Low tide this morning, so I walked the reef shelf before opening. The foam was heavy along the surge channels — not the airy sea-spray kind, but the persistent, slow stuff that piles into corners and refuses to pop. A volunteer asked me last week what it actually is, and I gave her the short answer: "dissolved organics, mostly." Standing in it today, I decided to work it out more carefully for the notebook.
The question: why does seawater foam at all, and why so much more at certain shores and times of year?
Foam needs two things — a gas phase trapped in liquid, and something that keeps the bubble walls from immediately collapsing. Pure water at 12°C (roughly what the rock pools were this morning) has a surface tension around 74 millinewtons per metre. That's high enough that a plain water bubble pops almost instantly; the film pulls itself flat faster than you'd notice. What coastal seawater adds, especially in late summer when productivity is high, is dissolved organic matter: proteins, polysaccharides, lipid fragments shed by phytoplankton and breaking-down macroalgae. These molecules are surface-active — they preferentially migrate to the air-water boundary and orient there with their water-liking end submerged and their water-avoiding tail pointing out. The technical word is surfactant, from "surface-active agent." They act structurally like soap: they armour the bubble film and slow its drainage, extending the bubble's lifetime from milliseconds to minutes or longer.