By Jacoba Charles, Estuary News Group
Aquatic plants tend to hide in plain sight, overlooked other than the occasional striking flower, or unnerving tendril brushing the leg of a summer swimmer. With a few exceptions—certain anglers, perhaps, or birders—people rarely notice details beyond a mass of green floating on the water. And of course, many species of aquatic plant don’t emerge from the surface at all, spending their lives out of human view.
And yet, the quiet existence of aquatic plants plays an outsized role on the life, and even the physical structure, of the water bodies they occupy—influencing currents, shorelines, and even which other life-forms will thrive in their company. And over the last several decades, a silent but staggering shift has taken place within this plant community, as native water plants have been nearly entirely lost from the Sacramento-San Joaquin Delta. At the same time, total aquatic plant cover of the same area has doubled—with a handful of invasive species covering far more area than a complex suite of native species ever occupied.
“Historically, the situation was very different than it is today,” says Shruti Khanna, a senior environmental scientist with the CDFW, who is a coauthor on special issue put out by the journal San Francisco Estuary and Watershed Science, reviewing the state of the science regarding invasive plant species ecosystem services—or, more often, disservices—within the Bay-Delta system. Khanna has been mapping invasive species in the Delta since the early 2000s. “Not only has the cover doubled over the past few decades, but it is mostly invasive.”
To get a very rough idea of the magnitude of this change, imagine your home refrigerator. How would your life would be impacted if its contents were suddenly replaced with foods you would never eat, at least not exclusively: say cotton candy and twinkies? In this scenario, there is twice as much of the new type of food as there was of the old. Not only are you lacking good ingredients for dinner, the fridge itself no longer works properly: the door doesn’t close, the contents lose their cool, and melting ice pools on the shelves (thought the cotton candy and twinkies don’t care about any of that; they are fine). And gradually, all the refrigerators in your area are being transformed in the same way.
Of course, estuaries are exponentially more complicated than refrigerators—and the impacts of the rapid change in the Delta’s species composition are correspondingly complex, as well as phenomenally disruptive to the denizens who have lived there for centuries.
“Aquatic vegetation affects every aspect of its environment,” says Maggie Christman, a program manager with the Delta Stewardship Council, and lead author of a paper in the SFEWS special issue. “It’s creating the environment that wildlife are living in, it’s affecting the food web, it’s affecting the physical environment, too. It’s very, very important.”
One of the single biggest differences of the new, invasive plant cover—besides the fact that there is twice as much of it—is that the invasive submerged species grow bushier, thus filling more of the water column from roots on up; by contrast, the native plants that have been crowded out were slimmer, and most of their leafy material was near the surface.
The bushy, invasive species act as a comb or a filter, capturing particles of mud and other sediment that float by in the water, causing it to settle to the bottom. One of these—Brazilian waterweed—was introduced to the Delta in 1946, and that one species now accounts for over half of the aquatic plant cover found in the area.
Filtered water may sound like a good thing, but Delta waters were historically sediment-rich and cloudy; this one characteristic alone influenced many aspects of the local ecosystem and landscape.
“We are just starting to see exactly how big of an effect aquatic vegetation has, and how dynamic that effect is,”Christman says, adding that studies examining the effects of invasive aquatic plants are all fairly recent. “The implications range from marsh restoration to resilience to sea level rise.”
As the invasive plants slow currents with their bulk, they collect sediment where they grow—trapping it before it can reach the shoreline shallows, where historically it built new mudflats and marshlands. And, as Khanna puts it, if the sediment is not ending up on the marsh, then how can the marsh keep up with the sea level rise that is coming with climate change?
But the effects of this sediment-trapping extend beyond the marsh. Many native species, such as the nearly-extinct Delta smelt, evolved to depend on such turbidity to hide from predators; today’s clearer waters often favor invasive species instead, contributing to the decline of the smelt and other native species. And the invasive plants also tend to favor invasive fish, which prefer their dense architecture for refuge, food, and a safe nursery space for their young.
A small bright spot is that a higher amount of carbon than in the past does also get trapped along with the sediment—but the overall ecosystem effects remain overwhelmingly negative.
The rapid shift from native to invasive species was sped up by human reshaping of the Delta—which today has 75% more open water than it did historically, according to another paper published in the SFEWS special issue. The Sacramento-San Joaquin Delta has undergone one of the most extensive conversions of wetlands to human uses known worldwide, as broad swathes of wetland were converted to a combination of agricultural fields and leveed, open-water channels.
Today, restoration efforts are attempting to restore a fraction of the complex ecosystem that was lost—even as new invaders continue to arrive and proliferate, and weather fluctuates between increasing extremes.
Understanding of these issues is a constantly moving target, Christman concludes. “There’s a lot we don’t know still, and that we’re just figuring out.”
