Inorganic and Organic Nitrogen Utilization in the Southern Ocean Mesoscale Iron Enrichment Experiment (SOFeX)

San Francisco State University, San Francisco CA

Investigators

Abstract

The results of mesoscale (in situ) and microscale (shipboard-bottle) iron enrichment experiments conducted in the high nitrate, low chlorophyll (HNLC) waters of the sub-arctic Pacific, equatorial Pacific and the Southern Ocean strongly suggest that the rate of phytoplankton growth and biomass accumulation are limited, at least in part, by the availability of iron. To directly assess this hypothesis without the restrictions of deck-board incubation experiments, an in situ mesoscale iron enrichment experiment will be conducted in the Southern Ocean - the largest HNLC region on Earth and arguably the most important area in regulating global climate. The SOFeX project, led by Moss Landing Marine Laboratories, will involve two iron enrichment experiments along the 170'W meridian during this austral summer (December-January). During one 45-day cruise they will fertilize waters in the nitrate-rich and silicate-rich waters south of the Polar Front, and in the silicate-poor, but nitrate-rich waters north of the Polar Front. This sub-project will investigate the nitrogen dynamics of the natural planktonic community in response to the iron enrichment. The 15 N stable isotope technique will be used to quantify nitrate, nitrite, ammonium and urea uptake within and outside of the iron-enriched sites, thus providing a direct and unambiguous measure of nitrogen uptake independent of anv observable changes in the ambient nitrogen concentrations within these waters. Since cell size is thought to be a major factor controlling the flux of material from the surface to deep waters (i.e. export production), the size- spectrum of nitrogen utilization (pico-, nano- and microplankton) will be determined by using different pore-sized filters to collect particulates after short-term 15 N incubations. To evaluate the factors controlling nitrogen preference (or inhibition) in this HNLC region, the activities of nitrate reductase (NR) and nitrite reductase (NiR) will be assessed, in concert with N uptake rate measurements, to determine if iron limitation directly regulates the ability of phytoplankton (both large and small) to assimilate ammonium versus nitrate and test whether iron-limited phytoplankton are Fe-nitrogen co-limited. Deck experiments are designed to determine the effects of irradiance on nitrate and ammonium uptake. During collaborative studies with MLML and Mark Brzezinski (UCSB), the scientists in this sub-project will determine the effects of various concentrations of iron and zinc on the growth of phytoplankton (in particular diatoms) and their use of nitrogen and silicate. To ensure that the rates of nitrogenous nutrition are an accurate measure of new and regenerated production, the experimental design includes corrections for the effects of isotopic dilution from 14 NH4, losses from release of 15 N in dissolved organic nitrogen (DON) and N02, and competition from heterotrophic uptake. These N uptake experiments are a key component in the proposed SOFeX study and their results are critical to a comprehensive understanding of the ecosystem response to iron enrichment in the Southern Ocean. Modest phytoplankton growth and the lack of significant depletion of macronutrients are two fundamental observations common to HNLC regions and still remain to be explained in the Southern Ocean. This study will directly answer the question 'does iron stimulate the uptake and assimilation of nitrate, and if so, what group is stimulated?' all within the context of an in situ ecosystem response to iron enrichment.