The weed in the Venice Lagoon is mostly sea lettuce, the flat green alga that lagoon fishers call salata, together with the fine red filaments they call pelo rosso, literally red hair. A study published in Marine Policy in 2026 interviewed 31 lagoon fishers and recorded 33 local names covering 26 different algae in 11 families, eight of which had not appeared in any earlier survey of algal impacts in the lagoon (Mendoza et al., 2026, p. 3).
That study is the reason this page can name what you are looking at. The interviews ran between February 2022 and April 2024 at fishing stations near Lido Malamocco, Lido Faro, Punta Sabbioni, Murano, Burano, Sant'Erasmo, Sant'Elena and Chioggia, with people who had been fishing for between six and more than sixty years (Mendoza et al., 2026, p. 2). Four boat trips were made to collect the algae, and the specimens are kept dried in the Ca' Foscari herbarium in Venice.
What follows is their account: which weed is which, why it fills the shallows, why it smells when it rots, what it does to nets and engines, and how the seagrass meadows it displaced were replanted in the northern lagoon.
What is the seaweed in the Venice Lagoon?
Most of the loose green weed in the lagoon is sea lettuce, Ulva australis and Ulva rigida, which fishers call salata or veli. It was the most widely named weed in the study: 13 of the 31 people interviewed mentioned it, more than twice as many as any other (Mendoza et al., 2026, pp. 3 to 5). After it come the sticky filamentous mats, pelo vischio and pelo oglio, each named by 6 people, the mucilage that fishers call mucillagine, named by 5, the fine green threads of Chaetomorpha linum called sea or cea, named by 4, the red Polysiphonia called alga rossa or alghe americane, named by 3, and Agardhiella subulata, called veleta, named by 2.
Those figures count people, not plants. They say how many interviewees brought a weed up in conversation, and nothing about how much of it grows in the water. The study is an ethnographic one, built from what fishers know rather than from measured cover, and its authors are explicit about that.
One local name can also cover several species. The paper gives the example of pelo rosso, which may refer to any of a number of Gracilaria algae unless the speaker adds a distinguishing detail (Mendoza et al., 2026, p. 3).
Salata, pelo rosso and pelo vischio: the names on the water
The Venetian names describe what the weed looks like and what it does. Salata is the lettuce one, flat and bright green, and one fisher described it in exactly those terms: green ones, like lettuce. Pelo rosso means red hair, and covers the fine red Gracilaria filaments. Pelo oglio, also written peo ojo or pelo verde, is the one fishers say forms like a green carpet over the salata beneath it.
Each name carries a use or a complaint. Veleta, Agardhiella subulata, grows everywhere and shelters shrimp and the small fish called acquadelle, Atherina boyeri, while also blocking nets and fouling boats. Gracilaria, under the names pelo rosso and barba di frati, was described as a hiding place, as a source of material for medicine and pharmaceuticals, and as the base of natural banks of clam seed and oysters. Sea or cea, Chaetomorpha linum, was the one weed named for cooking (Mendoza et al., 2026, pp. 3 to 5).
Two of the best known invaders appear under their own names. Sargassum muticum is alga giapponese, the Japanese weed. Together with Undaria pinnatifida it grows on the stonework of the historic centre, where the paper notes it creates significant navigational difficulties, though its effect is small beside that of the two Ulva species (Mendoza et al., 2026, pp. 7 to 8).
Why the lagoon smells when the weed rots
The smell visitors notice on a hot, still day in a back canal is the weed decomposing. Of the 31 interviewees, 5 said salata produces a bad smell as it rots, 2 that its decomposition makes the water rot, and 2 that it takes oxygen out of the water (Mendoza et al., 2026, pp. 3 to 5). Again, those are counts of people who raised the point.
The mechanism they describe is straightforward. A thick mat of weed cuts light and water exchange, then dies back and is broken down by bacteria that consume the dissolved oxygen as they work. Fishers said that alga rossa, peo oglio and salata leave the lagoon floor muddy and sterile, and that mucilage suffocates everything, allowing less oxygen into the sediment and harming the seagrasses underneath (Mendoza et al., 2026, p. 7). The authors add from the wider literature that the two Ulva species can trigger anoxic events, meaning stretches of water with almost no dissolved oxygen left.
This is a summer problem in shallow, poorly flushed water. The lagoon averages about 1.2 m deep, and currents run at 100 to 120 cm per second in the inlets but only 0 to 10 cm per second in the smaller canals, so weed accumulates where the water barely moves (Mendoza et al., 2026, p. 2).
What the weed does to lagoon fishing
Asked what the algae had cost them, the interviewees raised effects on fishing methods and activities 42 times, on equipment 35 times and on the people in the trade 28 times (Mendoza et al., 2026, p. 7). Within the first group, a decline in catch came up 13 times and changes in fishing method 10 times. Within the third, the decline in the number of fishers came up 17 times and the loss of boat builders twice.
