Now suppose that the warm zone is a fetid sewage-laden or chemical contaminated water, or just a sterile desert-like water zone. The warming ( also increasing rotting of organic waste ) inducing the flow towards cool zones, then the same dispersion will occur and increase pollutants and contamination dispersion, resulting in polluted zone expansion.
Note that fetid waters or just life-unfriendly waters can by just being in contact with healthy life-rich waters impact them and be a source of pollution. A knock-on effect is very possible because polluted waters emit pollutants and harsh chemical conditions that impact other zones ( gazes, pH increase, saturation of - likely decomposing- organic matters,...).
Note also that even if only some species disappear from the sea (due to specific pollutants or just over-fishing ), this will break equilibrium in ecosystem, resulting in disappearance of macro fauna , then expansion of micro organisms, that will in the end die from nutrient exhaustion if not due tobpollution, and the result is a fetid water mass stretching in a global marine mass that is 70% of the earth surface.
Did you ever go by a mass of algae rotting in a new badly fitted hard slopy beach in modern 21st ac hyperwisdomlands?