Effects of ancient allochthonous and contemporary autochthonous organic carbon on the growth and reproduction of lake zooplankton

Abstract

Abstract Carbon with a radiocarbon age above 200 years before present (bp) generally is referred to as ancient carbon. Ancient organic carbon (OC) stored in glaciers, permafrost and unperturbed soil has been released with increasing temperatures resulting from climate warming and anthropogenic activities. Evidence suggests that ancient terrestrial OC can be incorporated by heterotrophic bacteria and consumers in many aquatic systems. However, it remains unknown whether ancient terrestrial OC promotes growth and reproduction of consumers. Ancient dissolved organic carbon (DOC) was extracted from frozen soils in the catchments of Lakes Tsochuolong (−216.9‰, 1,960 years bp) and Zhangnaitso (−384.1‰, 3,895 years bp) on the Tibetan Plateau, and was further degraded by heterotrophic bacteria. Degraded ancient DOC (containing bacteria) and Auxenochlorella (−161.6‰) were used exclusively or in combination as food (total 6 mg C/L) for Daphnia magna (D. magna) to compare their body length, mortality rate and offspring production. The mixed diets consisted of ancient DOC converted by bacteria at 4 mg C/L and Auxenochlorella at 2 mg C/L. The radiocarbon isotope (∆14C) value of D. magna fed on mixed diets was −293.0‰ for Lake Tsochuolong (group CA) and −265.9‰ for Lake Zhangnaitso (group CB), which was depleted than those fed exclusively on Auxenochlorella (−187.3‰, group 6C). This result suggests that 14C‐depleted DOC can be assimilated by zooplankton. Daphnia magna in group CB had the highest intrinsic rate of population increase (0.33/day) and number of neonates per individual (15.97/ind). The higher growth rate and offspring production in group CB than in CA (0.21/day and 7.68/ind) probably were because D. magna utilised more Auxenochlorella and phosphorus from diets in CB. However, there was no significant difference in the two indicators between CA and 6C (0.24/day and 12.30/ind) or between CB and 6C. Daphnia magna fed exclusively on degraded DOC plus bacteria had the lowest growth rate and offspring production. The results demonstrate that in the presence of algae with high nutritional value, degraded ancient DOC could support the growth and reproduction of zooplankton. Our findings extend the understanding of carbon sources and carbon cycling in food webs in lakes, particularly in high‐altitude and polar lakes in the context of climatic and environmental changes.