About

Raymond G. Najjar is a Professor of Oceanography with a joint appointment in the Department of Geosciences at Penn State. His research specialty focuses on climate and oceanography. He holds the title of Wilson Faculty Fellow and is involved in research related to climate and oceanographic processes. His work emphasizes understanding the interactions between the atmosphere and oceans, contributing to the broader field of climate science.

Research topics

• Environmental science
• Geology
• Ecology
• Biology
• Oceanography
• Atmospheric sciences
• Geography

Selected publications

• A fundamental trade‐off among resilience, resistance, efficiency, and redundancy in tidal wetlands

  Ecology · 2026-01-01

  articleOpen access

  In an era of change, the survival and adaptability of ecosystems will be tested. An optimal ecosystem would be both resistant and resilient to negative disturbance but also efficient and redundant in its growth when given positive subsidies. However, initial evidence has suggested that these properties cannot all be maximized at the same time, and so we sought to quantitatively assess whether there are fundamental trade-offs between them at the ecosystem level. To achieve this aim, we used a 250-m resolution NASA MODIS dataset of gross primary productivity (GPP) to monitor 145,871 tidal wetland locations across the conterminous United States every 16 days from March 2000 to December 2020. We quantified the size and duration of the perturbation events in tidal wetland GPP (n = 13,754,386) and modeled their frequency distributions. Event sizes and recurrence intervals were exponentially distributed and event durations were closely modeled by an inverse power law. This scale-free manner through which tidal wetlands dissipated perturbations to their GPP flux provided them with long-term stability across a wide range of geography. We also found that a tidal wetland's positive event responses traded off between properties of efficiency and redundancy, its negative events traded off between resistance and resilience, and that all four properties were orthogonally related to one another. We then constructed a conceptual model to help understand the potential mechanism behind this four-quadrant trade-off. The trade-off appeared to be driven by a feedback between the waiting time and magnitude of positive and negative events, the duration of their effects, and the environmental and physical constraints limiting an ecosystem's growth and productivity. In summary, we detail an emergent pattern of trade-offs and constraints associated with how tidal wetland ecosystems respond to both positive and negative perturbations in carbon flux.

  PublisherDOI

• Identification and Characterization of Surface Water Intakes on the Chesapeake Bay

  2026-04-22

  article1st authorCorresponding

  Even though surface water intakes in coastal regions are increasingly threatened by saltwater intrusion and river salinization, there are no comprehensive databases of these intakes. Here, using information from state agencies, we identified and characterized in a consistent manner the surface water intakes on the Chesapeake Bay, a large, coastal plain estuary of the Mid-Atlantic region of the United States, for the period 2016–2020. We identified 291 intakes in six use types: 156 irrigation and agriculture; 76 industrial, commercial, and manufacturing; 28 municipal; 19 fossil power; 10 mining; and 2 nuclear power. The nuclear and fossil power intakes accounted for 67.2% and 28.6%, respectively, of the water volume withdrawn (348 m 3 s –1 ); most of the remainder was due to industrial, commercial, and manufacturing (2.5%) and municipal (1.5%), with very small contributions from irrigation and agriculture (0.1%) and mining (0.04%). There are intakes across a wide salinity (S) range, but many occur in the low-salinity waters threatened by saltwater intrusion—specifically tidal fresh (S < 0.5 g kg –1 ) and oligohaline (0.5 g kg –1 < S < 5 g kg –1 ) , which have, respectively, 37% and 21% of the intakes and 11% and 28% of the water withdrawal. Our findings suggest a large potential threat of salt contamination to surface water intakes in tidal waters and the need for national databases identifying and characterizing these intakes to facilitate adaptation planning.

