Chapter 2: The Effects of Macrobenthos on Chemical Properties of Marine Sediment and Overlying Water Robert C. Aller This chapter reviews the effects of macrobenthos on the chemical properties of marine sediment and overlying water. It discusses the balance between material transport processes and chemical reactions in the environment. In marine sediments, large bottom-dwelling animals play a major role in mass transport by moving particles and fluids during feeding, burrowing, tube construction, and irrigation. These biogenic transport processes have significant direct and indirect effects on sediment composition and overlying water chemistry. The chapter covers diagenetic reactions, which are important in sedimentary deposits. These reactions involve the decomposition of organic matter and other biogenic components such as CaCO3 and SiO2-nH2O. These reactions influence pH and oxidation-reduction potential, and cause the depletion or buildup of reactants or products in both the fluid and solid phases of a deposit. Table I presents idealized decomposition reactions, including aerobic respiration, nitrate reduction, manganese reduction, iron reduction, sulfate reduction, methane production, and fermentation. These reactions illustrate the complex chemical processes occurring in marine sediments and their impact on the chemical properties of the environment. The chapter also discusses the influence of macrofauna on sediment-water exchange rates, reaction rates, and the chemistry of burrow habitats. It explores spatial and temporal patterns in sediment chemistry and summarizes the key findings of the chapter.Chapter 2: The Effects of Macrobenthos on Chemical Properties of Marine Sediment and Overlying Water Robert C. Aller This chapter reviews the effects of macrobenthos on the chemical properties of marine sediment and overlying water. It discusses the balance between material transport processes and chemical reactions in the environment. In marine sediments, large bottom-dwelling animals play a major role in mass transport by moving particles and fluids during feeding, burrowing, tube construction, and irrigation. These biogenic transport processes have significant direct and indirect effects on sediment composition and overlying water chemistry. The chapter covers diagenetic reactions, which are important in sedimentary deposits. These reactions involve the decomposition of organic matter and other biogenic components such as CaCO3 and SiO2-nH2O. These reactions influence pH and oxidation-reduction potential, and cause the depletion or buildup of reactants or products in both the fluid and solid phases of a deposit. Table I presents idealized decomposition reactions, including aerobic respiration, nitrate reduction, manganese reduction, iron reduction, sulfate reduction, methane production, and fermentation. These reactions illustrate the complex chemical processes occurring in marine sediments and their impact on the chemical properties of the environment. The chapter also discusses the influence of macrofauna on sediment-water exchange rates, reaction rates, and the chemistry of burrow habitats. It explores spatial and temporal patterns in sediment chemistry and summarizes the key findings of the chapter.