Low tide this morning, cold enough that my breath showed. I spent twenty minutes crouched at the outer rock pool watching the foam — not wave-surge foam, but the persistent kind sitting in the corners like dirty snow, barely moving while everything else drained back. The question: why does it stay?

Regular bubbles don't. Blow through a straw into fresh water and they pop in seconds. The film thins, drains under gravity and capillary pressure, and ruptures. Seawater behaves differently, and the reason is chemical rather than physical. Phytoplankton, bacteria, and decomposing algae release dissolved organic compounds — long-chain proteins, polysaccharides, lipids — that are surface-active. The textbook term is surfactant: a molecule that preferentially sits at the air-water interface and lowers surface tension there. The Marangoni effect then acts as a restoring force: if the film thins locally, surface tension rises at the thin patch, and that gradient drives fluid back toward it. The film resists rupture.

The rough scale: surface tension in clean seawater runs around 72 mN/m. With enough dissolved organics it can fall to 40–50 mN/m. That roughly 30% reduction is, apparently, enough to extend a bubble's lifetime from milliseconds to minutes.

Where my certainty stops: I can't tell you the specific composition of the foam at any given pool. It varies with season, local bloom history, and how much recent wave action has churned things. The mechanism — surfactant stabilisation — I'm confident about. Whether last week's phytoplankton bloom or last night's swell explains this foam right now, I genuinely don't know.

It's a useful habit: catch yourself assuming you understand something, then check. This one, mostly yes.

#tidepools #marine #notebook #observation