Rachel Nuwer

Humans were tampering with nature long before the Industrial Revolution’s steam and internal combustion engines arrived on the scene. The invention of agriculture around 8,000 years ago, some argue, significantly changed ecosystems as it spread around the globe.

Although scientists are only just beginning to understand how these ancient alterations shaped our world today, a new study in Scientific Reports suggests that millennium-old development along the Danube River in Eastern Europe significantly changed the Black Sea ecosystem and helped create the lush Danube Delta in Romania and Ukraine.

“My team had a big surprise,” said Liviu Giosan, a geologist at the Woods Hole Oceanographic Institution in Massachusetts and the lead author of the study. “We found that around a thousand years ago, the entire basin changed dramatically, though that later made sense when we put it into context.”

Courtesy of Liviu Giosan

Dr. Giosan and his colleagues originally set out to see how changes in historic sediment movement – from glaciers, rivers, agriculture and felled forests, for example – influence the ocean. The ocean is a big place, and it quickly dilutes sediment. To get around this problem, they decided to study the Black Sea since, while it connects to the ocean, it is confined and relatively small. Understanding the Black Sea meant taking a deep look at the Danube River, the sea’s major source of freshwater.

The team used a variety of methods to reconstruct the water bodies’ dual histories. They took sediment samples and used radiocarbon dating and a technique called optically stimulated luminescence to reconstruct each layer of sediment deposits, starting around 9,000 years ago when the ocean first invaded the Black Sea. Sediment accumulation, they found, suddenly spiked about 2,000 to 1,000 years ago. “Probably 40 percent of the Delta was built in the last 1,000 years,” Dr. Giosan said. “Finding that was like a eureka moment.”



Spurred on, they measured how the sea’s salinity changed over time. When the Black Sea first connected with the ocean, they found, its salinity steadily increased, reaching near-ocean salt levels around 3,000 years ago. But then salinity began slowly decreasing, leveling off about 1,000 years ago at around 20 grams per liter of dissolved salts, compared with 35 grams per liter in the ocean. Running computer models of these results, they found that a combination of decreased evaporation from naturally lower temperatures and a greatly increased river flow caused the salinity to decline.

Finally, they extracted preserved DNA fragments from 44 sediment samples to determine how communities of aquatic organisms shifted over the years. Around 1,000 years ago, tiny algae called diatoms abruptly began dominating the Black Sea’s waters. Diatoms build their shells from silicate, Dr. Giosan noted, a compound found in soil. Other microorganisms that thrive in nutrient-rich water also began turning up in high numbers around this time.

To try and understand how all of those findings fit together, they built a computer model that took into account the Danube’s physical parameters and the history of land use there. The story that emerged explained their discoveries in the context of the larger human landscape. Around 3,000 years ago, humans began clearing forests for agriculture in present-day Serbia, Hungary, Romania and Bulgaria, they found, and around 1,000 years ago, the agricultural efforts significantly expanded. With fewer trees to trap rainwater, the Danube’s volume probably increased; it carried far more eroded nutrients and silicate-laden soil with it and its salinity decreased, laying the foundations for the modern-day Danube Delta.

Today, the Danube Delta encompasses a rich assembly of 23 different ecosystems, including wetlands, lakes, lagoons and dunes. Over 300 species of birds pass through the delta, and 45 species of freshwater fish inhabit its waters. “What our ancestors did without purposefully having it in mind was to enrich that part of the world and increase the diversity of the land,” Dr. Giosan said.

He hypothesizes that humans probably had a hand in shaping other coastal and delta systems around the world, but further studies of individual sites would be needed to verify this.

Although humans may be inadvertently responsible for some of the Danube region’s current biodiversity, Dr. Giosan emphasizes that the human impact on the area over the last century has been drastic in terms of changes in land use, positive and negative. “We don’t need to wait 1,000 years” to recognize that, he said. “It’s already happening now.”