School of Graduate and Postdoctoral Studies

Venice really is sinking. Sort of. The home of gondolas, bridges and canals is built across a collection of islands, and the sediments beneath them are gradually compacting, causing the ground to subside. But while the land moves down, climate change is pushing the water up – making the shallow lagoon surrounding the Italian city an increasingly vulnerable environment. To understand what that change means for the lagoon, a team of Western researchers led by Earth sciences professor Alina Shchepetkina dug into the mud this summer, studying the murky waters surrounding the historic city. Shchepetkina, along with graduate students Martina Duarte and Tess Gates-Flaherty, studied tiny organisms that burrow, feed and move through the sediment, leaving behind traces of their lives and providing a record of how a changing environment is affecting the ecosystem. These clues also help scientists understand how similar changes played out in Earth’s not-so-distant past. “These tiny organisms are very sensitive to their environment, so changes in their behaviour can give us clues about changes in the ecosystem,” said Duarte, a geology master’s student. “By analyzing sediment cores layer by layer, we can look back roughly 70 years and see how those changes are recorded in the traces they leave behind.” During 12 weeks in Italy, the research team collected and analyzed sediment cores, working layer by layer with University of Padua professor Massimiliano Ghinassi and the scientists, technicians and students in his lab to reconstruct how the lagoon’s environment and its organisms have changed over the past several decades. This work was supported by a Mitacs Globalink Research Award. The field study is a continuation of research that began last year in Canada, when Shchepetkina and Gates-Flaherty studied sediments and burrowing organisms in the Bay of Fundy on the east coast, between Nova Scotia and New Brunswick. The two locations could hardly be more different. The Bay of Fundy is one of the world’s great tidal environments, with a difference of as much as 16 metres between high and low tide in some areas. The Venice Lagoon is microtidal, with a tidal difference closer to one metre. But that contrast is exactly what makes the two places useful for this research. “The Venice Lagoon is much warmer, with finer, muddier sediment and more extensive marshes, while the Bay of Fundy has stronger waves, much more sediment movement and a wider range of sediment, from mud to pebbles and gravel,” said Gates-Flaherty, a geology master’s student. “We found some similar organisms in both places, but also clear differences in the species and the traces they leave behind.” https://www.youtube.com/watch?v=DVSCozqq1z4

A lagoon under pressure

The Venice Lagoon is particularly vulnerable to climate-driven environmental change. It is shallow, connected to the sea and heavily influenced by human activity. Rising sea levels are putting additional pressure on an ecosystem already altered by centuries of development and more recent industrial activity. The result is a landscape where the city appears to be moving down as the water moves up. “People say Venice is sinking, but that’s only partly true,” said Shchepetkina. “The sediments beneath the city are compacting over time, so the ground is gradually moving down. And that was made worse in the mid-20th century when people were extracting groundwater and gas from beneath the city.” Meanwhile, climate change is pushing sea levels higher, adding another challenge for the lagoon and the city built around it. Recent heat events have also changed conditions in the shallow lagoon. During the team’s fieldwork, water temperatures reached about 32 C in some areas. Warmer water holds less oxygen and can affect salinity, pH and nutrient levels – all of which can put stress on animals living in the sediment. “These organisms are very sensitive to their environment, so as conditions change, we expect to see changes in their burrowing behaviour, some species may disappear altogether and new ones may arrive,” said Shchepetkina. “We even saw large blue crabs, which are a very recent arrival in the lagoon, and we don’t yet know what kind of traces they will leave behind. That’s part of what makes this research so interesting.”

Reading the mud

The organisms Shchepetkina and the Environmental Sedimentology and Ichnology team studies are known as infauna, which are tiny organisms that live within sediment. They include worms and other small creatures that burrow through the mud in search of food, shelter or a place to reproduce. The shape, size and orientation of those burrows can tell researchers something about how the organisms behaved and, importantly, the conditions in which they were living. “If they’re in a non-stable environment, they form escape traces,” said Duarte. “If they are in a calm environment, they behave in different ways. The traces are not simply fossils of animals. They are evidence of animal behaviour often when no other evidence of life exists, and that distinction matters. If scientists can also find that evidence in modern environments, they can then compare to what’s preserved in ancient sedimentary rocks. It turns the mud into a kind of environmental archive. And it’s largely gone unexplored in the Venice Lagoon. “To our knowledge, no one has really looked at the shape and morphology of the burrows in the Venice Lagoon,” said Gates-Flaherty. “There have been studies looking at how organisms move sediment, but not at the actual traces they leave behind. That gives us a new way of looking at how these organisms are responding to their environment and how those responses might be preserved through time and what they mean for physicochemical conditions.” In the field, that means moving through the lagoon by boat, stopping at different environments and collecting sediment cores. The landscape may seem repetitive at first – shallow water, mud, vegetation and channels – but the researchers quickly learned to look for the details. “When you’re out on the boat, you start to see the lagoon differently. You’re looking at the channels, the marshes, the changing water and the different sediments, and you’re asking what makes one spot worth investigating over another,” said Shchepetkina, an expert in environmental sedimentology, ichnology, petrography, and climate change.

Sea change

In the lab at the University of Padua, the sediment cores offer another way to read the lagoon. As layers accumulate over time, they preserve a vertical record of changing environmental conditions – and the traces left by the organisms living within them. By working their way down through the layers, the researchers can look back through the lagoon’s recent history. Gates-Flaherty is examining roughly the past 70-80 years in the lagoon (the time of climate change acceleration) looking at how burrows and the behaviour of organisms have changed through time, while Duarte is studying three different marsh environments, including the transition between tidal channels and marshes as rising water levels reshape the landscape. “Working in the field alongside people who are deeply knowledgeable about the Venice Lagoon has been an incredible experience, exposing Martina and Tess to new techniques, equipment and ways of approaching scientific questions,” said Shchepetkina. “That kind of hands-on collaboration is invaluable because it gives students the chance to learn science by doing it – and to build relationships and skills they’ll carry with them long after the project is finished.”