Low productivity ecosystems occur within water-saturated permeable rock strata or layers of buried unconsolidated sediments. Life in these dark, confined or connected ecosystems is almost completely comprised of microbial and small invertebrate detritivores and their protozoan consumers. Diversity and productivity decline with depth and connectivity to surface waters. Although groundwater ecosystems occur almost everywhere in the near surface crust, they are prominent below surface water and in artesian basins.