Hypoxia: the condition of extremely low dissolved oxygen concentrations in a body of water. Eutrophication: the process of nutrient enrichment, increased production of organic matter, and subsequent ecosystem degradation. Cycles: shape the landscapes and guide the flow of key chemical elements and compounds that support life and regulate climate. System: network of relationships among parts, elements, or components that interact with/influence one another through the exchange of energy, matter, or information. Open systems: systems that receive inputs of energy and matter and produce outputs of both. Closed systems: systems that receive inputs and produce outputs of energy but not matter. N energy inputs to earth = solar radiation, heat released by geothermal activity, organismal metabolism, human activities. Informational inputs = visual, olfactory (chemical), magnetic, thermal signs. N matter inputs = when chemicals/physical material move along systems. Feedback loop: a system"s output serving as an input to that system.