About

Bess B. Ward is the William J. Sinclair Professor of Geosciences and a faculty member at The High Meadows Environmental Institute at Princeton University. Her research focuses on the marine and global nitrogen cycle, utilizing molecular biological investigations of marine bacteria and bacterial processes, particularly nitrification and denitrification. She measures the rates of nitrogen transformation processes using various isotope approaches. Her ongoing research includes studying nitrogen cycling in suboxic zones of the world ocean such as the Arabian Sea, Eastern Tropical North and South Pacific, as well as in Chesapeake Bay and Great Sippewissett Salt Marsh. Additionally, her work explores nitrogen assimilation by phytoplankton and the functional diversity of eukaryotic phytoplankton in the ocean, along with the diversity of bacterial functional guilds involved in the nitrogen cycle of aquatic systems.

Research topics

• Chemistry
• Environmental chemistry
• Environmental science
• Ecology
• Biology
• Oceanography
• Earth science
• Geology

Selected publications

• Prokaryotic bias in surface ocean particles.

  UNC Libraries · 2026-04-10

  articleOpen access

  While the ocean's photosynthetic production of organic matter rivals that on land, a combination of heterotrophy and sinking prevents significant accumulation of particulate organic matter (POM) in open ocean surface waters. The origins and fates of POM in ocean surface waters are unclear, in part due to the dominance of nonliving, altered material. From the natural nitrogen isotopic composition of chlorophyll and its degradation products, we estimate the fraction of particles from eukaryotic vs. prokaryotic phytoplankton. In subtropical gyres and along the eastern North Pacific margin, the eukaryotic-to-prokaryotic ratio in particles matches that of living phytoplankton. However, in the North Atlantic outside its subtropical gyre, particles have a lower eukaryotic-to-prokaryotic ratio than do the living phytoplankton. This discrepancy at least partly arises from preferential sinking of eukaryotic biomass, consistent with the canonical but disputed paradigm that cyanobacteria disproportionately fulfill the energetic demands of the upper ocean microbial community while eukaryotes drive export production. The prokaryotic bias in surface ocean particles may also result from slow decomposition of specific components of prokaryotic biomass, a possible bottleneck in the ocean's microbial loop. The different fates of organic matter produced by eukaryotic and prokaryotic phytoplankton affect the productivity of the surface ocean, carbon export to the interior, and the signals recorded in deep-sea sediments.

  PublisherDOI

• Prokaryotic bias in surface ocean particles

  Proceedings of the National Academy of Sciences · 2026-04-01

  articleOpen access

  While the ocean's photosynthetic production of organic matter rivals that on land, a combination of heterotrophy and sinking prevents significant accumulation of particulate organic matter (POM) in open ocean surface waters. The origins and fates of POM in ocean surface waters are unclear, in part due to the dominance of nonliving, altered material. From the natural nitrogen isotopic composition of chlorophyll and its degradation products, we estimate the fraction of particles from eukaryotic vs. prokaryotic phytoplankton. In subtropical gyres and along the eastern North Pacific margin, the eukaryotic-to-prokaryotic ratio in particles matches that of living phytoplankton. However, in the North Atlantic outside its subtropical gyre, particles have a lower eukaryotic-to-prokaryotic ratio than do the living phytoplankton. This discrepancy at least partly arises from preferential sinking of eukaryotic biomass, consistent with the canonical but disputed paradigm that cyanobacteria disproportionately fulfill the energetic demands of the upper ocean microbial community while eukaryotes drive export production. The prokaryotic bias in surface ocean particles may also result from slow decomposition of specific components of prokaryotic biomass, a possible bottleneck in the ocean's microbial loop. The different fates of organic matter produced by eukaryotic and prokaryotic phytoplankton affect the productivity of the surface ocean, carbon export to the interior, and the signals recorded in deep-sea sediments.

