By which mechanism is nutrient residence time different in rivers compared to ponds?

Nutrient residence time refers to the amount of time that nutrients remain within a water body before being processed, either through biological uptake, sedimentation, or export. In rivers, nutrient residence time is typically shorter due to the continuous flow and movement of water. The velocity of river water tends to carry nutrients downstream quickly, resulting in less time for organisms to utilize or transform those nutrients.

In contrast, ponds are often characterized by static or slow-moving water, which provides a longer residence time for nutrients. This allows for more extensive biological processes, such as microbial decomposition, sedimentation, and nutrient cycling, to occur. The relative tranquility of ponds enables nutrients to be absorbed more efficiently by the surrounding ecosystem, often leading to higher concentrations of nutrients being retained for more extended periods.

Thus, the correct answer highlights that in rivers, the mechanism of flow leads to a shorter nutrient residence time compared to the more stagnant conditions found in ponds, which supports a longer nutrient residence time.