To speak openly, I don't really see how you should be able to do this except by stealth.
And you should be aware that you may be subject to fines or even to having your mom's ashes retrieved and legally buried at your expense (not sure how they would do that with ashes scattered in a lake, but be aware of the risk).
There are two legal issues to be considered here. For one thing, it's the previously mentioned "Friedhofszwang" which means that ashes may only be buried in designated places like cemeteries or special burial forests. This law has not been renounced even by Bremen; they have only designated some additional areas.
There is another law which requires a coffin or an urn for burial. Even in a burial at sea the ashes are not simply scattered, they remain in an urn. Now the state of Bremen has in fact slackened this law and allows scattering of the ashes, but only in those designated areas. I do not assume that the lake you are considering happens to be located in those designated areas within the state of Bremen (there aren't a whole lot of lakes in Bremen to begin with, and nothing at all is "remote" in Bremen).
There has been a lot of discussion on these laws, and I don't really want to enter into all the details. Just to make a small case that these laws are not pure chicanery: Basically, the intent of these laws is to protect the dignity of the deceased. Grandmother's urn on the mantlepiece can so easily get into the way at some point and end up in a very impious spot in the garbage can. Or just consider how many families are terribly at war after daddy's death. So then, who gets to keep the urn while everybody is fighting over the inheritance? And why should the other children not even have the right to a place for their grief and memories?
If you want more advice, I wouldn't look for an attorney, I would look for a reputable funeral home. They have all the experience and can tell you what is possible and what is not.
And if I may add just one more thought: Honoring your mom's wishes certainly is an honorable thing in itself. However, you may also want to consider this: Was your mom aware of the difficulties involved? Would she have wanted you to take illegal steps in order to fulfill her wishes?