Chapter 6 - Radioecology of Tropical Freshwater Ecosystems: Mechanisms and Kinetics of Bioaccumulation and the Importance of Water Chemistry

Abstract

Radionuclides in (sub-) tropical freshwater systems generally behave in a predictable manner, based on what is known from lakes and streams in the better studied temperate climes. Within the water column, the fate and behaviour of radionuclides and their stable element analogs are typically governed by key physicochemical variables such as pH, redox potential, the concentrations of dissolved ions and the presence and type of organic matter. The chemical form (or speciation) of a radionuclide or stable element is generally of greater biological importance (i.e. bioavailability) than the total concentration. This concept is currently being integrated into mechanistic frameworks (e.g. biotic ligand and bioaccumulation models) by national regulators for protecting freshwater ecosystems. Given that the volume of fresh surface waters (i.e. rivers and lakes) is relatively small (0.01%) in comparison to seas and oceans, then the biota living within them can also substantially influence the chemistry of a radionuclide, by effectively acting as “large particles” available for surface complexation. For example, wetlands serve as effective biofilters to remove radionuclides from the water column. Once radionuclides have been accumulated by organisms, their behaviours generally reflect their similarities to essential (macro and micro nutrients) and non-essential elements. Metabolic mechanisms tending toward homeostasis typically make internal organism chemistry less dynamic than that in the external water column. Again, this is to be expected and radionuclide biokinetics generally follow the patterns observed for freshwater organisms in temperate climates. This constancy has enabled models to be developed to describe the rate and extent of radionuclide bioaccumulation. These models can be adapted to include various uptake pathways (from water, food or sediment) via gills, skin or gut and modes of excretion or dilution (e.g. diffusion, egestion, moulting and growth) and different parameters can be estimated for each radionuclide and organism. Key biotic factors known to influence radionuclide bioaccumulation are size, age and gender. There are also differences within and between species that reflect the natural variability within any system. Despite the similarities that exist between tropical and temperate freshwater systems, it should be noted that there is still a paucity of data for (sub-) tropical freshwater organisms and systems, and hence there is the chance for exceptions to the consistency to exist. Data are presented that show that the uptake of Sr and Cs by tropical freshwater fishes is much lower than would be expected based on the studies of freshwater temperate fishes. These observations point to the need to undertake additional site- and species- specific investigations on the radioecology of key radionuclides, whenever future nuclear developments in tropical systems are initiated.