Resilient ecosystems maintain a similar species diversity and composition despite extreme environmental variation, whereas ecosystem collapse can be characterized by high rates of mortality and a decrease in species diversity. Can aquatic macrophytes (submerged and floating) produce extreme chemical fluctuations and rapid ecosystem collapse in a small floodplain pond in the temperate zone? We measured fish survival from spring to fall and used mesocosms to measure diel fluctuations of dissolved oxygen (DO) and pH during the spring, summer and fall in patches of aquatic macrophytes (metaphyton, Chara, Potamogeton foliosus, and Lemna minor). Submerged macrophytes (metaphyton, Chara, P. foliosus) dominated in the spring and summer before being overgrown by a floating macrophyte in fall (Lemna minor). Out of 4 species of fish, only western mosquitofish survived from spring to fall. In spring, high rates of photosynthesis in submerged macrophytes produced long periods of supersaturation (12 – 16 h/day) and basicity (2h to 10 h/day of pH > 9.0). High rates of microbial respiration in summer were fueled by high temperatures (≈ 35° C), high nutrients (average TN and TP = 4.4 mg/L and 0.17 mg/L) and decaying submerged macrophytes resulting in long periods of hypoxia (4.8 h to 19.6 h/day of DO < 1.0 mg/L). In fall, cool temperatures (≈ 10° C) reduced extreme chemical fluctuations under the floating mat of L. minor. All macrophytes died-back in winter. This newly described seasonal cycle of colonization during spring run-off and ecosystem collapse in summer in floodplain wetlands was not a transition to an alternative stable state dominated by floating macrophytes as in other lentic ecosystems.