Understanding Modern Ocean Nutrient Cycles
The soft-tissue of marine organisms (i.e., organic matter) consist mainly of carbon and the major nutrients nitrogen (N) and phosphorous (P). Carbon is ubiquitous in the ocean so that growth of autotrophic phytoplankton is commonly limited by the availability of dissolved nutrients and sunlight as the energy source. Heterotrophic organisms ultimately depend on the production of organic matter by these phytoplankton as their food source. In the sunlit ocean surface nutrients are rapidly recycled to build organic matter that is then respired so as to release the nutrients back to the water were they become once again available to the phytoplankton. However, some of the organic matter sinks into the dark ocean interior before it is decomposed, thereby removing nutrients from the surface and sequestering nutrients and carbon at depth until ocean overturning slowly feeds those chemicals back to the surface. Some heterotrophic organisms adapted to low oxygen conditions can use the N-bearing nutrients nitrate, nitrite and ammonia (NO3 -, NO2 -, NH4 +) instead of oxygen to respire organic matter via denitrification and anammox, thereby depleting the ocean of bioavailable N. This N loss is mainly balanced by organisms capable of nitrogen fixation, which can break the strong triple bond of N2 to produce bioavailable N. All these biochemical transformations leave an imprint in the isotopic composition of the involved N-bearing compounds (organic matter, NO3 -, NO2 -, NH4 +). The isotopic composition of nitrate (NO3 -) is of particular importance of for the study of the modern ocean’s nutrient cycle because it is the dominant N-bearing nutrient and it carries both a nitrogen isotopic signal (nitrate-d15N) and an oxygen isotopic signal (nitrate-d18O) – the dual isotopes of nitrate.
As the available data on the ocean distribution of nitrate dual isotopes is growing it becomes increasingly clear that ocean biogeochemical models are required to deconvolve the various processes from the isotopic data. To this end I have implemented the N-cycle and its isotopes into the CYCLOPS ocean box model.
- Sigman, D.M., DiFiore, P.J., Hain, M.P., Deutsch, C., Wang, Y., Karl, D.M., Knapp, A.N., Lehman, M.F. and Pantoja, S. (2009) The dual isotopes of deep nitrate as a constraint on the cycle and budget of oceanic fixed nitogen, Deep Sea Research Part I: Oceanographic Research Papers, 56(9), 1419–1439, doi:10.1016/j.dsr.2009.04.007
- Sigman, D. M., DiFiore, P.J., Hain, M.P., Deutsch, C., Wang, Y., Karl, D.M., Knapp, A.N., Lehman, M.F. and Pantoja, S. (2009) Sinking organic matter spreads the nitrogen isotope signal of pelagic denitrification in the North Pacific, Geophysical Research Letters, 36, L08605, doi:10.1029/2008GL035784
- Sigman, D.M., and Hain M.P. (2012) The Biological Productivity of the Ocean, Nature Education, Vol 3, Issue 6.