The short version
Estuaries are early warning systems for climate and land-use change, but few studies compare many of them with the same instruments. Reinl and colleagues pulled together more than 250 million measurements, taken from 2002 to 2022 at 129 stations in 29 of the 30 National Estuarine Research Reserves, a network that includes the Waquoit Bay Reserve. At the 99 stations with long enough records, chlorophyll-a (a measure of floating algae) rose at 70% of sites, water temperature at 66%, and dissolved oxygen fell at 49%. Changing nutrient levels, not warming, best explained the rise in algae, while warming and more algae best explained the oxygen loss. Oxygen has not yet fallen to harmful levels at most sites.
What the researchers did
The team used standardized monitoring data collected by the reserve network's System-Wide Monitoring Program, then looked for groups of similar sites, long-term trends and the factors behind them.
- Assembled sensor readings (temperature, oxygen, salinity-related conductivity, pH, cloudiness) taken every 15 or 30 minutes, plus monthly water samples for nutrients and chlorophyll-a, from 129 stations in 29 reserves, 2002 to 2022.
- Sorted the stations into four groups with similar water chemistry, using principal component analysis and k-means clustering (statistical ways of grouping sites that resemble each other).
- Calculated yearly trends in each measurement at the 99 stations with more than 10 years of data (27 reserves), after removing the seasonal cycle.
- Tested which factors best predicted each site's trend in chlorophyll-a and in dissolved oxygen, by comparing and averaging many candidate statistical models.
- Also tracked how often oxygen dropped below 2 mg/L, the usual threshold for hypoxia (dangerously low oxygen).
What they found
- Algae and warmth up at most sites. Significant increases in chlorophyll-a appeared at 70% of the 99 trend sites and in water temperature at 66% (Reinl et al., 2026).
- Oxygen down at half. Dissolved oxygen declined significantly at 49% of sites. Even so, only eight sites showed more time spent below 2 mg/L, so the declines have not yet produced widespread hypoxia (Reinl et al., 2026).
- The Northeast stands out. Some of the fastest warming and steepest oxygen declines were at Northeast reserves, including Wells, Great Bay, Waquoit Bay and Narragansett Bay. Warming there ran 0.06 to 0.08 °C per year, in line with other regional estimates (Reinl et al., 2026).
- Nutrients, not heat, best explained the algae increase. Trends in ammonium, phosphate and turbidity (cloudiness) were the best predictors of rising chlorophyll-a. Falling ammonium went with more algae, which the authors suggest may reflect algae using it up as they grow, possibly spurred by rising phosphate. Nitrate trends ranked last (Reinl et al., 2026).
- Heat and algae together explained oxygen loss. Trends in temperature and chlorophyll-a were the two strongest predictors of falling oxygen. Warm water holds less oxygen, and decaying algae use it up, though solubility alone did not account for the changes (Reinl et al., 2026).
- A large share is local. The chlorophyll-a model explained 19% of the variation in trends from the shared predictors alone, and 71% once differences between reserves were included. The authors say site-specific drivers account for much of what remains unexplained (Reinl et al., 2026).
- Four kinds of estuary. The sites fell into four groups. One, mostly in the Northeast, had the coolest water, highest oxygen and lowest algae, yet included many of the fastest-warming sites (Reinl et al., 2026).
What it means for Waquoit Bay today
The study covers the whole national reserve network, and the Waquoit Bay National Estuarine Research Reserve is one of the 29 reserves in it. One author works at the Waquoit Bay Reserve. The paper names the Waquoit Bay station among the Northeast sites with the highest warming and steepest oxygen declines, but it gives the 0.06 to 0.08 °C per year range for the Northeast group, not for Waquoit alone. Station-by-station numbers are in supplementary data we did not read. The paper also cites the 1992 Waquoit Bay study that tied land nitrogen to the bay's seaweed, algae and oxygen crashes (see how nitrogen gets from homes into Waquoit Bay).
For Falmouth and Mashpee, the result adds to a point made by other work: warming alone does not explain algae growth, so nitrogen cuts still matter in a warming bay. Narragansett Bay offers an example in which nitrogen reductions brought fewer severe algae events and fewer low-oxygen events even as the water warmed (see Narragansett Bay's fifteen summers). Local warming has also been measured off the Cape's south shore (see the Nantucket Sound warm spot). The authors note limits: they could only analyze inorganic nutrients, the monitoring avoids storm periods, and samples are taken near the bottom. Their national models explain only part of each site's trend, so what happens in Waquoit Bay depends on local nitrogen loads too.
Read the paper
Reinl, K. L., Dunn, R. P., Cressman, K. A., Collins, T., DeBose, J. L., Friedrichs, C. T., Goldstein, J. S., Helms, A. R., Kinkade, C., Krask, J. L., Parrish, D. B., Nicklay, H. N., Ridge, J. T., Sanger, D. M., Cianci-Gaskill, J. A., Dix, N. G., Grothues, T. M., McMurray, S. E., & Peter, C. (2026). In situ observations reveal continental-scale warming, oxygen decline, and eutrophication in U.S. estuaries. Communications Earth & Environment, 7, 716. https://doi.org/10.1038/s43247-026-03934-w
License: This article is open access under a Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 (CC BY-NC-ND 4.0) license, which allows non-commercial sharing of the unmodified article, so we host the PDF here and also link to the publisher's copy. This summary is based on the full text. The card image is a title card made from the citation.
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