Back in April I commented on how little green I saw in Manhattan (see “A walk along the High Line”); however, I think that Venice probably beats Manhattan by some distance as the most built-upon city I have visited. So close together are the buildings crammed onto the islands that comprise the city that there is very little space for any greenery or vegetation at all. There is, moreover, nowhere equivalent to Central Park where locals can escape the urban fabric. I’ll go one step further and make the bold suggestion that Venice is the only city in the world where the most prolific type of vegetation is algae.
The algal growths are conspicuous at water level all around Venice: green lettuce-like fronds covering the steps, as in my picture below (taken close to Piazza San Marco). I could recognise this as the genus Ulva, which we have encountered in other posts (see “The River Wear in Summer”) but not the species, though a quick hunt on the internet suggests that the most common species in the brackish water of the Venice Lagoon is Ulva rigida, a species that we do have in the UK. I also found an estimate that a million tonnes of U. rigida biomass is produced each year in the Venice Lagoon, largely as a result of the nutrients that enter the lagoon. Venice’s long tradition as a trading centre has resulted not just in the wonderful architecture and art that surrounded me throughout this trip but also in a densely-populated industrialised hinterland, the Veneto, as well as the city itself. Much of the domestic and commercial waste made its way into the Venice Lagoon, leading to high concentrations of many pollutants.
Ulva cf rigida coating steps near Ponte di Paglia, Venice, September 2014.
Phosphorus is an important limiting nutrient for algae, as I have explained in earlier posts, and I was interested to read that the city has banned the sale of phosphorus-containing detergents, in an effort to reduce concentrations washed down the drains and into the lagoon. However, a final twist to the story is that there are also long-term plans to build a flood barrier across the entrance to the lagoon in an effort to reduce the number of occasions when the city is flooded. Yet, when this is completed (expected in 2017) it will not only stop water from the Adriatic coming into the city, it will also inhibit the movement of polluted water from the lagoon into the open sea.
Cuomo, V. Peretti, A., Palomba, I, Verde, A. & Cuomo, A. (1995). Utilisation of Ulva rigida biomass in the Venice Lagoon (Italy): biotransformation into compost. Journal of Applied Phycology 7: 479-485.