Recording the occurrence of trophic level changes in the lagoon of Venice over the '90s

Four areas of the Venice lagoon, placed near the Malamocco mouth (Alberoni, st. A), in the Lido watershed (Sacca Sessola, st. B) and near the mainland under the influence of freshwater and urban (San Giuliano, st. C) or industrial (Fusina, st. D) effluents were monitored in two periods: 1989–92 and 1998–99 in order to quantify some environmental changes (macroalgal and phytoplankton biomass, nutrient concentrations, physico-chemical variables) occurred in both the water column and the surface sediment over the '90s. Stations B and C, two areas particularly affected by macroalgae exhibited a biomass of ca. 20 and 8 kg WW m-², respectively, during 1989–90. In 1998–99 maximum densities decreased to ca. 0.3 and 0.01 kg WW m-². During that period, phytoplankton also decreased significantly both in peaks (Chl a: from 58–86 to 4.0–3.5 μg dm-³) and mean values (Chl a: from 9.1–10.3 to 1.3–1.4 μg dm-³), especially at sts. C and D. As far as nutrient concentrations are concerned, a different trend was observed in the water column and in the surface sediment, mainly because of the reduction of the primary producers and the disappearance of anoxic crises. In 1998–99 reactive phosphorus (RP) in the water column was up to ca. 3 times as high as in 1989–92. Conversely, dissolved inorganic nitrogen (DIN) was not significantly changed. In 1998–99 the 5 cm sediment top layer at sts. B, C, displayed a significant total nitrogen (TN) and total phosphorus (TP) decrease (TN annual mean: from 1.29 and 2.79 mg DW g-¹ to 0.69 and 1.47 mg DW g-¹, respectively; TP: from 401 and 626 μg DW g-¹ to 360 and 455 μg DW g-¹). A different result was found at st. A which in 1998–99 was colonised by the seagrass Zostera marina L. That station showed a TN sediment increase from 0.25 to 0.67 mg DW g-¹ and a TP decrease from 455 to 350 μg DW g-¹. Station D, which did not show any macrophyte biomass coverage either in 1992–93 or in 1998–99, exhibited negligible differences. Besides the monitoring of biomasses and nutrients, significant changes related to oxygen concentration, water transparency, pH and Eh and sediment rates were also recorded.