the transfer of contaminants from aquatic to terrestrial ecosystems and the effects of various classes of contaminants on terrestrial insectivores living near contaminated freshwaters.

Model predictions varied among contaminant classes.  Contaminants that are less bioaccumulative, excreted during metamorphosis, and more toxic to aquatic insects, such as trace metals, tend to affect terrestrial insectivores by reducing production of adult aquatic insects on which they prey. Conversely, highly bioaccumulative contaminants, like polychlorinated biphenyls (PCBs), that are retained during aquatic insect metamorphosis are often non-toxic to aquatic insect larvae at concentrations commonly measured in the environment. However, export of these contaminants from aquatic ecosystems in the bodies of emerging adult aquatic insects exposes terrestrial predators to potentially toxic levels of contaminants.

The conceptual model provides a foundation and baseline set of predictions as a simple and practical starting point to understand the transfer of contaminants from aquatic to terrestrial ecosystems and the effects of exposure on terrestrial insectivores. Although the model has limitations, it is a useful decision tool to begin to understand the effects of contaminant exposure on linked aquatic and terrestrial ecosystems.  These predictions can be used to shape experimental design and support efficient allocation of resources for research designed to inform management decisions.