One of the murkiest questions surrounding the Chesapeake Bay cleanup effort seems like it should be the easiest to answer: Is the water getting clearer?
For decades, widely used data indicate that, overall, water clarity is getting worse.
Earlier this year, for instance, the Chesapeake Bay Report Card released by the University of Maryland Center for Environmental Science reported that 2024 water clarity was “very poor” and that “water clarity scores continue to show a significant decline over time.”
One of the major goals of the state-federal Bay Program partnership is to reduce the amount of sediment and nutrients entering the Bay to improve clarity so that underwater grass beds can get enough light to survive.
The region has spent billions of dollars to control sediment and nutrient-fueled algae blooms that cloud the water — seemingly without significant results.
Yet underwater grass beds in the Bay have expanded even as data seem to show that the water is murkier. The amount of submerged aquatic vegetation, or SAV, increased from 38,227 acres in 1984 to 78,451 acres last year.
If the water is cloudy, how are grasses getting enough light to expand?
A recent analysis published in the Annual Review of Marine Science came up with an answer, though it is murky too: The amount of light available for plants is improving, even if it doesn’t always look that way.
“For a long time, the story was that we’ve been cleaning up the watershed, but clarity is not improving,” said Jessie Turner, an assistant professor in the Department of Ocean and Earth Sciences at Old Dominion University, who was the lead author of the journal article.
“That has switched, but it was hard to untangle things,” she said.
Turner has been trying to sort out the story for nearly a decade, first as a student at the Virginia Institute of Marine Science and then in her current position. Former colleagues from VIMS and the University of Delaware are co-authors of the article.
It turns out that how far we see into the water — how visibly “clear” it is — is not the same thing as how much light is passing through that water.
The Bay goal is to get more light to underwater plants, but the main tool for measuring clarity has been the Secchi disk — a black and white disk that is lowered into the water until it disappears. The Secchi disk is cheap and easy to use and has been relied upon for decades by researchers and citizen scientists.
But it measures visual clarity, not the amount or quality of light that might be reaching plants on the bottom.
Bay water is filled with tiny particles. Some are bits of sediment, but many are tiny algae cells and microscopic bits of detritus from organic material that is breaking down in the water.
Those organic particles limit visibility, but they don’t block light waves. Instead, they scatter them, reflecting light through the water column. Turner likens it to headlights in a fog bank. The headlights brighten the fog, which is made up of tiny water particles, but a driver can’t see very far into it.
“You can have a lot of light getting to your eyeball in the fog, but the visibility is very poor,” Turner said. “In the water, that would look like a very shallow Secchi depth reading. But you still have enough light for something like seagrass.”
“What ultimately matters to something like SAV is how much light is getting to the bottom,” she said.
When Turner and her colleagues examined historical data gathered with specialized light sensors, they found a different trend than those seen with Secchi disks.
Data from those sensors, which assess the amount of sunlight that is penetrating the water, including the specific wavelengths that are important for plant photosynthesis, show improvements since around 1990.
Many factors affect clarity and light availability, and their relative importance varies from place to place. Sorting them out is complex. For instance, the amount of sediment in the water has slowly declined over time. That has helped clear the water, but clearer water allows for more algal growth, Turner said, which in turn contributes to more tiny particles of organic material. The particles gradually settle to the bottom but are easily resuspended.
That can cloud the water from a Secchi disk perspective, but the increased amount of tiny organic particles, rather than larger sediment particles, can improve light.
Trying to understand all those factors, Turner said, “is a little bit of a maze.”
Water clarity is still important, she noted. Someone diving in the Bay wants to be able to see where they are going, and someone throwing a fishing line into the water wants a fish to be able to spot bait at the end of the line.
Future nutrient reductions should further reduce algae production, and over time that could improve clarity. That might be happening — Secchi disk readings in the last decade do show a slight improvement.
But it’s hard to predict whether Bay clarity goals will be met, Turner said. That’s because the Bay system has been fundamentally changed over the decades by things such as marsh loss, shoreline hardening and the transformation of its watershed to meet the needs of a growing population.
“A recovered Chesapeake Bay with improved water clarity may not resemble the ecosystem that we predict or expect,” Turner wrote in the paper. As a result, the future Bay may have a mix of different particle types and sizes than it did in the past, which means expectations about future clarity may be altered as well.
The good news is that even if clarity goals are not fully attained, continued light improvements will have ecological benefits, especially for grass beds, which will result in healthier ecosystems over time and more habitat for fish and crabs.
“We may get to a point where you still can’t see the bottom when you go swimming in a lot of places,” Turner said. “But if the ecosystem is healthy in terms of what’s living in it, then maybe that’s a success story in and of itself.”
By Karl Blankenship, The Chesapeake Bay Journal
January 2, 2025
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