The case studies

Eight aquatic ecosystems are used to understand the importance as an indicator of the eco-exergy to empower (emergy flow) ratio. Two of these ecosystems (called "control pond" and "waste pond") are in North Carolina (USA) and are part of a group of similar systems built to purify sewage. Near the town of Morehead City, six artificial lakes were created: three control ponds fed with estuary water and "clean" water from the local sewage treatment plant, and three "waste" ponds fed with estuary water mixed with more "polluted" (i.e., richer in nutrients) effluent (Odum, 1989). Plant and animal species were introduced in and around the lakes to colonize the new areas and eventually produce new ecosystems by natural selection.

The third ecosystem is the Lagoon of Caprolace in Latium, Italy, at the edge of the Circeo National Park. The Lagoon of Caprolace is an ancient natural system fed by rainwater and farmland runoff rich in nitrogen, phosphorus, and potassium.

The fourth ecosystem is Lake Trasimeno. This is the largest lake in peninsular Italy (area 124 km2), it is shallow (mean depth 4.7m, maximum 6.3 m) and accumulation processes are favored. The water level of the lake shows strong fluctuations with respect to meteorological conditions; hydrological crises occur after several years with annual rainfall < 700 mm.

The fifth system is the Lagoon of Venice. With a surface area of about 550km2, it is the largest Italian lagoon. The sea and the lagoon are connected through three inlets. The average daily volume of water that enters the lagoon from the sea is about 400 million m3, while 900 million m3 of fresh water flow into the lagoon every year from the drainage basin.

The sixth system is an artificial one, located in the central part of the Lagoon of Venice, i.e., the Figheri basin. Fish farming basins consist of peripheral areas of lagoon surrounded by banks in which local species of fish and crustaceans are raised. Salt water from the sea and freshwater from canals and rivers are regulated by locks and drains. The fishes of highest demand raised in basins are Dicentrarchus labrax (bass) and Sparus auratus. Various types of mullet are also raised, as well as eels and mollusks.

Two ecosystems are located within the Esteros del Iberá (northeastern Argentina), one of the most pristine and largest wetlands of South America (13,000km2). This subtropical wetland is located between 27°36'-28°57'S and 58°00'-57°30' W. The macrosystem consists of a mosaic of marshes, swamps, and open water bodies. It is located between three large rivers, the Rio Paraná alto, the Rio Paraná medio, and the Rio Uruguay, with a single outlet to the Rio Corrientes that feeds into the Parana Medio (Loiselle et al., 2001; Bracchini et al., 2005).

The Galarza Lagoon is a mesotrophic, round-shaped lake with an area of 14 km2 and averages 2 m in depth. The lagoon is fed by a small stream that originates in the large marsh area (200 km2) directly above the lagoon and feeds into another small stream that leads to another large shallow lagoon. The water then flows out of this second lagoon into another large marsh area.

Laguna Iberá (area 58 km2, mean depth 3.2 m), has a more irregular morphology and an eutrophic status. This lake is divided into two basins by a narrow passage that acts as a barrier reducing the interchange of wave energy and water masses. A small river (Río Miriñay) feeds the southern basin.

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