Final answer:
Lowering nutrients in a reef system from 0.075 ppm to 0.015 ppm could help return it to normal by reducing algal blooms that threaten corals, but without specific data, the effect of this change can't be confirmed.
Step-by-step explanation:
Lowering the nutrient level from 0.075 parts per million (ppm) to 0.015 ppm may lead to a return of the reef system to its normal state if nutrient enrichment is a primary stressor affecting the ecosystem. The measurement of necessary metrics, such as species diversity, water quality, and rate of coral cover growth, would provide evidence for recovery. For instance, reduced nutrient levels have been linked to lower algae growth, which can smother corals, and a more balanced ecosystem.
Without the actual data from the experiment on nutrient reduction, it's challenging to say definitively that this decrease in nutrient concentration caused the reef's return to a normal state. However, generally, excess nutrients can contribute to algal blooms that can harm coral reefs. Clear evidence would be observable in empirical data showing improved reef health metrics concurrent with the nutrient reduction.