Freshwater ecosystems face intensifying pressure from nutrient enrichment, urbanization, and climate variability, creating an urgent need for reliable, low-cost tools to monitor ecological condition. Plankton communities—phytoplankton and zooplankton—respond rapidly to physicochemical change because of their short generation times, direct sensitivity to nutrient and light regimes, and position at the base of aquatic food webs. This paper reviews the current scientific literature on the use of plankton diversity as a bioindicator of freshwater quality, with particular attention to seasonal variation and ecological significance. Evidence from lakes, reservoirs, rivers, and wetlands across multiple continents demonstrates that diversity indices such as the Shannon-Wiener index, along with pollution-tolerance indices such as the Palmer index, correlate consistently with trophic state and anthropogenic disturbance. Seasonal patterns show that plankton diversity typically peaks during pre-monsoon or spring transitional periods and declines during periods of nutrient loading, thermal stratification, or monsoonal dilution. The paper concludes that plankton-based biomonitoring, when combined with conventional physicochemical assessment, offers a cost-effective, ecologically integrative approach for tracking freshwater health, though standardization of sampling and identification protocols remains a challenge for broader adoption.
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