Carbonate-System Perturbation by Invasive Bivalves Threatens Drinking-Water Security in Calcium-Limited Catchments.

Invasive bivalve ecosystem engineers pose an under-recognized threat to drinking-water security. In the Waikato River, New Zealand, rapid establishment of Corbicula fluminea (Asian clam) since their 2023 incursion has depleted dissolved calcium ∼24% below historical baselines through biogenic CaCO3 fixation at ∼14 ± 14 tonnes day-1 (2024-2025) while simultaneously raising alkalinity ∼15-20% through catabolic NH4+/HCO3- excretion. Residence-time mass balances yield a basin-scale population density of 370 ± 323 individuals m-2, consistent with independent field surveys (557 ± 435 individuals m-2). Across a 20 year water treatment plant (WTP) operating record, arsenic-removal efficiency increases with Ca2+ and decreases with alkalinity, identifying Ca2+ as the dominant control and alkalinity as a negative covariate of coagulation chemistry. During a month-long 2024 anomaly, finished-water arsenic breached the 0.01 mg L-1 maximum acceptable value at five downstream WTPs, rising 6.4-fold at the monitored plant, despite increased alum dosing, while inlet arsenic stayed seasonal, indicating coagulation failure from selective Ca2+ depletion against a rising alkalinity background. A secondary threat arises when thermal stratification and attenuated flow in upstream hydroelectric reservoirs (τ > 6 days, Lake Kara̅piro) drive clam-mediated sediment anoxia, mobilizing reduced As3+ toward intakes. As C. fluminea expands globally, biological carbonate-system perturbation threatens drinking-water security in calcium-limited catchments.