Sampling method influences zooplankton diversity estimates in humic lakes: A case study with implications for ecological assessment

Humic lakes often exhibit pronounced habitat heterogeneity, low primary production and pronounced pelagic-littoral gradients, which complicate reliable biodiversity assessment. Despite their conservation importance, sampling methodology in zooplankton-based ecological assessment remains poorly understood. This study compared three sampling approaches (Random, Transect and Shore) to evaluate their effects on zooplankton diversity estimates, community composition and indicator performance in a humic lake. Environmental parameters showed limited spatial and temporal variability, whereas sampling method had a strong and consistent effect on species richness, community structure and indicator detection. A diverse assemblage was recorded, including seven rare species (>11% of taxa) and multiple dystrophic specialists (e.g., Gastropus minor , Lecane lunaris f. crenata , Trichocerca longiseta , T. simonei ). Random sampling maximised cumulative species richness and captured rare taxa across microhabitats, providing the closest approximation to an unbiased representation of the lake-wide community. Transect sampling, although yielding lower total richness, consistently identified pelagic rotifer indicator species relevant for ecological monitoring. Shore sampling substantially underestimated zooplankton diversity, particularly pelagic rotifers, due to near-shore avoidance and limited habitat representation, indicating a strong downward bias. Multivariate analyses confirmed that sampling method explained more variation in zooplankton assemblages than environmental variables. Methodological design is therefore a key determinant of perceived biodiversity patterns in humic lakes. Random sampling appears most suitable for biodiversity inventories, whereas Transect sampling provides greater diagnostic value for ecological monitoring. Shore-based sampling alone is inadequate for reliable assessment of humic lake zooplankton. Method-sensitive and standardised sampling protocols are essential to ensure reliable monitoring and assessment of humic lakes. • Sampling method strongly structured zooplankton communities in a humic lake. • Random sampling captured the highest observed and estimated species diversity. • Shore sampling underestimated diversity due to pelagic rotifer shore avoidance. • Transect sampling yielded all significant indicator species identified by IndVal. • Method-sensitive protocols are essential for reliable assessment of humic lakes.