David Timothy, Mark Altabet, Matt McIlvin and Peng Feng

1. d15N Systematics NO3- concentration d15NO3- Northern Patch High NO3, Low Si NO3- (mM), d15N Southern Patch High NO3, High Si Instantaneous d15N-PON Accumulated d15N-PON u (fraction of NO3- utilized) Changes in d15N of nitrate and particulate nitrogen during a mesoscale iron fertilization experiment in the Southern Ocean David Timothy, Mark Altabet, Matt McIlvin and Peng Feng School for Marine Science and Technology U. Massachusetts Dartmouth 706 S. Rodney French Blvd. New Bedford, MA 02744-1221 Background Deep-sea sedimentary d15N has been used to estimate past changes in relative nutrient drawdown in Fe-limited HNLC regions. In a number of settings, corroboration between surface-water d15N systematics (Fig 1) and modern sedimentary d15N has been used to infer greater nutrient drawdown and hence less CO2 degassing to the atmosphere during glacial periods. While the relationship between nutrient drawdown and d15N of sinking particulates is robust with a fairly consistent fractionation factor for nitrate uptake, release from Fe-stress may produce modifications either through changes in species composition and/or phytoplankton physiological state. The Southern Ocean Fe Experiment (SOFeX; Fig 2) provided an opportunity to study the relationship between nitrogen isotope biogeochemistry and nutrient drawdown during Fe-enhanced phytoplankton growth. SOFeX was a collaborative effort involving two Scripps vessels (the R/V Revelle and the R/V Melville) and a Coast Guard ice breaker, the Polar Star, to extend the monitoring of the biogeochemical response to meso-scale Fe addition. Iron was added to waters north and south of the Antarctic Polar Front. While both sites have HNLC conditions, [SiO4] was low at the northerly site and high at the southern location (Fig 2). The most extensive coverage occurred at the southern site and is the focus of this presentation. Here, FeSO4 was added four different times over a ~two week period (Fig 4). Results In polar HNLC waters, plankton d15N is primarily determined by the degree of NO3- drawdown and the fractionation factor for NO3- uptake by phytoplankton, assuming little influence from recycled N. An expectation was that Fe-stimulated drawdown of NO3- would cause an increase in d15N in both NO3- and particles. An important question is whether the fractionation factor would significantly change in Fe-replete conditions. 1) Particulate nitrogen in the mixed layer increased by a factor of 2-3 in response to the Fe addition in the southern patch (Fig 3). 2) PN accumulation and NO3- drawdown were both 1-2 µM during the occupation of the bloom (Fig 4), suggesting retention of particulates within the mixed layer of the southern patch. 3) d15N of PN and of NO3- increased by 1-2‰ as [NO3-] decreased, and there is a clear contrast between in- and out-patch stations with respect to particulate d15N (Fig 4). The isotopic fractionation factor for NO3- was near 5-6‰ and appears to have been unaffected by Fe fertilization, although further analyses of d15NO3- on collected samples are needed. 4) The > 54 µm size fraction was typically lighter than the 1-54 µm size fraction by about 0.5 ‰. In the south patch, this difference increased as the bloom progressed, and with increasing PN concentration (Fig 5). This result may have been caused by several factors. Large chain-forming diatoms responded to the Fe addition and were likely isotopically lighter than smaller flagellates. Also, there was a surprising lack of zooplankton in the larger size fraction - their presence would have caused these samples to be isotopically more heavy. 5) The relationship between d15N of the two size fractions in and out of the southern patch (Fig 5) is currently unexplained. Analysis of other size-fractionated samples collected in and out of the southern patch may clarify these results. Fig. 3 Vertical profiles of particulate nitrogen in and out of the southern patch. ~10L samples were collected with Niskin bottles and the particulates were collected on GF/F filters. There was a 2-3 fold increase in PN within the mixed layer. Fig. 2 Location of SOFeX sites (Jan - March 2002). This presentation focuses on the southern patch where high nitrate and high silicate concentrations occur. Figures courtesy of Ken Johnson. SIZE-FRACTIONATED IN SITU LVP COLLECTIONS Acknowledgements: Rekha Singh and John Andrews provided technical assistance. Ken Buesseler provided in situ pump samples. Funding is from the DOE Carbon Sequestration Program and NSF. Contact:maltabet@umassd.edu, dtimothyn@umassd.edu or http://www.cmast.umassd.edu/ Fig. 1 Theoretical changes in d15N with NO3- drawdown as prescribed by Rayleigh fractionation and assuming a constant fraction factor of 8‰. The model for ‘accumulated product’ assumes no loss of particulate nitrogen from the euphotic zone. The ‘instantaneous product’ model assumes efficient PN removal. Mass balance is satisfied as the accumulation of isotopically light PN balances the diminishing pool of isotopically heavy NO3-. Fig. 5 Results from size-fractionated in situ large volume pump collections. Consistent with the bottle samples, “in” station samples were somewhat heavier than “out” (top and as Fig 4) and the >54 µm size fraction was typically ~0.5‰ lighter than particles 1-54 µm. This difference increased with PN concentration in the south patch (bottom). Samples are courtesy of Ken Buesseler. Fig. 4 Southern Patch time series of average [NO3-] and PN in the mixed layer, and their d15N. The accumulation of PN approximately balances NO3- drawdown. The difference between d15NO3- and d15N-PON showed little change over the study, suggesting a constant fractionation factor for NO3- near 5-6 ‰.