Scientists from the University of Plymouth and Plymouth Marine Laboratory have found that the global ocean is darkening, with light penetration shrinking dramatically over the past two decades. Their study shows that nearly 10% of the world’s oceans, from vast open-sea and polar regions to coastal zones, have lost significant depth in the light-filled photic zone. This darkening poses serious risks to marine life.
A new study from Plymouth reveals that natural light is penetrating less deeply into the seas, shrinking the zone that sustains nearly most of marine life.
What is the photic zone?
Light plays a crucial role in marine life. The only natural light sources in the ocean are the sun and the moon, but their lights do not penetrate below a certain depth. The zone illuminated by daylight, known as the photic zone, typically extends through the upper 200 meters of the water column and is estimated to contain about 90% of all marine life.
Sunlight and moonlight also drive the largest daily migration of biomass, which is essential for maintaining invertebrate populations. Darkening means a reduction in the photic zone, either in area or in depth, meaning that light no longer penetrates as far or as widely as it used to.
How is the ocean getting darker?
Several factors contribute to ocean darkening. In coastal areas, nutrient loading, runoff, and upwelling increase biological activity, which makes waters darker. In the open ocean, rising sea surface temperatures disrupt currents and increase chlorophyll production in upper layers.
Over the past 20 years, the depth of the photic zone has decreased by over 10% across 10% of the ocean. The scale of these losses is so profound that the authors suggest they represent one of the largest habitat reductions on the planet.
Compared to daytime, the losses in photic zone depth at night were smaller, but they remain biologically significant. Many nocturnal animals are adapted to very low light intensities, so even small variations can affect survival and reproduction.
Without sufficient light, marine organisms that rely on it will be forced to migrate vertically into increasingly smaller zones, exposing them to greater competition and predation.
How do we measure ocean darkening ?
To calculate the darkening of the ocean, researchers used data collected by the NASA satellite Aqua, launched in 2002. This satellite is specifically designed to study the water cycle and continuously measures parameters such as temperature, chlorophyll concentration, and light attenuation.
Scientists in Plymouth gathered global data annually between 2003 and 2022 to measure variations in light attenuation in seawater. They caution against drawing regional conclusions and emphasize that the 20-year timeframe may not fully capture natural variability. However, they do highlight that ocean darkening is partially, or totally caused by anthropogenic sources.
Source : Davies, T. W., & Smyth, T. (2025). Darkening of the Global Ocean. Global Change Biology, 31(5). link