The Biosphere 2 ocean was built to simulate a Caribbean coral reef ecosystem. The large tank has a surface area of 35 x 20 meters (100 x 60 feet) and is 7 meters (24 feet) at its deepest end, with a shallow lagoon, partially separated by a fringing reef, leading up to a sandy beach. The mesocosm is housed within a high grade, corrosion resistant stainless steel container. During construction this steel was coated with a paint epoxy resin to hinder corrosion and to reduce leaching of metals into the ocean water. The total water volume of the Biosphere 2 ocean is roughly 2,600,000 liters (676,000 gallons). The system was initially populated with about 25 species of hard corals, 15 species of fish, 25 species of algae, and over a hundred invertebrate species.
Unlike most aquaria, animals are not fed. In fact this ocean is big enough to allow all organisms to rely on the internal food web supported by algae and coral photosynthesis. Light levels are relatively low due to a combination of latitude (32.5° North outside of the tropical reef belt) and shading by the overhead glass and space frame structure. Nonetheless, the measured rates of primary productivity, calcification and nutrient uptake were comparable to those of tropical reefs. Survival of hundreds of species with minimal husbandry over almost a decade, demonstrated that the system and its food web were largely self-sustaining.
Four principal types of substrate were incorporated into the reef: Arizona calcium carbonate-rich clay, Arizona limestone boulders and rocks, Caribbean limestone rubble, and Caribbean aragonite sand. Crushed oyster shells fill the cracks and crevices between large rocks. Aragonitic sand covers the foundation sediments the deep ocean, the lagoon, and the beach.
Numerous mechanical systems simulate or substitute for natural environmental processes. A wave generator and a pair of air-lift pumps circulate water within the tank. Only the wave generator is in continuous use. A series of centrifugal pumps circulate water from the ocean to a separate room where the water can be supplied to large experimental flow-through tanks and to heat exchange units. Two heat exchange units control the temperature of the ocean. They consist of large tubs containing plastic coils fed with hot or cold water from the Biosphere 2 Energy Center. Manually operated ball valves control the flow of hot and cold water through the coils. A series of filtration systems were initially installed, including algal turf scrubbers, protein skimmers, plate filters, and rotary drum filters. Although the ocean biota is very efficient at recycling nutrients, mechanical filtration is also utilized.
Between control and complexity: opportunities and challenges for marine mesocosms . Sagarin, R.D., Adams, J., Blanchette, C.A., Brusca, R.C., Chorover, J., Cole, J.E., Micheli, F., Munguia-Vega, A., Rochman, C.M., Bonine, K., van Haren, J. and Troch, P.A. (2016): Frontiers in Ecology and the Environment 14(7): 389–396.