  PublisherDOI

• Long-Term Trends and Interannual Variability in Tidal Wetland Gross Primary Production Across the Conterminous United States

  2026-01-20

  articleOpen access

  Tidal wetlands are critical carbon sinks, yet their response to ongoing environmental change remains uncertain across the conterminous United States. To address this gap, we quantified long-term trends and interannual variability in tidal wetland gross primary production (GPP; g C m⁻² d⁻¹) using a 20-year (2001–2020) satellite-based dataset. We also examined regional differences and the relative influence of climate drivers versus vegetation canopy on GPP dynamics. At the continental scale, GPP increased by approximately 6% over two decades, with the strongest gains in the South Atlantic and Gulf regions. Gulf wetlands exhibited the highest productivity, while Pacific and northern Atlantic wetlands were substantially lower, reflecting climatic gradients. Decomposition analysis indicates that rising shortwave radiation and air temperature are the primary drivers of productivity increases, outweighing declines in vegetative canopy coverage and apparent greenness. Interannual variability was modest overall but greatest in the Western Gulf, where episodic disturbances such as hurricanes and drought exert strong influence. These findings suggest that recent productivity gains are driven largely by climate forcing rather than vegetation changes, underscoring the need to incorporate climatic drivers into tidal wetland carbon models and management strategies.

  PublisherOA PDFDOI

• Freshwater faces a warmer and saltier future from headwaters to coasts: climate risks, saltwater intrusion, and biogeochemical chain reactions

  Biogeochemistry · 2025-03-10 · 16 citations

  articleOpen access

  Abstract Alongside global climate change, many freshwater ecosystems are experiencing substantial shifts in the concentrations and compositions of salt ions coming from both land and sea. We synthesize a risk framework for anticipating how climate change and increasing salt pollution coming from both land and saltwater intrusion will trigger chain reactions extending from headwaters to tidal waters. Salt ions trigger ‘chain reactions,’ where chemical products from one biogeochemical reaction influence subsequent reactions and ecosystem responses. Different chain reactions impact drinking water quality, ecosystems, infrastructure, and energy and food production. Risk factors for chain reactions include shifts in salinity sources due to global climate change and amplification of salinity pulses due to the interaction of precipitation variability and human activities. Depending on climate and other factors, salt retention can range from 2 to 90% across watersheds globally. Salt retained in ecosystems interacts with many global biogeochemical cycles along flowpaths and contributes to ‘fast’ and ‘slow’ chain reactions associated with temporary acidification and long-term alkalinization of freshwaters, impacts on nutrient cycling, CO 2 , CH 4 , N 2 O, and greenhouse gases, corrosion, fouling, and scaling of infrastructure, deoxygenation, and contaminant mobilization along the freshwater-marine continuum. Salt also impacts the carbon cycle and the quantity and quality of organic matter transported from headwaters to coasts. We identify the double impact of salt pollution from land and saltwater intrusion on a wide range of ecosystem services. Our salinization risk framework is based on analyses of: (1) increasing temporal trends in salinization of tributaries and tidal freshwaters of the Chesapeake Bay and freshening of the Chesapeake Bay mainstem over 40 years due to changes in streamflow, sea level rise, and watershed salt pollution; (2) increasing long-term trends in concentrations and loads of major ions in rivers along the Eastern U.S. and increased riverine exports of major ions to coastal waters sometimes over 100-fold greater than forest reference conditions; (3) varying salt ion concentration-discharge relationships at U.S. Geological Survey (USGS) sites across the U.S.; (4) empirical relationships between specific conductance and Na + , Cl − , SO 4 2− , Ca 2+ , Mg 2+ , K + , and N at USGS sites across the U.S.; (5) changes in relationships between concentrations of dissolved organic carbon (DOC) and different salt ions at USGS sites across the U.S.; and (6) original salinization experiments demonstrating changes in organic matter composition, mobilization of nutrients and metals, acidification and alkalinization, changes in oxidation–reduction potentials, and deoxygenation in non-tidal and tidal waters. The interaction of human activities and climate change is altering sources, transport, storage, and reactivity of salt ions and chain reactions along the entire freshwater-marine continuum. Our salinization risk framework helps anticipate, prevent, and manage the growing double impact of salt ions from both land and sea on drinking water, human health, ecosystems, aquatic life, infrastructure, agriculture, and energy production.

