Biological response to iron fertilization in the eastern equatorial Pacific (IronEx II). I. Microplankton community abundances and biomass

Abstract

During the IronEx II experiment in the eastern equatorial Pacific (May to June 1995), the response of the microplankton community to mesoscale iron fertilization was followed using a combination of marker-pigment, microscopical and flow cytometric techniques. Phytoplankton standing stock bloomed dramatically over a period of 6 d following 3 iron additions of 2 and 1 nM, respectively. Carbon biomass in the patch increased by a factor of 4, chlorophyll a by about a factor of 16 and diatoms by > 70-fold relative to contemporaneous levels in the ambient community. The bloom then plateaued sharply and remained at a more or less constant level for 4 d, despite the addition of more iron (1 nM) and physiological indices (low C:chl a ratio and elevated photochemical quantum efficiency) suggesting that the cells were healthy and growing rapidly. Relatively large pennate diatoms (Nitzschia spp., median length 20 to 24 µm) dominated the patch bloom, with smaller pennate species and centric diatoms declining in relative importance. Heterotrophic bacteria increased at a slow rate (0.08 d -1 ) for >10 d during the experiment, as did heterotrophic nanoflagellates. There were also indications of enhanced cell size, cellular pigment content and possibly growth rates of small phytoplankton. Nonetheless, little difference was observed between the ambient community and the peak patch bloom with respect to the size composition of auto-and heterotrophic populations <10 µm in cell size. The relative constancy of the smaller size fractions, the sharp curtailment of net growth of the bloom after 6 d, and > 3-fold increase in large heterotrophic dinoflagellates and ciliates suggest that protistan grazers may have played an active role in controlling the phytoplankton response to increased iron availability.KEY WORDS: Diatom · Bloom · Phytoplankton pigments · Carbon:chlorophyll · Size structure · Heterotrophic protists Resale or republication not permitted without written consent of the publisher

Significant CO2 fixation by small prymnesiophytes in the subtropical and tropical northeast Atlantic Ocean

“…However, as potential light respiration of fixed carbon by phytoplankton cells could occur, the reported specific rates of CO 2 fixation into cell biomass should be treated as lower or net estimates. The total CO 2 fixation rates reported here ( Figure 4, Table 2) are comparable to previous measurements (for example, see Landry et al, 2000;Pérez et al, 2006;Morán, 2007). Moreover, the large contribution of small phytoplankton (that is, Prochlorococcus, Synechococcus, Euk-A and Euk-B) to CO 2 fixation reported here (Figure 4) is also in agreement with previous size-fractionation studies (for example, see Marañ ón et al, 2001;Pérez et al, 2006).…”

Biological response to iron fertilization in the eastern equatorial Pacific (IronEx II). III. Dynamics of phytoplankton growth and microzooplankton grazing

“…These samples were analyzed as described below for abundances, biovolumes and carbon biomass of ciliates and dinoflagellates. Heterotrophic nanoflagellates (2 to 20 µm) and other small protists were quantified from epifluorescence microscopical analyses of gluteraldehyde preserved samples (25 to 50 ml) taken from the same depth (15 m), but typically from a hydrocast 1 to 2 h before water was collected for dilution experiments (methods as described by Landry et al 2000).…”

The relative roles of coastal and oceanic processes in determining physical and chemical characteristics of an intensively sampled nearshore system

“…However, there is no evidence of this in our data and rank statistics did not improve the correlation among variables. Alternatively, the relative abundance of nitrogen may lead to variance in phytoplankton abundance that is unrelated to upwelling or the water column nutrients that we estimated, and is perhaps more a function of turbulent mixing (Smetacek and Passow 1990), other micronutrients such as iron (Coale et al 1996;Hutchins et al 1998), or zooplankton grazing (Landry et al 2000). A lack of correspondence between Chl a and nitrate and phosphorus was also reported by Menge et al (1997) for coastal Oregon and we note that our Chl a estimates were in a range similar to theirs.…”