  PublisherOA PDFDOI

• Prokaryotic bias in surface ocean particles

  Repository for Publications and Research Data (ETH Zurich) · 2026-04-07

  otherOpen access

  While the ocean’s photosynthetic production of organic matter rivals that on land, a combination of heterotrophy and sinking prevents significant accumulation of particulate organic matter (POM) in open ocean surface waters. The origins and fates of POM in ocean surface waters are unclear, in part due to the dominance of nonliving, altered material. From the natural nitrogen isotopic composition of chlorophyll and its degradation products, we estimate the fraction of particles from eukaryotic vs. prokaryotic phytoplankton. In subtropical gyres and along the eastern North Pacific margin, the eukaryotic-to-prokaryotic ratio in particles matches that of living phytoplankton. However, in the North Atlantic outside its subtropical gyre, particles have a lower eukaryotic-to-prokaryotic ratio than do the living phytoplankton. This discrepancy at least partly arises from preferential sinking of eukaryotic biomass, consistent with the canonical but disputed paradigm that cyanobacteria disproportionately fulfill the energetic demands of the upper ocean microbial community while eukaryotes drive export production. The prokaryotic bias in surface ocean particles may also result from slow decomposition of specific components of prokaryotic biomass, a possible bottleneck in the ocean’s microbial loop. The different fates of organic matter produced by eukaryotic and prokaryotic phytoplankton affect the productivity of the surface ocean, carbon export to the interior, and the signals recorded in deep-sea sediments.

  PublisherDOI

• Nitrification

  Elsevier eBooks · 2025-01-01

  book-chapter1st authorCorresponding
  PublisherDOI

• Phytoplankton community succession and biogeochemistry in a bloom simulation experiment at an estuary-ocean interface

  2025-03-13

  preprintOpen accessSenior author

  Abstract. Phytoplankton blooms, especially diatom blooms, account for a large fraction of marine carbon fixation. Species succession and biogeochemical parameters change rapidly over a bloom, and determine the resulting biological productivity. This study implemented daily sampling of a 24–L microcosm bloom simulation experiment to assess changes in assemblage and biogeochemical processes while excluding changes due to advection. 15NO3- and H13CO3- tracer incubations were performed alongside pigment and DNA sampling to compare temporal trends in community composition and primary productivity (nitrogen (N) and carbon (C) transport rates). Rapid drawdown of nutrients and maximum C and N transport rates corresponded with peak chlorophyll–a and fucoxanthin pigment concentrations. Fucoxanthin, typically associated with diatoms, was the dominant diagnostic pigment, with very low peridinin (dinoflagellate) and zeaxanthin (cyanobacteria) concentrations, indicating a diatom bloom. 18S rRNA gene analysis showed clear community succession throughout the duration of the bloom and multiple species of diatoms co–occurred, including during the bloom peak. The presence of metazoan 18S, high carbon–to–chlorophyll ratios, and a model analysis provide evidence of grazing in the latter half of the bloom. A traditional bloom framework suggests that species succession occurs as the bloom progresses and that phytoplankton diversity reaches a minimum of just one or two dominant species when phytoplankton productivity is at its maximum. However, this study produced a negatively monotonic productivity–diversity relationship with relatively high minimum diversity values. This 18S–based analysis therefore presents a more complex relationship between bloom progression and phytoplankton diversity.

  PublisherDOI

• Phytoplankton community succession and biogeochemistry in a bloom simulation experiment at an estuary–ocean interface

