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Ecosystems fall into two general categories: terrestrial and aquatic. Terrestrial ecosystems, such as deserts, rainforests, and tundra, are categorized by annual rainfall and temperature. Aquatic ecosystems are divided into freshwater and marine environments, with freshwater comprising less than two percent of Earth's surface while marine ecosystems cover approximately 75 percent.

Terrestrial biomes are classified based on climate characteristics including annual temperature and rainfall. Rainforests near the equator have high temperatures and high rainfall, supporting diverse habitats. Deserts have high temperatures but low rainfall. Tundra ecosystems exhibit very low temperatures and minimal rainfall, resulting in the lowest biodiversity among major terrestrial biomes.

Freshwater ecosystems contain three distinct zones. The littoral zone is the warm, shallow area near edges with abundant producers and consumers. The limnetic zone is the well-lit deep-water surface dominated by plankton. The profundal zone is deep water where little light penetrates, inhabited mainly by organisms that consume dead material and do not photosynthesize.

Shallow ocean ecosystems host the highest marine diversity because sunlight penetrates the water, supporting photosynthetic organisms and coral reefs. The deep ocean receives minimal light, limiting photosynthesis and supporting fewer, often unusual organisms adapted to darkness and high pressure. However, the deep ocean surface does support plankton, which performs approximately 40 percent of Earth's photosynthesis.

Wetlands are shallow standing water areas like marshes, swamps, and bogs characterized by hot, moist environments. These conditions support the highest biodiversity of all freshwater ecosystems, including diverse plant, animal, fungi, and bacterial life. Their unique environmental characteristics create ideal conditions for numerous organisms to thrive.

All ecosystems, regardless of type, share the same dynamic processes of energy and matter transformation. Ecosystems are powered by constant solar energy that flows through them, while matter is recycled through biogeochemical cycles. These processes occur through photosynthesis and feeding relationships among organisms in the environment.

Water depth significantly influences aquatic ecosystems by affecting temperature and sunlight penetration. In freshwater systems, depth determines which organisms inhabit each zone, from the warm littoral zone to the dark profundal zone. In marine ecosystems, depth creates distinct zones: shallow water with high diversity, the well-lit surface supporting plankton, and the deep ocean with minimal light and unique adapted species.