Last updated on January 5th, 2026 at 11:51 am
Seawater is a complex solution dominated by dissolved salts, gases, nutrients, trace metals, and organic compounds. Despite geographical variation in salinity, the relative proportions of major ions remain remarkably constant worldwide, a concept known as the Principle of Constant Proportions. The majority of seawater salinity is contributed by chloride, sodium, sulfate, magnesium, calcium, and potassium.
Regional differences mainly occur in semi-enclosed basins and high-evaporation zones (e.g., Arabian Gulf, Red Sea), while open-ocean regions across North America, Europe, and South America exhibit more stable concentrations.
Major ion composition of seawater (mg/L):
Representative concentration ranges (mg/L) by global region
Values reflect typical seawater at ~35 PSU; higher-evaporation regions show slightly elevated ion levels.
| Major Ion | West Asia & Middle East | North America | North Africa | Europe | South & Central America | Other (Global Open Ocean) |
|---|---|---|---|---|---|---|
| Chloride (Cl⁻) | 19,000 – 21,000 | 18,500 – 19,500 | 18,800 – 20,000 | 18,600 – 19,600 | 18,500 – 19,400 | ~19,000 |
| Sodium (Na⁺) | 10,500 – 11,600 | 10,200 – 10,800 | 10,400 – 11,200 | 10,300 – 10,900 | 10,200 – 10,800 | ~10,500 |
| Sulfate (SO₄²⁻) | 2,700 – 3,000 | 2,600 – 2,800 | 2,650 – 2,900 | 2,600 – 2,850 | 2,600 – 2,800 | ~2,700 |
| Magnesium (Mg²⁺) | 1,300 – 1,450 | 1,250 – 1,350 | 1,280 – 1,400 | 1,260 – 1,360 | 1,250 – 1,340 | ~1,300 |
| Calcium (Ca²⁺) | 380 – 450 | 380 – 420 | 380 – 430 | 380 – 420 | 380 – 410 | ~400 |
| Potassium (K⁺) | 360 – 420 | 360 – 400 | 360 – 410 | 360 – 400 | 360 – 390 | ~380 |
| Bicarbonate (HCO₃⁻) | 120 – 160 | 110 – 140 | 115 – 145 | 110 – 140 | 110 – 135 | ~130 |
| Bromide (Br⁻) | 60 – 75 | 60 – 70 | 60 – 72 | 60 – 70 | 60 – 68 | ~65 |
| Strontium (Sr²⁺) | 7 – 10 | 7 – 9 | 7 – 9 | 7 – 9 | 7 – 9 | ~8 |
| Boron (as B) | 4 – 6 | 4 – 5 | 4 – 5 | 4 – 5 | 4 – 5 | ~4.5 |
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West Asia & Middle East values trend slightly higher due to higher evaporation basins (Arabian Gulf, Red Sea).
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North America, Europe, and South/Central America reflect stable open-ocean chemistry.
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Minor variation occurs due to river discharge, temperature, seasonal mixing, and local salinity.
Source: Water Conditioning & Purification Magazine, January 2005
