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Design and evaluation of benthic mesocosms for coastal lagoons

Abstract:

We present the design of mesocosms built to run experiments in a coastal lagoon of the southern Atlantic Ocean, in Uruguay. Cylindrical mesocosms (1.7 m high, 2.26 m diameter) were built using supporting structures of iron grating (15 x 15 cm) to avoid damage from strong winds. The iron structures were still in good condition for over four years. Within the iron grating, four types of materials were tested for the walls. A single galvanized iron sheet, painted, and covered with thermic polyethylene on both sides proved to be the most suitable design under conditions of low water levels. This design remained in good conditions for more than 6 months. In the short and medium term, i.e. weeks to months, light, Secchi disk transparency, temperature and dissolved oxygen were similar inside and outside the mesocosms, indicating that the experimental set-up did not cause any artifacts. However, very short-term experiments may be affected by the water heating due to its confinement, while in the long-term, i.e. more than one year, the enclosures' walls suffer major deterioration. A wind-driven agitator was tested to simulate the water mixing within the enclosures. However, light penetration was highly reduced with the mixing system. Temperature profiles inside and outside the polyethylene enclosures showed that they were vertically mixed. Differences in dissolved oxygen concentration between surface and bottom within the enclosures as well as between enclosures and the lagoon were small. © Asociación Española de Limnología.