Hi Ruta

Ok I found the cause, but I don't under stand the reason why I have this problem.

I'm trying to render this image at high res.
http://www.renderosity.com/mod/gallery/index.php?image_id=1921297

The material used is the "Clear Water" preset, and the atmosphere is again another preset. Outside the image in the top right I have very large Chrome Cube to refelect light back into the image and from what I can see this is clearly the cause.

I can verify this by setting the material of the cube to a Mirror material the problem goes away, when I set it back to Chrome the problem comes back again.

So I know that's the cause, but the marks left behind are from the render bucket. It's not a refelection from the cube.

Any ideas ?
This problem can be seen in every render mode I've tried from Final right through to use mode.

Regards
James