Nutrients associated with terrestrial dissolved organic matter drive changes in zooplankton:phytoplankton biomass ratios in an alpine lake

Summary Dissolved organic matter ( DOM ) is increasing in many lakes due to climate change and other environmental forcing. A 21‐day microcosm experiment that manipulated terrestrial DOM was used to determine the effect of DOM on zooplankton:phytoplankton biomass ratios (z:p). We predicted that if DOM additions increase the amount of fixed carbon available for higher trophic levels through stimulation of the microbial loop and hence zooplankton, the z:p will increase. However, if DOM additions increase other nutrients besides fixed carbon, we predict stable or decreasing z:p due to nutrient stimulation of phytoplankton that subsequently enhances zooplankton. The effects of experimental additions of terrestrially derived DOM on zooplankton, phytoplankton, z:p and zooplankton net grazing were assessed in microcosms (sealed bags) incubated in the epilimnion (shallow; 1.5 m) and hypolimnion (deep; 8.0 m) strata of an alpine lake. DOM addition treatments (DOM+) had a 6.0‐ to 7.5‐fold increase in phytoplankton biomass relative to controls, but only a 1.3‐ to 1.5‐fold increase in zooplankton biomass, on day 21 of the experiment. The z:p was, thus, lower in the DOM + treatments (ratios: 2.3 deep and 4.4 shallow) than in the control treatments (ratios: 13.4 deep and 17.5 shallow), providing evidence that DOM additions provide nutrient subsidies besides fixed carbon that stimulate phytoplankton biomass accumulation. The increase in zooplankton biomass during the experiment was similar in magnitude to the total amount of dissolved organic carbon ( DOC ) in the DOM added in the sealed bags at the beginning of the experiment, which suggests zooplankton biomass stimulation due to increased phytoplankton biomass, and not from DOM through the microbial loop, which would have greater trophic transfer losses. The consumer net grazing effect in the DOM + treatments was reduced by 2.8‐fold in the shallow stratum and by 2.9‐fold in the deep stratum relative to the control treatments, indicating that zooplankton were unable to exert strong top–down control on the primary producers. The role of nutrients needs to be considered when examining the response of pelagic ecosystems to inputs of terrestrial DOM , especially in lakes with lower DOC concentrations.