Research

How does the ocean garden grow?

Just like the plants growing in your garden, algae, or small plants growing in the surface ocean, need specific nutrients to grow. Plant growth in the ocean can be limited by the availability of nitrogen (N), phosphorus (P), and trace metals such as iron (Fe). When algae lack these nutrients, photosynthesis in the ocean, and thus the transfer of carbon from the atmosphere to the ocean, can be limited. Because of the potential for nitrogen to limit marine photosynthesis, and thus to influence the global carbon cycle, I study the processes that add nitrogen to, remove it from, and cycle it in the ocean.

Much of my research involves quantifying the importance of different sources of "new" nitrogen to phytoplankton growing in marine surface waters:

"1" represents the flux of nitrate up from the subsurface, which is the dominant source of new N to most of the surface ocean, including at Bermuda.

"2" represents the biologically-mediated process of di-nitrogen fixation. Much of my work has involved looking for geochemical signatures of this process, especially by measuring the stable isotopic composition (d15N) of the surface ocean dissolved organic nitrogen (DON) pool as well as of thermocline nitrate.

"3" represents atmospheric deposition of N to the surface ocean. While a relatively small mass flux, atmospheric deposition of N is interesting because it has geochemical signatures similar to those of N2 fixation, specifically a high N:P ratio and a low d15N signature. At Bermuda, the d15N of total nitrogen in wet deposition is -2.3 per mil.

Dissolved organic nitrogen (DON) in the ocean

Another aspect of my work involves measuring the stable isotopic composition of dissolved organic nitrogen (DON d15N) to infer mechanisms of surface ocean nitrogen cycling:


Average profiles from the North Atlantic (red) and North Pacific (blue) ocean show that: 1) the d15N of DON is higher than the d15N of subsurface NO3- in both basins, and, 2) the d15N of DON is higher in the North Pacific than 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 d15N of DON is due to fractionation upon breakdown, and can explain the low d15N of surface ocean suspended particulate organic nitrogen. From Knapp et al., 2011, Global Biogeochemical Cycles

Related to my work on bulk DON d15N, I have also measured the d15N 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 d15N from a marine environment; specifically, we measured samples from the surface waters of the North Atlantic Ocean and north of Australia (above; from Knapp et al., 2012, Marine Chemistry).	We found that LMW DON d15N < HMW DON d15N, 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.

Nitrogen cycling in the Eastern Tropical South Pacific

In 2010 and 2011 I was a Co-PI on two cruises to the Eastern Tropical South Pacific on a NSF-funded project with Co-PIs Doug Capone, Will Berelson, Masha Prokopenko, and Karen Casciotti. We used a variety of biological and geochemical tools to quantify the relative and absolute importance of both subsurface nitrate and N2 fixation as sources of new nitrogen to phytoplankton in surface waters, as well as rates of net and gross primary productivity. We presented preliminary results at the 2012 Ocean Sciences Meeting in Salt Lake City. Final results coming soon!

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