An aquatic ecosystem refers to the community of organisms and their interactions within water-based environments, encompassing both freshwater and marine systems. These ecosystems are characterized by unique structural and functional components that sustain life in dynamic equilibrium. Understanding their ecological composition and dynamics involves exploring the interplay of abiotic and biotic elements, food webs, energy flow, nutrient cycling, and regulatory mechanisms that shape their stability and productivity. The foundation of any aquatic ecosystem lies in its abiotic environment. Key abiotic factors include water chemistry, temperature, dissolved oxygen, salinity, pH, light penetration, and nutrient availability. These parameters directly influence the distribution, abundance, and productivity of aquatic organisms. For instance, light penetration determines the depth to which photosynthetic organisms can thrive, defining the euphotic zone in oceans and lakes. Similarly, dissolved oxygen levels regulate the diversity and density of aerobic organisms. Stratification in freshwater lakes into epilimnion, thermocline, and hypolimnion exemplifies how temperature and density differences shape habitat conditions, influencing biological activity and nutrient cycling. In marine systems, salinity gradients and tidal fluctuations create niches for species adapted to varying osmotic pressures. The biotic composition of aquatic ecosystems includes producers, consumers, and decomposers, each playing a pivotal role in energy transfer and matter recycling. Primary producers in aquatic systems are predominantly phytoplankton, algae, and aquatic plants. In marine ecosystems, microscopic phytoplankton such as diatoms, dinoflagellates, and cyanobacteria form the primary photosynthetic base, contributing significantly to global oxygen production and carbon fixation. In freshwater bodies, macrophytes like water lilies, hydrilla, and submerged grasses contribute alongside phytoplankton to primary production.