Abstract
Understanding variation in zooplankton assemblages provides insight into freshwater food webs that can inform management decisions. Monitoring zooplankton depends on using methods capable of efficiently generating comparable estimates across time and space. Despite the importance of zooplankton, routine monitoring remains challenging. Traditional enumeration methods used to describe zooplankton assemblages are accurate but are time consuming and require a high degree of taxonomic expertise. As such, I evaluated the efficiency and accuracy of a novel sieving technique for estimating zooplankton biovolume and size structure. Zooplankton were sampled from 64 sites across 22 lakes and reservoirs in Idaho during summer 2024. Traditional and rapid (sieve-based) techniques were applied to samples collected from systems that varied in size, type (i.e., natural, reservoir), and trophic state to identify potential methodological constraints. Estimates of biovolume and size class distributions from both methods were then used to compare overall analytical performance, including the ability of the sieve-based method to reproduce patterns in zooplankton abundance, size structure, and volumetric estimates generated by the traditional technique. Zooplankton body width was a better predictor of sieve retention than body length, and width-based size distributions exhibited minimal overlap among sieve fractions. A zooplankton size distribution (ZSD) was estimated for four size classes (1,000, 850, 710, and 500 µm) as 100 times the volume in a sieve size of interest divided by the total volume 150 µm and larger sieves. Positive linear relationships were observed between sieve-based and traditionally derived ZSD estimates. Model performance improved progressively from ZSD–1000 (r = 0.49) to ZSD–500 (r = 0.77). Method performance exhibited taxon-specific limitations in systems dominated by Holopedium gibberum. Additionally, a ZSD was estimated using dry weight and compared to proportional biovolumes to assess whether volumetric estimates reflected differences in total sample biomass. Zooplankton biovolume varied between tow types and across systems. Mean sample processing time was 70.4 ± 30.1 minutes using the traditional method compared to 26.1 ± 11.5 minutes using the sieve-based method, representing a reduction of more than 60%. The results from this research identified important methodological considerations and provide guidance for the application of a rapid zooplankton monitoring method in lakes and reservoirs.