  Biogeosciences · 2025-09-17 · 1 citations

  articleOpen accessSenior author

  Abstract. Phytoplankton blooms, especially diatom blooms, account for a large fraction of marine carbon fixation. Species succession and biogeochemical parameters change rapidly over a bloom, and determine the resulting biological productivity. This study implemented daily sampling of a 24 L microcosm bloom simulation experiment to assess changes in assemblage and biogeochemical processes while excluding changes due to advection. 15NO3- and H13CO3- tracer incubations were performed alongside pigment and DNA sampling to compare temporal trends in community composition and primary productivity (nitrogen (N) and carbon (C) transport rates). Rapid drawdown of nutrients and maximum C and N transport rates corresponded with peak chlorophyll a and fucoxanthin pigment concentrations. Fucoxanthin, typically associated with diatoms, was the dominant diagnostic pigment, with very low peridinin (dinoflagellate) and zeaxanthin (cyanobacteria) concentrations, indicating a diatom bloom. 18S rRNA gene analysis showed clear community succession throughout the duration of the bloom and multiple species of diatoms co-occurred, including during the bloom peak. The presence of metazoan 18S, high carbon-to-chlorophyll ratios, and a model analysis provide evidence of grazing in the latter half of the bloom. A traditional bloom framework suggests that species succession occurs as the bloom progresses and that phytoplankton diversity reaches a minimum of just one or two dominant species when phytoplankton productivity is at its maximum. However, this study produced a negatively monotonic productivity–diversity relationship (PDR) with relatively high minimum diversity values. This 18S-based analysis therefore presents a more complex relationship between bloom progression and phytoplankton diversity.

  PublisherOA PDFDOI

• Supplementary material to "Denitrification as the predominant process in nitrous oxide production in the water column of two eutrophic reservoirs"

  2025-10-29

  articleOpen access
  PublisherDOI

• Similar Oxygen Sensitivities of Different Steps of Denitrification in Estuarine Waters

  Environmental Science & Technology · 2025-04-04 · 5 citations

  articleSenior author

  Hypoxia is observed and projected to expand in many aquatic environments, largely due to excess anthropogenic nutrient inputs and climate change, thus influencing biogeochemical processes. Denitrification, generally an anaerobic process, removes bioavailable nitrogen and produces nitrous oxide (N2O). However, limited observations of the effect of oxygen on denitrification restrict our ability to estimate changes in the amount of bioavailable nitrogen and N2O emissions under anthropogenic perturbations and climate change. Here, we show that all denitrification steps increased, while the N2O production yield from denitrification decreased with decreasing oxygen in Chesapeake Bay – the largest estuary in the United States. The different steps of denitrification responded similarly to oxygen changes in Chesapeake Bay, unlike open ocean oxygen minimum zones, with implications for the accumulation or depletion of denitrification intermediates such as nitrite and N2O. Our observations also suggest that current model parametrizations of denitrification in Chesapeake Bay likely overestimate denitrification and nitrogen removal in the presence of oxygen, which would bias the evaluation of nutrient cycling, ecosystem productivity, and the extent of hypoxia. Overall, our newly derived oxygen sensitivities of denitrification could be used to improve model parametrizations of denitrification and constrain the nitrogen budget and N2O emissions in estuarine and coastal environments experiencing hypoxia.

  PublisherDOI

• Mechanistic understanding of nitrate reduction as the dominant production pathway of nitrous oxide in marine oxygen minimum zones

  Nature Communications · 2025-10-07

  articleOpen access

  Nitrous oxide (N2O), a potent greenhouse gas and ozone-depleting agent, is produced intensely in oxygen minimum zones (OMZs) predominantly through nitrate reduction $$\left({{{{\rm{NO}}}}}_{3}^{-}\to {{{{\rm{N}}}}}_{2}{{{\rm{O}}}}\right)$$ . However, mechanisms and controls of this pathway remain unclear. Here, we investigate the microbial ecology governing this pathway using experiments and an ecosystem model. We experimentally confirm a critical hypothesis: most $${{{{\rm{NO}}}}}_{3}^{-}\to {{{{\rm{N}}}}}_{2}{{{\rm{O}}}}$$ denitrifiers do not utilize extracellular nitrite, an intermediate of the pathway. Model results demonstrate that the $${{{{\rm{NO}}}}}_{3}^{-}\to {{{{\rm{N}}}}}_{2}{{{\rm{O}}}}$$ pathway is compatible with oxygen, and that its response to oxygen is heterogeneous because it is governed by niche partitioning of distinct microbial types and thus may not follow a smooth curve. Lastly, experiments demonstrate that this pathway is sensitive to the type of organic matter, its electron acceptor, in addition to organic matter availability. These findings advance our mechanistic understanding of the primary N2O production pathway, necessary for predictions of marine N2O emissions. Experiments and modelling unravel mechanisms of nitrous oxide production from nitrate, the primary nitrous oxide production pathway in oxygen minimum zones. Results highlight the critical role of microbial interactions in regulating this pathway.

