Doing more with less? Balancing sampling resolution and effort in measurements of protistan growth and grazing-rates

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The dilution-method has been key in establishing the role of protistan-grazing in marine foodwebs. Yet its laborious application limits the sampling-resolution achieved. We assessed the reliability of an abbreviated method known as the 2-point by analyzing 77 dilution-experiments performed using 4–5 dilutions in diverse biotic and abiotic conditions. Our aim was to inform practitioners on how experimental design and nonlinear feeding behaviors affect the accuracy of 2-point rate-estimates. We found good agreement between rate-estimates of both phytoplankton growth (μ) and grazer-induced mortality (g) from either method, even though the comparison included experiments with nonlinear feeding-responses. The accuracy of 2-point estimates was similar to the inherent standard deviation of dilution-series estimates (± ∼ 0.1 d−1). Nonlinear feeding-responses did not alter overall rate-estimates, negating the need for more than two dilution-levels. Decreasing dilution resulting in an increase in biomass from 10% to 40% resulting in increased biomass increased the median difference between 2-point and dilution-series estimates ∼ 3-fold and increased 2-point estimates' variance ∼ 2-fold, both for μ and g. Recognition of these biomass vs. accuracy tradeoffs enables practitioners to choose whether to procure more biomass at the expense of constraining estimate variance. Using duplicate bottles at each dilution level doubled average accuracy of 2-point estimates. The reduction in effort and water-needs afforded by the 2-point design facilitates acquisition of higher-resolution data of predation-rates across seasons, latitudes, and in response to diverse environmental conditions in the ocean, which is critically needed to decipher abiotic and biotic drivers of protistan-grazing and to parameterize protistan herbivory in global biogeochemical models.

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Limnology and Oceanography: Methods