Marine dark waves: The consequences of a dark bottom

The seafloor loses light nearly overnight, and coastal water can occasionally change from clear to hazy. Scientists can evaluate the timing, duration, and intensity of these underwater blackouts across regions thanks to a novel surveillance system. These blackouts can swiftly damage kelp and seagrass.

The University of Waikato has issued new regulations that treat underwater blackouts as observable occurrences with distinct beginning and ending locations. Francois Thoral, a postdoctoral researcher who is investigating how foggy water alters ecosystems, oversaw the project.

The group came to a consensus on a common definition of a marine dark wave, which is a brief but severe decrease in underwater light. According to Thoral, "light is a fundamental driver of marine productivity, but up until now we have not had a consistent way to measure extreme reductions in underwater light."

Why does light vanish?

Seafloor populations may experience abrupt darkness if noon light is blocked by sediment, algae blooms, and dissolved organic materials. Turbidity rises as rivers or waves fill the water with particles. Sunlight scatters and less light reaches below the surface of the water as it gets cloudier.

As freshwater carried more organic debris that absorbed light offshore over decades, several Norwegian fjords became dimmer. A storm or bloom can still cause a darkness that strikes quickly, but that creeping murkiness establishes the scene.

Monitoring darkness over time

The team used extended records of seabed light from California and New Zealand to capture infrequent blackouts. For sixteen years, sensors at the University of California, Santa Barbara (UCSB) recorded light every day from a depth of roughly twenty-one feet.

For ten years, New Zealand moorings tracked light at around 23 and 66 feet, and for twenty-one years, satellites covered the East Cape. Because seabed light varies with the seasons and only a distinct deviation indicated a real event, such lengthy baselines were essential.

Dark waves appear in several ways.

Dark waves lasted anywhere from five days to almost two months at each location, and some of them almost completely eliminated seabed light.

The scientists examined duration and intensity across locations and depths after treating each incident as a block of lost light.

The researchers found between 25 and 80 occurrences along the East Cape of New Zealand starting in 2002, frequently after periods of severe weather that decreased underwater light. Reports must include criteria before comparing locations or years because the outcomes were dependent on how stringent the cutoff was.

Plants become less energetic

Dark waves can harm an ecosystem from the bottom up because many coastal food webs are based on light-dependent plants. Less sunshine caused kelp and seagrass to slow down their photosynthesis, which reduced the amount of sugar they produced and depleted their reserves.

A 63% decrease in light was associated with a 95% decrease in productivity in one kelp research. According to Thoral, "even brief periods of reduced light can impair photosynthesis in kelp forests, seagrass, and corals."

Animals have difficulty seeing

In nearshore waters, dark waves also altered the environment for animals that hunt or conceal themselves by sight. Because suspended particles dispersed light and reduced contrast, murkier water reduced many fish's optical range.

This reduction of visibility may alter who finds food first and interfere with cues related to migration, mating, and schooling. Behaviour is still a major unanswered subject because researchers knew far less about these animal reactions than they did about plant stress.

Darkness is caused by land occurrences.

Runoff frequently started on land when soil was disturbed by fire or rain, sending plumes into coastal waters. Steep slopes can release debris flows during wildfires, and the muddy surge increases cloudiness in nearshore areas.

The scientists intended to evaluate the effects of mudslides and fires on kelp because UCSB has one of the few seabed light records. According to this link, even before any ocean-based restoration takes place, watershed management and erosion control can safeguard marine habitat.

Satellites monitor dark waves

Scientists can use satellites to monitor entire coastlines for dark waves, much beyond a few sensor lines. Computers calculated the amount of light that reached the bottom at each point by monitoring variations in surface brightness and sea colour.

The seafloor is divided into pixels on the maps that are roughly 0.3 miles wide, yet important days are still hidden by clouds and glare. The same things that discolour water can also obstruct the satellite image since storms frequently produce heavy clouds.

Making preparations for marine dark waves

Resource managers and conservation organisations can approach low-light occurrences as quantifiable risks rather than infrequent surprises in field notes if there is a common definition of marine dark waves.

The concept of marine heatwaves—long stretches of abnormally warm ocean water—was defined as a result of similar thinking, and this helped coordinate global monitoring initiatives.

Compound stress on reefs could be more easily identified by combining dark wave tracking with signals for heat, low oxygen, or acidity. Since many coasts lack long-term sensors, data is the largest constraint.

A common definition transforms sudden bursts of seabed darkness into evidence that can direct action. Communities can expect brief, severe light losses combined with warmer seas with increased monitoring.