Sampling location for ...

Sampling location for monitoring

Because of the time since the initial sampling, that location is the most important in the lake. It is immaterial whether it is the right locations or not, it provides historical data which allows changes over time to be evaluated. As others have said, what is the purpose of the monitoring. If it is a comparison for assessing climate change or land use management effects on the lake then use the historical site. If you now want to understand how the lake works, then use the historical site as the reference point and add another site (or more) at locations that will show how different lake processes, especially hydrodynamics, affect the water quality parameters being measured at the historical monitoring site.