I would look into putting a good aeration system in. Make sure it is done by someone who knows what they are doing. You will need to run it slowly at first if your pond has fish in it so that you don't kill them. The smell will be strong at first where the bubbles come up from the bottom but as you add disolved oxygen to the water and decompose that materila at the bottom in a rich oxygen environment the smell will go away. Again, be careful as you bring it on line. You may need to run it 4 hours a day for a few weeks and then 8 and etc. until you can run it 24 X 7. You'll be amazed at how it will improve the water quality and deal with the smelly bottom mud. Good Luck.