Where Rivers Meet the Sea: Two Mediterranean Deltas Under Pressure

For centuries, Mediterranean deltas were considered hostile and unfit for human settlement. Today, they are shaped by the symmetrical lines of intensive agriculture and aquaculture. These territories embody many of the environmental challenges of our time: from soil contamination to increasingly industrialized farming and rising sea temperatures.

Acting as receptors of both urban and rural pollution, rivers carry contaminants towards the sea, where they are funnelled into coastal ecosystems. In this edition of Lapilli+, we travel through the shifting environments of the Po Delta in Italy and the Ebro Delta in Spain.

We hope you’ll enjoy the read and happy holidays!

Inside his laboratory at the Delta Institute of Applied Ecology in Ferrara, Italy, biologist Edoardo Turolla has been breeding clams, mussels and other bivalve mollusks for decades. Pointing to containers filled with water in varying shades of green, he explains which microalgae are used to feed the thousands of mollusks that he and his team are hatching for a number of national and international research and development projects.

Well known throughout the Po Delta area, Turolla is the person locals call whenever something unusual is found in the delta’s waters. In 2015, he was lucky enough to come across a rare, golden-colored oyster south of the Po Delta in the Sacca di Goro. That same week, a fisherman discovered a second specimen of the same rare oyster, but of the opposite sex. “From those two specimens, we were able to breed thousands of other oysters, which we named the Golden variety,” Turolla explains.

Goro is the leading center for Manila clam (Ruditapes philippinarum) production across Europe. More than half of Italy’s clam production and around 40 percent of Europe’s is concentrated within the municipality’s roughly 2,000-hectare lagoon. However, Goro’s shellfish farming community has recently begun to diversify. Facing rising temperatures, anoxic events and the spread of blue crabs, local shellfish farmers are now complementing their established clam beds with oyster (Ostrea edulis) farming, in an effort to broaden local production and reduce dependence on a single species. Oysters, in fact, are better able than mussels and clams to adapt to changes in water temperature, particularly in the shallow waters of the Adriatic Sea.

Some 1,200 kilometres (745 miles) to the west, in Fangar Bay in Catalonia, Spain, biologist Eve Galimany and two of her colleagues from the Institute of Marine Science’s Renewable Marine Resources Department are assessing the health of mussels on a humid morning in early July. The researchers are visiting an aquaculture facility to examine the condition of mussels in the Ebro Delta. Their project is coordinated with colleagues in Vigo, Galicia, where more than 90 percent of Spain’s mussel production — and around 40 percent of Europe’s total — is concentrated. The study aims to help prevent and mitigate the damage associated with rising temperatures in Galicia by observing conditions in Mediterranean colonies.

“Here, in the Ebro Delta, we have the same mussel variety as in Galicia,” Galimany explains. “The two environments, however, are very different. In the Mediterranean, temperatures are much higher, but in recent years, Atlantic waters have also been experiencing episodes of unusually elevated temperatures.” Higher sea temperatures increase the risk of mussel mortality. “What we are seeing here is what could happen in Galicia in the near future,” Galimany says.

In the Catalan bay, mussel mortality begins when water temperatures reach around 28 degrees Celsius (82 degrees Fahrenheit), while in Galicia, which is a much colder environment, mortality starts at just 17 degrees (62.5 Fahrenheit). In recent years, Galimany explains that these temperature peaks have been occurring earlier than in the past, meaning that mussels now need to be harvested as early as the beginning of summer.

The researchers record how many liters of water some mollusks can filter, and at what rate. Under optimal conditions, a mussel filters around four liters (one gallon) of water per hour. Under high-temperature conditions, however, this figure drops to about half a liter (0.13 gallons) per hour, as mussels stop feeding — and filtering — from heat-induced stress.

Thanks to their findings, the Institute of Marine Science researchers hope to contribute to the development of tools for sea farmers, to better identify when mussels enter thermal stress and to determine the optimal moment for harvesting them before mortality occurs. Yet, the future of mussel farming in the bays of the Ebro Delta — like that of clam farming in the waters of the Po Delta — remains uncertain if average sea temperatures continue to rise.

In both deltas, rising temperatures are not the only concern. In the Sacca di Goro lagoon, the freshwater, which is naturally rich in nutrients, flows from the Po Delta and meets the Adriatic Sea’s salt water, creating an environment that has become ideal not only for mollusk growth, but also for a highly invasive species: the blue crab.