Assessing microbial responses to iron enrichment in the Subarctic Northeast Pacific: Do microcosms reproduce the in situ condition?

• Published in: Deep-sea research. Part II, Topical studies in oceanography Topical studies in oceanography, 2006-10, Vol.53 (20), p.2182-2200
• Main Authors: Scarratt, M.G., Marchetti, A., Hale, M.S., Rivkin, R.B., Michaud, S., Matthews, P., Levasseur, M., Sherry, N., Merzouk, A., Li, W.K.W., Kiyosawa, H.
• Format: Article
• Language: English
• Subjects: Bacillariophyceae; Bacteria; Bottle effects; Dimethylsulfide; Iron; Marine; Microcosms; Phytoplankton
• ISSN: 0967-0645; 1879-0100
• DOI: 10.1016/j.dsr2.2006.05.035

A microcosm experiment was conducted in the NE Pacific in July 2002 to compare the microbial response between microcosms and the Subarctic Ecosystem Response to Iron-Enrichment Study (SERIES) in situ iron-enrichment experiment. Seawater microcosms (20 L) were incubated aboard ship under natural light using three treatments: (1) low-iron seawater amended with 4 nmol l −1 FeSO 4 (+Fe); (2) low-iron seawater amended with 4 nmol l −1 FeSO 4 and 86 nmol l −1 GeO 2 (+Fe+Ge); (3) seawater collected from the in situ Fe-enriched patch (PW). The +Fe+Ge treatment used germanium to control diatom growth to assess the role of diatoms in dimethylsulfoniopropionate (DMSP) production. The following variables were measured in the microcosms and in situ: chlorophyll a (chl a), nitrate ( NO 3 - ), silicic acid (Si(OH) 4), phytoplankton abundance and species identification, bacterial abundance (including estimates of low- and high-DNA bacteria), bacterial production, bacterial specific growth rate, particulate and dissolved DMSP and dimethylsulfide (DMS) concentrations. There was little or no significant difference (ANCOVA) in the response of most variables between the +Fe and PW microcosms, but large differences were observed between both these treatments and the in situ data from the enriched patch. Chl a in all microcosms increased from ambient levels (approx. 0.5–1 μg l −1) to approx. 4.5–6.2 μg l −1 after 11 d incubation, when NO 3 - was fully depleted from all microcosms. During this same period, in situ chl a increased more slowly to a maximum of 2.9 μg l −1 on day 11. Nanophytoplankton and picophytoplankton were more abundant in the microcosms relative to the in situ community, which became dominated by large diatoms. Bacterial abundance was similar in the microcosms and in situ, but bacterial production was significantly higher in the microcosms. While neither DMSP d nor DMS accumulation showed significant differences between the microcosms and in situ , particulate DMSP concentrations increased significantly faster in the +Fe and PW treatments. These differences represent bottle effects resulting from the containment of the microcosms, which suppresses grazing, alters community and food web structure, enhances iron and nutrient regeneration, and isolates the community from physical transport and export processes including sinking. Thus during this experiment, the microcosms were not a good model for the in situ system in terms of the effects of iron on the phytoplan