Microbial carbon production and transfer across trophic levels is affected by solar UVA and phosphorus

Abstract

Processes and environmental factors that alter carbon fixation and fluxes are key to understanding ecosystem function and impacts of global change stressors. We tested the effects of ultraviolet A radiation (UVA) and nutrients on the 14C production of particulate (bacteria, algae and zooplankton size fractions) and dissolved organic carbon. Experiments were carried out in situ on two occasions during the ice-free period of a high mountain lake (Sierra Nevada, Spain). The production of organic carbon was strongly modulated by nutrients and moderately by UVA. While UVA generally reduced primary production, this effect was alleviated by nutrient enrichment. Nutrients had opposite effects to UVA on basal trophic levels by inhibiting primary production and stimulating the bacterial incorporation of algal carbon exudates in the midsummer experiment, and vice versa in the early fall experiment. Also, nutrients had a pronounced effect stimulating particulate carbon in zooplankton only in midsummer when organisms were actively growing. These results suggest that, while UVA effects are restricted to the basal trophic levels of primary producers and bacteria, effects of nutrients transfer up the food web. However, higher carbon accrual in zooplankton during midsummer may not result into higher growth due to inefficient carbon absorption.