I agree with all of the ...

I agree with all of the above: why the sampling is being undertaken meaning what are you trying to establish. Is it is to gain an understanding of the lake's structure/stratification eg temp, ph, nutrient load, sources of pollutants, suitability of specific uses? Depth with impact on nutrient loading temp potentially pH. vicinity to inlet outlet, turbidity will impact on the "quality" of water. I would continue to use the previous sites with potentially new ones to have something to compare and to establish a future baseline with a summary of reasoning behind site choice as you mentioned you dont know why the previous sites were chosen - it may have been accessability, politics, vicinity to a then existing factory any number of reasons.