Different reactions of Southern Ocean phytoplankton size classes to iron fertilization

Abstract

During the European Iron Fertilisation Experiment (EIFEX), performed in the Southern Ocean, we investigated the reactions of different phytoplankton size classes to iron fertilization, applying measurements of size fractionated pigments, particulate organic matter, microscopy, and flow cytometry. Chlorophyll a (Chl a) concentrations at 20-m depth increased more than fivefold following fertilization through day 26, while concentrations of particulate organic carbon (POC), nitrogen (PON), and phosphorus (POP) roughly doubled through day 29. Concentrations of Chl a and particulate organic matter decreased toward the end of the experiment, indicating the demise of the iron-induced phytoplankton bloom. Despite a decrease in total diatom biomass at the end of the experiment, biogenic particulate silicate (bPSi) concentrations increased steadily due to a relative increase of heavily silicified diatoms. Although diatoms .20 mm were the main beneficiaries of iron fertilization, the growth of small diatoms (2-8 mm) was also enhanced, leading to a shift from a haptophyte-to a diatom-dominated community in this size fraction. The total biomass had lower than Redfield C : N, N : P, and C : P ratios but did not show significant trends after iron fertilization. This concealed various alterations in the elemental composition of the different size fractions. The microplankton (.20 mm) showed decreasing C : N and increasing N : P and C : P ratios, possibly caused by increased N uptake and the consumption of cellular P pools. The nanoplankton (2-20 mm) showed almost constant C : N and decreasing N : P and C : P ratios. Our results suggest that the latter is caused by a shift in composition of taxonomic groups.

Diatom δ13C, δ15N, and C/N since the Last Glacial Maximum in the Southern Ocean: Potential impact of Species Composition

“…Iron enrichment experiments in the Southern Ocean indicate that inputs of iron to surface waters here shifts the structure of the phytoplankton community toward domination by large diatoms (Fragilariopsis, Thalassiothrix, and Odontella spp. ), and increases the rates of growth and carbon uptake by these diatoms [Gall et al, 2001a[Gall et al, , 2001bHoffmann et al, 2006;Assmy et al, 2007], all things that should drive the d 13 C of both bulk POM and diatom organic matter up and not down. Because all of the organic matter within a diatom cell (including that occluded within the opal) is produced during photosynthesis, there should be a direct link between bulk cell and opal bound organic matter values.…”

Algal pigments in Southern Ocean abyssal foraminiferans indicate pelagobenthic coupling

“…Some samples with highly dense pigments were diluted with 90% acetone prior to HPLC analysis. All samples were analysed based on the HPLC method of Barlow et al (1997), as detailed in Hoffmann et al (2006). This method was adapted to our instrument (Waters 600, Waters, USA) and quality controlled as described in detail in Taylor et al (2011).…”

Evidence for marine production of monoterpenes

“…For phytoplankton pigment analysis, approximately every 3 h, 1-2 L of water from the ships seawater line (7 m below the ship), were filtered through GF/F filters, which were stored at −80 • C until they were analysed by high performance liquid chromatography (HPLC), details are described in Hoffmann et al [29] The taxonomic structure of the phytoplankton communities was derived from photosynthetic pigment ratios using the CHEMTAX program [30] applying the input matrix of Veldhuis and Kraay. [31] The phytoplankton group composition is expressed in chlorophyll a concentrations.…”