These dark aquatic systems buried below ground have varied levels of connectivity to surface waters, which influence their traits and processes profoundly. Microbial mats composed of bacteria and aquatic fungi on submerged rock surfaces are major food sources for protozoans and invertebrates, especially in systems that are isolated from surface waters. Larger water bodies support predatory fish. When connectivity is strong, inflow brings a supply of nutrients and organic matter, as well as itinerant biota.