Bettina Sohst

Developing standards for dissolved iron in seawater

In nearly a dozen open- ocean fertilization experiments conducted by more than 100 researchers from nearly 20 countries, adding iron at the sea surface has led to distinct increases in photosynthesis rates and biomass. These experiments confirmed the hypothesis proposed by the late John Martin [Martin, 1990] that dissolved iron concentration is a key variable that controls phytoplankton processes in ocean surface waters. However, the measurement of dissolved iron concentration in seawater remains a difficult task [Bruland and Rue, 2001] with significant interlaboratory differences apparent at times. The availability of a seawater reference solution with well- known dissolved iron (Fe) concentrations similar to open- ocean values, which could be used for the calibration of equipment or other tasks, would greatly alleviate these problems [National Research Council (NRC), 2002]. The Sampling and Analysis of Fe (SAFe) cruise was staged from Honolulu, Hawaii, to San Diego, Calif., between 15 October and 8 November 2004 to collect data and samples that were later used to provide this reference material. Here we provide a brief report on the cruise results, which have produced a tenfold improvement in the variability of iron measurements, and announce the availability of the SAFe dissolved Fe in seawater standards.

Basin-scale transport of hydrothermal dissolved metals across the South Pacific Ocean.

Hydrothermal venting along mid-ocean ridges exerts an important control on the chemical composition of sea water by serving as a major source or sink for a number of trace elements in the ocean. Of these, iron has received considerable attention because of its role as an essential and often limiting nutrient for primary production in regions of the ocean that are of critical importance for the global carbon cycle. It has been thought that most of the dissolved iron discharged by hydrothermal vents is lost from solution close to ridge-axis sources and is thus of limited importance for ocean biogeochemistry. This long-standing view is challenged by recent studies which suggest that stabilization of hydrothermal dissolved iron may facilitate its long-range oceanic transport. Such transport has been subsequently inferred from spatially limited oceanographic observations. Here we report data from the US GEOTRACES Eastern Pacific Zonal Transect (EPZT) that demonstrate lateral transport of hydrothermal dissolved iron, manganese, and aluminium from the southern East Pacific Rise (SEPR) several thousand kilometres westward across the South Pacific Ocean. Dissolved iron exhibits nearly conservative (that is, no loss from solution during transport and mixing) behaviour in this hydrothermal plume, implying a greater longevity in the deep ocean than previously assumed. Based on our observations, we estimate a global hydrothermal dissolved iron input of three to four gigamoles per year to the ocean interior, which is more than fourfold higher than previous estimates. Complementary simulations with a global-scale ocean biogeochemical model suggest that the observed transport of hydrothermal dissolved iron requires some means of physicochemical stabilization and indicate that hydrothermally derived iron sustains a large fraction of Southern Ocean export production.

Iron supply and demand in an Antarctic shelf ecosystem

The Ross Sea sustains a rich ecosystem and is the most productive sector of the Southern Ocean. Most of this production occurs within a polynya during the November–February period, when the availability of dissolved iron (dFe) is thought to exert the major control on phytoplankton growth. Here we combine new data on the distribution of dFe, high-resolution model simulations of ice melt and regional circulation, and satellite-based estimates of primary production to quantify iron supply and demand over the Ross Sea continental shelf. Our analysis suggests that the largest sources of dFe to the euphotic zone are wintertime mixing and melting sea ice, with a lesser input from intrusions of Circumpolar Deep Water and a small amount from melting glacial ice. Together these sources are in approximate balance with the annual biological dFe demand inferred from satellite-based productivity algorithms, although both the supply and demand estimates have large uncertainties.

Assessing Phytoplankton Nutritional Status and Potential Impact of Wet Deposition in Seasonally Oligotrophic Waters of the Mid‐Atlantic Bight

To assess phytoplankton nutritional status in seasonally oligotrophic waters of the southern Mid-Atlantic Bight, and the potential for rain to stimulate primary production in this region during summer, shipboard bioassay experiments were performed using natural seawater and phytoplankton collected north and south of the Gulf Stream. Bioassay treatments comprised iron, nitrate, iron + nitrate, iron + nitrate + phosphate, and rainwater. Phytoplankton growth was inferred from changes in chlorophyll a, inorganic nitrogen, and carbon-13 uptake, relative to unamended control treatments. Results indicated the greatest growth stimulation by iron + nitrate + phosphate, intermediate growth stimulation by rainwater, modest growth stimulation by nitrate and iron + nitrate, and no growth stimulation by iron. Based on these data and analysis of seawater and atmospheric samples, nitrogen was the proximate limiting nutrient, with a secondary limitation imposed by phosphorus. Our results imply that summer rain events increase new production in these waters by contributing nitrogen and phosphorus, with the availability of the latter setting the upper limit on rain-stimulated new production. Plain Language Summary Human activities have substantially increased the atmospheric loading and deposition of biologically available nitrogen, an essential nutrient, to the surface ocean. Such atmospheric inputs to the ocean will likely impact on oceanic primary production by phytoplankton, and thus the marine ecosystem and ocean carbon cycling, although the scale and spatial distribution of such impacts are not well known. In this study, we used shipboard experiments, observations, and laboratory measurements to assess the potential impacts of atmospheric nitrogen deposition in rainfall on oceanic waters of the Mid-Atlantic Bight, off the U.S. eastern seaboard, during the summer. We find that the growth of phytoplankton in these waters is limited by the availability of nitrogen during summer, such that nitrogen added to the ocean by summer rain events can considerably stimulate phytoplankton primary production. However, the biological impact of these rainwater nitrogen inputs appears to be limited by the availability of another essential nutrient, phosphorus, which is present at relatively low concentrations in rainwater. This is the first study to directly examine the nutritional status of phytoplankton in relation to the impacts of rainwater nitrogen addition on primary production in oceanic waters off the U.S. East Coast.