The importance of a reef-forming polychaete, Mercierella enigmatica fauvel, in the oxygen and nutrient dynamics of a hypereutrophic subtropical lagoon

The introduction and growth of the calcareous reef-forming polychaete, Mercierella enigmatica Fauvel, during the past 50 years has greatly aggravated the eutrophication processes in Lac de Tunis, a broad, shallow lagoon on the North African coast. To quantify the polychaete's importance in the lagoon's oxygen and nutrient dynamics, portions of a reef were isolated by large in situ plexiglas enclosures (open to the atmosphere) during six experiments between December 1976 and April 1977. The concentrations of oxygen, ammonia, nitrite, nitrate, and phosphate were measured every 2 h for 24 h in a reef enclosure, in a control enclosure, and in the open water to allow calculation of exchange rates. Fourier series are used as explicit functions describing net productivity and net nutrient exchange rates in each enclosure. Reef respiration represented a large oxygen sink averaging 5·8 g O₂ m⁻² day⁻¹. The polychaetes fed on organic particulates and excreted large amounts of inorganic nutrients during specific periods of activity. This rapid nutrient recycling mechanism contributes to a state of active producer metabolism in the lagoon.