Research: Dissolved organic nitrogen (DON)
The δ15N of DON records the production and
consumption of DON in the surface ocean.
 | The absolute
value of the δ15N of DON has been linked to the The
δ15N of subsurface NO3- in
both the North Atlantic and North Pacific, with the δ15N of DON in
the North Pacific higher than that in the North Atlantic. From
Knapp et al., 2011, Global Biogeochemical Cycles) |
 | Cartoon representing surface ocean nitrogen cycling based on
data from the North Pacific and North Atlantic. The data suggest
that the high δ15N of DON is due to fractionation upon breakdown,
and can explain the low δ15N of surface ocean suspended particulate
organic nitrogen. From
Knapp et al., 2011, Global Biogeochemical Cycles |
Recently published work shows new evidence for DON production
and consumption with an isotope effect in the upper 50 m of the eastern tropical South
Pacific:
 |
 |
| Samples were collected in 2010 and 2011 in the eastern tropical South Pacific | We found evidence for
DON production in the upper 50 m that was highly correlated with
surface ocean chlorophyll a concentration
measurements. |
|  |
| We also found the upper 50 m integrated DON stock was highly
negatively correlated with its δ15N . |
Using the upper 50 m DON
concentration and δ15N data we were able to estimate an isotope
effect for the consumption of bulk DON of 5.5 +/-1.2‰.
|
Related to my work on bulk DON δ15N, I have also measured the
δ15N of both low and high molecular weight (LMW and HMW,
respectively) DON in the surface ocean:
 |
 |
|
| This work includes
the first report of LMW DON δ15N from a marine environment;
specifically, we measured samples from the surface waters of the
North Atlantic Ocean and north of Australia (above; fromKnapp et
al., 2012, Marine Chemistry). |
We found that LMW DON δ15N < HMW DON δ15N, indicating that LMW DON comes from
PON, not HMW DON. Above is a schematic of LMW and HMW DON cycling
in the surface ocean, including isotope effects for DON production
and consumption. From
Knapp et al., 2012, Marine Chemistry. |
Where?
Studies of DON δ15N in the Sargasso Sea have shown no evidence for the
accumulation of nitrogen from N2 fixation in the DON
pool, but in the Western Tropical South
Pacific have shown evidence for diazotrophs releasing dissolved N that
subsequently supports export production in large-volume mesocosm
experiments. In the Eastern Tropical South
Pacific DON concentration and δ15N measurements
show evidence for photosynthetic production as well as consumption
in the upper 50 m with an isotope effect.
Publications:
Knapp, A.N., K.L. Casciotti, and M.G. Prokopenko, Dissolved
organic nitrogen production and consumption in eastern tropical
South Pacific surface waters, 2018, Global Biogeochemial Cycles, 32,
doi:10.1029/2017GB005875
Knapp, A.N., S.E. Fawcett, A. Martinez-Garcia, N. Leblond,
T. Moutin, and S. Bonnet, Nitrogen isotopic evidence for a shift
from nitrate- to diazotroph-fueled export production in the VAHINE
mesocosm experiments, 2016, Biogeosciences, 13, 4645-4657,
doi:10.5194/bg-13-4645-2016
Letscher, R.T., A.N. Knapp, A.K. James, C.A. Carlson, A.E. Santoro,
and D.A. Hansell, 2015, Microbial community composition and nitrogen
availability influence DOC remineralization in the South Pacific
Gyre, Marine Chemistry, 177, 325-334, doi:10.1016/j.marchem.2015.06.024
Letscher, R.T., D.A. Hansell, C.A. Carlson, R. Lumpkin, and
A.N. Knapp, 2013,
*Dissolved organic nitrogen in the global surface ocean: Distribution and fate,
Global Biogeochem. Cycles, 27, doi/10.1029/2012GB004449/
Knapp, A.N., D.M. Sigman, A.B. Kustka,
S.A. Sanudo-Wilhelmy, and D.G. Capone, 2012, The distinct nitrogen isotopic compositions of low and high
molecular weight DON, Marine Chemistry, 136-137, 24-33, doi:10.1016/j.marchem.2012.05.001
Knapp, A.N., D.M. Sigman, F. Lipschultz, A.B. Kustka, and
D.G. Capone, 2011, Interbasin isotopic correspondence
between upper-ocean bulk DON and subsurface nitrate and its
implications for marine nitrogen cycling, Global
Biogeochemical Cycles, 25, GB4004, doi:10.1029/2010GB003878
Knapp, A.N., D.M. Sigman, and
F. Lipschultz, 2005, N isotopic composition of dissolved organic
nitrogen and nitrate at the Bermuda Atlantic Time-series Study site, Global Biogeochem. Cycles, 19, GB1018, doi:10.1029/2004GB002320
This personal Web page is not an official Florida State University
Web page.
