Until the 1970s, the Orbetello Lagoon ecosystem was mesotrophic, richer in nutri-ents than the sea, but without excesses. Marine angiosperms were widespread and fish, especially eels, were abundant (Lenzi et al., 1998). However, effluent from do-mestic wastewater treatment plants and four fish farms flowing into the lagoon and the Albegna River (a tributary that drains a vast area of agricultural land) increased the system’s eutrophic status. High nutrient input in these years led to the increasing-ly frequent proliferation of opportunistic, fast-growing macroalgae (Bombelli and Lenzi, 1996; Lenzi et al., 2003). Traditional fishing using fixed nets and installations in seaward channels was hard hit, which is reflected in a sharp decline in the catch of eels (Anguilla anguilla) and mullet (Mugil cephalus) (Lenzi and Solari, 2002). This prompted the lagoon fishing company to begin intensive aquaculture of sea-bream (Sparus aurata) and sea-bass (Dicentrarchus labrax) in order to survive financially.
In an eutrophic environment with low water turnover –an environment common to most Mediterranean lagoons– the excessive proliferation of macroalgae is not con-trolled by primary consumers, due to the rapid turnover of vegetation. Thus energy does not flow in the grazing chain and most of the biomass ends up as detritus on the bottom. The detritus chain also suffers, since organic detritus accumulates too fast to become available to higher trophic levels. As a result of these bottlenecks, organic detritus accumulates in sediments, favouring anoxia, the sulphate-reductive mineralisation of organic matter, and ultimately dystrophic conditions (Fig. 3.14).
Fig. 3.14 Possible management interventions at different levels of the lagoon ecosystem.
176
What can be done to counteract the effects of eutrophication? The first step should be to reduce the external input of nutrients (Fig. 3.14, point 1), which is not always possible, especially when the area is intensively settled. In the case of Orbetello Lagoon, phyto-treatment ponds were built between the nutrient sourc-es and the lagoon. Thsourc-ese ponds reduced nitrogen and phosphorus in urban wastewater by about 80% (Fig. 3.15) after a mean residence time of about 30 days (Lenzi et al., 1998). Reductions of only 10-15% were recorded for fish-farm wastewater treatment ponds, because of low mean residence times ranging from 0.3 to 2.5 days (Porrello et al., 2003a,b, 2006; Gennaro et al., 2006). Local au-thorities later built a pressurised pipeline to dispose of treated urban wastewater offshore. This solution was not accepted unanimously, because the nutrients could upset the local trophic balance of the sea.
Fig. 3.15 Phyto-treatment pond (a specifically separated marginal area of the lagoon) for urban waste-water. The green colour is due to blooming of the cyanobacteria Spirulina sp. and Lyngbya aestuarii.
Another solution which was immediately implemented was the annual harvesting of algal masses (Fig. 3.14, point 2) using special boats. Harvesting only removed 5-10% of the major spring-summer macroalgal standing crop: four boats with a capacity of 2 tonnes each worked 8 hours a day, 6 days a week (Lenzi and Mattei, 1998; Lenzi, 1998; Lenzi et al., 2003) (Fig. 3.16). This biomass was disposed of in landfills after partial dehydration. Unfortunately, no industrial use has yet been found for it. Its low value makes handling and processing uneconomic.
Other solutions for preventing dystrophic crises were pumping sea water into the lagoon and excavating submerged channels which were intended to counteract
177
the accumulation of organic matter in the sediment, and to improve water col-umn parameters (Fig. 3.14, point 3). Pumping was an effective but expensive management solution. It created a one-way current towards one of the outlets which counteracted stagnation in the lagoon centre. Submerged channels con-veyed pumped water into central areas, and are thought to provide a refuge for wildlife during critical periods. However, since they clogged with organic mat-ter, silt and clay faster than they could be dredged, after a few years the channels became sources of nutrients and hydrogen sulphide.
A purely biological management solution was to increase primary consumers (Fig. 3.14, point 4). Being low in the food chain, feeding on algae, small crusta-ceans, molluscs and organic detritus, grey mullet and sea bream could transfer energy that would otherwise enrich sediment with usable biomass. Sea bream fry were purchased and raised intensively in nets in a special pond until they weighed 80-100 g. They were released into the open lagoon in spring, where they could grew to commercial size (400-500 g) over 6-7 months while the fish-eating birds were absent. They were then caught in the fixed installations at the lagoon outlets. This system of combined intensive and extensive aquaculture is known as integrated aquaculture.
Environmental crises were especially severe in the 1990s, and the eel and mullet populations were decimated. Even after 15 years of remediation, the fisheries are not recovering, although the introduction of more than 500,000 bream fry has finally put fishing back on its feet and saved the fishing com-pany. Bream has now become the main species, but the imbalance in fish popu-lations indicates that the environment has still to be fully restored. Although the lagoon environment affects fish populations, the opposite is also true, and the fact that environmental quality influences lagoon populations and vice versa can be directed in a positive way by appropriate management. Work is currently being done on the introduction of grey mullet, a species which is more difficult to reproduce artificially, and whose fry are not commercially available. In their search for species that interact at low levels of the food chain, the researchers are also evaluating the possibility of introducing two al-lochthonous (Red Sea) herbivores: the rabbit-fish species Siganus luridus and Siganus rivulatus. These species consume macroalgae and could first be tested in the phyto-treatment ponds of the fish farms, where macroalgae are prolific.
If compatible, they could be introduced directly in the lagoon. Siganus species are not of commercial interest, but once they have completed their ecological task as primary consumers, they could be transformed into fishmeal for fish farms. This would make intensive aquaculture more ‘eco-compatible’; it is currently energy-intensive and depletes marine fish stocks using fish-meal that could be consumed directly by humans. Integrated aquaculture has been wel-comed by the fishermen of the Orbetello Fishermen’s Cooperative, but it has proved hard to dissuade them from introducing the fry of carnivorous fish,
178
such as sea bass, which command higher prices. This would be bad manage-ment, as carnivorous fish would cause a decline in primary consumers, de-creasing the flow of energy from autotrophs to higher trophic levels and in-creasing the risk of dystrophic events.
Finally, Lenzi et al. tested an innovative sediment management system that influenc-es nutrient release (Fig. 3.14, point 5). The rinfluenc-esuspension of sediment occurs fre-quently in shallow lagoons where relatively large boats are active. It was found that these boats disturb the upper 5 cm of sediment (Lenzi et al., 2005). Sediment resus-pension leads to the oxidative mineralisation of organic detritus, decreasing the risk of dystrophic events (Lenzi et al., 2010). During resuspension, orthophosphates are blocked as insoluble salts, and may be adsorbed by clays and carbonates and bind to ferric oxy-hydroxides (De Jonge and Villerius, 1989; Dodge et al., 1984; Golter-man, 2001). The removal of orthophosphates limits the phosphorus available to al-gae, while the decrease in algae and reduced risk of dystrophic events favours the return of angiosperms, which are not limited by nutrients since they have roots.
Fig. 3.16 Macroalgal harvesting using special boats.
The solutions applied have helped overcome the environmental crisis, to maintain a local economy based on fish, and to allow a traditional activity to continue, albeit with methods modified to combat eutrophication and in line with the changing fish market (the wholesale prices of bream and bass have halved in Italy since the 1990s).
Besides adopting new methods such as intensive aquaculture to provide a quantity of fish beyond the natural limits of the lagoon, new tools for environmental manage-ment and research, and integrated aquaculture with its positive environmanage-mental ef-fects, a new marketing approach has also been tried, with fishermen selling direct to consumers and fish processing being undertaken to add value to the final product.
179