Role of macroalgal biomass and clam fishing on spatial and temporal changes in N and P sedimentary pools in the central part of the Venice lagoon

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.