Description: Nutrient availability and temperature are important drivers of phytoplankton growth and stoichiometry. However, the interactive effects of nutrients and temperature on phytoplankton have been analyzed mostly by addressing changes in average temperature, whereas recent evidence suggests an important role of temperature fluctuations. In a laboratory experiment, we grew a natural phytoplankton community under fluctuating and constant temperature regimes across 25 combinations of nitrogen (N) and phosphorus (P) supply. Temperature fluctuations decreased phytoplankton growth rate (rmax), as predicted by nonlinear averaging along the temperature–growth relationship. rmax increased with increasing P supply, and a significant temperature × P × N interaction reflected that the shape of the thermal reaction norm depended on nutrients. By contrast, phytoplankton carrying capacity increased with N supply and in fluctuating rather than constant temperature. Higher phytoplankton N:P ratios under constant temperature showed that temperature regimes affected cellular nutrient incorporation. Minor differences in species diversity and composition existed. Our results suggest that temperature variability interacts with nutrient supply to affect phytoplankton physiology and stoichiometry at the community level.

Description: Resource use efficiency (RUE) is an ecological concept that measures the proportion of supplied resources, which is converted into new biomass, i.e., it relates realized to potential productivity. It is also commonly perceived as one of the main mechanisms linking biodiversity to ecosystem functioning based on the assumption that higher species numbers lead to more complementary and consequently more efficient use of the available resources. While there exists a large body of literature lending theoretical and experimental support to this hypothesis, there are a number of inconsistencies regarding its application: First, empirical tests use highly divergent approaches to calculate RUE. Second, the quantification of RUE is commonly based on measures of standing stock instead of productivity rates and total pools of nutrients instead of their bioavailable fractions, which both vary across systems and therefore can introduce considerable bias. Third, conceptual studies suggest that the relationship between biodiversity, productivity and RUE involves many more mechanisms than complementary resource use, resulting in variable magnitude and direction of biodiversity effects on productivity. Moreover, RUE has mainly been applied to single elements, ignoring stoichiometric, or metabolic constraints that lead to co-limitation by multiple resources. In this review we illustrate and discuss the use of RUE within and across systems and highlight how the various drivers of RUE affect the diversity-productivity relationship with increasing temporal and spatial scales as well as under anthropogenic global change. We illustrate how resource supply, resource uptake and RUE interactively determine ecosystem productivity. In addition, we illustrate how in the context of biodiversity and ecosystem functioning, the addition of a species will only result in more efficient resource use, and consequently, higher community productivity if the species' traits related to resource uptake and RUE are positively correlated.