Biological weed control takes many forms. One of the simplest ways to limit weed growth is to shade it out, thus planting trees along the southern bank of a river, for instance, can be a long term strategy for reducing weed growth.
Barley straw can successfuly limit the growth of filamentous algae, as long as it is introduced into the water early in the year. The leachate created as the lignin in the straw breaks down restricts the algae from forming new cell structures. The straw needs to remain aerobic, and positioned so that the leachate is carried through the main body of the water by the natural flow. For smaller ponds a concentrated form of this leachate is now commercially available through Kingcombe Aquacare.
Other biological control measures include stocking with grass carp (subject to DEFRA and Environment Agency consents) or even planting with preferred species to compete with the problem plants.
The common factor is that most biological control measures tend to be much slower acting and somewhat unpredictable, so they are normally used in combination with other more short term measures.