About

Michael Piehler is the Cary C. Boshamer Distinguished Professor and serves as the Director of the Institute for the Environment at the University of North Carolina at Chapel Hill. He holds faculty appointments in the Department of Earth, Marine and Environmental Sciences, the Department of Environmental Sciences and Engineering, and the Environment, Ecology and Energy Program. His research and teaching focus on the connection between human activity and the function of natural systems, particularly at the interfaces of land and water. His research group employs field, laboratory, and data science methods to understand the contribution of natural systems to ecosystem and economic resilience. Piehler advises governments, non-government organizations, and private sector organizations on issues related to sustainability and the environment. His research program is supported by a diverse portfolio of federal, state, and regional sponsors.

Research topics

• Environmental science
• Geology
• Biology
• Ecology
• Chemistry
• Environmental chemistry
• Botany
• Geography
• Environmental engineering
• Meteorology

Selected publications

• Chronic enrichment affects nitrogen removal in tidal freshwater river and estuarine creek sediments

  UNC Libraries · 2025-01-25

  articleOpen accessSenior author

  Population growth in coastal areas increases nitrogen inputs to receiving waterways and degrades water quality. Wetland habitats, including floodplain forests and marshes, can be effective nitrogen sinks; however, little is known about the effects of chronic point source nutrient enrichment on sediment nitrogen removal in tidally influenced coastal systems. This study characterizes enrichment patterns in two tidal systems affected by wastewater treatment facility (WWTF) effluent and assesses the impact on habitat nitrogen removal via denitrification. We collected intact sediment cores from prevalent habitats in a tidal freshwater river (TFZ; swamp forest) and a tidal estuarine creek system (EST; salt marsh) upstream and downstream of a WWTF outfall, and quantified dissolved gas fluxes across the sediment-water interface during wet conditions in early summer and dry conditions in late summer. Data collected during two synoptic water quality monitoring campaigns complimented laboratory experiments to provide environmental context for biogeochemical processing. The two systems exhibited different enrichment patterns such that the river-dominated TFZ system was characterized by consistently elevated nitrate + nitrite concentrations downstream of the WWTF, whereas precipitation and tidal influence affected nutrient distributions in the EST creek. Downstream sediments in TFZ exhibit an apparent saturation response, while upstream rates may be limited by other factors, such as labile organic matter availability. In contrast, downstream sediments in EST denitrify at higher rates than upstream during wet conditions that may enhance transport of effluent. This work provides information on ecosystem functioning in human-influenced environments and can be of use in developing nature-based solutions, such as water treatment wetlands, for nitrogen removal.

  PublisherDOI

• Coastal squeeze reduces nitrogen removal services provided by wetlands: insights from an interdisciplinary framework

  Environmental Research Letters · 2025-09-29

  articleOpen accessSenior author

  Abstract Natural landscapes provide valuable ecosystem services that increase community resilience to environmental change. We present a novel interdisciplinary framework to quantify and spatially evaluate the value and fate of coastal wetlands in the context of sea level rise (SLR) and future land use (FLU) plans. We apply our framework in New Bern, NC, USA, where we project changes in nitrogen removal ecosystem services provided by wetlands and undeveloped open spaces during heavy rainfall events under current sea levels and with 0.15–1.5 m (0.5–5 ft) of SLR. These landscapes currently provide $90 000 USD worth of nitrogen removal ecosystem services annually. Areas currently designated for conservation are especially valuable, contributing 53% of annual services despite making up only 13% of New Bern’s total land area (107 km 2 ). We show that these Conservation designations are expected to lose over 60% of their wetlands with 0.90 m (3 ft) of SLR, reducing New Bern’s expected annual benefit by 56%. Wetland migration to higher elevations is inhibited largely by existing urban development, though we locate potential wetland migration corridors that extend into Developed and Urban Transition FLU designations. Application of our framework can help to maintain ecosystem services and reduce the pressures of coastal squeeze across changing coastal landscapes.

