It is generally assumed that the larger the bottle volume, the longer the duration of phytoplankton microcosm experiments. We hypothesize that volume and duration are independent, as volume does not regulate the extension of the exponential growth phase. We conducted two microcosm experiments using 1, 2, and 8 L bottles, inoculated with phytoplankton collected in the Ria Formosa lagoon (SE Portugal) and incubated for 1, 2, 4, and 8 days. Phytoplankton net growth rates were estimated using chlorophyll a concentration and cell abundance, determined with epifluorescence and inverted microscopy. Results show that the experimental duration significantly affected net growth rates, independently of volume, with decreasing net growth rates with time. Regarding volume, we found significant, but weak, differences in net growth rates, and significant two-way interactions only for the larger-sized cells. No significant differences in net growth rates across the different volumes were detected for the smaller, most abundant taxa and for the whole assemblage. We conclude that duration, not volume, is the main factor to consider in microcosm experiments, and it should allow the measurement of responses during the exponential growth phase, which can be detected through daily sampling throughout the duration of the experiment.
