Hi Norm.
Here's my best thought (guess?) on the sterile sand pits. I think they are sterile here because of the groundwater flow. If the water volume is replaced at too frequent an interval, there just isn't time to get the algae community developed, I suspect. If you test water quality, the water still has plenty of alkalinity (plenty of nutrients), just as do our lakes around here. I think there is a water exchange rate that keeps the pits from reaching their potential. Thus, they tend to be less productive (perhaps mesotrophic). Adding more nutrients (fertilizer) would not make a difference in this situation, as there already are enough nutrients in the water, and anything you add probably just heads "downstream" through the groundwater.
So, that brings up the question of why are some more fertile than others? Could some be in a location with "less" groundwater flow? Could others be older, and perhaps have more silt on the bottom, and perhaps this has slowed the water movement? I don't know, but suspect these type of answers are probably the explanation.
Dave
