TY - JOUR T1 - A revised dilution methodology and implications for estimates of rates of plankton mortality JF - bioRxiv DO - 10.1101/072231 SP - 072231 AU - Stephen J. Beckett AU - Joshua S. Weitz Y1 - 2016/01/01 UR - http://biorxiv.org/content/early/2016/08/30/072231.abstract N2 - Author contribution statement: SJB and JSW designed the research questions and approach. SJB wrote the code and performed the experiments. SJB and JSW wrote the paper.Scientific significance statement: Zooplankton grazing is an important driver of plankton mortality and is a core component of the microbial loop. The dilution method is the prevailing tool used to measure the impact of zooplankton in marine microbial communities. However, the theoretical model underlying the interpretation of experimental measurements using this method does not account for niche competition between plankton. As a consequence, we show that the dilution method may conflate the effects of grazing with those of niche competition, implying that previously reported grazing rates could be overestimated. We propose modifying the classical dilution method by diluting only the zooplankton and then measuring the response of plankton. This “Z-dilution” method explicitly accounts for the effects of niche competition in limiting net plankton growth. We find the Z-dilution method provides robust grazing rate estimates in theory and suggest ways in which the relative strength of niche competition to zooplankton grazing could be measured by combining the Z-dilution and classical dilution methods.Data availability statement: Code is available from http://github.com/WeitzGroup/DilutionMethod-NicheCompetition and is archived on Zenodo at http://dx.doi.org/10.5281/zenodo.61196 (Beckett and Weitz, 2016).Abstract The dilution method is the principal tool used to infer in situ microzooplankton grazing rates. However, grazing is the only mortality process considered by the theoretical model underlying the interpretation of dilution method experiments. We show an alternative interpretation arises when there is concurrent niche competition within the plankton community. We find that grazing rates may be overestimated – the degree of overestimation is related to the importance of niche competition relative to zooplankton grazing. Thus, we propose a modification to the dilution method to disentangle the effects of niche competition and zooplankton grazing. Our theoretical results suggest the revised “Z-dilution” method can robustly infer grazing mortality, regardless of the dominant plankton mortality driver. Further, we show it is possible to independently estimate both grazing mortality and niche competition when the classical and Z-dilution methods are used in tandem. We discuss the significance of these results for quantifying plankton mortality rates. ER -