Original:  It is this upper sunlit ocean that creates the food supply that ultimately sustains most of the ocean ecosystem  . Light only penetrates to a  few hundred meters depth , so the remaining ocean below this is cold and dark.
Revised:  It is this upper sunlit ocean that creates the food supply that ultimately sustains most of the ocean ecosystem . Light only penetrates to a few hundred meters depth , so the remaining ocean below is cold and dark.
Edit intent: clarity