Nutrients and warm temperatures explain a seasonal cycle of colonization and ecosystem collapse in a floodplain pond

Resilient ecosystems maintain similar species diversity and composition despite extreme environmental variation, while ecosystem collapse can be characterized by high mortality rates and decreased species diversity. Submerged aquatic macrophytes often support species diversity by providing habitat, shelter, food, and oxygen for fish and invertebrates. Can submerged macrophytes cause extreme chemical fluctuations, high mortality, 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 assess diel fluctuations of dissolved oxygen (DO) and pH during spring, summer, and fall in patches of aquatic macrophytes (metaphyton, Chara , Potamogeton foliosus , and Lemna minor ). Submerged macrophytes (metaphyton, Chara , and P. foliosus ) dominated in spring and summer before being overgrown by a floating macrophyte in fall ( L. minor ). Of the four fish species, only western mosquitofish survived from spring to fall. In spring, high rates of photosynthesis in submerged macrophytes led to long periods of supersaturation (12–16 h/day) and basicity (2–10 h/day of pH >9.0). High rates of microbial respiration in summer were driven by elevated temperatures (≈35°C), high nutrient levels (average TN and TP = 4.4 and 0.17 mg/L), and decaying submerged macrophytes, resulting in long durations of hypoxia (4.8–19.6 h/day of DO < 1.0 mg/L). In fall, cooler temperatures (≈10°C) diminished extreme fluctuations in DO and pH beneath a floating mat of L. minor . All macrophytes died back in winter. This seasonal cycle of colonization during spring runoff and ecosystem collapse in summer was not a transition to an alternative stable state dominated by floating macrophytes as in other lentic ecosystems. Warm temperatures and nutrient enrichment can produce extreme diel fluctuation in DO and pH in patches of submerged macrophytes, resulting in high rates of mortality in floodplain ponds, one of the most common types of wetlands in the world.