ALBERT

Description: The Southern Ocean is largely a High Nutrient Low Chlorophyll (HNLC) region where macronutrient concentrations are high and phytoplankton productivity is low. However, there are productive 'hot spots' that sustain large phytoplankton blooms. These areas, maintained by natural iron (Fe) fertilization, are important for the Southern Ocean ecosystem and for driving carbon export. Fe addition on-deck bioassay experiments were conducted on two cruises to the Scotia Sea region of the Southern Ocean (austral spring 2006 and summer 2008) to better understand how Fe controls the microplankton (20-200μm) community structure on a seasonal basis. Light microscopy and fast-repetition rate fluorometry were used to examine changes in the species composition and physiological status of the microplankton community. Bioassays were carried out in three contrasting regions of the Scotia Sea: (1) a naturally Fe-fertilized, high chlorophyll area downstream (north and northwest) of the Islands of South Georgia (DSG); (2) a low Fe, low chlorophyll area upstream (south) of the Islands of South Georgia (USG); and (3) a naturally Fe-fertilized area north of the South Orkney Islands (SOI). Multivariate statistics were applied to the light microscopy results, showing significant differences between the initial microplankton communities for each of the bioassays. These differences were primarily spatial (between regions) and secondarily temporal (between seasons). Significant microplankton community shifts occurred in three of five bioassays, those in spring and summer USG and in summer DSG only. In summer, USG community responses increased significantly in medium (100-1000pgCcell -1) and large (>1000pgCcell -1) diatom species in response to Fe addition. Such a response was consistent with relief from in situ Fe limitation, which favours larger microplankton species with higher Fe requirements and subject to lower grazing pressures. The largest biomass increase in Fe-treated bioassay bottles was in Pseudonitzschia spp., which suggests that this genus may be a particularly important member of the microplankton community in the Scotia Sea. © 2011 Elsevier Ltd.

Description: Marine N2 fixing microorganisms, termed diazotrophs, are a key functional group in marine pelagic ecosystems. The biological fixation of dinitrogen (N2) to bioavailable nitrogen provides an important new source of nitrogen for pelagic marine ecosystems and influences primary productivity and organic matter export to the deep ocean. As one of a series of efforts to collect biomass and rates specific to different phytoplankton functional groups, we have constructed a database on diazotrophic organisms in the global pelagic upper ocean by compiling about 12 000 direct field measurements of cyanobacterial diazotroph abundances (based on microscopic cell counts or qPCR assays targeting the nifH genes) and N2 fixation rates. Biomass conversion factors are estimated based on cell sizes to convert abundance data to diazotrophic biomass. The database is limited spatially, lacking large regions of the ocean especially in the Indian Ocean. The data are approximately log-normal distributed, and large variances exist in most sub-databases with non-zero values differing 5 to 8 orders of magnitude. Reporting the geometric mean and the range of one geometric standard error below and above the geometric mean, the pelagic N2 fixation rate in the global ocean is estimated to be 62 (52–73) Tg N yr−1 and the pelagic diazotrophic biomass in the global ocean is estimated to be 2.1 (1.4–3.1) Tg C from cell counts and to 89 (43–150) Tg C from nifH-based abundances. Reporting the arithmetic mean and one standard error instead, these three global estimates are 140 ± 9.2 Tg N yr−1, 18 ± 1.8 Tg C and 590 ± 70 Tg C, respectively. Uncertainties related to biomass conversion factors can change the estimate of geometric mean pelagic diazotrophic biomass in the global ocean by about ±70%. It was recently established that the most commonly applied method used to measure N2 fixation has underestimated the true rates. As a result, one can expect that future rate measurements will shift the mean N2 fixation rate upward and may result in significantly higher estimates for the global N2 fixation. The evolving database can nevertheless be used to study spatial and temporal distributions and variations of marine N2 fixation, to validate geochemical estimates and to parameterize and validate biogeochemical models, keeping in mind that future rate measurements may rise in the future. The database is stored in PANGAEA (doi:10.1594/PANGAEA.774851).