Can anyone explain this phenomenon?

Hoping this is the right group for this. I was on a shell sand beach on North Uist in the outer Hebrides today. Lovely white sand, lapping water at just about full tide. There was some black sand in bands but in one part of the beach, the darker material was concentrated in regular strips in the water. For the life of me, I couldn’t work out how this could occur. Would love to know. For clarity, the dark areas are in the water just below the tide line.