Silver nanoparticles (AgNPs) are increasingly used in consumer products, biotechnology, and medicine, and are released into aquatic ecosystems through wastewater discharge. This study investigated the phytotoxicity of AgNPs to aquatic plants, Egeria densa and Juncus effusus by measuring physiologic and enzymatic responses to AgNP exposure under three release scenarios: two chronic (8.7 mg, weekly) exposures to either zerovalent AgNPs or sulfidized silver nanoparticles; and a pulsed (450 mg, one-time) exposure to zerovalent AgNPs. Plant enzymatic and biochemical stress responses were assessed using superoxide dismutase (SOD) and peroxidase (POD) activity, malondialdehyde (MDA) concentrations and chlorophyll content as markers of defense and phytotoxicity, respectively. The high initial pulse treatment resulted in rapid changes in physiological characteristics and silver concentration in plant tissue at the beginning of each AgNPs exposure (6 h, 36 h, and 9 days), while continuous AgNP and sulfidized AgNP chronic treatments gave delayed responses. Both E. densa and J. effusus enhanced their tolerance to AgNPs toxicity by increasing POD and SOD activities to scavenge free radicals but at different growth phases. Chlorophyll did not change. After AgNPs exposure, MDA, an index of membrane damage, was higher in submerged E. densa than emergent J. effusus, which suggested that engineered nanoparticles exerted more stress to submerged macrophytes.