  PublisherDOI

• Chronic enrichment affects nitrogen removal in tidal freshwater river and estuarine creek sediments

  Journal of Environmental Quality · 2025-01-13 · 4 citations

  articleOpen accessSenior author

  Population growth in coastal areas increases nitrogen inputs to receiving waterways and degrades water quality. Wetland habitats, including floodplain forests and marshes, can be effective nitrogen sinks; however, little is known about the effects of chronic point source nutrient enrichment on sediment nitrogen removal in tidally influenced coastal systems. This study characterizes enrichment patterns in two tidal systems affected by wastewater treatment facility (WWTF) effluent and assesses the impact on habitat nitrogen removal via denitrification. We collected intact sediment cores from prevalent habitats in a tidal freshwater river (TFZ; swamp forest) and a tidal estuarine creek system (EST; salt marsh) upstream and downstream of a WWTF outfall, and quantified dissolved gas fluxes across the sediment-water interface during wet conditions in early summer and dry conditions in late summer. Data collected during two synoptic water quality monitoring campaigns complimented laboratory experiments to provide environmental context for biogeochemical processing. The two systems exhibited different enrichment patterns such that the river-dominated TFZ system was characterized by consistently elevated nitrate + nitrite concentrations downstream of the WWTF, whereas precipitation and tidal influence affected nutrient distributions in the EST creek. Downstream sediments in TFZ exhibit an apparent saturation response, while upstream rates may be limited by other factors, such as labile organic matter availability. In contrast, downstream sediments in EST denitrify at higher rates than upstream during wet conditions that may enhance transport of effluent. This work provides information on ecosystem functioning in human-influenced environments and can be of use in developing nature-based solutions, such as water treatment wetlands, for nitrogen removal.

  PublisherOA PDFDOI

• Comment on egusphere-2023-292

  2023-09-06

  peer-reviewOpen accessSenior authorCorresponding

  <strong class="journal-contentHeaderColor">Abstract.</strong> Sustaining water quality is an important component of coastal resilience. Floodwaters deliver reactive nitrogen (NO_x) to sensitive aquatic systems and can diminish water quality. Coastal habitats in flooded areas can be effective at removing reactive nitrogen through denitrification (DNF). However, less is known about this biogeochemical process in urbanized environments. This study assessed the nitrogen removal capabilities of flooded habitats along an urban estuarine coastline in the upper Neuse River Estuary (NRE), NC, USA under two nitrate concentrations (16.8 &micro;M and 52.3 &micro;M NO_x, respectively). We also determined how storm characteristics (e.g., precipitation and wind) affect water column NO_x concentrations and consequently DNF by flooded habitats. Continuous flow-through sediment core incubation experiments quantified gas and nutrient fluxes across the sediment-water interface in marsh, swamp forest, undeveloped open space, stormwater pond, and shallow subtidal sediments. All habitats exhibited net DNF. Additionally, all habitats increased DNF rates under elevated nitrate conditions compared to low nitrate. Structured habitats with high sediment organic matter had higher nitrogen removal capacity than unstructured, low sediment organic matter habitats. High precipitation-high wind storm events produced concentrations significantly lower than other types of storms (e.g., low precipitation-high wind, high wind-low precipitation, low wind-low precipitation), which likely results in relatively low DNF rates by flooded habitats and low removal percentages of total dissolved nitrogen loads. These results demonstrate the importance of natural systems to water quality in urbanized coastal areas subject to flooding.

