HunterDave provides good advice on the matter. Many lakes support freshwater mussel (not actually clams) populations but you should not consume them.
Mussels filter feed rather indiscriminately and ingest whatever planktonic algae exist in the water column. Mussels from low nutrient lakes feed primarily on harmless algae and likely wouldn't make you sick.
However, mussels from higher nutrient lakes with planktonic algae dominated by cyanobacteria (blue-green algae) are another matter.
Cyanobacterial toxins can accumulate to a small extent. More importantly though, the mussels actually don't digest most cyanobacteria and rather compact the healthy cyanobacteria cells/colonies into pseudofeces that are excreted. When consuming mussels collected from cyanobateria impacted lakes, you are also likely to consume the cyanobacteria in the gut of the mussels and the toxins contained within. The liver toxin, microcystin, that is commonly produced in Alberta Lakes is stable to heat and will not degrade or breakdown with boiling.