Application of Zooplankton Indicators for Assessing Ecological Functioning Across an Urbanization Gradient

Abstract

Urbanization is one of the most pervasive drivers of freshwater ecosystem degradation, altering hydrology, nutrient loading, and habitat structure in ponds, lakes, and reservoirs embedded within growing metropolitan landscapes. Zooplankton communities, owing to their short generation times, sensitivity to water-quality change, and central position in pelagic food webs, are increasingly recognized as effective bioindicators of ecological functioning. This study examined zooplankton assemblages across eight standing-water sites distributed along a rural-to-highly-urban gradient, quantified by percentage impervious surface cover in the surrounding catchment (4-81%). Zooplankton density, taxa richness, Shannon-Wiener diversity (H'), Pielou evenness (J'), and the rotifer-to-crustacean abundance ratio were used as indicators of ecological condition, alongside supporting water-quality variables (total phosphorus and chlorophyll-a). Diversity and richness declined sharply and significantly with increasing imperviousness (r = -0.94 and -0.97, respectively; p < 0.001), while the rotifer-to-crustacean ratio increased more than five-fold from rural to highly urban sites (r = 0.96, p < 0.001), reflecting a shift toward small, tolerant taxa under nutrient enrichment and hydrological instability. Zooplankton density exhibited a unimodal response, peaking at intermediate urbanization before declining in the most impervious catchments, consistent with a subsidy-stress model in which moderate nutrient enrichment initially increases secondary production before habitat degradation suppresses it. These results support the use of zooplankton community metrics, and in particular the rotifer:crustacean ratio and Shannon diversity index, as sensitive and cost-effective indicators for urban water-body monitoring programs and management decision-making.