The short version
Eelgrass, the underwater meadow plant that Waquoit Bay has largely lost, does best in water between about 10 and 25 °C. A team led by a National Park Service scientist pooled years of eelgrass monitoring from eight sites between New Hampshire and Maryland, including two on Cape Cod, and matched it with local summer water temperatures. They found that after a warmer-than-normal summer, eelgrass was less likely to be present at a monitoring spot the following year, while slightly cooler summers went with the best odds. Where eelgrass did survive, how thick it grew did not track the previous summer’s temperature.
What the researchers did
The study drew on SeagrassNet, a worldwide program that measures seagrass the same way everywhere, so sites can be compared.
- Eight sites: Great Bay (NH), Salem Sound, Duck Harbor Beach and Pleasant Bay (MA, the last two on Cape Cod), T-Dock and Fort Getty (RI), Fire Island (NY) and Tingles Island (MD).
- Eelgrass cover was measured each year in permanent quarter-square-meter plots at the season’s peak, over 9- to 17-year stretches between 2003 and 2021. Only the shallowest line of plots with eelgrass was used, to limit differences in light.
- For each site they calculated the average June–August water temperature, from loggers in the beds or nearby stations, and compared each summer with that site’s own long-term average.
- A statistical model then asked whether the previous summer’s temperature predicted whether eelgrass was present the next year, and how much of it there was.
What they found
- Hot summers, fewer plants next year. Last summer’s temperature was a statistically strong predictor of whether eelgrass showed up. Once a summer ran about 0.5 °C (roughly 1 °F) or more above a site’s normal, the predicted chance of finding eelgrass the next year fell steadily as temperatures climbed.
- Slightly cool summers were best. Summers up to about 0.5 °C below a site’s average went with the highest odds of eelgrass being present.
- Thickness didn’t follow temperature. Where eelgrass was present, its percent cover was not linked to the prior summer’s temperature. The authors note that a plant has to survive before it can grow, so the presence result matters most.
- Wide range of conditions. Average summer water ran from 15.7 °C at Salem Sound to 27.0 °C at Tingles Island. Eelgrass cover averaged 34.5% across all sites and years, and some sites had chronically little or none.
- Heat and murky water add up. Eelgrass needs more light when it is warm, so the authors say reducing other stresses, including poor light from nutrient pollution and turbidity, may help meadows cope. They also suggest restoration could use heat-tolerant plants; Great Bay, NH, for example, runs about 5 °C warmer in summer than Salem Sound despite being farther north.
What it means for Waquoit Bay today
Waquoit Bay was not one of the eight sites; two Cape Cod sites were, and the message carries over: eelgrass presence on a regional scale is threatened by warming waters
(Plaisted et al., 2022). Waquoit is shallow and already warm in summer, and the Reserve’s own records show the water warming over the past two decades (see the Reserve’s monitoring trends, 2000–2022). The bay lost its eelgrass mainly because nitrogen pollution clouded the water (see why Waquoit Bay lost its eelgrass); this study suggests that hot summers will make any comeback harder.
We can’t cool the bay, but we can clear it. The study’s point that warm eelgrass needs more light is a practical argument for cutting nitrogen: clearer water gives plants more margin in a hot year. Any future eelgrass restoration in the bay should also plan for warm summers, for example by choosing source plants from warm local meadows. The study tracked presence and cover only, not why plants disappeared, so it shows a strong pattern rather than a proven cause.
Read the paper
Plaisted, H. K., Shields, E. C., Novak, A. B., Peck, C. P., Schenck, F., Carr, J., Duffy, P. A., Evans, N. T., Fox, S. E., Heck, S. M., Hudson, R., Mattera, T., Moore, K. A., Neikirk, B., Parrish, D. B., Peterson, B. J., Short, F. T., & Tinoco, A. I. (2022). Influence of rising water temperature on the temperate seagrass species eelgrass (Zostera marina L.) in the Northeast USA. Frontiers in Marine Science, 9, 920699. https://doi.org/10.3389/fmars.2022.920699
License: © 2022 the authors. Open access under the Creative Commons Attribution 4.0 (CC BY 4.0) license. The full PDF below is reproduced unmodified from the publisher, Frontiers. The analysis was funded by the US National Park Service’s Northeast Coastal and Barrier Network. The card image is the paper’s first page (CC BY 4.0).
Drafted with AI assistance from the linked source.