  PublisherOA PDFDOI

• Microplastic Polymer Accumulation, Distribution, and Toxicity in Sediment of a Freshwater Tidal Marsh

  SSRN Electronic Journal · 2025-01-01

  preprintOpen access
  PublisherDOI

• The Emerging Global Threat of Salt Contamination of Water Supplies in Tidal Rivers

  Environmental Science & Technology Letters · 2025-07-02 · 10 citations

  reviewOpen access

  Salt contamination of water supplies in tidal rivers is a global problem, but it has received little attention beyond site-specific studies. Drought, sea-level rise, navigation channel dredging, and watershed land-use change increase the risk of salinization and threaten drinking water supplies, agricultural irrigation, and infrastructure (via corrosion). The emerging issue of salt contamination of water supplies in tidal rivers and its diverse impacts highlight the critical need for interdisciplinary research that must integrate knowledge from oceanography, hydrology, and water resource management. Here we elucidate oceanic and hydrological processes regulating saltwater intrusion into estuaries and tidal rivers as well as watershed processes driving enhanced chemical weathering and export of watershed salts into rivers. By synthesizing studies around the world, we discuss how sea-level rise, prolonged drought, and increasingly extreme weather events in a changing climate are driving more frequent saltwater intrusion events that threaten water security globally. We propose a convergent research agenda toward the development of a decision support tool for salinity management. Specifically we recommend making ion-specific measurements and developing hydrological-hydrodynamic models to simulate the transport of major salt ions. These models can then be combined with artificial intelligence algorithms and enhanced monitoring to explore management strategies with stakeholders.

  PublisherDOI

• Salinity Trends in Chesapeake Bay Program Stations

  Open MIND · 2025-01-01

  datasetOpen access

  Salinity data synthesized and summarized from both EPA and USGS publicly available data

  Publisher

• Ecosystem Metabolic Rates Estimated from Diel Oxygen Measurements in Two Subtropical Estuaries

  Estuaries and Coasts · 2025-08-07 · 1 citations

  articleOpen access

  Abstract Subtropical estuaries worldwide are facing increasing pressure from human population growth, development, and climate change. Carbon is a useful currency for understanding how estuaries respond to these pressures and yet relatively little is known about carbon cycling in subtropical estuaries. Here we compute gross primary production (GPP), ecosystem respiration (ER), and net ecosystem production (NEP) from the diurnal cycle in dissolved oxygen measured during 38 week-long individual deployments over three years in two estuaries in the southeastern United States, Biscayne Bay and Tampa Bay. On average for both estuaries, GPP and ER nearly balance, with NEP about an order of magnitude smaller. Even though production in Tampa Bay and Biscayne Bay is dominated by different primary producers and limiting nutrients, mean GPP was the same, about 190 mmol O 2 m –2 d –1 (570 g C m –2 y –1 ). Our GPP estimates for Biscayne Bay are more than an order of magnitude greater than the only other productivity estimates available for this system, which are planktonic net primary productivity measurements from the late 1970s. GPP was strongly correlated with water temperature in Biscayne Bay ( r = 0.60) but had the strongest correlation with salinity in Tampa Bay ( r = 0.39). These findings highlight the importance of more frequent production measurements in these complex estuaries, especially in the face of a changing climate.

  PublisherOA PDFDOI

• Benthic macrofaunal carbon fluxes and environmental drivers of spatial variability in a large coastal-plain estuary

  Biogeosciences · 2025-12-08 · 1 citations

  articleOpen access

  Abstract. While the importance of carbon cycling in estuaries is increasingly recognized, the role of benthic macrofauna remains poorly quantified due to limited spatial and temporal resolution in biomass measurements. Here, we ask: (1) To what extent do benthic macrofauna contribute to estuarine carbon cycling via respiration and calcification? and (2) How well can routinely collected environmental variables predict their biomass? We analyzed data from 8128 benthic samples collected from the Chesapeake Bay between 1995 and 2022 and estimated associated carbon fluxes using empirical relationships. We then used generalized additive models to relate observed and modeled environmental variables to the biomass. Biomass was highest in the upper mainstem of the Bay (Upper Bay) and upper Potomac River Estuary, the largest tidal tributary of the Bay. In the Upper Bay, benthic macrofauna respired 18 %–45 % of the estimated organic carbon supply. Calcification-driven alkalinity reduction reached 6.31 ± 2.84 mol m−2 yr−1 in the Potomac River Estuary, aligning with prior estimates of alkalinity sinks in the tributary and highlighting the potential importance of calcifying fauna in alkalinity dynamics. Estimated CO2 production in the Upper Bay from benthic respiration and calcification (151 g C m−2 yr−1) also exceeded observed air–sea CO2 fluxes (74.5 g C m−2 yr−1). Generalized additive models revealed that low salinity, moderate dissolved oxygen, and elevated nitrate best predicted high-biomass zones, with the three predictors explaining 52 % of biomass deviance. These predictive relationships offer a pathway to estimate macrofaunal biomass and associated carbon fluxes in systems where direct biomass measurements are sparse. Our findings demonstrate that benthic macrofauna play a substantial and spatially structured role in estuarine carbon cycling. Incorporating their contributions into estuarine biogeochemical models will improve predictions of ecosystem responses to environmental and anthropogenic change.

