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.
The gas itself mostly comes from wave action. Waves break, entrain air, and the turbulence in the surge channels churns that air into small bubbles. The smaller the bubble, the greater its surface area relative to volume, so surface-active compounds coat small bubbles very efficiently. My rough sense is that we're dealing with bubble diameters in the tens to hundreds of micrometres — too small to see individually, which is why the foam looks solid.
What I find genuinely interesting is the seasonal signal. This morning's foam was noticeably thicker than it was in May, and that tracks. Summer and early autumn are when phytoplankton blooms peak, macroalgae senesce, and dissolved organic carbon concentrations in nearshore water climb. I don't have a local measurement to quote, but a standard oceanography text puts coastal dissolved organic carbon at roughly one to three milligrams per litre, and even that small concentration is enough to meaningfully reduce surface tension when conditions for accumulation are right — a calm cove, a slow-draining rock pool, a surge channel with little flushing.
I should be honest about where I'm standing here. The basic surfactant chemistry is textbook-level and I'm confident in it. What I'm less sure about is whether the composition of the organics matters much to the foam quality, or whether it's mostly a matter of total concentration. I've seen references suggesting that specific polysaccharides from certain diatom species are particularly effective surfactants, but I haven't read into that carefully enough to say more. For now: the foam is there because the sea is productive and the channels are sheltered. That much I can say with reasonable confidence.
The volunteer seemed satisfied with "dissolved organics." I think next time I'll add: the sea is foaming because it's alive and the tide has been working it.
#marine #science #notebook #tides