When Hurricane Harvey made landfall in Texas and Louisiana in August 2017, it dumped so much rain on the Houston metropolitan area that the National Weather Service had to add two new colors to its precipitation maps, for 20 to 30 inches and greater than 30 inches of rain. Much of that water eventually flowed into the part-saltwater, part-freshwater Galveston Bay — and in just over three and a half days, all the salt water had been flushed out, leaving the bay entirely full of freshwater.
The amount of freshwater that flowed into the bay was enough to fill the entire bay five times over, said Robert Hetland, Ph.D., professor in Texas A&M University’s Department of Oceanography, who recently coauthored an article in the journal Estuaries and Coasts about the event.
The bay’s oysters, which contribute $43 million to Texas’ economy each year on average and which need at least some saltwater to survive, were devastated by the influx of freshwater.
“It happened so fast. This is the thing that just boggles my mind — we estimated the outflow of Galveston Bay to be bigger than the Mississippi River during flood. You could just imagine water streaming out of the mouth of Galveston Bay,” Hetland said.
Galveston Bay is part of the largest estuary in Texas and the seventh-largest estuary in the United States. Estuaries are mixing zones where the ocean meets the land, with freshwater, nutrients and sediments delivered from rivers and saltwater delivered from the ocean.
Estuaries are one of the most productive ecosystems in the world, providing vital breeding and nursery grounds, habitat and food, said Paul Montagna, Ph.D., chair for HyrdoEcology at Texas A&M University-Corpus Christi’s Harte Research Institute for Gulf of Mexico Studies.
“Coastal areas are these really narrow bands along the shore where freshwater is diluting seawater. It’s basically where 95% of all marine life is being created in the whole world,” he said.
Thanks to lots of sediment coming in from the rivers and tiny tidal ranges, Texas’ estuaries are uniquely shallow and gently sloped, said Russell Feagin, Ph.D., professor in Texas A&M’s Department of Ecology and Conservation Biology.
“You have this really low slope that goes miles and miles and miles, so you just have a huge area over which the estuary’s mixing is happening,” he said.
That shallowness, plus the protection of barrier islands, “makes for an enormous area of very, very rich habitat,” Montagna said.
Turning the dials
Everything in an estuary is a balancing act — between saltwater and freshwater, sediment accumulation and sea level rise, and too many nutrients and too few, among others.
“If you think of every one of those components as being like a dial, each of those dials has different, almost infinite settings. So the combination of those things means every bay and every estuary is unique,” Montagna said.
Salinity is “one of the largest drivers” in estuaries, Feagin said. Knowing the salinity of water can tell you about the amount of water coming from different sources and the species that live in the water.
Salinity also affects the estuary’s residence time, or the amount of time it takes for nutrients and anything else in the water column to be cycled out of the estuary into the ocean. Texas estuaries have relatively long residence times, Hetland said, meaning that they can take longer to recover from damaging events such as the influx of freshwater from Hurricane Harvey.
Estuaries also rely on sediment balance, Montagna said. If an estuary is like a bank account, he said, sediment is like a paycheck, while erosion and sea level rise are like spending money.
“You get paid, you have money come in, and then you go to the grocery store and money goes out,” he said. “And if you go to stores more often than money comes in, you wind up with a negative balance.”
Getting off balance
As humans change the environment through land use change — converting land to roads and cities — and through climate change, Montagna said estuaries’ delicate equilibrium is being upset. With only 2.5% of the planet’s water fresh, and only 1% of that accessible for drinking, more and more freshwater is being diverted.
“There's a lot of water in the world, but you can die of thirst out in the middle of the ocean. Not a lot of it is drinkable,” he said. “Farms, cities, industry and the environment — we all have to share.”
With less freshwater coming into estuaries, nutrients and sediment can’t reach estuaries as easily, and there’s not enough freshwater to balance out the saltwater, making estuaries less productive.
Land use changes also make estuaries — and the rest of the environment — more vulnerable to climate change impacts, Feagin said.
“Climate change impacts are going to be different everywhere. Some places are going to rain more, and some places are going to get drier,” he said. “And that totally influences the freshwater input to estuaries.”
Whether it’s getting wetter and fresher or saltier and dryer, neither is good for estuaries’ equilibrium, he said, or for the animals, plants, people and economies that rely on them.
When water gets saltier, it mixes less and has longer residence times, so they take longer to “forget” changes from droughts or storms and can’t recover as fast, Hetland said. Saltier, warmer water also doesn’t hold oxygen as well, Montagna said, so marine creatures can have insufficient oxygen and die.
Because estuaries are the nursery grounds for many species that require a narrow range of salinity to survive, not having the right balance of fresh and saltwater can have impacts for generations of fish, shrimp, oysters and more.
“Baby marine species literally follow the fresher water. That’s how they find the nursery habitats to grow up in,” Montagna said. “So if you don’t have a healthy nursery, you can’t grow adult species either.”
Texas already has some hypersaline — extra, extra salty — estuaries, such as the Laguna Madre, and climate change could result in more, Feagin said. Hypersaline estuaries have more algal flats and less vegetation, which has implications for their ability to capture carbon from the atmosphere, another benefit of estuaries.
“There's no organic matter that's being deposited into the soil and building a nice rich peat, so you're not sinking carbon,” he said.
Already “sediment-starved” from land use changes, estuaries are more vulnerable to erosion from rising sea levels and more intense storms, Montagna said. Because the land is sinking while the sea level is rising, the Texas coast has some of the highest apparent sea level rise rates in the world.
Increased salinity, decreased carbon storage and rising sea levels are all self-perpetuating problems, he said.
“The environment is less resilient,” he said. “We’ve built this feedback in the system where the problems make themselves worse.”
The big picture
Three years after Hurricane Harvey, Hetland said that Galveston Bay’s oysters are starting to recover. But slow-moving, wet storms like Harvey are becoming more common. So estuaries remain in a state that he calls “punctuated disequilbrium,” like Sisyphus eternally pushing the boulder up the hill in the underworld.
“They’re always moving towards some steady state that they’ll will never reach, because some event like Hurricane Harvey happens,” he said. The key will be making estuaries more resilient to those events.
Solving these problems, Montagna said, will require taking a whole-system view. He recalled that when he first moved to Texas in 1986, he was asked how much water would need to flow into the San Antonio Bay in order to maintain a healthy ecosystem.
“It took me 20 years to figure out how to ask that question the right way. At first, we were trying to look at the parts in isolation, and we couldn't get the right answers,” he said. “But now that we understand how the parts interact, the domino effects between the parts, we're finally understanding how the system really works. And now we also know what we need to do to solve the problems.”
Montagna said the good thing is that solutions, from better water use planning to restoring estuaries, can create feedback loops just like problems do.
“You can do two or three little things and all of a sudden that can have a big effect. So I think we can chip away at it,” he said.
“As a scientist, I believe the world will be constantly changing, and there's no way to stop that. So we can drive change in directions that benefit us or directions that can harm us.”