Angela Knapp
Texas A&M University-College Station, TX·Earth & Environmental Science
Tiny ocean plants called phytoplankton form the base of the marine food web. Like crops, they need fertilizer, especially nitrogen.
- Fields
Earth & Environmental Science
Chemistry
Ecology & Evolution
- Topics
nitrogen fixation
N2 fixation
dissolved organic nitrogen
- Grants
- 3 known grants, through Jul 2028
What they work on
- Measure dissolved organic nutrients and nitrogen and oxygen isotopes in southwest Pacific seawater to build seasonal delta 15N budgets, isotope-based accounts of how much newly fixed nitrogen sinks to the deep ocean.
- Test whether newly fixed nitrogen accumulates in the surface dissolved organic nitrogen (DON) pool, and whether this depends on which nitrogen-fixing microbes are present, using community data from French HOPE collaborators.
- Test whether submarine groundwater discharge, seawater-bound groundwater seeping through the seafloor, is the main source of usable DON and iron on the West Florida Shelf, using radium isotope mass balances and isotopic fingerprints.
- Determine whether groundwater relieves iron stress in Trichodesmium, a nitrogen-fixing cyanobacterium, and so raises nitrogen fixation, using groundwater-addition incubations and measurements of iron-stress gene expression.
- Map where DON is produced and consumed across the subtropical North Pacific on a San Diego–Honolulu cruise, using DON isotopes alongside productivity estimates from oxygen/argon ratios and triple oxygen isotopes.
- Add a semi-labile DON tracer, representing slowly used coastal nitrogen, to a global ocean biogeochemistry model to estimate how much of open-ocean productivity this nitrogen source supports.
More about the research
Across huge stretches of the ocean, the water is so poor in nutrients that scientists cannot fully explain how life there grows as well as it does. Some nitrogen comes from microbes that perform nitrogen fixation, turning nitrogen gas into a form living things can use. Other nitrogen may arrive from the coast, either in groundwater or carried sideways by ocean currents as dissolved organic nitrogen (DON), meaning nitrogen bound up in dissolved organic molecules. This group of projects uses chemical tracers to follow where this nitrogen comes from and where it goes. The main tracers are nitrogen and oxygen isotopes, which are slightly heavier or lighter forms of the same atoms that act as fingerprints of a nitrogen source. The projects work in three nutrient-poor regions: the southwest Pacific, the West Florida Shelf, and the subtropical North Pacific.
Why it matters
Without knowing where and how much nitrogen reaches phytoplankton, scientists cannot accurately predict how many fish the ocean can support or how fish stocks will cope with environmental change. This work could improve the nutrient estimates used in ocean models that underpin fisheries projections. It could also clarify how groundwater and river inputs shape coastal ecosystems, both in Florida and in similar coastal regions of Australia, India and Africa.
Known grants
From NIH and NSF research grants. Labs often have other funding we can't see.
Geochemical constraints on the rates and fates of nitrogen fixation in the southwest Pacific
NSF · Aug 2025 – Jul 2028
Collaborative Research: Contribution of allochthonous DON to biological nitrogen demand in the subtropical North Pacific
NSF · Jul 2024 – Jun 2027
Collaborative Research: Linking iron and nitrogen sources in an oligotrophic coastal margin: Nitrogen fixation and the role of boundary fluxes
NSF · Nov 2024 – Apr 2027
Closest labs
Oscar Sosa
University of Puget Sound, WA·Earth & Environmental Science
Test whether methane made from methylamine and methane made from methylphosphonate in high-molecular-weight dissolved organic matter carry distinct stable carbon isotope signatures.
Angelicque White
University of Hawaii at Manoa, HI·Earth & Environmental Science
Sustain time-resolved hydrographic, biogeochemical and biological measurements at Station ALOHA through ten five-day cruises per year plus moored platforms, extending the North Pacific Subtropical Gyre climatology to 40 years.
Winifred Johnson
University of North Carolina Wilmington, NC·Earth & Environmental Science
Identify which marine microbial groups produce which osmolytes under nitrogen stress by pairing osmolyte measurements with transcriptomics in laboratory cultures.
The school
Texas A&M University-College Station: 23 labs on PI Pages, $1.4B a year on research, 768 research doctorates a year.
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