Effects of increased allochthonous dissolved organic carbon on the growth of planktonic biota in freshwater ecosystems: A meta-analysis

Abstract

Water browning, induced by allochthonous dissolved organic carbon (DOC) input, has become a widespread phenomenon in boreal lakes over the past decades. Directly quantifying aquatic organisms' responses to increased DOC concentrations is essential for projecting carbon cycle processes in freshwater ecosystems. In this study, we assessed the impacts of DOC addition on the growth of three freshwater planktonic groups: phytoplankton, zooplankton, and bacteria, and explored potential drivers behind variations in effect size. Background DOC concentrations vary between 0.5 and 25 mg L⁻¹, while total phosphorus concentrations span from 0.0003 to 1.55 mg L⁻¹. Based on a meta-analysis of 804 observations from 47 publications, we found that DOC addition had a significant positive effect on bacteria, while it had a small but negative impact on both phytoplankton and zooplankton. In different climate zones, DOC addition often stimulated bacterial growth, but it exerted either positive or negative effects on phytoplankton and zooplankton. Additionally, the effect sizes of both phytoplankton and zooplankton showed a significant negative relationship with the magnitude of DOC enrichment, while bacteria exhibited positive responses. Furthermore, the effect sizes of these three taxa correlated negatively with background total phosphorus concentrations and positively with the DOC : total phosphorus ratio. A significant negative correlation between effect size and experimental duration was observed for bacteria. In summary, this synthesis indicates that excessive DOC loading can inevitably inhibit phytoplankton and zooplankton growth. Future studies should focus on the interactions between DOC addition and global change factors to improve forecasts of carbon-climate feedback in aquatic ecosystems.