Biofilms are the greenish-brown “slime” that coats rocks, plants, and solid surfaces in a stream. They’re made up of algae, bacteria, and other organisms in a polysaccharide (carbohydrate) matrix—the slime. Although they may be slimy, biofilms play a critical ecological role, providing energy and organic matter to fish and to aquatic invertebrates, such as miniature crustaceans and damselfly larvae, that are an essential part of the food chain.

In a study of 46 small California streams, scientists detected more pesticides in biofilms than in bed sediment. On average, four times as many current-use pesticides were detected in biofilm at a stream site as in sediment. In particular, pyrethroid insecticides, which are highly toxic to both terrestrial and aquatic insects, were detected more frequently in biofilm than in sediment.

Biofilms also may provide more insight than sediment into whether pesticide occurrence is affecting stream health. Four of six models of different measures of invertebrate community structure (e.g., number of species present) were improved when the concentration of a pesticide in biofilm was included as a variable. The link between pesticide occurrence and the health of stream biota may be missed if only sediments are monitored.