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Palmer Drought Severity Index values for the time span of this study (panel A) and gene abundances (panel B). In panel A, asterisks indicate months when samples were collected. In panel B, gene abundances are overlaid onto the PDSI values. Error bars represent one standard deviation. Credit: Anne Bernhard
A prolonged drought in southeastern Connecticut reduced the stability of microorganisms responsible for a critical step in the nitrogen cycle in a coastal salt marsh, according to research led by a Connecticut College scientist and published in Estuaries and Coasts. The study was led by Anne Bernhard, professor of biology at Connecticut College. Bernhard and her co-author analyzed microbial communities in a salt marsh at the Barn Island Wildlife Management Area in Stonington, Connecticut, from 2006 to 2019. The period included a severe regional drought from 2013 to 2018.
Researchers measured the abundance of microbial groups involved in nitrogen and carbon cycling. While most groups declined during dry periods, ammonia-oxidizing archaea and ammonia-oxidizing bacteria showed the largest fluctuations.
Archeal amoA gene abundances were nearly 35 times higher in wet conditions than in dry conditions. Over the course of the study, abundances of ammonia-oxidizing archaea and bacteria varied by as much as 30,000-fold and 9,500-fold, respectively. Both groups showed lower temporal stability during dry conditions compared with other microbes measured.
Nitrification—the conversion of ammonium to nitrate—plays an important role in regulating nitrogen in coastal ecosystems. Salt marshes buffer storm surge, store carbon and provide habitat for fish and shellfish.
After drought conditions eased in 2018 and 2019, abundances of ammonia-oxidizing archaea and bacteria returned to levels more similar to those observed before the drought.
Palmer Drought Severity Index values for coastal southeastern Connecticut from 1980 to 2022, as reported by NOAA (https://climatedataguide.ucar.edu/climate-data/palmer-drought-severity-index-pdsi). Values < −2.0 are considered to represent drought conditions, with values < −4.0 considered extreme drought. Shaded area denotes the current study period. Credit: Anne Bernhard
Temporal stability values for microbial populations with all samples combined (panel A), and separated into wet and dry samples (panel B). Stability was calculated by µ/s, according to Tilman (1999), where greater values represent lower temporal variation around the mean. Bac16S = bacterial 16S rRNA genes, Arch16S = archaeal 16S rRNA genes, AOA = archaeal amoA , AOB = bacterial amoA , nirS = denitrifiers, Com = comammox amoA , pmoA = methane oxidizers. Credit: Anne Bernhard
Gene abundances for bacterial 16S rRNA (A), archaeal 16S rRNA (B), archaeal amoA (C), bacterial amoA (D), comammox amoA (E), nirS (F), and pmoA (G) for samples under wet (PDSI > 0) or dry (PDSI < 0) conditions. Means are displayed above the whiskers. Credit: Anne Berhard
The findings provide long-term, field-based evidence that extended dry conditions can alter the stability of microbial communities central to nitrogen cycling in coastal marshes.
More information Anne Bernhard et al, Decreased Stability in Ammonia-Oxidzing Archaea and Bacteria during Dry Conditions in a Salt Marsh, Estuaries and Coasts (2026). DOI: 10.1007/s12237-025-01666-2
— Source: Phys.org (https://phys.org/news/2026-03-prolonged-drought-linked-instability-key.html)
