Water in Tampa Bay is getting hotter and fresher. That could spell trouble for seagrass.

Marcus Beck plunged into Tampa Bay with a pizza box in his arms.Snorkel in mouth, he slowly opened the box to reveal its contents. Instead of a cheesy pie, there was a copy of his team’s latest research paper. Stamped on top of the study, in bold red letters, were three words: Hot and Fresh! Beck and a group of researchers at the Tampa Bay Estuary Program used the creative slogan in the title of their recent, urgent findings: Florida’s largest open-water estuary, Tampa Bay, is getting warmer and less salty over time, according to their analysis of decades of temperature and salinity data.“We really liked ‘Hot and Fresh’ as an expression. It gets right to the point and it’s more intuitive to people: Tampa Bay is getting hotter and fresher,” said Beck, a program scientist at the Tampa Bay Estuary Program. Beck volunteered to dive into the bay, with the pizza box, for a video to announce the study. “It makes the science a lot more accessible.”Tongue-in-cheek expressions aside, the findings underscore a difficult truth: Future efforts to restore the thousands of acres of lost Tampa Bay seagrass will be complicated by harsher conditions. In the face of hotter and rainier days, spurred by global climate change, ecosystem restorers will have to work harder to maintain a healthy bay, researchers say.Over the past half-century, Tampa Bay’s average water temperature has increased by up to 3 degrees Fahrenheit, according to the study, published earlier this month in the peer-reviewed coastal research journal Estuaries and Coasts. That equates to an increase of roughly 0.07 degrees per year.Scientists have found seagrass growth is slowed when water temperature eclipses 86 degrees. Compared to 1975, there are now 48 more days per year when temperatures exceed that threshold, researchers found.It’s likely the bay will continue to heat up as humans burn fossil fuels and the planet warms: “Trends are very likely to continue to push seagrasses further outside of their tolerance ranges,” the group of nine scientists wrote in their report.As waters have heated, they’ve also become less salty. Why does that matter? Tampa Bay’s salinity levels determine whether marine life can thrive.Take oysters as an example: The shelled animals can handle short bursts of fresh water, but they’ll die if exposed for too long. The same is true if water becomes too salty. Long periods in overly salted water can lead to weakened oysters that die quicker. Seagrasses respond similarly to changes in salinity.Scientists track salinity in “parts per thousand,” or the ratio of dissolved salts compared to liters of water. Lower Tampa Bay, closest to the Gulf of Mexico, is unsurprisingly the saltiest part of the bay. Salinity there usually ranges between 25 and 38 parts per thousand. But farther north, the bay freshens up. Where the Hillsborough River empties into the bay, salinity ranges between 15 and 30 parts per thousand, according to the estuary program.Over the past five decades, Tampa Bay’s salinity has decreased by up to 2.6 parts per thousand, according to the study’s findings. For the analysis, which took roughly a year, scientists gathered three historical datasets from Hillsborough and Pinellas county environmental experts and Florida’s wildlife commission.There are five species of seagrass in Tampa Bay, from the tubelike manatee grass to the flat shoal grass, and each has a different tolerance to changes in salt levels. Overall, water in the bay is becoming more fresh by about 0.06 parts per thousand every year.“It’s your typical climate change story: We know that it’s getting hotter,” Beck said. But on the bay getting less salty, “That was an interesting finding that we didn’t expect.”As the ocean continues to warm and expand its way into new, fresher waterways, sea-level rise and other human influences — like redirecting water from where it naturally flowed — are expected to bring saltier water along with it. At the southern end of Florida’s Everglades lies Florida Bay, where a lack of rainfall and diverted freshwater flows in 2015 made parts of Florida Bay twice as salty as ocean water. It ultimately killed swaths of seagrass.But unlike systems like Florida Bay that lack direct freshwater inputs, Tampa Bay is connected to the Hillsborough River, where decades of increased rainfall have meant more fresh water empties into the bay. These northern stretches of the bay, typically more shallow than other areas, have seen the largest changes in temperature and salinity levels over the past half-century, researchers found.Since 1975, Tampa Bay has seen an annual increase of 37 days per year when salinity drops below 25 parts per thousand.They bay getting fresher, not saltier, “was opposite of what we’ve studied,” Beck said. “Our explanation for that was, yes, we are seeing sea-level rise — but the increase in precipitation over the years has counteracted that effect.”Seagrasses are the canaries in an aquatic coal mine.Scientists consider the marine plants an “indicator species”: If once-lush beds of seagrass are now weak and withered, it’s usually a sign there are larger environmental issues at play. Since 2016, the bay has lost nearly a third of its seagrass, or more than 11,500 acres, according to a recent analysis by the Southwest Florida Water Management District.There’s some evidence that hotter and fresher conditions have contributed to widespread seagrass die-offs in the bay. But just how much remains unclear, according to researchers.Reining in pollution dumping into Tampa Bay from rainfall runoff remains a major priority to protect seagrasses and the marine life that depend on them. Historically, it was human wastewater that caused most of the bay’s pollution problems. But upgrades to sewage plants curbed a major portion of the bay area’s wastewater issues and, today, it’s the water running off of land, collecting fertilizer and pesticides along the way, that cause most of the issues in the bay.At least 50% of the modern nitrogen pollution comes from stormwater runoff, according to the estuary program. Solving the pollution problem is made more difficult when you throw hotter and fresher water into the mix, according to the researchers.“The things that we have done in the past are not going to be as effective to protecting water quality as they have been,” Beck said. The bad news is that these changes are happening. The good news is that scientists are learning about it now, not years from now. “Doing something about it now, rather than later, is going to be easier than 10 or 20 years from now.”Usual methods for restoring Tampa Bay, including limiting the amount of nutrients flowing into the water, should continue, the paper’s authors note. But climate change may make those efforts less successful in the future. The scientific results should inform ecosystem restorers in the region that the estuary is likely becoming less resilient with climate change.Last year, the Tampa Bay Times joined researchers at the estuary program as they checked water temperatures in Old Tampa Bay, the estuary’s northwestern section. Each year over the next five years, researchers will monitor water temperatures from July through October as the heat is peaking.Data collected from that multiyear effort will offer scientists a clue about how temperature might hinder seagrass restoration, and it aims to answer the questions teed up by the team’s most recent analysis on temperature and salinity, according to Maya Burke, an assistant director of the Tampa Bay Estuary Program.The biggest takeaway from the study, according to Beck?“It’s the urgency of how climate change is influencing our environment — and the importance of acting while we can, before it’s too late.”

Water in Tampa Bay is getting hotter and fresher. That could spell trouble for seagrass.

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