TECHNIQUES IN THE ENVIRONMENTAL RESTORATION PROGRAMME OF THE VENICE LAGOON:THE PALUDE DELLA ROSA PILOT PROJECT 1

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1 ANDREINA ZITELLI, PIERLUIGI ROSSETTO* TECHNIQUES IN THE ENVIRONMENTAL RESTORATION PROGRAMME OF THE VENICE LAGOON:THE PALUDE DELLA ROSA PILOT PROJECT 1 The aim of the Pilot Project was to set up an experiment to recover the environment of a sub-basin of the Venice Lagoon. The Palude della Rosa is a shallow brackish marsh in the transition zone of the Northern Venice Lagoon. The final goal was to arrest the dystrophic processes and encourage the spontaneous productivity of the ecosystem through guided restoration actions. The programme was formulated in successive phases. One preliminary step was carried out from May 1991 until June 1992 in order to identify the priorities in the general goals and to plan the monitoring programme and experiments, to organise a data-base of the pre-existing scientific knowledge and to design structures and equipment for the experimental phases. The first experimental phase started in winter 1992 and was concluded in December This entailed acting on the hydrodynamics of the chosen area and setting up the monitoring programme on the physical, chemical and biological parameters to describe the natural conditions of the ecosystem (water column and sediments). Indeed an extensive study of the ecological characteristics of the Palude della Rosa and of the surrounding area was considered necessary to verify the response of the environment to the restoration actions. A second operational phase followed directly on from the first from December 1993 to June From December 1993 to February 1994 the preliminary rehabilitation action was carried out. This involved mechanical treatment of the sediments including ploughing, sand addition and tilling. March to June 1994 saw the substantial extension of an experiment in phanerogam transplanting to reintroduce the almost extinct native Zostera noltii species. During this second operational phase, concluded in June 1995, the monitoring study was continued and developed. The effects of the demonstration restoration project were complemented by specific research on the benthic fluxes and nitrogen balance in the brackish marsh supported by the Ministry of Universities and Scientific and Technological Research (MURST: Venice Lagoon System Project). As well as monitoring the spontaneous evolution of the ecosystem, future work may also include extending experiments in biological restoration with a * Andreina Zitelli, professor at DAEST (Department of Economic and Social Regional Analysis) of the IUAV (Venice Architecture School). Pierluigi Rossetto, technical manager of the Consorzio Thetis, Venice, Italy, a consortium of italian companies and istitutions for the application of new technologies to marine environment 1 The Venice Water Authority received a proposal from DAEST-IUAV, for an experimental pilot project for the biological recovery of the Venice Lagoon. The proposal was drawn up by Prof. Andreina Zitelli of DAEST, in charge of the scientific aspects of the project, Dr. Valentino Fossato of CNR Venice - Institute of Marine Biology; Dr. Roberto Bertaggia from the Department of Ecology and the Environment, Veneto Region and Dr. Massimo Cardinaletti from ASPIV, Venice. In 1988, this was presented to the Interministerial Committee as per Article 4 of Law 798/84. Following a decision taken by the Committee on October 4th 1989, the programme is being implemented by the Consorzio Venezia Nuova, who have appointed the Consorzio Thetis to carry out the work, together with DAEST-IUAV with the involvement of the academics who drafted the proposal 1