  PublisherOA PDFDOI

• Co‐occurrence and successional patterns among diatoms, dinoflagellates, and potential parasites in a coastal upwelling experiment

  Limnology and Oceanography · 2025-04-08 · 3 citations

  articleOpen accessSenior authorCorresponding

  Abstract Diatom‐dominated blooms in coastal upwelling systems contribute disproportionately to global primary production. The fate of carbon captured during a diatom bloom is often influenced by species‐specific ecological differences. However, successional patterns that take place during a diatom bloom are often oversimplified, and the diversity of diatom adaptations to different stages of a bloom remains poorly characterized. To improve our understanding of diatom specificity to certain conditions within a bloom, we employed microscopy, 18S rRNA amplicons, and biogeochemical analysis within a simulated upwelling mesocosm experiment. We successfully simulated a diatom bloom and found that diatoms bloomed during early and late phases of the bloom. Surprisingly, the relative abundance of congeneric diatoms with the Thalassiosira , Chaetoceros , and Pseudonitzschia displayed opposing patterns that were consistent among experimental mesocosms. The late stage of the bloom was especially interesting because some diatoms continued to bloom among mixotrophic dinoflagellate genera Akashiwo , Heterocapsa , and Prorocentrum . Additionally, Syndiniales putative parasites were correlated with several diatoms, especially in the initial phase of the bloom. The novel observations of consistent rapid successional changes within our mesocosms reflect the ability of diatom and dinoflagellate genera to occupy bloom conditions that fall outside traditional expectations. Syndiniales parasite co‐occurrence with blooming diatoms may be important to successional trends of coastal diatom populations, and this parasitic interaction deserves further study in coastal upwelling systems. This study indicates there are underlying diatom traits and biotic interactions that should be considered when estimating their contribution to productivity and carbon cycling within upwelling systems.

  PublisherOA PDFDOI

Recent grants

• Collaborative Research: Microbial Ecology of Nitrogen Cycling in the Oxygen Minimum Zone of the Arabian Sea

  NSF · $396k · 2004–2008

• Dimensions: Collaborative Research: Functional Diversity of Marine Eukaryotic Phytoplankton and Their Contributions to the C and N Cycling

  NSF · $1.4M · 2012–2016

• Collaborative Research: Mechanisms and Controls of Nitrous Oxide Production in the Eastern Tropical North Pacific Ocean

  NSF · $699k · 2017–2021

• Eukaryotic Phytoplankton Functional Diversity: Dynamics of Phytoplankton

  NSF · $800k · 2005–2009

• Environmental control of microbial N20 fluxes and DIN loss in salt marsh sediments

  NSF · $599k · 2010–2014

Frequent coauthors

• Amal Jayakumar

  Princeton University

  56 shared

• Xin Sun

  Carnegie Institution for Science

  52 shared

• Claudia Frey

  University of Basel

  36 shared

• Xianhui Wan

  Nanjing University of Aeronautics and Astronautics

  30 shared

• Andrew R. Babbin

  Massachusetts Institute of Technology

  29 shared

• Sarah E. Fawcett

  University of Cape Town

  26 shared

• Osvaldo Ulloa

  22 shared

• Andreas Oschlies

  GEOMAR Helmholtz Centre for Ocean Research Kiel

  20 shared

Labs

• Vecchi Research GroupPI