You can observe the effects of turbidity on water by standing at the harbor’s edge following a significant downpour. The harbor, which was rather clear the day before, is now brown-grey and thick with suspended soil that has been carried by the watershed’s accumulated grit and washed from upstream riverbanks and streets. When light strikes a surface, it doesn’t go very far. Visibility becomes almost nonexistent a few centimeters in. The water appears to be opaque. This is the most obvious form of turbidity; it is transient, coastal, and quite simple to see.
A larger-scale variant of this phenomenon occurs in the open ocean, with effects that reach well below the surface. The portion of the ocean that lies below the sunny euphotic zone but above the completely lightless deep is known as the twilight zone, which is the mesopelagic layer that extends from about 200 to 1,000 meters below the surface. Although it’s not completely dark, the light at those depths is already filtered and faint, and the further you go, the more so. The animals that inhabit it, such as lanternfish, bristlemouths, small crustaceans, and gelatinous invertebrates that make up a biomass that is roughly equal to all ocean fish combined, have evolved to low light levels. They rely on the supply of organic matter that falls from above, known as “marine snow,” which transports carbon and food from the productive surface layers downward.
The productivity of the surface is necessary for such supply. The entire system is driven by phytoplankton in the sunlit zone, which either photosynthesize, generate organic matter, die, and sink, or are consumed and expelled, and the resulting particles sink. What is consumed by the twilight zone depends on the volume of the sinking material. Additionally, the availability of light directly affects phytoplankton productivity. The effective depth to which photosynthetically useful light penetrates decreases when turbidity in the surface and near-surface layers increases, whether due to storm-driven sediment runoff, algal blooms dense enough to shade themselves, or suspended particle clouds produced by dredging and deep-sea mining operations. The productive zone gets thinner. There is less food produced. less carbon sinks.
Contrary to popular belief, there is a direct correlation between open-ocean turbidity and coastal land use. The sediment loads carried by river systems that drain agricultural landscapes are significantly higher than they were prior to intensive farming’s modification of watershed hydrology. Depending on current and settling dynamics, the silt remains suspended in the water column in coastal and eventually open ocean habitats. Algal blooms are fueled by nutrient runoff, specifically nitrogen and phosphorus from fertilizers, which cause the water they inhabit to become more murky. The impacts of these blooms can last for weeks in stratified coastal waters, and they can be dense enough to prevent light from reaching even moderate depths.
Although mechanically straightforward, the deep-sea mining dimension is more confined. Sediment plumes, which are clouds of small particles disturbed from the seafloor that rise into the water column and drift with ambient currents before settling, are produced by mining vehicles operating on the abyssal plain. These plumes increase the turbidity of mid-water environments, particularly the lower boundary of the twilight zone, in the immediate vicinity of active mining operations. The direction of the effect is certain, but it is still unclear how far these plumes travel and how much they restrict light penetration in the mesopelagic above them.
This relates to more comprehensive climate accounting because of the effects of compromised biological pump operation on the carbon cycle. The productivity of phytoplankton and the subsequent sinking of that organic matter contribute to the ocean’s ability to extract carbon dioxide from the atmosphere. Because less biomass arrives, less carbon travels to depth in a form that remains there, so a twilight zone that receives less food from above also sequesters less carbon. Climate models have always viewed this feedback as reasonably steady, but the data from areas affected by turbidity indicates that this assumption needs closer examination.