2 view to reorganising some levels of the trophic chain and to management of the restored ecosystem. Programme objectives Palude della Rosa Deterioration of the Venice Lagoon is known to have many causes. The main ones are the higher concentrations of pollutants, nutrients and organic material in the water and sediment, partly as a result of their granulometric and chemical-physical characteristics. Modified lagoon hydraulics, morphology and hydrodynamics and poor maintenance of the channels ensuring peripheral circulation are all factors that accentuate environmental deterioration. This has repercussions on the plant and animal communities with simplification of the trophic chains and the loss of levels of ecosystem complexity. The programme of measures to restore the Venice Lagoon is inevitably complex and far reaching. Based above all on the concept of management, priority must be given to intermediate objectives involving recovery and environmental quality. The measures are aimed at developing operational and management techniques and methods to correct a number of the system's physical and chemical characteristics, so reorienting the high level of primary productivity towards greater biological diversification and more trophic chains. A number of factors influenced the choice of the Palude della Rosa as the experimental site for this pilot project demonstrating the application of operational technologies and models that can be applied to recover Mediterranean coastal zones affected by eutrophication. An experimental multi-year programme of environmental restoration actions was then implemented. The programme included an integrated monitoring campaign, measures to improve the morphological and hydrodynamic condition of the chosen environment and environmental manipulation aimed at encouraging the reintroduction of native plant and animal species. The demonstration site, selected on the basis of its specific characteristics, is known as the Palude della Rosa. It is situated to the north of the island of Torcello in the north of the Venice Lagoon. This 3.5 km 2 pool of salt water has retained the morphological characteristics typical of a transitional area in the north lagoon. It is a shallow open basin surrounded by salt marshes (barene, particular land habitats with typical halophilic vegetation) and mud flats (velme, particular land habitats within the tidal range). The number and type of species depends on the maturity of the salt marsh and the degree of water salinity and is influenced by freshwater coming from the Canale Silone which drains a large basin known as the Bonifica Veronese. Limited sections of the salt marsh at the start of the Canale Silone are eroding 2

3 allowing the fresh water in the canal to mix with the water in the Palude della Rosa. In the south east the marsh is connected to the rest of the lagoon affected by tidal expansion via the main Torcello channel. It communicates with the most northerly areas by means of five tidal creeks (ghebi). These channels are gradually silting up starting from the point where they flow into the marsh and this has altered the original depth to the point where in some cases all traces have been wiped out. In recent years, accumulating organic substances, seaweed and debris and the inflow of nutrient rich fresh water have accentuated eutrophication, typified by the exceptional bloom of seaweed (in particular Ulva rigida). There has been a drop in both the quality and quantity of fish life. Just ten years or so ago this marsh was famous as one of the lagoon's most productive areas. Now secondary productivity in the marsh (with commercially interesting crustacea, molluscs and fish) is in danger of exhaustion, although it still supports the traditional fishing activities of fisherman of the island of Burano. The chosen area does not claim to be representative. However, the fact that its natural characteristics are still so well preserved justifies its choice as a pilot site. The demonstration environmental restoration project can thus benefit from a morphological and environmental base which is still recognisable and above all free from the significant intervention of man. The Palude della Rosa is a well defined sub-basin communicating with the rest of the lagoon through easily identifiable channels. It therefore lends itself well to monitoring of water quality and quantity, chemical and biological exchanges and tidal variations. During an initial development phase, the preliminary proposal presented to the Interministerial Committee as per Law 798/84 was translated into an executive plan. Most of the work carried out in the programme to date has been of a demonstrative nature and can be divided into basic categories as follows: Implementation Information gathering and monitoring aimed at: acquiring bibliographic information on the historical and environmental evolution of the site; surveying of current bathymetry and the sections of the main channels flowing into the basin; monitoring the hydrological, hydrodynamic, sedimentological and biological parameters. Environmental restoration aimed at: containing the water in the Canale Silone along the western side of the marsh by constructing piling to coincide with eroding salt marshes; 3

4 modifying local hydrodynamics by remodelling the Torcello tidal creek which flows into the marsh; treating the surface layer of sediment by ploughing and tilling the shallows and modifying granulometry by adding sand. Biological restoration aimed at: reintroducing native aquatic plants in suitable areas. As mentioned above, this demonstration project is complemented by research carried out as part of the Ministry of Universities and Scientific Research project Venice Lagoon System. To provide a complete picture of the current programme, mention must also be made not just of work completed to date, but also of long term monitoring and the management activities aimed at maintaining and extending work already initiated. Preliminary phase Site knowledge (1993) Preliminary work entailed bibliographic research to collect available scientific information on the environmental evolution of the Palude della Rosa and neighbouring areas of the lagoon. To ensure the success of an environmental recovery project by restoring or moderating exchange conditions, priority is usually given to modifying the hydrodynamic conditions on the site. Between the Palude della Rosa and the rest of the lagoon, it was decided to improve exchange with the sea by remodelling the channels and tidal creeks which link the marsh with the eastern section of the open lagoon receiving water via the Bocca di Lido. In March 1993, the Torcello tidal creek was dredged, restoring the depth to the bathymetric levels of This encouraged water exchange between the marsh and the open lagoon. An artificial barrier was also constructed along the side of the Canale Silone where the salt marshes were exposed to erosion to canalise the fresh water, rich in nutrients, outside the marsh and set up deposition to compensate for the erosion caused by the water in the canal. The objectives of this phase were firstly to gain information on the hydrodynamics and chemical-physical, sedimentological and biological characteristics of the Palude della Rosa and secondly to create a reference against which to evaluate the effects caused by the experimental sediment treatment. The phase involved five monitoring campaigns (in March, June, July, September and December 1993). The results were then processed to calculate total values for the main parameters considered. Brief monthly campaigns were also carried out involving the phytobentos and seaweeds. 4

