Decline of Ulva growth in the lagoon of Venice

Trophic Conditions in the Waters of the Venice Lagoon (Northern Adriatic Sea, Italy)

“…Its environmental conditions are frequently monitored and deeply studied in order to prevent dystrophic crisis occurrences, which might damage the fishing activity and threaten human health. After the experience of the U. rigida proliferation in the 1980s several actions were taken to reduce its growth and development (biomass harvest, phosphorus ban), even though the climatic changes appeared to have been the main hampering factor [12,16]. Some significant changes of the lagoon conditions were already observed back in 1998, when, in the central lagoon some parameters, which are strongly related to the macrophytes (oxygen saturation, pH and E h ) decreased progressively in the water column [12,27].…”

“…At the end of the 1990s, in fact, the nitrate load in the Venice lagoon was estimated to be over 3000 t y -1 (~ 75% from the drainage basin and 25% from atmospheric deposition), the ammonium ~1000 t y -1 (~40% from the drainage basin and 60% from atmospheric deposition), the total phosphorus ~300 t y -1 and the phosphate ~80 t y -1 (80% from the drainage basin and 20% from atmospheric deposition) [15]. During the 1990s also the environmental conditions of the lagoon changed again, as seaweed blooms declined [12,16], mainly because of climatic changes, the increase of sediment re-suspension and the above cited reduction of nutrient loads. [14] late 1990s 4000 380 All sources [15] 1999 1050 150 Industrial [13] This paper aims at investigating temporal, seasonal and spatial variations in the lagoon trophic status following the implementation of environmental legislation; this was achieved via the measures of nutrients, chlorophyll a and macrophyte coverage which were carried out once in the whole lagoon and monthly in two stations.…”

Green and golden seaweed tides on the rise

“…Thus, although water quality in Tokyo Bay has improved, Ulva green tides have increased; the species responsible overwinters as unattached thalli drifting at the sediment surface of shallow waters 22 . The notorious Ulva blooms of Venice in the late 1980s, however, are no longer such a nuisance 23,24 , even though nutrient concentrations -in particular nitrate -have not significantly diminished in the subsequent decades 24 . As biomass accumulation is a function of the seed population multiplied by its growth rate, spreading seed banks of overwintering, free-floating strains of local Ulva could obviate the effects of reducing eutrophication because they are protected from the many Ulva grazers (such as snails and crustaceans) that live on the sea floor 25,26 .…”

“…From satellite images, an estimated one million tonnes wet weight of Sargassum is exported to the Atlantic each year 36 . For comparison, about the same mass of Ulva was collected and disposed of on land during the green tide of the 2008 Olympics 3 , and a similar amount was estimated to have accumulated in the Venice lagoon during the peak outbreak 23 .…”

“…If carried out in the early stages of the bloom, this technique could mitigate the magnitude of the beaching events. In other regions, techniques to collect Ulva masses in shallow water from special ships by rakes, nets or suction pipes could be developed and one technique is already in operation in the Venice lagoon 23 . Transporting the Ulva biomass to processing factories from harbours should be much cheaper and easier than using rakes and tractors on beaches; the quality of the raw material will also be superior because it will be fresher and contain less sand.…”