Nutrient concentrations have been investigated in the Venice lagoon for three scenarios: abnormal growth of nuisance macroalgae, decrease of macroalgal dominance and intense catching of the bivalve Tapes philippinarum Adams and Reeve by means of hydraulic and mechanical dredges. Total nitrogen and organic and total phosphorus monitored in June 1987, 1993 and 1998 showed significant changes both in concentrations and distribution. The disappearance of macroalgal blooms and the starting of intense clam fishing activities affected mainly organic phosphorus concentrations which, between 1987 and 1998, decreased from 104 ± 42 to 59 ± 31 µg cm-3 , accounting for 27% (1987) and 16% (1998) of total phosphorus, respectively. Peak values also decreased significantly: they changed from 246 to 124-146 µg cm-3. Similar results were recorded both for the mean total nitrogen concentrations, which decreased from 1.21 ± 0.60 to 0.93 ± 0.48 mg cm-3 , and the highest total nitrogen values which collapsed from 2.98 to 1.37 mg cm-3. It was also observed that spatial nutrient distribution was more homogeneous because of the high sediment re-suspension and spreading over the whole lagoon. The mean values of nutrient concentrations monitored one to three times per month per year in 1989-1990 (first scenario) and 1998-1999 (third scenario) at three stations located in the lagoon inlet (Alberoni, station A), in the central lagoon (Sacca Sessola, station B) and in proximity of the mainland (San Giuliano, station C) confirm the nutrient decrease recorded in the surface sediments of the whole central lagoon. At station B, an area characterised by a macroalgal biomass up to 20 kg m-2 , wet wt., in 1989, the mean total nitrogen concentration decreased from 1.43 ± 0.42 mg cm-3 in 1989-1990 to 0.75 ± 0.15 mg cm-3 in 1998-1999. Similarly, organic phosphorus decreased from 106 ± 39 to 62 ± 23 µg cm-3. Station C showed similar changes, whereas at station A, recently colonised by seagrasses, the opposite results were found.
The goals of conservation and sustainable use of environmental ecosystems have increased the need for detailed knowledge of ecological evolution and responses to both anthropogenic pressures and recovery measures. The present study shows the effects of natural processes and planned intervention in terms of reducing nutrient inputs in a highly exploited coastal lagoon, describing its evolution over a 16-year period from the late 1980s (when eutrophication was at its peak) until 2003. Changes in nutrient and carbon concentrations in the top layer of sediments were investigated in parallel with macroalgal and seagrass biomass in the most anthropized basin of Venice Lagoon in four surveys conducted in accordance with the same protocols in 1987, 1993, 1998, and 2003. A pronounced reduction in trophic state (mainly total nitrogen, organic phosphorus, and organic carbon concentrations) and macroalgal biomass was recorded, together with the progressive expansion of seagrass meadows. General considerations are also made on the effects of Manila clam farming and the shift from illegal to managed clam farming.
established by the Italian law were not rare and they were found close to the mainland and the main river outflows. The comparison with previous data to update the knowledge on the trophic conditions of the lagoon showed that the overall trophic load is reducing. Only nitrate concentrations did not change.The seasonal samples allowed to confirm the above observations highlighting the behaviour of nutrients in relation to the weather conditions and to the primary producer fluctuations.
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