Effective treatment of flowing water systems depends on what aspect relating to time?

The correct answer is contact time, as it refers to the duration that a pesticide remains in contact with the target organism in various aquatic environments. In flowing water systems, this concept is crucial because the movement of water can dilute and carry away the treatment agent, which can affect its efficacy.

Sufficient contact time allows the pesticide to effectively act on unwanted aquatic species before being washed away. If the contact time is too short, the treatment may not achieve the desired level of pest control since the organisms may not absorb enough of the pesticide to be affected.

While elapsed time and retention time also deal with time-related factors, they pertain to different aspects of pesticide application. Elapsed time generally refers to the overall duration since treatment but does not specifically address the effectiveness of interaction with the pest. Retention time is more commonly associated with how long a substance remains in a particular location, such as in a pond or lake, rather than flowing water. Time of day could have some relevance in certain ecological contexts, but it does not specifically relate to the effectiveness of aquatic treatment in the same direct manner as contact time does. Thus, understanding contact time is vital for achieving successful pest management in flowing water systems.