Estuarine, Coastal and Shelf Science (2002) 54, 773–792 doi:10.1006/ecss.2001.0848, available online at http://www.idealibrary.com on Changes in Abundance and Composition of Phytoplankton and Microphytobenthos due to Increased Sediment Fluxes in the Venice Lagoon, Italy C. Facca a , A. Sfriso a , G. Socal b a Department of Environmental Sciences, Calle Larga Santa Marta 2137, 30123 Venice, Italy b Consiglio Nazionale delle Ricerche, Istituto di Biologia del Mare, Castello 1364/A, 30122 Venice, Italy Received 8 August 2000 and accepted in revised form 28 May 2001 By comparison with measurements carried out in 1990–91, phytoplankton biomass in 1998–99 appears strongly reduced in the central basin of the Venice lagoon. Chlorophyll a concentrations, which in the past easily reached 100 g dm 3 , reached only 5 g dm 3 during this investigation. Phaeopigment concentrations were close to the detection limit. The introduction of the bivalve Tapes philippinarum Adams and Reeve in the lagoon and its harvesting by hydraulic and mechanical dredges strongly increased the amount of sediment resuspension and settlement. This phenomenon caused a marked increase in water turbidity and the disruption of the benthic microlayer of the lagoon bottoms composed by bacteria, microphytobenthos and small macrophytes. Significant correlations between microphytobenthos abundance and the suspended and settled sediment, and with the underwater light availability were found. In the areas affected by the highest sedimentation fluxes significant negative correlations were also found between the abundance of the microphytobenthos and phytoplankton communities. Many benthic taxa such as Amphora, Cocconeis, Navicula, Nitzschia, Pleurosigma and Thalassiosira, were found in all the water column samples and they were more abundant than exclusively planktonic diatoms.  2002 Elsevier Science Ltd. All rights reserved. Keywords: Phytoplankton, microphytobenthos, diatoms, sediment resuspension, Venice lagoon Introduction In the last four decades the lagoon of Venice has undergone marked changes in the composition of its primary producers. Since 1960 the seagrass meadows which covered the whole lagoon have begun to dis- appear from the central basin (Sfriso et al., 1987; 1992a,b and references therein). This area is affected by inputs both from the mainland (chemicals and nutrients from the industrial area and the agricultural drainage basin and urban sewage from the city of Mestre), the city of Venice and its major islands. Moreover (10–20 m) deep and (100–200 m) wide canals were dredged for commercial ships to gain the access to the industrial area of Porto Marghera (Sfriso et al., 1992a; Pavoni et al., 1992; Marcomini et al., 1995). The shallow bottoms of the lagoon favoured a marked increase of nitrophilic macroalgae, especially Ulva rigida C. Ag., which, between 1960 and 1990, was the main producer in the central part of the Venice lagoon (Sfriso et al., 1992a; Sfriso & Marcomini, 1996). During this period phytoplankton showed high concentrations especially near the industrial area (Sfriso & Pavoni, 1994) and in the deepest canals, especially around Venice, where macroalgal growth was prevented by light reduction. In these areas phytoplankton showed chlorophyll a (Chl. a) concentrations ranging from 10 to 60 g dm 3 (Sfriso & Pavoni, 1994; Socal et al., 1999). The highest blooms (especially Skeletonema costatum and Cylindrotheca closterium) were usually observed in the late winter, before the quick increase of Ulva biomass, and after its collapse that occurred in May–July (Sfriso et al., 1988; Bianchi et al., 2000). After macroalgal collapse Chl. a concentrations exceeding 100 g dm 3 (Sfriso et al., 1987; 1992a and references therein) were frequently found. Since 1990 the distribution of macroalgae has decreased, mainly due to climatic changes (unfavour- able weather conditions extending winter-like con- ditions up to May–June) and other synergystic factors (Sfriso, 1996; Sfriso & Marcomini, 1996). As re- ported by these authors unfavourable growth con- ditions monitored in late spring during consecutive years have reduced the distribution and production of Ulva rigida C. Ag. in the Venice lagoon. The a To whom correspondence may be addressed (sfrisoad@unive.it) 0272–7714/02/050773+20 $35.00/0  2002 Elsevier Science Ltd. All rights reserved.