No one makes this assertion. The author does refute the researchers, but she’s refuting their assertion that P. brevis is the cause of the die-off. The researchers never assert anything about the general tendency of dolphins to inhabit—or not inhabit—areas with P. brevis. (While it’s stated that in this particular case, the dolphins did inhabit an area where P. brevis isn’t common, they’re not asserting that this “tends” to happen, or that dolphins also don’t inhabit areas were P. brevis is common.)

The author doesn’t consider the effects of synthetic pollutants on the Gulf of Mexico dolphins. She only looks at the effects (or lack thereof) of P. brevis—a naturally-occuring algae species—on those dolphins.

The researchers believe that P. brevis can contribute substantially to dolphin die-offs; their hypothesis is that the brevetoxin released by P. brevis was the root cause of the deaths. The author casts doubt on that belief by referring to dolphins in the Gulf of Mexico: there, we see the researchers’ proposed cause (P. brevis) but we don’t see the effect (dolphin die-off). This is a classic move for weakening causal hypotheses: show that the supposed cause can occur without producing the associated effect.

The author doesn’t suggest that either population of dolphin—whether in the Gulf of Mexico or the Atlantic—live in “relatively pollution-free waters.” She believes the Atlantic dolphins live in polluted waters and has nothing to say about pollution in the Gulf of Mexico.

This is the opposite of why the author refers to the Gulf dolphins. She’s providing evidence against the researchers’ argument that P. brevis was responsible for the Atlantic dolphin die-off. She claims that in the Gulf of Mexico, P. brevis is common, and yet we don’t see dolphin die-offs. This is a classic move for weakening causal hypotheses: the author is showing that the supposed cause (P. brevis) can occur without producing the associated effect (dolphin die-off).