The reliability of grazing rate estimates from dilution experiments

The reliability of grazing rate estimates from dilution experiments

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Ocean Science Discussions, 1, 21–36, 2004 www.ocean-science.net/osd/1/21/ SRef-ID: 1812-0822/osd/2004-1-21 European Geosciences Union Ocean Science Discussions The reliability of grazing rate estimates from dilution experiments: Have we over-estimated rates of organic carbon consumption? J. R. Dolan and K. McKeon Marine Microbial Ecology Group, Laboratoire O´eanographique de Villefranche, CNRS UMR 7093, Universit´e Paris VI, Station Zoologique, F-06230 Villefranche-Sur-Mer, France Received: 4 November 2004 – Accepted: 17 November 2004 – Published: 18 November 2004 Correspondence to: J. R. Dolan (dolan@obs-vlfr.fr) © 2004 Author(s). This work is licensed under a Creative Commons License. 21 Abstract According to a recent global analysis, microzooplankton grazing is surprisingly invari- ant, ranging only between 59 and 74% of phytoplankton primary production across systems di ering in seasonality, trophic status, latitude, or salinity. Thus an important biological process in the world ocean, the daily consumption of recently fixed carbon, 5 appears nearly constant. We believe this conclusion is an artefact because dilution experiments are 1) prone to providing over-estimates of grazing rates and 2) unlikely to furnish evidence of low grazing rates. In our view the overall average rate of micro- zooplankton grazing probably does not exceed 50% of primary production and may be even lower in oligotrophic systems. 10 1 Introduction Recently Calbet and Landry (2004) presented a global analysis of the impact of mi- crozooplankton grazing based on the results of ’dilution’ grazing experiments. They found that microzooplankton grazing was surprisingly invariant, ranging only between 59 and 74% of phytoplankton primary production across systems di ering in seasonal- 15 ity, trophic status, latitude, or salinity. If 64% of the carbon fixed photosynthetically per day is consumed by microzooplankton there appears to be little left for any forms of carbon export or fueling whatever type of food web co-occurs with the microzooplank- ton, from bacteria to nekton. We believe this conclusion is an artefact because dilution experiments are 1) prone to providing over-estimates of grazing rates and 2) unlikely 20 to furnish evidence of low grazing rates. The dilution approach relies on the reduction of encounter rates between phyto- plankton and their microzooplankton grazers. Natural water samples are amended with varying proportions of filtered seawater creating a dilution series, and grazing rate is estimated as the increase in apparent phytoplankton growth rate with dilution factor. 25 Microzooplankton grazing rate is estimated as the slope of a regression of apparent 22
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phytoplankton growth in the various dilutions against dilution factor. Growth rate of the phytoplankton is estimated as apparent growth rate extrapolated to 100 % dilution (growth in the absence of grazers).
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The reliability of grazing rate estimates from dilution experiments

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