5 In January 1994, experimental sediment treatment work began. Three neighbouring areas of about 7 hectares were identified within the marsh for the sediment treatment trials involving: ploughing, ploughing-tilling, ploughingcovering with sand-tilling. Wet ploughing on the lagoon bed and other operations were carried out using specially designed equipment and systems and methods compatible with the shallows in the chosen area. The aim of the treatment was to achieve a lasting change in the contact zone between sediment and water to improve exchanges and interface kinetics. Marine phanerogams are plants that play an important role in recolonisation and are indicators of the successful rehabilitation of coastal areas. According to the oral traditions of local fishermen Zostera noltii was once present on a large scale in the Palude della Rosa although it had been completely absent for at least a decade. An initial experiment carried out in the spring of 1993 into the transplantation of Zostera noltii had demonstrated that plants could be re-introduced into the demonstration area. In the spring of 1994, a large scale follow-up to the transplantation trials began. Zostera noltii was transplanted into various parts of the marsh starting from pre-existing areas and ending with experimental and untreated areas to assess the survival of the species on the various sites. Transplantation was carried out using turfs taken from neighbouring donor sites. The hydrological, biological and sedimentological parameters were monitored up to June 1995 as follows: monitoring before and after the ploughing and tilling to verify short term effects; five medium term monitoring campaigns to verify the effects induced by treatments with respect to the natural trend of the system (March, June, July, September and December 1994) and short monthly campaigns involving phytobentos and macroalgae; two additional campaigns in spring-summer The two years of monitoring of parameters relevant to hydrology, hydrodynamics, sediments and life have considerably extended the knowledge of the Palude della Rosa system. The research focused on the global behaviour of the system, in terms of the exchanges of the Palude with the outside area, of the mass balances and of the mechanisms that characterise its behaviour. Environmental restoration work Monitoring the effects of the measures ( ) Results and future developments Analysis of the large amount of collected data has demonstrated that the Palude della Rosa, together with the network of channels and tidal creeks that feed it and determine its hydrodynamic and trophic condition, is a 5

6 significant subsystem of the northern Venice Lagoon. The setting up of the wooden palisade between the Palude and the Silone canal and the dredging of the Torcello channel enabled the Silone fresh waters, the largest contribution in nutrients from the mainland, to be channelled outside the Palude, and allowed a longer time of residence of the waters with an higher salinity inside the Palude. The sediment treatments produced the overturning of the surface layers of the sediment and increased the surface of the sediment exchanging with the water column by over 40%. Modifications induced by the treatments on the physical and chemical characteristics of the sediment (ph, redox, porosity), on the oxygen demand, on the activity of the enzymes and on the pore water chemical species, are considered positive results. Though carried out in a small demonstration area, the experiment proved the capability of the treatments to contribute to the rehabilitation of sediment quality. Transplantation of Zostera noltii gave positive results both in the survival capacity of transplanted units and in the recolonization capability of transplanted sods. Modifications induced by Zostera noltii via their roots in the physical and chemical characteristics of the upper layer of the sediment (particularly redox and ammonia profiles) have been studied and described, together with the positive contribution of transplanted sods on sedimentation. In conclusion, results so far suggest that the objectives of the next phase of the project should include a general restoration of the water circulation of the Palude area, through the dredging of channels that feed the Palude from the sea front, the reconstruction of marshlands in the erosion areas and the interception of fresh waters carried into the northern basin of the Palude by small channels, all with the aim to eliminate the causes of the deterioration process while creating the basic conditions for future wide scale restoration treatments and measures to be turned to permanent effect. 6