Martin, can you point me to any references to lager yeast and low Ca?
Having lived and traveled around Germany, the water can be variable. Some is pretty hard. Where I lived it had a mineral taste, but that was more wine country.
The reference for low calcium in Southern Bavaria will be published in the Mar/Apr 2014 issue of Zymurgy. The water of Southern Bavaria is quite similar...hard and alkaline. That makes it easily softened by pre-boiling or lime softening. Either of those options have been available for over a hundred years. So yeasts from that region are likely acclimatized to low calcium conditions.
However, places like Jever and Dortmund have water with more permanent hardness. I expect that yeasts used in those places do not have to have low calcium content to perform well. An extensive article on this calcium/yeast issue should be published in Zymurgy, probably in May/June.