Scientists have discovered sudden underwater blackouts that can plunge the seabed into darkness and threaten marine life.
Clouds, smoke and fog can darken the sky above us; but underwater, a different set of forces can completely block sunlight.
Sediment runoff, algal blooms and organic matter can drastically reduce the amount of light reaching the ocean floor, sometimes creating a almost total darkness.
To better understand and compare these events, an international team of scientists has for the first time developed a scientific framework to help identify marine blackouts.
The research, presented in a published last week in the journal Communications Earth & Environmentdefines a new phenomenon known as marine black wave.
These events are brief but severe periods of underwater darkness that can disrupt kelp forests, seagrass beds and other marine ecosystems that depend on lightexplains .
“We have long known that light levels are critical for photosynthetic organisms such as algae, seagrass and coral, and that factors that reduce light on the seafloor can affect them,” he said. Bob Miller, biologist at the University of California and co-author of the article. “This study creates a framework for comparing such events, which we call black waves“.
The aim of the project was establish a shared system that allows scientists to compare extreme light loss events in different coastal regions. Until now, researchers have not had a consistent way to evaluate how severe or widespread these underwater blackouts could be.
“Light is a fundamental driver of marine productivitybut until now we haven’t had a consistent way to measure extreme reductions in underwater light,” said the paper’s lead author, François Thoralpostdoctoral researcher at the University of Waikato and Earth Sciences New Zealand.
To build the new framework, the team analyzed long-term observations of multiple locationswhich included 16 years of data collected at the Santa Barbara Coastal Long-Term Ecological Research (LTER) Site and 10 years of measurements from coastal sites in the Hauraki Gulf, New Zealand.
The researchers also examined 21 years of light estimates on the seafloor derived from satellite imagery along the East Cape of New Zealand.
In these regions, black sea waves varied widely in duration. Some lasted just a few days, while others continued for more than two months. In the most extreme casessunlight reaching the seabed was almost completely blocked.
The analysis identified between 25 and 80 dark wave events along the Eastern Cape since 2002. Many of these episodes coincided with powerful storms and major weather systems, including Cyclone Gabrielle.
For many years, scientists focused on slow declines and long-term water clarity as a major threat to coastal ecosystems. The new findings suggest that sudden black wave events can be equally harmful.
“Even short periods of reduced light can harm photosynthesis in kelp forests, seagrasses and corals,” said Thoral. “These events can also influence the behavior of fish, sharks and marine mammals. When the darkness persists, ecological effects can be significant“.
The marine black wave framework adds a new dimension to existing systems used to track marine heat waves, ocean acidification and deoxygenation.
“Together, these tools give coastal communities, resource managers and conservation groups a clearer way to identify when marine ecosystems are experience intense short-term stress“, concludes Thoral.
