A slimy source of information on pesticides in streams

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Analysis of stream biofilms reveals the presence of a wider array of pesticides than analysis of bed sediment, reports a new study by the U.S. Geological Survey Regional Stream Quality Assessment. Biofilm analysis may also be a better predictor of potential adverse effects on the community of organisms that live on and in the streambed.

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.

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Concentrations of pesticides in sediment in 82 streams and in biofilms in 54 streams in California in 2017

Hydrophobic (sediment-associated) pesticides were measured in sediment samples collected from 82 wadeable streams and in biofilm in 54 of those streams in the Central California Foothills and Coastal Mountains ecoregion.115 current-use and 3 legacy pesticides were measured in stream sediment; 93 of the current-use pesticides and the same 3 legacy pesticides were measured in biofilm. On aver...