Papers by Andres Gutierrez
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The carbon flux through major phytoplankton groups, defined by their pigment markers, was estimat... more The carbon flux through major phytoplankton groups, defined by their pigment markers, was estimated in two contrasting conditions of the Northwestern Mediterranean open ocean ecosystem: the spring bloom and post-bloom situations (hereafter Bloom and Post-bloom, respectively). During Bloom, surface chlorophyll a (Chl a) concentration was higher and dominated by diatoms (53% of Chl a), while during Post-bloom Synechococcus (42%) and Prymnesiophyceae (29%) became dominant. The seawater dilution technique, coupled to high pressure liquid chromatography (HPLC) analysis of pigments and flow cytometry (FCM), was used to estimate growth and grazing rates of major phytoplankton groups in surface waters. Estimated growth rates were corrected for photoacclimation based on FCM-detected changes in red fluorescence per cell. Given the 30% average decrease in the pigment content per cell between the beginning and the end of the incubations, overlooking photoacclimation would have resulted in a 0.40 d À 1 underestimation of phytoplankton growth rates. Corrected average growth rates (m o ) were 0.90 7 0.20 (SD) and 0.40 7 0.14 d À 1 for Bloom and Post-bloom phytoplankton, respectively. Diatoms, Cryptophyceae and Synechococcus were identified as fast-growing groups and Prymnesiophyceae and Prasinophyceae as slow-growing groups across Bloom and Post-bloom conditions. The higher growth rate during Bloom was due to dominance of phytoplankton groups with higher growth rates than those dominating in Post-bloom. Average grazing rates (m) were 0.58 7 0.20 d À 1 (SD) and 0.31 70.07 d À 1 . The proportion of phytoplankton growth consumed by microzooplankton grazing (m/m o ) tended to be lower in Bloom (0.69 7 0.34) than in Post-bloom (0.80 7 0.08). The intensity of nutrient limitation experienced by phytoplankton indicated by m o /m n (where m n is the nutrientamended growth rate), was similar during Bloom (0.78) and Post-bloom (0.73). Primary production from surface water (PP) was estimated with 14 C incubations. A combination of PP and Chl a synthesis rate yielded C/Chl a ratios of 34 721 and 168 7 75 (g:g) for Bloom and Post-bloom, respectively. Transformation of group-specific Chl a fluxes into carbon equivalents confirmed the dominant role of diatoms during Bloom and Synechococcus and Prymnesiophyceae during Post-bloom.
Phytoplankton distribution is relatively constant in large areas of the surface ocean. In order t... more Phytoplankton distribution is relatively constant in large areas of the surface ocean. In order to maintain this apparent stability, phytoplankton production and losses have to be balanced. Indeed, growth (μ o ) and grazing (g) rates obtained simultaneously with the dilution technique are often tightly coupled. One problem with this approach is that growth and grazing are not independent in the ecological model on which the method is based (net growth rate = μ o -g). We evaluated to which extent this methodological artefact may influence the correlation between μ o and g estimated using the dilution technique. Following a Monte-Carlo approach, we show that the methodological correlation can be substantial depending on: (1) the % error in the measurement of the state variable N D (e.g. chlorophyll a) and (2) the range (± SD) of the μ o and g considered. As long as the error of N D is small (< 10%), the measured correlation between growth and grazing closely reflects a true ecological relationship. For large errors, the dilution technique can yield a substantial correlation between both variables, regardless of their ecological relation. The influence of this methodological correlation decreases as the range of growth and grazing rate values increases. We developed a procedure to evaluate the ecological versus the methodological nature of the correlation observed between μ o and g. The application of this procedure to a data set obtained from a coastal site revealed that the high correlation observed (r S = 0.881, p < 0.0001) reflected a true ecological relationship.
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Papers by Andres Gutierrez