Coral reefs provide habitat to some of Earth's most diverse ecosystems - from sea horses to reef sharks to teeny tiny snails, coral reefs can even make rainforests seem a bit boring. However, in addition to the well studied threats to coral reefs like warming oceans, storm frequency, and ocean acidification, scientists have described an additional threat to reef survival: Dredging.
Humans have been dredging, or digging up the bottoms of rivers and bays, since 4000 BC when the Nile River's floodwater was channeled to feed a rapidly growing population. Similarly, most dredging operations today serve to provide large vessels access to bays and rivers that would otherwise be too shallow.
While good for commerce, dredging can be quite destructive to surrounding habitats. In efforts to relocate vast amounts of submerged earth, sand and mud are quickly suspended, forming an underwater dust storm and blocking light until the dust settles.
For marine life like fish, crabs, and turtles, this may not be a big problem - they can swim out of the dust storm, and into better conditions. But for organisms that are not as mobile, like corals and sea anemones, there is risk of getting buried.
Underwater 'dust storms' are not just a man-made phenomenon - they happen naturally too, particularly during hurricanes, so many immobile critters have adapted ways to dust themselves off. Corals, for example, secrete a mucus sand trap, shrugging the sandy layer off in an elegant bundle of mucus. So dredging might not be an issue for corals either, right?
Unfortunately, the story is not quite this simple. Scientists at the Australian Institute of Marine Science studied the effects of an extensive dredging project off of Barrow Island, a large island off of Australia's west coast and Australia's largest producer of oil. The researchers found the reaction of corals to the dredging-induced sand infiltration depended on two key factors: the thickness of the suspended sand cloud, and the temperature of the water.
The study was conducted for a year-and-a-half and monitored 1,500 corals from within a half mile of the dredging site to over 20 miles away, making it one of the largest studies on the effects of dredging on coral reefs to ever be conducted.
The dredging project began concurrently with an ocean 'heat wave', meaning the local ocean was much warmer than average. Heat waves like this often stimulate coral bleaching, where corals release their microscopic algal symbionts, which can be a precursor to coral death. The reason corals expel their symbionts under stress is not well understood.
In the context of this heat wave, the effects of dredging on local coral reefs depended on the severity of the sand's light-blocking and dust cover. When the water was warm, the moderate light reduction caused by the sand storm decreased coral bleaching, particularly for 'branching' species like Elkhorn coral.
However, when the sand storm caused severe light-blocking and coral burial, the stress of the sand outweighed the sand's ability to prevent bleaching. Additionally, distance from the dredging site impacted coral survival. Corals within a few hundred feet of the dredging site were less likely to survive, particular when dredging coincided with the heat wave.
In other words, the underwater upheaval of sand ended up benefiting some corals during unseasonably warm conditions, but overall the combination of the the heat wave with dredging activity resulted in greater coral mortality than dredging during more typical temperatures. Taken together, the results of this work suggest the management of local environmental pressures, such as ocean heat waves, may have the capacity to reduce the impact of endeavors like dredging.
