As u/AncientHistory has already pointed out (and r/Italy has been snickering about for some time) it would seem that Reddit and Twitter have combined two viral social media posts into one story: surprisingly clear venetian canals, and the sighting of dolphins in the port of Cagliari.

But this doesn't change the fact that human activity has had an enormous impact on the venetian lagoon, especially given the demands of modern transport and consumption. The city's environmental authorities work very hard to safeguard the flora and fauna of the lagoon and preserve its delicate hydrography. While delicate, the city could not have developed as it did without the lagoon and its life, and the city's eponymous Republic (which lasted over a thousand years) certainly could not have existed without this very distinctive geographic feature.

The lagoon is not a homologous body of water. The rivers feeding the lagoon, many of which Venetians redirected over the centuries, cut deep channels in the otherwise shallow sandy bed. High sandbanks just under the surface of the water characterize the lagoon's northern third, while farther to the south are deep basins. The lagoon's salinity is also highly variable, with lows in the summer months and highs in the winter. The different conditions within the lagoon allows for a wide variety of aquatic flora and fauna, both migratory and permanent, ranging from flounder to sea horses to sturgeon and sea bass. There is also a great variety of bird life in the lagoon, from aquatic birds like herons, marsh harriers, and kingfishers, as well as birds not typically associated with aquatic environments, like turtle doves and starlings.

Human habitation in the lagoon has long had a substantial impact on its flora and fauna. While the homes of the early Venetians were built on existing sandbars and the densest communities in Republic's early history were on the lagoon edge, already in eleventh century the early Venetians exploited the lagoon to practice aquaculture, seasonally closing off narrow channels with fine nets in order to capture migrating fish for consumption. In the 19th century, some of these "fishing valleys" ("valli da pesca") were permanently closed off, with new fish only introduced artificially.

Thus human activity in the lagoon was dependent on the hydrography of the region for centuries: the lagoon was a vital source of seafood for the growing city, while the surrounding rivers would be used to develop rice paddies. Birds would be hunted in the lagoon’s reeds, and the marshy soil of the lagoon’s shores and islands (while not particularly nutritious) allowed for the growth of orchards.

The modern city of Venice is, itself, a great act of human engineering. Few of the sandbars on which the city is built would have seemed suitable for human habitation by Venice's early residents. Indeed, for much of the Republic’s early history people lived in scattered communities mostly located in the northern lagoon's firmer ground, in and around the island of Torcello. The issue was that Torcello's firm ground rises up out of the shallow northern lagoon where the marshy terrain can't quite decide if it's land or water; and marshy ground breeds malaria, dangerous for a growing city, and importantly the northern lagoon is also unfit to dock increasingly large mediterranean galleys.

Thus modern Venice grew from a fortress policing navigable channels located at the lagoon's center. A few houses on the "High Bank" (the Rio Alto, or Rialto) overlooking a canal and wide basin would soon become the city's economic center (and that canal could become the Grand Canal, and the basin the Basin of St. Mark) while over the centuries stone pavement would come to cover the city's marketplaces, church squares, and streets (more or less in that order). Organic refuse from human activity in the city might have attracted new breeds of scavenger birds and fish, however on the whole the paving of sandbars and canal-banks was ultimately detrimental for aquatic life, destroying tidal habitats and impeding the growth of reeds and seaweeds.

It's certainly easier to unload people and goods from boats onto a paved surface instead of a sandy bank, and besides, walling in the banks of canals with bricks guarantees a swifter flow of water. Indeed, still water is very dangerous to humans: stagnant water breeds parasites and harmful diseases. But the Venetians soon found they would have to do much more to keep the water of the lagoon flowing.

The venetian lagoon has been on the earth for about the same time as mankind. In geological terms, it is ephemeral. This became clear to Venetians as early as the 14th century, when a solution was deemed necessary to eliminate accumulation of sediment from branching delta of the river Brenta. The same river whose flow had carved the grand canal and whose sediment had built the Rialto was now dangerous to the increasingly dense community, swamping canals in the most literal sense of the word. Thus the river's branches were directed into a single estuary some ten miles to the south, away from the city.

Over the next two centuries the river Brenta would be further subject to a series of branching canalizations as the Republic of Venice came to govern the mainland, turning the river into an aquatic highway linking the city to its hinterland. But in the 16th century, almost concurrently with Europe's economic activity moving westward and away from Venice, so did the lagoon proceed to become more hostile to its inhabitants. Thus a host of other rivers (even a branch of the Po delta, which threatened to overflow into the Adige River) were all redirected. Rivers to the north of the lagoon were directed into the flow of the Piave and the Sile, while in the south, they were directed into the Adige. These three rivers were themselves canalized and directed into the Adriatic Sea (as well as the aforementioned branch of the Po).

The reduction of freshwater inflow certainly had an impact in terms of the kind of wildlife the lagoon could support. But it would be other artificial developments which would have an even greater environmental impact, this time from across from the lagoon. It was since the unity of Italy in 1861 that discussions of revitalizing Venice began, thoroughly unsuitable as the city was for the needs of a modern industrial economy. In 1917, a group of shrewd entrepreneurs took advantage of wartime mobilization (and the sentiment that, "Something Ought To Be Done For Venice") to collaborate with the local and national government in order to create a large industrial harbor just across the lagoon from Venice, in the community of Marghera. The ambitious project was served by two new railway trunk lines and a new maritime canal dredged through the heart of the lagoon. The project also entailed constructing massive dockyards jutting into the water, pouring cement over the lagoon's sandy banks and dredging deep canals. Over the next ten years, the harbor of Marghera became one of the region's premier industrial hubs. While it was heavily bombed during the Second World War, Marghera was nonetheless chosen to become a regional hub for Italy's postwar state-sponsored entities, with expanded rail yards and new shipyards, as well as a series of petro-chemical plants which would become notorious for lapses in worker safety. In the 1960s, a new deepwater canal was dredged through the lagoon, purposefully measured to accommodate modern oil tankers. The 1960s also saw the expansion of Venice's airport by infilling parts of the lagoon, as well as the dredging of a new canal connecting said airport to the city.

Given all this, since the Second World War the lagoon's historic flora and fauna has found it difficult to survive in and around the city, among the oil tankers, cruise ships, taxi-boats, and pleasure craft. But on the edges of the lagoon, away from the dredged canals and human activity, plant and animal life still survives. The northern third of the lagoon, thanks to its very shallow bed difficult to navigate for all but the most seasoned venetian boaters as well as its sparsely inhabited communities, is the part of the lagoon that has retained its most naturalistic character. When Ernst Hemingway wrote the novel Across the River and into the Trees, it is the north part of the lagoon that he has his characters visit for a hunting trip, and the author's terse prose is nonetheless particularly appreciative of the lagoon's flora and fauna. This part of the lagoon most resembles where the first Venetians would have lived over a thousand years ago.

Just for everyone's edification, the photos going around are from Cagliari, Sardinia, not Venice. Answers should address what marine life has historically existed in the canals of Venice.