Trace metals and nitrogenous nutrition of Antarctic phytoplankton: experimental observations in the Ross Sea

Abstract

As part of the US JGOFS Antarctic Environment Southern Ocean Process Study (AESOPS), shipboard incubation experiments were conducted to investigate the effects of iron and zinc addition on ammonium (NH 4 + ) and nitrate (NO 3 − ) uptake by natural assemblages of Antarctic phytoplankton. Samples were collected from the Ross Sea during austral summer (January–February, 1997) within the mixed layer at two stations: a continental shelf station—Orca, and a high NO 3 − , low chlorophyll (HNLC) station located further offshore—Blue. Twenty-liter bottle incubations were sampled repeatedly during an 8-day period at Orca, and a 13-day period at Blue for short-term 15 N-tracer uptake experiments. Ambient concentrations of NO 3 − were elevated at both locations. Biomass, measured as chlorophyll a (chl a ), was relatively high at Orca (chl a =2.31 μg l −1 ), whereas true HNLC conditions were observed at Blue (chl a =0.52 μg l −1 ). All Fe enrichments produced an acceleration in NO 3 − decline, and accumulation in biomass. Chl a increased 2–3-fold at Orca and 4–5-fold at Blue; these increases were directly correlated with increasing Fe enrichment. Unambiguous responses to zinc addition were not evident at Blue, whereas increased total biomass accumulation and nitrate drawdown were observed at Orca. Rates of biomass (particulate nitrogen) specific NO 3 − -uptake in the Fe-enriched samples were up to 2-fold greater than un-enriched controls at both sites. There were no significant changes in specific uptake rates of NH 4 + at the HNLC site, and only a 30–40% increase at Orca with the highest Fe enrichments. These results clearly indicate that Fe additions resulted in faster rates of NO 3 − consumption per unit phytoplankton biomass at both sites. The N-uptake response to zinc enrichment was not as evident as with Fe, presumably due in part to the relatively high dissolved concentrations of zinc (ambient concentrations=ca. 2 nM). However, during the later sampling periods, zinc addition resulted in a 40% increase in the specific uptake rates of NO 3 − but not NH 4 + , whereas the specific uptake rates of NO 3 − and NH 4 + increased by 4–16% and 18–49%, respectively, at Orca. Absolute uptake rates of NO 3 − and NH 4 + (corrected for isotopic dilution) were 4–5 and 2–3 times greater, respectively, than the controls at Blue, whereas at Orca both NO 3 − - and NH 4 + -uptake rates doubled as a result of Fe enrichment. Post-incubation, size-fractionation uptake experiments demonstrated that the higher rates of nitrogen uptake were primarily due to larger phytoplankton (>5 μm). The f -ratio [ f =NO 3 − -uptake/(NO 3 − +NH 4 + -uptake)] increased from ca. 0.7 to 0.8 as a result of Fe enrichment at the HNLC site, but declined from an average of 0.84 to 0.52–0.69 in both the controls and the Fe-amended samples collected further onshore at Orca . This decrease may be due to the inhibition of NO 3 − -uptake by elevated NH 4 + concentrations resulting from increased heterotrophic remineralization within the carboys over time. Based on the apparent half-saturation constant ( K s ) of 0.09 nM Fe estimated for community planktonic NO 3 − -uptake, the availability of dissolved Fe (ambient concentration=0.03–0.04 nM) limits the uptake of NO 3 − by phytoplankton at the HNLC and continental shelf regions of the western Ross Sea during austral summer.