Temperature, growth metrics, and the unimodal size scaling of phytoplankton growth

Abstract

Previous studies have shown that phytoplankton growth rates peak at intermediate cell sizes, contrary to predictions by the metabolic theory of ecology. It has been suggested that the unimodal relationship between phytoplankton cell size and growth rate may arise because the smallest species are adapted to warm temperatures. A related issue is the possibility of this size scaling pattern being sensitive to the specific metric used to determine growth rate. To test whether temperature affects the unimodal size scaling pattern, we used a double approach: 1) growing batch cultures of several species at two different temperatures (18ºC and 25ºC) to measure their growth rate, and 2) using thermal response models available in the literature to estimate the growth of each species at different temperatures. To assess the sensitivity of growth rate estimates to the metric used, we calculated growth rates based on experimental data of abundance, fluorescence, and chlorophyll, carbon and nitrogen content. Our results show that phytoplankton growth rates peak at intermediate sizes at both 18ºC and 25ºC. The unimodal pattern prevails irrespective of the growth metric employed. However, growth rates estimates based on fluorescence and chlorophyll tend to exceed those based on carbon and nitrogen, which reflects consistent patterns in the variability of cellular composition.