, Volume 518, Issue 1-3, pp 189-200

Features of hypereutrophic Molino lagoon ecosystem dominated by sedentary polychaetes

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Ecological study of the Molino lagoon situated in lowland area at the NW Adriatic coast was undertaken to expose the causes of an extreme sulfide contamination of its bottom sediments. The lagoon is connected with the sea and experiences an intense tidal water exchange. Its trophical status corresponded to the hypereutrophic in accordance with basic parameters thus measured. The diel fluctuations of dissolved oxygen in the near-bottom layer approached 10 to 14 mg O2 l−1. Dissolved inorganic phosphorus content attained 3 to 12 μM, Ptot −6 to 25 μM l−1, suspended organic carbon −6 to 25 mg l−1 and labile organic matter −6 to 16 mg C l−1. The wet phytoplankton biomass was over 10 g m−3 during the whole period summer, reaching 40 to 80 g m−3. The biomass of bacterioplankton varied between 1 to 2 g m−3. The bottom was covered by semi-liquid black mud, which contained 600 to 800 mg S dm−3 of acid volatile sulfides. The bottom surface emanated free H2S up the water column and to the atmosphere. However, the ecosystem of this lagoon preserved a quasi-normal efficient food web and escaped nuisance monospecific blooms, unlike most of hypereutrophic lagoons in this region. An explanation of the above mentioned phenomena we see in absolute predomination of sedentary serpulid polychaetes as the key ecosystem component. The polychaetes represented a single macrobenthos group in the lagoon. Their meat biomass was 100 to 400 g m−3 with the share of serpulids over 90% in areas covered by black mud. The polychaete community formed in the lagoon a powerful biofilter. Its long-term functioning resulted in hyper-accumulation of organic matter and nutrients and in formation of sulfide mud on its bottom. The functional parameters of food web components were assessed. A tentative energy balance in this hypereutrophic, but quasi ``healthy'' lagoon ecosystem was calculated, and its specific features were delineated in relation to the functioning of polychaetes biofilter. The efforts for its melioration undertaken in 1996–1997 are also exposed and discussed.