Identify a major consequence of heavy metal pollution in aquatic systems.

Heavy metal pollution in aquatic systems leads to significant toxicity in aquatic organisms, making this choice the most accurate. Heavy metals such as lead, mercury, arsenic, and cadmium can accumulate in the tissues of fish and other water-dwelling organisms. These metals disrupt biological processes and can cause a range of health issues, including reproductive failures, neurological damages, and even death. The persistence of heavy metals in the environment means they can remain in sediment and bioaccumulate in the food chain, impacting not only individual species but also entire ecosystems.

While options like oxygen depletion, turbidity increase, and decrease in salinity levels can affect aquatic environments, they are not direct consequences of heavy metal pollution. For example, oxygen depletion is often linked to organic pollution and nutrient overloads while turbidity might be more related to sediment or algae blooms. Salinity changes are typically associated with water flow alterations or climate factors rather than heavy metals directly. Therefore, the primary and most pressing impact of heavy metal pollution is indeed the toxicity it poses to aquatic life.