Shock loading refers to:

Shock loading refers to a situation where a significant influx of a harmful substance occurs rapidly, leading to severe environmental consequences. In option C, the description of "several months' accumulation of acid quickly melting into a spawning area" accurately captures the essence of shock loading. This phenomenon points to a sudden and critical increase in acidity in a specific environment, which can drastically alter the conditions needed for aquatic life to thrive, particularly during sensitive reproductive periods like spawning.

The rapid influx of acid can result in detrimental effects on the pH levels of the water, impacting fish and other aquatic organisms crucial for maintaining a balanced ecosystem. Such sudden changes can lead to high mortality rates among fish eggs and larvae, ultimately affecting the overall population and health of the aquatic community.

In contrast, the other options refer to biological events or environmental changes that do not align with the concept of shock loading, which emphasizes the swift and drastic addition of pollutants rather than gradual changes or reproductive patterns in aquatic species.