THE ROLE OF CRAYFISH IN TERRESTRIAL AQUATIC METAL BIOGEOCHEMICAL CYCLES

Crayfish account for large percentages of aquatic biomass in almost all freshwater ecosystems, making their participation in the cycling of metals of particular importance to sediment quality and biogeochemical cycling of metals. Trace elements both enter and leave a stream or river in the form of soluble, colloidal, and particulate matter. Crayfish are agents of metal cycling; eating and re-suspending metals attached to particulate matter. By processing sediments, organic matter, algae, and phytoplankton, and through uptake of metals through the gills, crayfish play an important role in the environmental biogeochemical cycling of metals in terrestrial aquatic ecosystems. Although their importance in biogeochemical cycling has been long established little is known of the fractionation of metals from food source to tissue. Understanding of elemental behavior at this level therefore provides critical insight into the role of sediment processors in the biogeochemical cycling of metals. Crayfish can potentially store metals in both soft and hard tissue. This has consequences for bioaccumulation. For example, if crayfish shed most accumulated metals through molting, and the exoskeleton is buried or otherwise left undisturbed, one can consider the exoskeleton a net sink for metals. To explore the partitioning of metals in crayfish tissue, crayfish were collected at the Spring River in Hardy, AR. Treatment groups were selected with each fed a diet consisting of known concentrations of metals as well as two control groups which were fed standard, non-spiked diets. Metal concentrations in the gills, muscle, and exoskeleton over the duration of the experiment were analyzed by DRC-ICP-MS. We show that the gill tissue chemistry is unfractionated when compared to the water chemistry. In the case of tissues there is a consistent and predictable relation between diet and tissue chemistry. Certain metals such as Ca and Pb were found in the exoskeleton in amounts proportional to that of the diet. Concentrations in the exoskeleton increased over time as expected. Our data suggests that biomagnification of metals in crayfish tissue is directly related to environmental concentrations with crayfish acting as passive “processors” rather than “fractionators” of metals.