for treatment with Barley Straw you need 1-2ozs. per sq. yard.
However at the risk being boring, you must sort out the symptoms of algal growth, to derive the greatest benefit from your pond, or are you modestly referring to a lake!! When you see what there is to do you may feel like filling it in but the project can be so satisfying.There are several companies selling both Barley Straw and lavender treatments,personally I would track down a grower of Barley,and get some that way( if the amount required is substantial-probably about 3 treatments a year.)