Progressive decoupling between phytoplankton growth and microzooplankton grazing during an iron-induced phytoplankton bloom in the Southern Ocean (EIFEX)

Abstract

Dilution experiments were performed to quantify growth and mortality rates of phytoplankton groups (as defined by pigment markers) for 5 wk in an iron-induced phytoplankton bloom during the European Iron Fertilization Experiment (EIFEX) conducted in the Southern Ocean. Rates could be reliably measured for the 2 main groups, diatoms and prymnesiophytes. Mean phytoplankton intrinsic growth rates were around 0.23 d ), leading to an increase in decoupling between phytoplankton growth and microzooplankton grazing. The decrease in grazing was correlated with the decrease in concentrations of small microprotozooplankton (< 60 µm). As a consequence, net growth in the dilution experiments increased from around 0 d −1 up to 0.13 d −1 in the last days of the experiment, 35 d after the initial iron fertilization. This pattern did not reflect the dynamics of net phytoplankton accumulation in the fertilized patch, which increased until Days 24 to 27 and decreased thereafter. The difference between experimental and natural phytoplankton net growth is the biomass that escapes microzooplankton grazing and does not accumulate in the surface mixed layer, i.e. the biomass that went to higher trophic levels plus that exported out of the mixed layer. It increased throughout EIFEX and suggests a shift from a predominantly recycling system towards a more exporting one.

Decoupling Between Phytoplankton Growth and Microzooplankton Grazing Enhances Productivity in Subantarctic Waters on Campbell Plateau, Southeast of New Zealand

“…Photoacclimation related artefacts during incubations can significantly affect pigment‐based growth rates due to uncoupling between pigment and cell growth (Brown et al, ; Selph et al, ; Worden & Binder, ), and specific measures should be adopted to quantify and correct for them (Gutierrez‐Rodriguez & Latasa, ; Landry, ). The photoacclimation response estimated in our study (mean ± sd (range), Phi nut = −0.050 ± 0.092, (−0.220–0.100) and Phi nonut = −0.057 ± 0.092, (−0.200–0.100)) was modest (~5% change in cell pigment content during the incubations) compared to other studies (Brown et al, ; Gutiérrez‐Rodríguez et al, ; Selph et al, ), suggesting that the mean daily light exposure in the deck experiments was consistent with the in situ mixed‐layer averaged irradiance (Latasa et al, ). Further support for the applied photoacclimation correction comes from the fact that the Phi factor obtained across experiments varied with MLD in a way that was consistent with our current understanding of photoacclimation dynamics (e.g., cell pigment decrease, and higher Phi, in experiments conducted with communities collected from deeper mixed layers and vice versa) (Figure S3).…”

When Mixed Layers Are Not Mixed. Storm‐Driven Mixing and Bio‐optical Vertical Gradients in Mixed Layers of the Southern Ocean

“…All profiles fit by some type of sigmoid that had the depth of the inflection point, and/or subsurface maximum below the MLD was considered homogeneous within the mixed layer. Upper bound estimates of biological timescales of restratification from these profiles ranged between 1 and 3 days (depending on seasonality of storm timescales; see Table ), which are shorter than typical phytoplankton doubling times inferred from field observations in the region (e.g., de Baar et al, ; Latasa et al, ; Martin et al, ; Nelson & Smith, ; Owens et al, ; Reay et al, ; Sakshaug & Holm‐Hansen, ; Smetacek et al, ) or satellite ocean color (Behrenfeld et al, ).…”

“…Biological timescales of restratification inferred from wind profile matchups (i.e., τ bio ∼0.6–2.8 days) are thus consistent with overall statistics (i.e., τ bio <1–3 days, inferred by combining wind and profile structure statistics), despite the inherent inaccuracies of daily‐averaged wind analyses (from 6‐hourly synoptic winds) assigned to the profile sampling times and the potentially short biological timescales of restratification. Both approaches imply that biological timescales for the formation of vertical bio‐optical gradients are shorter than biological timescales for phytoplankton growth inferred for the region from field experiments (from a variety of techniques; e.g., Nelson & Smith, ; Reay et al, ; Sakshaug & Holm‐Hansen, ), including in situ iron‐addition experiments (e.g., de Baar et al, ; Latasa et al, ; Martin et al, ; Owens et al, ; Smetacek et al, ) when phytoplankton growth is expected to be enhanced, satellite observations (Behrenfeld et al, , ) as well as those simulated by state‐of‐the‐art Earth System models (Rohr et al, ). This suggests the possibility that biological timescales for phytoplankton growth in the Southern Ocean might be shorter than commonly thought, in agreement with findings of unusually high maximal growth rates (i.e., between ∼0.3 and 1 day −1 ) both in the sea ice zone (Smith et al, ; Spies, ) and in open ocean environments (Priddle et al, ; Timmermans et al, , ).…”

Major constrains of the pelagic food web efficiency in the Mediterranean Sea

“…Other authors reached higher resolution assessing ingestion rates and potential production for different classes of preys through pigment analysis (HPLC, flow cytometry), counts at the microscope of microphytoplankton, counts in epifluorescence microscopy of autotrophic and heterotrophic nano-and picoplankton (e.g. Fonda Umani and Beran, 2003;Fonda Umani et al, 2005, 2012Modig and Franze, 2009;Gutiérrez-Rodríguez et al, 2010Lie and Wong, 2010;Selph et al, 2011;Di Poi et al, 2013;Latasa et al, 2014;Liu et al, 2014).…”