Small creatures play a part in asking big questions.

The 2024 preprint Environmental Factors Drive the Composition of Diatom Communities Across a Range of Northern Hardwood Streams by Lindsey Sahlmann, et. al. examines the sensitivity of diatom communities to environmental changes in forested headwaters. According to the authors, these algae act as “sensitive environmental indicators, providing crucial insights into ecosystem health and resilience of streams flowing through northern hardwood forests in the face of environmental stressors.”

This research centers on the Hubbard Brook Experimental Forest (HBEF) in New Hampshire. Since the 1960s, the HBEF has maintained detailed ecological records involving water chemistry, forest structure, and atmospheric conditions. This long-term dataset offers a window into how forested watersheds adapt to anthropogenic pressures, including rising temperatures and shifting precipitation patterns. Understanding diatom responses within these headwater streams contributes to broader knowledge about New England’s climate resilience.

Diatoms as Indicators of Change

Diatoms are single-celled algae encased in intricate silica shells. They represent essential components of aquatic ecosystems because they form the base of the food web, cycle nutrients, and reflect subtle fluctuations in water chemistry. When temperatures rise, nutrients become limited, or precipitation regimes shift, diatom assemblages respond by altering which species appear. These assemblage changes provide data on water quality and stream health.

The authors state that “diatoms respond rapidly to shifts in water quality, such as nutrient levels, pH, light availability, and temperature,” underscoring their value as barometers of forest and stream conditions. This is significant in areas like New England, where changing winter snowfall and more frequent extreme rainfall events affect forest structure and hydrology. Diatom communities offer a clear signal of how such pressures play out at the microscale.

Hubbard Brook’s Place in the Story

The Hubbard Brook Experimental Forest is a central point of reference for ecological research in the northeastern United States. Its streams drain a 3,157-hectare area characterized by dense canopies, steep topography, and acidic soils typical of the White Mountain region. Research at the HBEF has addressed topics such as acid rain, forest management, and nitrogen cycling. Records include some of the earliest measurements of streamflow and water chemistry on a watershed scale.

However, the authors observe that “there are no published data concerning in-stream algal assemblages” at the HBEF. This project addresses that gap by analyzing diatom communities on artificial moss substrates in small weir ponds. These substrates mimic the natural bryophytes found in cold-water streams, stabilizing algae growth during variable streamflow.

Hubbard Brook’s significance extends to New England’s broader environmental context. Many northern hardwood forests share parallels in temperature regimes, canopy density, and acid deposition histories. The patterns observed in Hubbard Brook streams may apply to other headwater ecosystems facing similar stressors.

Light Availability and Diatom Diversity

Light is one of the key variables influencing diatom distribution at Hubbard Brook. The authors refer to “a significant positive correlation between species richness and cumulative irradiance measured at stream sites over two weeks prior to sampling.” Brachysira confusa showed a strong association with higher light levels, while several Eunotia species thrived under intermediate light conditions.

Small canopy gaps caused by natural disturbances or pest outbreaks increase sunlight penetration. Over time, these gaps can shift conditions in headwater streams from dark, oligotrophic environments to more light-exposed systems. This effect is relevant to climate resilience. As pest-related defoliation or partial harvesting persist in New England’s forests, higher sunlight in streams could alter the makeup of diatom species, thereby changing nutrient cycling and food web dynamics.

Broader Connections to Climate Resilience

New England’s forests are undergoing transformations due to pest invasions, diseases, and altered snowfall patterns. Each factor can influence the amount of light that reaches streams and the timing or volume of snowmelt. Subtle increases in daily sunlight or nutrient flux in headwater streams have implications for diatom communities and, by extension, aquatic ecosystems. Changes in diatom composition can affect primary production and stream metabolism, which relate to water quality and biodiversity.

Hubbard Brook’s long data record stands out as a lens for understanding these dynamics in northern hardwood ecosystems. The research findings reveal how diatoms respond to pH, dissolved organic carbon, nitrogen, and light. These observations extend to various forest-management scenarios, further connecting scientific data to effective decision-making about forest health and watershed protection.

Looking Ahead and Taking Action

Ongoing observations of diatom communities in places like Hubbard Brook present an opportunity to track responses to disturbance and environmental shifts. Researchers point to the need for continuous measurement of light, nutrients, and canopy openings to understand the long-term trajectories of these streams. This work contributes to a framework for more effective forest and water management practices aimed at sustaining biodiversity and ecosystem services across New England.

Understanding diatom ecology allows scientists to demonstrate how headwater streams respond to seasonal events, climate trends, and forest dynamics. This knowledge contributes to resilience planning in the face of environmental change. By emphasizing the responses of these microscopic organisms, the study underscores the tangible connections between forest canopies, stream ecosystems, and the future of New England’s watersheds.

 

Read the preprint version of Environmental Factors Drive the Composition of Diatom Communities 2 Across a Range of Northern Hardwood Streams

Authors: 

Lindsey Sahlmann, Rubenstein School of Environment and Natural Resources, University of Vermont, Burlington, Vermont, USA

Mark B. Edlund, St. Croix Watershed Research Station, Science Museum of Minnesota, Marine on St. Croix, Minnesota, USA

Audrey N. Thellman, Department of Earth, Marine and Environmental Sciences, University of North Carolina, Chapel Hill, North Carolina, USA

Christopher T. Solomon, Cary Institute of Ecosystem Studies, Millbrook, New York, USA

Ana M. Morales‐Williams, Rubenstein School of Environment and Natural Resources, University of Vermont, Burlington, Vermont, USA

William Scott Keeton, Rubenstein School of Environment and Natural Resources, University of Vermont, Burlington, Vermont, USA

William B. Bowden, Rubenstein School of Environment and Natural Resources, University of Vermont, Burlington, Vermont, USA

 

Carlson, Lindsey Sahlmann and Edlund, Mark B. and Thellman, Audrey N. and Solomon, Christopher T. and Morales‐Williams, Ana M. and Keeton, William S. and Bowden, William B., Environmental Factors Drive the Composition of Diatom Communities Across a Range of Northern Hardwood Streams. Available at SSRN: https://ssrn.com/abstract=5020173 or http://dx.doi.org/10.2139/ssrn.5020173