Comment: unimodal relationship between phytoplankton-mass-specific growth rate and size: a reply to the comment by Sal and López-Urrutia (2011)

Sal and López-Urrutia (2011) had two criticisms of our analysis on the relationship between community-based, temperature-corrected, mass-specific growth rates of natural phytoplankton assemblages under nutrient-enriched conditions and average size based on data from dilution experiments in surface waters of the global ocean (Chen and Liu 2010). Two individual data sources (Chen et al.2009; Sherr et al. 2009) used photoacclimation-corrected growth rates, while other data were not corrected for photoacclimation. If uncorrected data in Chen et al. (2009)were used in the analysis, the quadratic term in the unimodal fit became insignificant, but the slope of the linear fit was still significantly positive, suggesting that larger phytoplankton grow faster. However, if a lower temperature coefficient (activation energy E = 0.32 eV instead of 0.41 eV) were used to normalize the temperature effect, there was no relationship between phytoplankton specific growth rate and average size, which is consistent with the prediction of the Metabolic Theory of Ecology (MTE) that cell-specific production rate should scale isometrically with cell size expressed in terms of carbon (López-Urrutia et al. 2006).