Why Is the Mediterranean Sea So Clear?

The Mediterranean’s legendary transparency comes from a combination of geography, chemistry, and biology:

1. Low Sediment Input: Few large rivers (Nile, Rhône, Po, Ebro) deliver silt; most watersheds are small or dammed.
2. High Salinity (~38 psu): Excess evaporation over precipitation concentrates salts, inhibiting many algae species that cloud water.
3. Deep Basins: Average depth 1,500 m; light penetrates deeper before hitting bottom.
4. Strong Circulation: Atlantic inflow at surface, dense Mediterranean outflow at depth flushes nutrients and particulates.
5. Efficient Grazers: Abundant herbivorous fish (salema, parrotfish) and zooplankton keep phytoplankton blooms in check.

The result: Secchi disk depths of 30–50 m (100–160 ft) are common—among the clearest large seas on Earth.