Threshold elemental ratios for carbon versus phosphorus limitation in Daphnia THOMAS R. ANDERSON* AND DAG O. HESSEN † *National Oceanography Centre Southampton, University of Southampton, Southampton, U.K. † Department of Biology, University of Oslo, Blindern, Norway SUMMARY 1. The transition from carbon (C) to phosphorus (P) limited growth in Daphnia depends not only on the C : P ratio in seston, i.e. food quality, but also on food quantity. Carbon is commonly believed to be limiting at low food because of the energetic demands of basal metabolism. The critical C : P ratio in seston (otherwise known as the threshold elemental ratio, TER) above which P is limiting would then be high when food is scarce. 2. A new model that differentiates between the C : P requirements for growth and maintenance is presented that includes terms for both C and P in basal metabolism. At low food the calculated TERs for Daphnia of around 230 are only slightly higher than values of 200 or so at high intake. Seston C : P often exceeds 230, particularly in oligotrophic lakes where phytoplankton concentration is low and detritus dominates the diet, indicating the potential for limitation by P. 3. The analysis highlights the importance of P, as well as C, in maintenance metabolism and the overall metabolic budget, such that food quality is of importance even when intake is low. Further measurements of C and P metabolism at low food, in particular basal respiration and excretion rates, are needed in order to improve our understanding of the interacting roles of food quantity and quality in zooplankton nutrition. Keywords: Daphnia, ecological stoichiometry, phosphorus limitation, threshold elemental ratio, zooplankton nutrition Introduction Considerable variability is seen in the C : P of lake seston (Hecky, Campbell & Hendzel, 1993; Elser et al., 2000), which is frequently dominated by detrital material rather than living phytoplankton cells (Hessen et al., 2003). Phosphorus content is often low indicating poor food quality for Daphnia, which has high P in biomass and is a non-selective filter feeder. Moreover, most oligotrophic lakes are characterised by edible seston in concentrations below the satur- ating level for intake (typically about 1 mg C L )1 ). Both food quality and quantity should therefore be taken into consideration when identifying elemental mismatches between producers and consumers, an important aspect of understanding community struc- ture, nutrient cycling and the fate of carbon in food webs (Sterner & Elser, 2002; Hessen et al., 2004). The potential for either C or P to be limiting can be usefully studied for any chosen scenario by identify- ing the critical C : P ratio (the threshold elemental ratio, TER) in seston, above which P limitation occurs, with C otherwise limiting. Two approaches are commonly used for estimating TERs. First, growth and egg production can be examined either over a gradient of food C : P or in response to dietary P additions. Second, calculations can be undertaken based on simple stoichiometric theory in which C : P ratios in food and consumers are compared and, taking into account growth efficiencies, the limiting element identified as the one in least supply relative to Correspondence: Thomas R. Anderson, National Oceanography Centre Southampton, University of Southampton, Waterfront Campus, Southampton SO14 3ZH, U.K. E-mail: tra@noc.soton.ac.uk Freshwater Biology (2005) 50, 2063–2075 doi:10.1111/j.1365-2427.2005.01450.x Ó 2005 Blackwell Publishing Ltd 2063