Suspended Particles: Their Role in Estuarine Biogeochemical Cycles

Suspended particles are instrumental in controlling the reactivity, transport and biological impacts of substances in aquatic environments, and provide a crucial link for chemical constituents between the water column, bed sediment and food chain. This article reviews the role of suspended particles in the chemical and biological cycling of trace constituents (trace metals, organo-metallic compounds and hydrophobic organic micropollutants; HOMs) in estuaries, with particular emphasis on the effects of and changes to particle reactivity and composition. The partitioning (or distribution coefficient, KD) and bioavailability of chemical constituents, and assimilation efficiency (AE) of such by bivalve suspension feeders, are identified as key parameters requiring definition for accurate biogeochemical modelling, and the discussion centres around the determination of and controls on these parameters. Particle-water interactions encompass a variety of physical, biological, electrostatic and hydrophobic effects, and are largely dependent on the character and concentration of suspended particles and salinity. The salinity-dependence results from the competing and complexing effects of seawater ions for trace metals, and the compression of water in the presence of dissolved seawater ions and consequent salting out of neutral solute (HOMs, organo-metallic compounds and some trace metal complexes). The extent of biological solubilization of chemical constituents from suspended particles is dependent on the nature of chemical components of the gastro-intestinal environment and their interactions with ingested particles, and the physiological (e.g. gut passage time) and chemical (e.g. redox conditions and pH) constraints imposed on these interactions. Generally, chemicals that associate with fine, organic-rich particles (or, for some HOMs, fine inorganic particles), and desorb at pH 5–6 and/or complex with digestive enzymes or surfactants are most readily solubilized in the gut. The extent of assimilation of solubilized chemical is then determined by its ability to pass the gut lining and partition into cytosolic material. In practice, KD and AE are determined experimentally by means of radiotracers added to contained suspensions or mesocosms, while operational measurement of bioavailability relies on in vitro chemical or biological (enzymatic) extraction of particles. What is lacking, however, and is identified as an ultimate goal of future research, is the ability to predict these parameters from theoretical principles and thermodynamic constants. Since many of the inherent interactions and mechanisms are controlled by particle composition and reactivity, a more immediate objective would be better characterization of the biogeochemical properties of suspended particles themselves. This includes chemical resolution of the bulk organic matter, definition of the abundance and synergistic effects of component sorbent phases, and determination of the effects of particle-seawater ion interactions on the reactivity of the particle surface.