A Preliminary Model of the Dispersal and Biological Effect of Toxins in the Tamar Estuary, England

Abstract

Harris, J.R.W., Bale, A.J., Bayne, B.J., Mantoura, R.F.C., Morris, A.W., Nelson, L.A., Radford, P.J., Uncles, R.J., Weston, S.A. and Widdows, J., 1984. A preliminary model of the dispersal and biological effect of toxins in the Tamar estuary, England. Ecol. Modelling, 22: 253–284. A preliminary one-dimensional model of the axial dispersal of a range of environmental toxins in a partially-mixed macrotidal estuary in the South-West of England is described. Aromatic hydrocarbons ranging from benzene to benzo( α )pyrene are considered, together with cadmium. As an indicator of biological impact, toxic effects on the mussel, Mytilus edulis , are simulated. Solute dynamics are treated as an advection-diffusion process. Modelling of particle dynamics incorporates deposition and resuspension and simulates the observed turbidity maximum at the limit to saline intrusion and the effect of runoff and tidal range on the suspended load of particulates. Hydrocarbon losses by volatilisation, photo-oxidation and bacterial action are incorporated. The partition of toxins between dissolved and particle-adsorbed phases is assumed in all cases to be reversible and to achieve equilibrium. Chemical speciation of cadmium is treated similarly; in this case competetive binding of humics and particulates by protons and major cations is taken into account. Toxic effects are simulated as acting on the energy balance of Mytilus edulis , via its rates of respiration and absorbtion. Examples are given of simulated spatio-temporal distributions of aromatic hydrocarbons and cadmium input towards the head and towards the mouth of the estuary; under winter and summer conditions in the case of the former, and summer only in the case of the latter. The distribution of toxic effects in mussels is similarly exemplified. The distribution of toxins within the simulated estuary depends critically on the balance between water flows and tidally-driven particle movement, and hence on the partition between dissolved and particle-adsorbed phases. It is concluded that considerable fundemental investigation is still needed to elucidate the impact and dispersal of toxins in estuaries.