The colour of the sea at Stonehaven

This is a view looking out to sea at Stonehaven near Aberdeen on the north east coast of Scotland. Standing there, it was obvious that the nearer water was blue and further out the water looked green. It is less obvious on the photograph, which is disappointing, but you can almost convince yourself if you enlarge it. I have blogged before about my conviction that the sea reflects the colour of the sky, hence the blue. But that doesn't explain the green. Then I noticed that two rivers flow out into the sea in the bay.

My theory is that the large amount of freshwater from heavy rain flowing into the bay pushed the salt water further out. Also, the freshwater is less dense so would ride over the salt water. I saw a hint that green colouration could well be biological and due to algae or plankton. The salt water containing these was pushed further out and so the green colour was further out.