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.”
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.”
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.
So they've been measuring the wrong thing for the last 35 years? Actual light, including spectrums, aren't that tough to measure.
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