Deciphering the Chronological Dynamics of Azolla invasion in Dal Lake, Srinagar: Insights from Sediment eDNA and ²¹⁰Pb/¹³⁷Cs Dating Techniques

Exploring the evolution of biological systems across temporal scales and diverse paleoenvironmental contexts is facilitated by the geological records. Recent advancements in DNA sequencing and sediment dating have enabled the reconstruction of past flora dynamics from ancient plant DNA extracted from lake sediment samples. In this study, we investigated the invasion of Azolla species within Dal Lake, India, employing an integrated approach of environmental DNA (eDNA) analysis with ²¹⁰Pb and ¹³⁷Cs dating. The introduction and accurate identification of Azolla in Dal Lake have posed persistent challenges amidst its considerable negative impact on the ecosystem. By employing this integrated methodology, we were able to construct the temporal trajectory of Azolla’s introduction within the Dal Lake ecosystem. Sediment coring was conducted in 2022 within Dal Lake, and to establish chronology, sediment dating was performed using ²¹⁰Pb and ¹³⁷Cs, revealing sedimentation rates of 0.63 cm/year and 0.86 cm/year, respectively. The eDNA analysis involved extracting Azolla DNA from lake sediments, its PCR amplification using rbcL primers, followed by DNA sequencing and BLAST (Basic Local Alignment Search Tool) search to refine taxonomic resolution. Analysis of sediment cores dating back to 1925 revealed the presence of Azolla DNA beginning around 1979, persisting to the present, indicating its continual presence in Dal Lake. DNA sequenced from the deepest lake sediment layers identified A. pinnata and A. filiculoides as the first invaders, while DNA of other Azolla species such as A. mexicana, A. microphylla, A. rubra, A. nilotica, and A. caroliniana, was detected from subsequent layers. This approach confirmed the first occurrence of Azolla in Dal Lake and provided insights into species composition changes over time. This study demonstrates a notable application of eDNA and ²¹⁰Pb and ¹³⁷Cs dating in Dal Lake, revealing the invasion timeline of a non-native species. Such interdisciplinary approaches set a precedent for future studies on paleo-ecological phenomena and invasive species management.