  PublisherDOI

• Comment on egusphere-2023-292

  2023-08-27

  peer-reviewOpen accessSenior authorCorresponding

  <strong class="journal-contentHeaderColor">Abstract.</strong> Sustaining water quality is an important component of coastal resilience. Floodwaters deliver reactive nitrogen (NO_x) to sensitive aquatic systems and can diminish water quality. Coastal habitats in flooded areas can be effective at removing reactive nitrogen through denitrification (DNF). However, less is known about this biogeochemical process in urbanized environments. This study assessed the nitrogen removal capabilities of flooded habitats along an urban estuarine coastline in the upper Neuse River Estuary (NRE), NC, USA under two nitrate concentrations (16.8 &micro;M and 52.3 &micro;M NO_x, respectively). We also determined how storm characteristics (e.g., precipitation and wind) affect water column NO_x concentrations and consequently DNF by flooded habitats. Continuous flow-through sediment core incubation experiments quantified gas and nutrient fluxes across the sediment-water interface in marsh, swamp forest, undeveloped open space, stormwater pond, and shallow subtidal sediments. All habitats exhibited net DNF. Additionally, all habitats increased DNF rates under elevated nitrate conditions compared to low nitrate. Structured habitats with high sediment organic matter had higher nitrogen removal capacity than unstructured, low sediment organic matter habitats. High precipitation-high wind storm events produced concentrations significantly lower than other types of storms (e.g., low precipitation-high wind, high wind-low precipitation, low wind-low precipitation), which likely results in relatively low DNF rates by flooded habitats and low removal percentages of total dissolved nitrogen loads. These results demonstrate the importance of natural systems to water quality in urbanized coastal areas subject to flooding.

  PublisherOA PDFDOI

• Storm characteristics influence nitrogen removal in an urban estuarine environment

  Natural hazards and earth system sciences · 2023-11-28 · 2 citations

  articleOpen accessSenior authorCorresponding

  Abstract. Sustaining water quality is an important component of coastal resilience. Floodwaters deliver reactive nitrogen (including NOx) to sensitive aquatic systems and can diminish water quality. Coastal habitats in flooded areas can be effective at removing reactive nitrogen through denitrification (DNF). However, less is known about this biogeochemical process in urbanized environments. This study assessed the nitrogen removal capabilities of flooded habitats along an urban estuarine coastline in the upper Neuse River estuary, NC, USA, under two nitrate concentrations (16.8 and 52.3 µM NOx, respectively). We also determined how storm characteristics (e.g., precipitation and wind) affect water column NOx concentrations and consequently DNF by flooded habitats. Continuous flow sediment core incubation experiments quantified gas and nutrient fluxes across the sediment–water interface in marsh, swamp forest, undeveloped open space, stormwater pond, and shallow subtidal sediments. All habitats exhibited net DNF. Additionally, all habitats increased DNF rates under elevated nitrate conditions compared to low nitrate. Structured habitats with high-sediment organic matter had higher nitrogen removal capacity than unstructured, low-sediment organic matter habitats. High-precipitation–high-wind-storm events produced NOx concentrations significantly lower than other types of storms (e.g., low-precipitation–high-wind, high-wind–low-precipitation, low-wind–low-precipitation), which likely results in relatively low DNF rates by flooded habitats and low removal percentages of total dissolved nitrogen loads. These results demonstrate the importance of natural systems to water quality in urbanized coastal areas subject to flooding.

  PublisherOA PDFDOI

• Reframing the contribution of pelagic Sargassum epiphytic N2 fixation

  UNC Libraries · 2023-09-07

  articleOpen access1st authorCorresponding

  Though nitrogen fixation by epiphytic diazotrophs on pelagic Sargassum has been recognized for decades, it has been assumed to contribute insignificantly to the overall marine nitrogen budget. This six-year study reframes this concept through long-term measurements of Sargassum community nitrogen fixation rates, and by extrapolating mass-specific rates to a theoretical square meter portion of Sargassum mat allowing for comparison of these rates to those of other marine and coastal diazotrophs. On 24 occasions from 2015 to 2021, rates of nitrogen fixation were measured using whole fronds of Sargassum collected from the western edge of the Gulf Stream off Cape Hatteras, North Carolina. Across all dates, mass-specific rates ranged from 0 to 37.77 μmol N g-1 h-1 with a mean of 4.156 μmol N g-1 h-1. Extrapolating using a mat-specific density of Sargassum, these rates scale to a range of 0 to 30,916 μmol N m-2 d-1 and a mean of 3,697 μmol N m-2 d-1. Quantifying this community’s rates of nitrogen fixation over several years captured the sometimes-extreme variability in rates, characteristic of marine diazotrophs, which has not been reported in the literature to date. When these measurements are considered alongside estimates of the density of pelagic Sargassum, rates of nitrogen fixation by Sargassum’s epiphytic diazotrophs rival that of their coastal macrophyte and planktonic counterparts. Given Sargassum’s wide and expanding geographic range, the results of this study suggest this community may contribute reactive nitrogen on a meaningful, basin-wide scale, which merits further study.

