Predictions for the Mobility of Elements in the Estuarine Environment

Abstract Folsom, B.R. and Wood, J.M., 1986. Predictions for the mobility of elements in the estuarine environment. In: P. Lasserre and J.M. Martin (eds), Biogeochemical Processes at the Land-Sea Boundary. Elsevier, Amsterdam. All chemical and physical processes, including biologically mediated ones, are governed by thermodynamic and kinetic principles. The composition and distribution of elements at the surface of the earth are far from those defined by equilibrium conditions. Stored potential and chemical energy is released as the various reactions proceed toward equilibrium states. Biological catalysis allows organisms to extract some of the energy from these spontaneous but slow processes. Living organisms themselves are thermodynamically unstable and require significant amounts of energy to allow for the production, compartmentalization, uptake and secretion on the many chemicals needed, along with energy demands for mobility, growth and reproduction. Due to the complexity of most natural systems, thermodynamic and kinetic models are difficult to use in describing the cause and effect of stresses on a specific system because of insufficient data concerning all relevant chemical species and reactions under the natural conditions of pH, pE, temperature and ionic strength. In part, some simplifications and general observations can be made which allow for less complete, but manageable modeling of natural systems under stress. Trends have been observed and documented for a number of changes in the physical environment. First, changes in pH can dramatically affect the solubility of many metal complexes. Acidification leads to the dissolution of Al, Mn, Fe and Zn from sediments and particulates in significant amounts. Changes in reduction potential genreally correlates to the dissolved oxygen content of the water which is mediated biologically through photosynthesis and respiration. Fe and Mn oxides/hydroxides become soluble under anoxic conditions with most metal ions being insoluble under oxic conditions. Another physical transition occurs in the estuarine environment. Water salinity increases as fresh water mixes with salt water. Transition metals bound to refractory organic materials, sediments and clays can be displaced by the fast exchange alkali and alkaline earth metals or can be recomplexed by inorganic ligands. Therefore, when predicting the effects resulting from changes in the physical environment on elemental mobility, general trends which follow established principles can be used as a guide. Additional effects can be predicted dependent upon the actions of the biological organisms which reside in the particular system. The ability of various organisms to compete is dependent upon the environment including the availability, chemical speciation and distribution of essential elements along with the physical characteristics of light, temperature, pH, pE and ionic strength. In turn, living organisms will modify their environment as a result of cellular metabolism. Photosynthetis leads to the consumption of CO 2 and nitrogen compounds with the production of oxygen, reduced organic compounds and an increase in pH. Respiration results in the consumption of oxygen, nitrate, sulfate and/or oxygen, the degradation of organic molecules to CO 2 with related changes in pE and pH. Because of the requirements for O, C, N, S and P, biological activity, metal mobility and the bulk elemental cycles are intimately linked.