Recording the occurrence of trophic level changes in the lagoon of Venice over the ’90s Sfriso Adriano * , Facca Chiara, Ceoldo Sonia, Marcomini Antonio Department of Environmental Sciences, University of Venice, Calle Larga, Santa Marta 2137, 30123, Venice, Italy Available online 12 July 2005 Abstract 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 À 2 , respectively, during 1989 – 90. In 1998 – 99 maximum densities decreased to ca. 0.3 and 0.01 kg WW m À 2 . During that period, phytoplankton also decreased significantly both in peaks (Chl a : from 58–86 to 4.0–3.5 Ag dm À 3 ) and mean values (Chl a : from 9.1–10.3 to 1.3–1.4 Ag dm À 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 À 1 to 0.69 and 1.47 mg DW g À 1 , respectively; TP: from 401 and 626 Ag DW g À 1 to 360 and 455 Ag DW g À 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 À 1 and a TP decrease from 455 to 350 Ag DW g À 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 E h and sediment rates were also recorded. D 2005 Elsevier Ltd. All rights reserved. Keywords: Macroalgae; Phytoplankton; Nutrient concentrations; Environmental variables; Venice lagoon 1. Introduction Over the last three decades, marine coastal ecosystems experienced a progressive environmental decay at a world scale (Schramm and Nienhuis, 1996; Morand and Briand, 1996). Lagoons, bays and estuaries were affected by abnormal inputs of pollutants and trophic substances such as phosphorus and nitrogen compounds which favoured an excessive growth of nuisance macroalgae (Fletcher, 1994; Bach et al., 1978; McComb et al., 1981; Birch et al., 1983; Lee and Olsen, 1985; Frederiksen, 1987; Ro ¨ senberg et al., 1990; Sfriso et al., 1992; Rijstenbil and Haritonidis, 1993; Den Hartog, 1994; Marcomini et al., 1995; Schramm and Nienhuis, 1996; Morand and Briand, 1996; Sfriso and Marcomini, 1996a, 1997, 1999; De Casabianca et al., 1997, 2002). The lagoon of Venice particularly suffered from this phenomenon especially between the ’70s and the ’80s. During that time, the excessive growth and summer decom- position of the chlorophycea Ulva rigida C. Agardh seriously affected nutrient and pollutant cycles and the biodiversity of 0160-4120/$ - see front matter D 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.envint.2005.05.009 * Corresponding author. Tel.: +39 041 2348529; fax: +39 041 2348584. E-mail address: sfrisoad@unive.it (S. Adriano). Environment International 31 (2005) 993 – 1001 www.elsevier.com/locate/envint