“Climate change is the biggest threat to the Great Barrier Reef and coral reefs around the world,” Australia’s Great Barrier Reef Marine Park Authority has declared. While that’s true, the world’s largest coral reef system faces other dangers, including dredging, a practice that has expanded in recent decades as marine shipping traffic across the region has boomed. To prevent ship groundings, seafloor sediment is routinely dug up and dumped out of the way – sometimes just dozens of kilometers from coral reefs, according to Laurence McCook, a former Great Barrier Reef Marine Park scientist and expert on marine ecosystem management.

While government policies require all dredging projects in the Great Barrier Reef to assess possible effects on coral reefs, seagrass beds and other “sensitive environments” before proceeding, a 2015 report identified a lack of baseline knowledge about the impact of dredging on the Great Barrier Reef as a major impediment in making the best decisions on where to dredge and dump.

Now scientists in Finland have devised a solution to obtaining such baseline knowledge and how it might be used to establish and monitor dredge sites. The scientists recently published research that suggests studying seafloor geology using two simple techniques – sonar mapping and analysis of sediment cores – could help coastal zone managers better monitor dredge dumping sites over time to prevent ecological harm. The study’s lead author, Joonas Johannes Virtasalo, a senior researcher at the Geological Survey of Finland, said assessments of seafloor geology could also be used to more carefully select dredge and dump sites as well as identify no-dredging zones.

Several years ago Virtasalo realized most dredging regulations in Finland focused on biological and water chemistry conditions at dredge sites but were ignoring the seafloor itself.

“More emphasis should be put on the determination of the geological integrity of a seafloor, with respect to its ability to provide a habitable substrate” for small bottom-dwelling organisms, said Virtasalo. Those organisms – which include worms, sponges, crustaceans and sea grasses – help to maintain healthy water and sediment quality on the ocean floor and should be protected from dredging whenever possible, he noted.

To help establish geological monitoring standards for dredging, Virtasalo and two other Finnish scientists spent several weeks in 2015 and 2017 analyzing the seafloor quality of two former Finnish dredge sites: one located 20km (12 miles) south of Helsinki in the Gulf of Finland and the other situated 9km west of the city of Uusikaupunki in the Baltic Sea, according to the study published in the journal Frontiers in Marine Science.

Virtasalo and his team used a sonar device to map seafloor geography and took dozens of sediment core samples at each site. Back in the lab, the scientists used carbon dating to determine the approximate age of the sediment and measured the sediment grain size in their core samples. They also interpreted acoustic data to characterize the seafloor geography of each dumping site.

While both sites showed slight signs of recovery since dredge dumping had been halted several years prior to the study, each had differing apparent rates of recovery and post-dumping characteristics. The Gulf of Finland site experienced less erosion and a weaker bottom current than the Baltic Sea site, which appeared more smoothed and recovered because a strong bottom current carried and deposited dumped sediment elsewhere.

“The most impacted are the least erosional or least depositional dumping sites, where little change in the seafloor takes place with time,” said Virtasalo. To avoid likely ecological harm, dredge dumping should be stopped or avoided at these sites, he said.

With that observation in mind, Virtasalo and his team created an assessment protocol for monitoring dredge sites with acoustic surveys and sediment core analyses.

“Careful selection of the dumping site is the key to minimizing the environmental impacts,” said Virtasalo. “In some special cases, dumping can even be directed to improve the seafloor environment.” He said Finland is now discussing whether dredge spoil should be dumped on soft anoxic seafloor – fine-grained sediments devoid of oxygen but rich in organic matter – to reduce the flux of nutrients leaking from those sites so as to improve the local water quality.

Researchers in China have also studied the effects of dredging on seafloor characteristics using acoustic data, and recommend that such information be used to establish dredging policies that strike a balance between economic development and maintaining the ecological health of the seafloor.

McCook, who is an adjunct principal research fellow at the Australian Research Council Centre of Excellence for Coral Reef Studies at James Cook University, said the Great Barrier Reef Marine Park Authority acknowledges in its research that there appears to be great variability of seafloor geology and sediment composition within its jurisdiction. But he said gaps in that knowledge remain, such as the extent of seafloor variability, and so how geology should be interpreted in choosing a dredge dumping site is up for debate. Standardized site evaluation methods like Virtasalo’s could provide the knowledge necessary to decide where is best – or not – to dredge and dump, and for monitoring areas where dredged sediment has been deposited, McCook added.

“Dredging activities can have a wide array of effects on coral reef ecosystems, as outlined in recent scientific reports, but that depends greatly on where, how and when,” a site is dredged, he said. “There is limited conclusive evidence about impacts on the Great Barrier Reef. For my part, I would say almost certainly the Great Barrier Reef has been affected by dredging, but how much and how seriously remains very much an open question scientifically.”

Major Queensland ports such as Townsville, Gladstone and Abbot Point are excluded from the Great Barrier Reef Marine Park, even though they are part of the Great Barrier Reef World Heritage Site. In a 2014 report on the Great Barrier Reef, experts highlighted changes in seafloor sediment and sediment-saturated waters as key risks to corals and other marine life. The dumping of waste at sea from capital dredging, which involves the scooping up enormous amounts of fine sediment deep to create or expand ship channels, was banned in mid-2015 within the marine park. (Spoils can still be dumped on land.)

But the prohibition doesn’t apply to the region’s major ports. In late 2015, the state of Queensland government approved the expansion of Abbot Point, a major coal shipping terminal, which would involve dredging and dumping more than 3 million cubic meters (106 million cubic ft) of dredged sediment into nearby waters. Sediment would not be dumped directly on the reef but close enough that hundreds of concerned scientists, including prominent coastal marine scientist Sarah Hamylton, called on the government to reconsider approval of the project as part of what they see as their responsibility to speak out in the “coal versus coral war” in Australia. While the government has permitted the Abbot Point project to move forward, it’s reduced the amount of sediment it’s allowed to dredge down to just under 1.2 million cubic meters (42.4 million cubic ft) of sediment, according to an Australian government fact sheet. All dredged sediment must also be dumped on land in a contained industrial area.

To reduce further possible harm, the Queensland Port Authority has also recently implemented a policy prohibiting dredging in some major ports off the Great Barrier Reef during turtle nesting and hard coral spawning seasons. In a 2017 paper, a group of Australian scientists recommend seagrass meadows also be considered when determining how to minimize ecological harm, as sediment dumping from dredging is severely harming seagrass health throughout the Great Barrier Reef Heritage Area.

According to the Great Barrier Reef Marine Park Authority, all applications for dredging in both the marine park and World Heritage Site “undergo a comprehensive environmental assessment” and require a government permit before moving forward. In a statement the Authority said it acknowledges dredging’s disruption of the natural marine ecosystem and as a result encourages project applicants to consider alternatives to dredge dumping at sea. Some “beneficial use” alternatives include reuse of dredge sediment to restore wetlands. It recommends toxic dredged sediments be disposed of in protected landfills to avoid contamination of the marine environment.

When dredge dumping can’t always be avoided, said Virtasalo, “careful selection of the dumping site is the key to minimizing the environmental impacts.” Studying seafloor geology could help coastal zone managers make better dredging decisions, reducing harm to sensitive marine ecosystems.