  PublisherDOI

• Environmental drivers of spatial variability in benthic macrofauna biomass and associated carbon fluxes in a large coastal-plain estuary

  2025-04-08

  preprintOpen access

  Abstract. Extensive datasets document the distribution and composition of benthic macrofauna in some estuaries, yet their impact on carbon cycling remains poorly quantified. To address this, we investigated (1) how water chemistry and sediment composition correlate with benthic biomass distribution and (2) the contributions of benthic macrofaunal carbon fluxes to estuarine carbon budgets. We analyzed 8,128 benthic samples collected from Chesapeake Bay (1995–2022) and used generalized additive models to relate observed and modeled environmental variables to the biomass. We also estimated their associated carbon fluxes (calcification and respiration rates) using empirical relationships. The highest biomass was found in the upper Potomac River Estuary and Upper Bay; moderate dissolved oxygen, low salinity, and high nitrate concentrations were the clearest predictors of these zones (explaining 52 % of the deviance in biomass). Low surface NO3- concentrations within the estuary coincide with high inputs of allochthonous particulate organic carbon (POC) from riverine sources; this POC be the primary food source supporting high biomass zones. In the oligohaline Upper Bay, benthic macrofauna respire 17–50 % of total organic carbon available in that region, whereas their contribution is lower downstream. Moreover, the estimated benthic macrofaunal CO2 production rates from respiration and calcification rates in the Upper Bay (205±70 g C m-2 yr-1) exceeds estimated outgassing (74.5 g C m-2 yr-1), suggesting benthic macrofauna contribute significantly to air-sea gas exchange. The explainable spatial distribution of biomass and major role in estuarine carbon cycling highlight the importance and feasibility of incorporating the impacts of benthic macrofauna into numerical models. Refining these models could improve predictions of estuarine responses to natural and anthropogenic changes.

  PublisherOA PDFDOI

Recent grants

• Collaborative Research: Estuarine Response to Climate Forcing

  NSF · $331k · 2010–2015

• Collaborative Research: Multiple Stressors in the Estuarine Environment: What drives changes in the Carbon Dioxide system?

  NSF · $180k · 2015–2020

• WSC-Category 1 Collaborative Proposal: Coupled Multi-scale Economic, Hydrologic, and Estuarine Modeling to Assess Impacts of Climate Change on Water Quality Management

  NSF · $120k · 2014–2018

• Collaborative Research: Impacts of atmospheric nitrogen deposition on the biogeochemistry of oligotrophic coastal waters

  NSF · $386k · 2013–2017

• Collaborative Research: Estuarine metabolism and gas exchange determined from dissolved oxygen time series: method development, field evaluation, and application to historical data

  NSF · $899k · 2020–2025

Frequent coauthors

• Marjorie A. M. Friedrichs

  43 shared

• Galen A. McKinley

  42 shared

• John Marshall

  Massachusetts Institute of Technology

  39 shared

• Kathleen J. Mackin

  37 shared

• Robin Clark

  Met Office

  37 shared

• Michael C. Morgan

  St. Vincent's Medical Center

  37 shared

• Todd D. Sikora

  Millersville University

  37 shared

• J. Botella

  Massachusetts Institute of Technology

  37 shared

Education

• Ph.D., Atmosphere and Ocean Sciences Program

  Princeton University

  1990

• Bachelor of Engineering, Mechanical Engineering

  Cooper Union for the Advancement of Science and Art

  1985