i dont actually believe it is referring to the pycnocline. there are plenty of simpler interpretations, but I am trying to be generous to the muslim argument.
If the pycnocline is observable, then the Muslim argument has literally nothing.
Nadir Ahmed uses the following quote to 'prove' that they are not observable
"The pycnocline forms an invisible boundary - between buoyant, downflowing river water and dense, inflowing saltwater."
But the pycnocline in question is a specific pycnocline at Chesapeake Bay, not pycnoclines in general. Even here it is not certain whether the author of the article really LITERALLY means invisible. It obviously wasn't the point of the sentence to state that you cannot see the pycnocline. Human authors do not have to mean every single word they say literally. It seems like Ahmed has more faith that humans mean what they say, than the author of the quran.
Anyway, let us assume that the pycnocline at Chesapeake Bay is invisible. There are still plenty of other pycnoclines that clearly are observable:
![](http://media.merchantcircle.com/57702/Cenote-Diving-Riviera-Maya_full.jpeg)
see how the rays of light refract at a different angle as they enter the saltier water
![](http://www.teamkarst.org/asgard19.jpg)
"Some two and a half feet below the surface of the Caribbean Sea at Tankah Bay, Mexico lies the undulating boundary between saltwater and freshwater. This boundary is visible as shimmering layer of water, similar to what cooking oil floating on top of water in a container might look like. This halocline is visible due to the different refractive indices of saltwater and freshwater."