Hypereutrophication events in the Ca’Pisani lagoons associated with intensive aquaculture

Abstract

Parameters of ecosystem structure and functioning were analyzed in three hypereutrophic lagoons of Ca’Pisani during the season of 2001. Lagoons are situated at wetlands of the NW Adriatic in the vicinity of the Porto Viro, Po River delta. They are associated with intensive fish culture enterprise and accept its wastewater. In June, the lagoons were found overloaded with the biomass of nitrophylic algae. At the end of July, a bloom of potentially toxic dinoflagellate Alexandrium tamarense occurred. Soon, it was supplanted by the picocyanobacterial assemblage, which arrived into the lagoons from the coastal Adriatic via the Marine channel. Wet biomass of this new picocyanobacterial bloom arrived in September attained 30–60 g m⁻³. Decrease of white disk water transparency down to 30–40 cm resulted in a gross mortality of macrophytes accompanied by spreading of floating saprobic alga Enteromorpha. Phytoplankton was dominated in June to July by small mixotrophic phytoflagellates with a wet biomass of 200–1300 mg m⁻³. Number of bacterioplankton ranged between 4 and 7 × 10⁶ ml⁻¹ and its wet biomass between 1.4 and 2.1 g m⁻¹. Its maximum of 18 × 10⁶ ml⁻¹ was observed in late August, when the mortality of macrophytes had occurred. Zooplankton and zoobenthos were found depleted in the lagoons especially during the blooms. Diel fluctuations of dissolved oxygen in the lagoons in June to July reached 150–200% of saturation. Photosynthetic oxygen production ranged between 15 and 30 g O₂ m⁻² d⁻¹. Water column deoxygenation rate was 1–1.5 mg O₂ l⁻¹ h⁻¹. Total photosynthesis production reached 3–8 g C m⁻² d⁻¹ by the share of phytoplankton 5–15%. Hyper-accumulation of total phosphorus in the water column and of toxic labile sulfides in the bottom sediments was documented. Content of inorganic phosphorus in water remained unusually high even by its intensive uptake by microplankton. The PO₄P uptake rate measured with ³²P-label ranged during the bloom of picocyanobacteria between 10 and 50 nM l⁻¹ min⁻¹, and the residence time of PO₄P between 15 and 50 min. The data were generalized via the calculation of energy balance and the deduction of the energy flow scheme in the ecosystem. Their analysis demonstrates the invalidation of ecosystems in hypereutrophic lagoons due to their overload with organic matter, with nutrients and with labile sulfides. After having depleted their animal food web, they are unable to decompose local plus external organic loading.