Abstract. A laboratory study was undertaken to determine the effect of food quality on feeding, respiration, reproduction and the resulting carbon budget of Temora longicornis. The stoichiometric ratios N : P, C : N and C : P of Rhodomonas salina were used as indicators of food quality. R. salina was grown in media with different inorganic nutrient concentrations to produce food for T. longicornis with particulate organic N : P ratios ranging from 10 : 1 to 23 : 1. Feeding rate was not affected by food quality. Maximum respiration (R), egg production rate (EPR), assimilation efficiency (AE), gross growth efficiency (GGE) and metabolic increment (MI) occurred when T. longicornis was fed on phytoplankton with a food quality of 16N : 1P. EPR, GGE and AE also decreased with decreasing C : N ratio and the energy required to produce eggs (CoE) decreased with decreasing N : P ratio, indicative of nitrogen-dependent production.

These data suggest that an algal composition of 16N : 1P defines the Threshold Elemental Ratio (TER) and is the optimum diet for T. longicornis. The variations in metabolic rates and the resulting carbon budget are proportional to the quality of food ingested. GGE was negatively affected at dietary ratios above and below 16N : 1P, which in the natural environment could lead to a decline in species biomass with detrimental consequences for fisheries and carbon export. Field data show that phytoplankton organic N : P ratios can change on decadal timescales, and that an increase in the food N : P ratio can co-occur with a shift to smaller sized zooplankton and a change in species abundance. Further research is required to assess how much of the change in zooplankton community structure and activity can be attributed to changes in food quality, rather than to changes in temperature and food quantity.