Why Are Oceans Becoming Darker? Exploring the Global Changes in Ocean Color

At dusk, a massive transfer of biomass occurs in the oceans, as trillions of tiny creatures like zooplankton, krill, and lanternfish rise from the depths, drawn by phytoplankton blooms. This nocturnal feeding frenzy is crucial for marine ecosystems, as these creatures avoid predators who hunt visually, diving back down at dawn.

Solar and lunar cycles dictate marine behavior, yet recent observations show that large areas of the ocean have darkened. Tim Smith, a marine scientist at the Plymouth Marine Research Institute, has been at the forefront of this research, studying the impact of global warming and land-use changes on ocean light dynamics.

Smith told New Scientist about the causes and implications of ocean darkening, exploring ways to enhance light penetration into underwater habitats.

Thomas Luton: How did you first notice the darkening of the ocean?

Tim Smith: We approached this issue from a unique perspective. For the last decade, I’ve collaborated with Tom Davis, focusing on the effects of artificial light pollution. Analyzing two decades of global satellite data revealed a consistent darkening pattern in the ocean, suggesting an increase in surface water opacity which affects well-connected expansive regions rather than isolated patches. About one-fifth of the world’s oceans have experienced some form of darkening.

What causes ocean darkening?

In coastal areas, river changes significantly impact ocean coloration. Alterations in land use directly influence what enters rivers, thereby transforming the optical properties of ocean water. Flood events can greatly increase the influx of suspended particulates and colored dissolved organic matter, contributing to the characteristic “steeped tea” color.

An additional driver of ocean darkening is nutrient loading, where fertilizers from agricultural runoff stimulate phytoplankton growth, reducing light penetration. Although coastal waters have been recognized as darkening for some time, the phenomenon is now extending into the open ocean.

What factors lead to changes in the open ocean?

These changes may correlate with the abundance of phytoplankton driven by climate change, such as rising ocean temperatures and increasing frequency of marine heatwaves. Climate alterations influence vast ocean circulation patterns significantly.

The proliferation of phytoplankton relies on a mix of light, nutrients, temperature, and water column dynamics. In winter, storms typically mix the ocean, but as spring arrives, a stable surface layer forms. These layers limit vertical mixing and enhance light and nutrient concentration, fostering phytoplankton growth.

I suspect that we’re witnessing a complex interplay between shifts in global circulation patterns and localized weather phenomena, such as clearer skies that promote phytoplankton growth. This combination may contribute to the widespread darkening of the open ocean.

What impacts does ocean darkening have on marine ecosystems?

To understand this better, consider the ocean’s food chain. Phytoplankton, the primary producers, experience the first effects of darkening. The next tier includes zooplankton, like Calanus copepods, which serve as a critical link in the food web and engage in diurnal vertical migration, moving up and down daily for feeding.

During the day, they dive to depths of 200 to 300 meters where light is scarce, eluding visual predators. By night, they ascend in search of food. This behavior represents the largest biomass transfer on Earth, as millions of zooplankton migrate invisibly through the water, significantly outnumbering the terrestrial migrations like the Serengeti wildebeest.

What occurs when light cannot penetrate deep underwater?

The existence of dark regions in the ocean restricts the vertical habitat for species, which could lead to heightened competition for food and space. Some species may expend less energy hunting, impacting predation dynamics and thus altering food webs and global fishery productivity.

Fish species that rely on sight, including both small schooling fish and large predators like tuna, will find their hunting zones confined to the shallows. Simultaneously, phytoplankton may face altered depths for photosynthesis due to decreasing light availability.

Is nighttime ocean darkness still a concern?

Absolutely. Beyond sunlight, moonlight plays a crucial role in nocturnal migrations of many marine creatures. While the ocean appears nearly black at night to humans, the moon’s dim glow has significant implications for guiding species during foraging and return to deeper waters.

Our lunar models indicate that as ocean clarity decreases, moonlight’s penetration diminishes, which may compress the nighttime habitat, dramatically shifting ecological interactions in darkness.

What is the global impact of these changes?

Ocean darkening could profoundly affect the carbon cycle as well. If zooplankton cannot dive as deeply to evade predators due to limited light, their efficiency in pulling carbon from the atmosphere diminishes. When zooplankton perish, they normally sink and trap carbon deep in the ocean; without the ability to dive, much of this carbon may remain in the upper layers, ready to be re-released into the atmosphere.

However, assessing how carbon moves from the illuminated surface to the ocean floor remains complex. Satellite data provides a global perspective, but it offers only a glimpse into dynamics at work.

Is there a way to combat ocean darkening?

In certain areas, yes. Coastal waters are especially vulnerable to terrestrial activities, particularly agricultural runoff. By managing land better, including practices such as reducing fertilizer usage, we could restore some clarity to coastal waters. Initiatives like the AgZero+ program led by the UK Center for Ecology and Hydrology encourage collaborative efforts with farmers to develop eco-friendly farming techniques, thereby minimizing runoff and enhancing water quality. Strategies like improved fertilizer management and agroforestry could substantially mitigate the darkening of coastal waters.

Nevertheless, addressing the drivers of darkening in the open ocean is far more challenging. Even if global emissions halt immediately, ecological responses would take decades, potentially centuries.

Is there hope for the seas?

Absolutely. Evidence shows that marine environments can exhibit remarkable resilience when given a chance. Protected marine ecosystems can recover swiftly. For instance, kelp forests off California rebounded rapidly in well-managed reserves after a severe marine heatwave between 2014 and 2016.

This resilience has led to a global push to expand marine protected areas, which can act as ecological refuge zones, helping to rebuild vital marine life and restore ecological equilibrium. Such measures are crucial in the face of climate stressors like heatwaves.

There is optimistic news: the ocean exhibits extraordinary self-repair capabilities. With adequate protection and time, marine ecosystems can respond swiftly, crucial for all life on Earth. The oceans, covering about 70% of the planet, play a significant role in climate regulation and carbon absorption, underscoring the need to protect this invaluable life-support system.