Organic Carbon Loading Governs Ecological Structure and Nitrogen Cycling in a Tropical Closed‐Basin Lake Over the Late Holocene

Abstract

This study reconstructs late Holocene changes in the biogeochemical cycling of Jiaming Lake, eastern Taiwan using diatom assemblages, stable carbon and nitrogen isotopes (δ ¹³ C, δ ¹⁵ N), total organic carbon (C) and nitrogen (N) contents, and C/N ratios. Elevated C/N ratios and more negative δ ¹³ C values indicate wetter intervals (3,800–2,400 and 1,600–800 cal yr BP) characterized by increased input of terrestrial organic matter driven by enhanced runoff. In contrast, lower C/N ratios and less negative δ ¹³ C values reflect reduced terrestrial input and a greater relative contribution from aquatic primary production, consistent with decreased runoff and relatively drier climatic conditions during 2400–1600 and 800–300 cal yr BP. The persistent dominance of acidophilic diatom taxa indicates that lake‐water conditions remained acidic over the past 3,800 cal yr BP, although reorganization of diatom assemblages occurred after 2,400 cal yr BP. These long‐term ecological changes were closely linked to variations in organic C loading and associated shifts in N‐cycling. During the wetter intervals, variations in organic C loading corresponded to differences in the intensity of anaerobic N‐loss processes, likely including denitrification, DNRA, and anammox. In the earlier wet phase, the strong expression of anaerobic N‐loss cannot be attributed solely to climate‐driven organic carbon input to the lake. Archaeological records from eastern Taiwan indicate elevated regional human activity during this period, suggesting that anthropogenic influences may have contributed to the observed N‐cycle dynamics. On the other hand, low organic C loading during the drier phases shifts N transformation pathways toward aerobic nitrification processes along with water‐column mixing.