Freshwater ecosystems, encompassing lakes, rivers, and wetlands, are globally recognized as critical centers of biological diversity, supporting a wide array of flora and fauna ((Dudgeon et al., 2006); Strayer & Dudgeon, 2010). These ecosystems provide essential services to both human communities and wildlife ((Dudgeon et al., 2006); (Green & Elmberg, 2014)). Located in North Sulawesi, Indonesia, Tondano Lake stands out as a prominent freshwater body of high ecological, social, and economic importance. It not only supports local fisheries and tourism but also serves as a crucial habitat for various waterbird species.

Waterbirds are considered indispensable components of healthy aquatic ecosystems due to their multifaceted ecological functions. They act as regulators of aquatic populations by preying on fish and invertebrates, contribute to the regeneration of riparian vegetation through seed dispersal, and, most importantly, serve as sensitive bioindicators of environmental health ((Amat & Green, 2009); (Green & Elmberg, 2014)). Changes in their diversity, abundance, and distribution patterns can provide early warning signals of ecosystem degradation, such as a decline in water quality or habitat loss ((Kelly, 1998); (Grzędzicka, 2020)).

Despite their ecological significance, Tondano Lake and its waterbird populations are facing formidable environmental pressures. A primary threat is the aggressive invasion of the exotic aquatic plant, water hyacinth (Eichhornia crassipes), which has been reported to cover up to 30% of the lake’s surface area (Pratama et al., 2023)(Hendratta et al., 2024). This unchecked proliferation forms dense floating mats that obstruct light penetration, induce eutrophication, and reduce dissolved oxygen levels, thereby disrupting the fundamental food web and negatively impacting species that depend on open, clear water for foraging, such as Himantopus leucocephalus and Egretta garzetta ((Toft et al., 2003); (Villamagna & Murphy, 2010)). Concurrently, the lake is experiencing accelerated sedimentation, or shoaling, primarily due to heightened erosion from agricultural and deforested areas in the surrounding watershed. This process reduces the lake’s depth, fundamentally altering habitat structure and decreasing the availability of prey, which is particularly detrimental to piscivorous species like Alcedo atthis ((Kelly, 1998); (Kumar & Pattnaik, 2012)).

Existing scientific literature on Tondano Lake has predominantly focused on water quality and fish populations (Onibala et al., 2025), leaving a significant research gap in studies that integrate the dynamics of waterbird communities with the specific, escalating environmental threats of water hyacinth invasion and sedimentation. This gap underscores the urgent need for a dedicated study on waterbirds as a key bioindicator group ((Amat & Green, 2009); (Green & Elmberg, 2014)). By investigating the relationship between waterbird populations and these environmental stressors, this research can offer a more holistic and nuanced understanding of the lake’s health and resilience.

This work is aligned with global conservation efforts and contributes to the realization of Sustainable Development Goals (SDGs) 6 (Clean Water and Sanitation) and 15 (Life on Land). The specific objectives of this study were: (1) to identify the species and abundance of waterbirds in Tondano Lake; (2) to calculate the species diversity index and relative abundance; and (3) to analyze the ecological role of waterbirds within the context of the identified environmental threats, specifically the invasion of water hyacinth and sedimentation. By addressing these objectives, this research aims to provide the scientific data necessary to inform and strengthen the management of the Tondano Lake ecosystem.

This study demonstrates that Tondano Lake supports a moderate level of waterbird diversity, as reflected by a Shannon-Wiener index of $H' = 1.92. The eight recorded species, primarily from the family Ardeidae, exhibit varied ecological responses to environmental conditions. This species richness and diversity represent a simplified community structure when compared to historical regional data (H' = 2.5; Harmoko et al., 2018), providing evidence of ongoing ecological decline. The dominance of the adaptable Egretta garzetta alongside the low abundance of the sensitive specialist Alcedo atthis provides a clear signal of ecological stress within the lake ecosystem.

Waterbirds function as key ecosystem guardians, contributing to ecological stability by regulating aquatic populations, dispersing plant seeds, and serving as sensitive bioindicators of environmental quality. The regression analysis revealed significant negative correlations between waterbird abundance and both water hyacinth density (r = -0.68, p = 0.032) and sedimentation (r = -0.72, p = 0.019), demonstrating the dual and interacting impacts of these environmental pressures. These findings establish a contribution to the body of research on Indonesian freshwater ecosystems by linking waterbird population dynamics with specific, integrated environmental threats.

The results emphasize that water hyacinth invasion and sedimentation are interconnected, forming a self-reinforcing feedback loop that accelerates ecological degradation. Ensuring the long-term sustainability of the Tondano Lake ecosystem therefore requires integrated conservation strategies that address both stressors simultaneously. This includes actions spanning multiple scales from direct lake management to the restoration of the surrounding watershed. Future research should extend the observation period to further capture migratory dynamics and incorporate additional water-quality parameters. Such comprehensive monitoring would deepen understanding of the complex interactions between waterbirds and their changing environment, thereby strengthening conservation planning for Tondano Lake and comparable tropical freshwater systems.

Acknowledgements and funding statements: The authors extend their sincere gratitude to the North Sulawesi BKSDA, the Minahasa Regency Government, and the local community members of Tondano Lake for their invaluable assistance with species identification. Logistical support was provided by the Universitas Negeri Manado. This research was funded by an internal research grant from the Universitas Negeri Manado in 2025.

Competing of interest: The authors declare that there are no financial or personal relationships that could be construed as a potential conflict of interest.

Author’s contributions: Rievo Djarang: conceptualization, research design, data collection, data analysis, and manuscript writing. Gerry Harindah: data analysis, critical revision of the manuscript, and final manuscript approval. Ni Wayan Suryani and Marthy Lingkan Stella Taulu: field support and literature review.

Generative AI: Not applicable

Data availability: The raw data supporting the conclusions of this article, including the monthly abundance counts, Shannon-Wiener index calculations, and environmental variable measurements (water hyacinth density and water depth) for Tondano Lake, will be made available by the authors, without undue reservation, to any qualified researcher.