Limnological Sampling Techniques and Monitoring Strategies for Ecological Health Assessment

Freshwater ecosystem integrity is facing a growing threat from human interventions such as agricultural effluent, industrial and domestic effluent, and climatic variations. An accurate evaluation of the ecological status of an aquatic ecosystem is dependent on the proper incorporation of well-designed limnological sampling methods. This paper describes a rigorous assessment of the use of multi-level sampling approaches conducted at five different locations in a tropical freshwater reservoir and inflowing stream system. The assessment includes physicochemical parameters, biological communities, habitat features, and environmental DNA (eDNA) sequencing analysis to develop the Ecological Health Index (EHI). Seasonal sampling during the pre-monsoon, monsoon, and post-monsoon periods was done. Physicochemical variables such as pH, dissolved oxygen, electrical conductivity, total dissolved solids, nitrate, and total phosphorus were determined in situ and later validated via lab work. Indices that include the Shannon diversity index and Pielou’s evenness index were calculated based on phytoplankton, zooplankton, and macroinvertebrates. The findings suggest significant spatial variation with respect to ecological status; wherein upstream sites are oligotrophic to mesotrophic (EHI: 70.3-82.9) while downstream agricultural impacted sites are eutrophic to hypertrophic (EHI: 35.5-48.2). The correlation between the composite index and individual biological/physicochemical measurements is highly significant (r² = 0.87, p < 0.01). These results emphasize the importance of using the multi-proxy approach in the ecological regulation process and reservoir management. In this study, an efficient and inexpensive methodology has been proposed for use in freshwater quality monitoring programs in tropical/subtropical conditions.