The equipment complaints are specific. Pelo rosso, the larger invaders such as Sargassum muticum and Undaria pinnatifida, and the red alga fishers describe as looking like a dragon's tail block nets and ruin boat engines, making it harder to fish for cuttlefish, brill and squid. Nets fill even in bad weather, and the remedy is to bring them ashore and clean them by hand and with a pressure washer (Mendoza et al., 2026, p. 8).
Some gave up on the lagoon altogether. One clam farmer told the interviewers that the algae reproduced so heavily that the farm was abandoned for sea fishing instead. Changes in practice of that kind, including moving from the lagoon to the open sea and fencing farms, came up 6 times, and gear maintenance 9 times.
The seagrass meadows the weed replaced
The weed matters most because of what it grew over. Interviewees said the lagoon's seagrasses, which they group under the name Posidonia and which are properly Cymodocea nodosa, Zostera marina and Nanozostera noltei, were very abundant before the 1980s, and listed what they did: they held the sediment and slowed erosion, fed migratory birds, put oxygen into the water, sheltered fish and shrimp, and served as spawning ground for cuttlefish (Mendoza et al., 2026, p. 7).
The decline is attributed to Ulva overgrowth and mucilage together with clam harvesting and nurseries. One fisher dated the spread of salata to 1993 and 1994 around the three islands of Poveglia, Sacca Sessola and Santo Spirito, where the cutting of the Malamocco to Marghera industrial canal fifteen years earlier had reduced the water exchange so that the channels no longer discharged as they had (Mendoza et al., 2026, p. 6).
A second account dates a collapse precisely. In July 2001 the water of the northern lagoon turned a strange green colour, and a Murano fisher described fish, plants, molluscs and mussels dying and the seagrass disappearing with them. From that year, he said, the fish were gone from that part of the lagoon.
- Before 1980Seagrass very abundantMeadows hold sediment and shelter fish
- 1993 to 1994Salata spreadsAround Poveglia, Sacca Sessola and Santo Spirito
- July 2001Green water in the northFish, plants and molluscs die
- 2002Ecosystem goneThe year the fisher dates the loss from
- 2014 to 2018LIFE SeResto replantingSeagrass transplanted in the northern lagoon
Sources: Mendoza, J. N., Pruse, B., Mattalia, G., Kochalski, S., Prakofjewa, J., Buosi, A., Ciriaco, A., Primavera, F., Martini, A., Bittner, M. V., Soukand, R., & Sfriso, A. (2026); European Commission (2026).
How the meadows came back in the northern lagoon
The replanting began with fishers rather than with scientists. A Murano interviewee described asking the interregional public works authority for Veneto, Trentino Alto Adige and Friuli Venezia Giulia for permission to plant hundreds of sods of seagrass, each 23 cm across, where the meadows had been before. The sods took. The initiative was taken to Ca' Foscari University, an expert there saw that it was working, and the result was a European project (Mendoza et al., 2026, p. 8).
That project is LIFE SeResto, reference LIFE12 NAT/IT/000331, coordinated by Ca' Foscari's environmental sciences department and running from 1 January 2014 to 30 April 2018. Its purpose was to restore priority habitat 1150, coastal lagoons, which covers about 3,660 hectares in the northern part of the lagoon, by transplanting submerged plants (European Commission, 2026). The official record notes that earlier rules had already cut the nutrient inflow from the watershed and regulated clam harvesting, but that poor water circulation in the choked parts of the lagoon stopped the meadows recolonising on their own.
The fisher's verdict was that the northern lagoon is back to how it once was, with go fish and eels returned, though he said flounder remain fewer, having been preyed on by cormorants. Two interviewees also reported less pollution than thirty years ago and less salata than there used to be.
How many seaweeds the lagoon actually has
The fishers' 26 algae are the ones that get in the way. The full botanical count is larger. A survey of the lagoon's photosynthetic life carried out in three campaigns between 2023 and 2025, and identified with DNA as well as by eye, found 81 taxa in all: 13 green algae, 11 heterokonts including the brown algae, 28 red algae and 29 vascular plants, the last group covering both the seagrasses and the salt marsh plants (Drigo et al., 2026). On those figures the lagoon holds roughly 52 seaweeds against 4 seagrasses.
That survey also found 9 of the 35 non-native species previously recorded in the lagoon, and recorded the yellow-green alga Vaucheria litorea for the first time. Its sharpest result concerns the cordgrasses of the salt marshes: the non-native Sporobolus anglicus proved more widespread than the native Sporobolus maritimus, and the hybrid Sporobolus x townsendii was confirmed in the central and southern lagoon. This work is a preprint and has not yet been peer reviewed, so its counts should be read as provisional.
The same source calls the Venice Lagoon the largest transitional environment in the Mediterranean Sea and an important biodiversity hotspot. For the plants rather than the weed, see the companion page on what grows on the barene and under the water.
