Organically Bonded Mg Clogging UF

For the past years we are struggling to solve the algae and organically-bonded magnesium problems in a lake in ​Saskatchewan, ​Canada. We use 3 banks of ultrafiltration (UF) filters followed by 0.5-micron filters before nanofiltration ( NF). The algae form even in winter months, under 3-feet-thick ice. The organics in water are non-biodegradable and Mg has bonded to them. This makes Mg basically impossible to oxide with any oxidizing agent that we have tried. Mg stays in a dissolved state so the NF has to remove the  Mg instead of the UF. That is why our reject rate is incredibly high (67% waste and it was designed to be 35%). And we need to replace the UF filters every 2 years while they should last 5-10 years. How can we prevent algae blooms, oxidize the organically bonded Mg and prolong UF life?