  PublisherDOI

• Ecosystem-based management for military training, biodiversity, carbon storage and climate resiliency on a complex coastal land/water-scape

  UNC Libraries · 2023-02-16

  articleOpen access

  The Defense Coastal/Estuarine Research Program (DCERP) was a 10-year multi-investigator project funded by the Department of Defense to improve understanding of ecosystem processes and their interactions with natural and anthropogenic stressors at the Marine Corps Base Camp Lejeune (MCBCL) located in coastal North Carolina. The project was aimed at facilitating ecosystem-based management (EBM) at the MCBCL and other coastal military installations. Because of its scope, interdisciplinary character, and duration, DCERP embodied many of the opportunities and challenges associated with EBM, including the need for explicit goals, system models, long-term perspectives, systems complexity, change inevitability, consideration of humans as ecosystem components, and program adaptability and accountability. We describe key elements of this program, its contributions to coastal EBM, and its relevance as an exemplar of EBM.

  PublisherDOI

• Reframing the contribution of pelagic Sargassum epiphytic N2 fixation

  PLoS ONE · 2023 · 16 citations

  Senior authorCorresponding
  • Biology
  • Ecology
  • Botany

  Though nitrogen fixation by epiphytic diazotrophs on pelagic Sargassum has been recognized for decades, it has been assumed to contribute insignificantly to the overall marine nitrogen budget. This six-year study reframes this concept through long-term measurements of Sargassum community nitrogen fixation rates, and by extrapolating mass-specific rates to a theoretical square meter portion of Sargassum mat allowing for comparison of these rates to those of other marine and coastal diazotrophs. On 24 occasions from 2015 to 2021, rates of nitrogen fixation were measured using whole fronds of Sargassum collected from the western edge of the Gulf Stream off Cape Hatteras, North Carolina. Across all dates, mass-specific rates ranged from 0 to 37.77 μmol N g-1 h-1 with a mean of 4.156 μmol N g-1 h-1. Extrapolating using a mat-specific density of Sargassum, these rates scale to a range of 0 to 30,916 μmol N m-2 d-1 and a mean of 3,697 μmol N m-2 d-1. Quantifying this community's rates of nitrogen fixation over several years captured the sometimes-extreme variability in rates, characteristic of marine diazotrophs, which has not been reported in the literature to date. When these measurements are considered alongside estimates of the density of pelagic Sargassum, rates of nitrogen fixation by Sargassum's epiphytic diazotrophs rival that of their coastal macrophyte and planktonic counterparts. Given Sargassum's wide and expanding geographic range, the results of this study suggest this community may contribute reactive nitrogen on a meaningful, basin-wide scale, which merits further study.

  PublisherDOI

• Non-Native Marsh Grass (Phragmites australis) Enhances Both Storm and Ambient Nitrogen Removal Capacity in Marine Systems

  Carolina Digital Repository (University of North Carolina at Chapel Hill) · 2023-03-03

  articleOpen access1st authorCorresponding
  PublisherOA PDFDOI

Recent grants

• Collaborative Research: Microbial Regulation of Greenhouse Gas N2O Emission from Intertidal Oyster Reefs

  NSF · $209k · 2012–2017

• Collaborative Research: the influence of predators on community structure and resultant ecosystem functioning at a biogeographic scale

  NSF · $174k · 2010–2015

• Collaborative Research: Linking Hydrogeomorphology And Denitrification in the Tidal Freshwater Region of Coastal Streams

  NSF · $55k · 2008–2012

Frequent coauthors

• Hans W. Paerl

  28 shared

• Suzanne P. Thompson

  25 shared

• Jonathan H. Grabowski

  Northeastern University

  24 shared

• Ashley R. Smyth

  18 shared

• Craig Tobias

  University of Connecticut

  18 shared

• Adam Gold

  Environmental Defense Fund

  17 shared

• A. Randall Hughes

  Northeastern University

  14 shared

• David L. Kimbro

  Northeastern University

  14 shared