The short version
Tampa Bay is the classic nitrogen cleanup success: after loads were cut by two-thirds, its seagrass meadows grew back to a record high in 2016. Since then the bay has lost 28% of that seagrass, even though the water has stayed clear enough for it to grow. Scientists at the Tampa Bay Estuary Program and partner agencies looked at about 50 years of data and found the bay has become steadily hotter and less salty. Their models suggest these changes are linked to the recent losses, though they could not prove they are the main cause.
What the researchers did
The team pulled together three long-term water monitoring programs with different designs, along with roughly 30 years of seagrass data.
- They tested for trends in water temperature and salinity in each part of the bay, by season, going back as far as 1975.
- They counted how many days each year the water was above 30 °C (86 °F) or below a salinity of 25 ppt.
- They used statistical models to see whether seagrass changes lined up with those stress days, after accounting for water clarity.
What they found
- A real comeback, then a reversal. Seagrass grew 79% between 1988 and 2016, to 41,655 acres, after nitrogen loads fell by two-thirds from a 1970s peak. By 2022 it was down to 30,137 acres, a loss of 11,518 acres.
- The upper bay was hit hardest. Old Tampa Bay lost 62% of its seagrass and Hillsborough Bay 80%, while the lower bay, which exchanges more water with the Gulf, held steady.
- Hotter water. Water temperature rose about 0.03–0.04 °C a year, or 1.3–1.7 °C over 50 years. On average across the bay, there are now 48 more days a year above 30 °C than in 1975.
- Fresher water. Salinity fell by 1.6–2.6 ppt over 50 years, with 37 more days a year below 25 ppt, in step with heavier rainfall.
- A link, but not proof. Taken together, the models showed recent losses were
somewhat associated with hotter and fresher conditions
(Beck et al., 2024), but the results were not consistent across datasets. Other possible factors include recurring toxic algae blooms in Old Tampa Bay, more seaweed, and disease.
What it means for Waquoit Bay today
Tampa Bay’s seagrasses are subtropical species, different from the eelgrass that once carpeted Waquoit Bay, so the specific temperature and salinity numbers don’t carry over. The broader warning does. Our own bay has already lost nearly all its eelgrass to nitrogen (see why Waquoit Bay lost its eelgrass), and eelgrass is a cool-water plant that struggles in heat. The Reserve’s records show Waquoit Bay warming too (monitoring trends, 2000–2022), and a regional study found eelgrass suffers after hot summers.
The authors say nutrient control should continue, but that climate change may weaken its payoff and call for added measures and steady monitoring. For Falmouth and Mashpee, clean water may be necessary but not sufficient to bring eelgrass back in a warmer climate, and recovery targets set decades ago may need a fresh look. For more on Tampa Bay, see our summary of the value of its water quality for recreation.
Read the paper
Beck, M. W., Flaherty-Walia, K., Scolaro, S., Burke, M. C., Furman, B. T., Karlen, D. J., Pratt, C., Anastasiou, C. J., & Sherwood, E. T. (2024). Hot and fresh: Evidence of climate-related suboptimal water conditions for seagrass in a large Gulf Coast estuary. Estuaries and Coasts, 47, 1475–1497. https://doi.org/10.1007/s12237-024-01385-0
License: This article is open access under a Creative Commons Attribution 4.0 (CC BY 4.0) license. The PDF below is reproduced unmodified from the publisher, Springer Nature. The card image is the article’s first page.
Drafted with AI assistance from the linked source.
