Chesapeake Ecotox Research Program: Research

Experimental Approaches

Mesocosms

CERP studies will employ mesocosms, aquaria that can be designed to mimic different types of aquatic conditions. Mescosm studies have become widespread in ecology in the last twenty years as biologists have tried to address the responses of populations to contaminants across a range of temporal and spatial scales (Resetarits and Bernardo 1998). Two ongoing studies involving the use of mesocosms are underway in the Chesapeake region, MEERC within the University of Maryland Center for Environmental Science (UMCES) and COASTES within the Academy of Natural Science Estuarine Research Center (ANSERC). In this project, a mesocosm-based experimental approach over four years will be used to quantify the relationships between sub-lethal contaminant exposure and ecological function. Use of mesocosms will permit scientists to manipulate and measure the effects of contaminant exposure on the ecological function of two surrogate species, Leptocheirus plumulosus and Fundulus heteroclitus. Studies will:

measure sediment-bound and dissolved levels of contaminants,
characterize the exposure of individual organisms by quantifying sub-organismal biomarkers of exposure and body residues,
quantify the ecological responses (growth, survival and reproduction) of populations of surrogate species in the mesocosms, and
develop population-level models that examine the consequences of contaminant exposure, assessed by biomarkers of exposure and changes in growth, survival, reproduction, and behavior, to the dynamics of the population.

During the years 2000 and 2001, mesocosm research will focus on defining the sediment ecotoxicology of Leptocheirus plumulosus when exposed to sediments from the regions of concern. During the years 2002 and 2003, research will focus on Fundulus heteroclitus using the same experimental design.

Sampled organisms will be examined for exposure and bioaccumulation of contaminants, biomarkers of exposure and effect, and the impact of exposure on behavior, survival, growth and reproduction.

Mechanisms of Exposure and Bioaccumulation

Exposure can be defined as the time-integrated rate of delivery of a chemical to an appropriate site within an organism. Because chemicals may follow several pathways to these sites, quantifying the total exposure requires a detailed description of each route. Exposure pathways are often divided as "external" – transport from sediments to organism – and "internal – disposition of the chemical within the organism. Exposure of a fish includes passive uptake of dissolved chemicals across the skin and gill surfaces, ingestion of contaminated prey, and filtration or ingestion of contaminated sediments. While the magnitude of total exposure is related to the total concentration of the chemical in the environment, the actual delivered dose depends strongly upon the physical and chemical properties of the contaminant, the physics of the local environment, and the behavior and physiology of the target organism.

For sediment bound chemicals to exert toxicological responses, they must first physically desorb (detach) from the matrix. Desorption rates depend upon the geochemical form of the chemical and the chemical properties and physical form of the sediments. Often, desorption is the slowest in a series of steps and thus it controls the overall bioavailability of sedimentary contaminants. Bioavailability of ingested solids, either prey or sediments, depends upon the rates of contaminant desorption and transport across the gut wall relative to the residence time of the solids in the digestive tract. Exposure of dissolved species by transport across membrane surfaces depends upon the permeability of the membrane, the diffusivity of the compound, and in the case of gills the organism's overall respiration rate.

While pharmokinetic models are used to describe contaminant uptake rates and resulting body burdens in simple laboratory systems, they generally do not include any feedbacks between the organism's behavior or physiology and exposure. Many biotic processes may alter either the magnitude of exposure or the bioavailability of sedimentary contaminants. For example, the residence time of a chemical in the gut of a deposit feeding organism is related to the efficiency of assimilation, which varies with the food quality. Organisms living in carbon-rich sediments may ingest less sediment (and thus consume less contaminant), but may extract chemicals more efficiently than those in carbon-poor environments. Also, the bioavailability of sediment-bound chemicals may change as a result of benthic organisms changing them. Dietary exposure to fish depends upon the non-linear, density-dependent feeding behavior, and the resulting burdens of chemicals in fish depend upon their growth rates, which is also related to their feeding rates. These types of feedbacks are not typically revealed in studies with individual organisms and are not included in pharmokinetic models. The mesocosm studies and dynamic population models will allow researchers to investigate how the behavior and physiology of an organism influences bioavailability and exposure of chemical contaminants.