Soda lake

The recipe is a basin with no outlet, carbonate-rich volcanic inflow, and ferocious evaporation. What remains is hot soda brine over trona flats, with fresher springs seeping in at the margins; lagoon temperatures run toward 104 °F. Nothing about this is ordinary lake water — it is closer to dilute washing soda.

Fish persist only at the edges, in the spring-fed lagoons, and they are unlike any others: the little Alcolapia tilapias of Magadi and Natron live at an alkalinity no other fish tolerates, and they manage it with re-plumbed physiology — excreting their nitrogen as urea, because ammonia cannot be shed into water this alkaline. Their entire species ranges are a handful of warm lagoons.

No aquarium lesson hides here, and the atlas offers none — a soda lake is not a care recipe. It matters as the boundary case: the demonstration of what 'alkaline' can mean, and of how far fish physiology can bend before fresh water stops being fresh.

The water of this character in the atlas

Sources

1. Obligatory Urea Production and the Cost of Living in the Magadi Tilapia (Wood et al., Physiological and Biochemical Zoology, 2002) — pH ~10, alkalinity ~380 mmol/L, Na ~350 mmol/L, salinity ~60% seawater
2. Lake Magadi Tilapia (Alcolapia grahami) — Ecological Risk Screening Summary (USFWS) — range, 40 °C hot springs, biology, cites Coe 1966; Seegers & Tichy 1999; Trewavas 1983
3. Remote sensing the hydrological variability of Tanzania's Lake Natron, a vital Lesser Flamingo breeding site under threat (Tebbs et al., Ecohydrology, 2013)