Did dilution limit the phytoplankton response to iron additi

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TitleDid dilution limit the phytoplankton response to iron addition in HNLCLSi sub-Antarctic waters during the SAGE experiment?AbstractAn in situ iron addition experiment (SAGE) was carried out in high-nitrate low-chlorophyll low-silicic acid (HNLCLSi) sub-Antarctic surface waters south-east of New Zealand. In contrast to other iron addition experiments, the phytoplankton response was minor, with a doubling of biomass relative to surrounding waters, with the temporal trends in dissolved iron and macronutrients instead dominated by physical factors such as mixing and dilution. The initial increase in patch surface area indicated a lateral dilution rate of 0.125d-1, with a second estimate from a model of the decline in peak SF6 concentration yielding a higher lateral dilution rate of 0.16-0.25d-1. The model was tested on the SOIREE SF6 dataset and provided a lateral dilution of 0.07d-1, consistent with previous published estimates. MODIS ocean colour images showed elevated chlorophyll coincident with the SF6 patch on day 10 and 12, and an elevated chlorophyll filament at the SAGE experiment location 3-4 days after ship departure, which provided additional lateral dilution estimates of 0.19 and 0.128d-1. Dissolved iron at the patch centre declined by 85% within two days of the initial infusion, of which dilution accounted for 50-65%; it also decreased rapidly after the 2nd and 3rd infusions but remained elevated after the fourth infusion. Despite decreases in nitrate and silicic acid from day 7 and 10, respectively, the final nutrient concentrations in the patch exceeded the initial concentrations due to supply from lateral intrusion and mixed-layer deepening. The low Si:N loss ratio suggested that the observed limited response to iron was primarily by non-siliceous phytoplankton. Algal growth rate exceeded the minimum dilution rate during two periods (days 3-6 and 10-14), and coincided with net chlorophyll accumulation. However, as the ratio of algal growth to dilution was the lowest reported for an iron addition experiment, dilution was clearly a significant factor in the SAGE experiment recording the lowest phytoplankton response to mesoscale iron addition. ©2010 Elsevier Ltd.AcknowledgementsWe thank the SAGE voyage leader Mike Harvey and scientific party, particularly Andrew Marriner, Bill Main and David Ho for assistance with the release, and the Officers and Crew of the R/V Tangaroa. We also thank Phil Boyd, Greg Foothead and NIWA Vessel Services for assistance with planning and mobilisation for the voyage, and Jill Schwarz and Matt Pinkerton for ocean colour image retrieval and analysis. SAGE was jointly funded through the New Zealand Foundation for Research, Science and Technology (FRST) programs(C01X0204) "Drivers and Mitigation of Global Change" and (C01X0223) "Ocean Ecosystems: Their Contribution to NZ Marine Productivity".

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1st AuthorLaw, C.AuthorLaw, C.Smith, M.Stevens, C.Abraham, E.Ellwood, M.Hill, P.Nodder, S.Peloquin, J.Pickmere, S.Safi, K.Walkington, C.Year2011JournalDeep-Sea Research Part II: Topical Studies in OceanographyVolume58Number6Pages786-799DOI10.1016/j.dsr2.2010.10.018URLhttps://www.scopus.com/inward/recor.....b32d49498cc5e6f90041918e1KeywordsAlgal growthAlgal growth rateChlorophyll accumulationData setsDilution limitsDilution rateDissolved ironIn-situInitial concentrationLoss ratioMesoscaleNew zealandNutrient concentrationsOcean-colourPhysical factorsSF6 tracerSignificant factorsSilicic acidsSub-Antarctic PacificSurface areaTemporal trends, Algae controlChlorophyllEstimationExperimentsPhytoplanktonSatellite imagerySulfur hexafluoride, Dilution, bioaccumulationchlorophyllconcentration (composition)data setdiffusiondilutionestimation methodexperimental studygrowth rateironMODISnitratenumerical modelnutrient cyclingocean colorphysiological responsephytoplanktonpopulation declinesilicic acidsurface water, Southern Ocean, algae, rank1Author KeywordsDiffusionDilutionIronIron addition experimentPhytoplanktonSF6 tracerSilicic acidSub-Antarctic Pacific

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TypeArticleCitationLaw, C., Smith, M., Stevens, C., Abraham, E., Ellwood, M., Hill, P., Nodder, S., Peloquin, J., Pickmere, S., Safi, K. and Walkington, C. (2011). Did dilution limit the phytoplankton response to iron addition in HNLCLSi sub-Antarctic waters during the SAGE experiment?. Deep-Sea Research Part II: Topical Studies in Oceanography, 58(6): 786-799