This lecture explores the impact of anoxia in surface environments, focusing on the formation and decay of organic matter, eutrophication in lakes, hypoxic coastal waters, water stratification, redox zonation, and the fate of sulfur and phosphorus in sediments. The instructor discusses the consequences of anoxia in the Gulf of Mexico, the Black Sea, and other aquatic systems, emphasizing the role of redox processes in shaping biogeochemical cycles. The lecture also delves into the microbial communities thriving in deep sediments and the energy gain processes associated with organic matter decomposition.