Phosphate availability is a critical constraint on prebiotic chemistry, yet natural environments rarely achieve concentrations compatible with origin-of-life experiments. Alkaline and saline lakes have emerged as potential solutions to this "phosphate problem," but the mechanisms controlling phosphate cycling in these extreme environments remain poorly understood. Here, I investigate phosphate dynamics in the alkaline Last Chance and Goodenough Lakes and the magnesium sulfate-rich Basque Lakes of British Columbia's Interior Plateau using oxygen isotope analysis of dissolved phosphate (δ¹⁸OPO₄) and lake waters (δ¹⁸OH₂O). I developed a modified silver phosphate precipitation protocol to enable isotopic analysis of phosphate in these chemically extreme waters. My results show that Last Chance Lake maintains phosphate concentrations up to 12 mmol/L through primarily abiotic processes, evidenced by significant deviations (up to +5.3 ‰) from oxygen isotopic equilibrium (denoted Δδ¹⁸OPO₄). However, neighboring Goodenough Lake's extensive microbial mats drive near-equilibrium signatures (Δδ¹⁸OPO₄ ≈ +2.2 ‰) while limiting phosphate accumulation. In contrast, my analysis of the Basque Lakes demonstrates how extreme brine chemistry can maintain phosphate far from equilibrium (Δδ¹⁸OPO₄ up to +10.7 ‰) through abiotic processes such as adsorption and phosphate mineral precipitation. These findings validate theoretical predictions of phosphate concentration mechanisms in alkaline environments and provide a framework for interpreting potential biosignatures in ancient Earth settings and extraterrestrial environments.