Water Clarity Study Sheds Light on Bering Sea Change

From NOAA Fisheries

Innovative research looks beneath the surface to reveal new insight on the ecosystem that supports the nation’s largest commercial fisheries and subsistence and coastal communities across Alaska.

NOAA Fisheries
Subsurface water clarity has a major impact on marine ecosystems and commercially important species like Alaska pollock.

In 2004, Alaska Fisheries Science Center biologists began attaching light sensors to Bering Sea survey bottom trawls to evaluate the effects of light on fish catchability. Fifteen years later, researchers looked at this unique dataset in a new light to reveal much more about the dynamic Bering Sea ecosystem.

NOAA Fisheries scientists collaborated with our partners to develop an automated process to translate these data into the first long time series of subsurface water clarity for the eastern Bering Sea.

“Until now, there was very little long term information on subsurface water clarity in the Bering Sea,”  said Sean Rohan, the NOAA Fisheries Alaska Fisheries Science Center biologist who led the study. “Working with the annual surveys provided unprecedented spatial coverage and resolution over a span of 15 years.”

Their approach provides a tool that expands possibilities for research in other regions. Their findings reveal patterns and trends in water clarity over depth and time that enhance our understanding of recent and future changes in the Bering Sea.

Water Clarity Reflects And Influences Ecosystem Structure And Change

Measures of water clarity provide information about how deep light can penetrate into the water column. It profoundly influences the structure of marine ecosystems, from plankton to predators.

Through its effects on heat and light in the water column, clarity affects primary production: production of plankton, which forms the base of the food web. Those changes propagate through the food web to affect productivity of higher trophic levels like fish.

Through its effect on visibility, water clarity influences predator-prey dynamics. Vision is critical to the success of predators who hunt by sight, and the survival of prey that depend on vision to evade predators. Water clarity can tip the balance of competition between visual predators and those that use other senses to detect prey.

In the eastern Bering Sea, water clarity depends on:

  • Amount and depth of plankton
  • Organic matter and sediment from rivers
  • Resuspension of seafloor sediment by currents and wind.

All of these factors can change over time with climate or other environmental change.

Monitoring Water Clarity Is Important—But Not Easy

Water clarity provides valuable information about commercially valuable fish and the ecosystem that supports them. But it can be difficult to monitor— especially in Alaska, where vast, remote seas and moody weather can make data collection a challenge.

Surface water clarity is widely recorded using satellite-based remote sensing. These measurements have shown changes in near-surface water clarity over decades. Previous studies provide insight into how these changes are related to changes in the ecosystem.

Frequent cloudy weather can make satellite coverage difficult to obtain in the Bering Sea. More importantly, surface clarity measured from satellites tells only part of the story. A lot of what goes on in the ocean happens beneath the surface.

Measuring subsurface light clarity has been particularly challenging.

“Nothing has been done before on this scale or this consistency. We had samples at about the same time every year from the same 376 stations, over the course of 15 years,” said Rohan.

Rohan says that the biggest challenge was coming up with a fully automated way to process the data. They needed to ensure that it is reproducible, consistent, and time-efficient in the long run. That also makes it an important tool beyond the Bering Sea.

“We are developing a method that opens doors to access subsurface clarity data in other Alaska regions where bottom trawl surveys are conducted,” said Rohan.

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