The author isn’t trying to assess the effects of anything—she’s assessing the cause of the dolphin die-off. The bacterial infection is just part of one theory on how the die-off occurred. It’s unknown whether there really was a bacterial infection, and the author’s interested in assessing two different hypotheses on exactly what caused the die-off.

Two problems. The author doesn’t suggest that the dolphins were ever correctly diagnosed—in fact, she rejects the explanation given by the research team that investigated the die-off. She also doesn’t discuss the research process much at all, other than to note that the dolphins were examined and tested for toxins. And she accepts the results of those tests without any assessment or critique. What she does assess, meanwhile, is the explanation the researchers offer to account for what those tests showed.

Two problems. Yes, the author exposes weaknesses in one hypothesis for the dolphin die-off, but she doesn’t end there. She goes on to propose an alternative hypothesis. This tells us that her purpose isn’t just to critique the first hypothesis—it’s to find a better explanation. Also, her critique of the first hypothesis isn’t over its methodology. She doesn’t discuss methodology much at all, other than to note that the dolphins were examined and tested for toxins. And she accepts the results of those tests. Her critique is all about the explanation offered to account for what those tests showed.

The author assesses two alternative explanations—the brevetoxin hypothesis in P3 and the synthetic pollutants hypothesis in P4—in an effort to account for the cause of the die-off.

The author doesn’t compare how deadly one pollutant is compared to another. Rather, she’s trying to find the correct cause of a single, large case of dolphin mortality. She does consider the kinds of effects that different pollutants can have on dolphins, but she doesn’t draw up a comparison between their relative effects—she just matches up the dolphins’ observed condition with the possible pollutant-based causes of that condition.