Agricultural pollution cuts out key shredders

Posted on June 26, 2010

Farming-related changes in land use have been shown to affect the structure and composition of aquatic communities and to reduce the efficiency with which organic matter is broken down and recycled in streams.  To relate aquatic community structure to ecosystem function, French researchers measured sediment microbial activity and rates of leaf litter breakdown at twelve stream sites that differed in terms of surrounding land use (forest, extensive agriculture, intensive agriculture).  Results from the four-month field program showed that microbial activity was positively correlated with sediment organic content and point-source nutrient pollution from local farms, but was unrelated to broader land-use patterns.  In contrast, the breakdown of beech leaf litter in coarse mesh bags was unrelated to local nutrient levels but strongly related to land use, showing a 75% decrease in intensively farmed areas compared with a forested reference site.  Litter breakdown was also directly related to the abundance of shredding invertebrates collected from the litter bags.  These shredders were dominated by gammarids (shrimp-like amphipods), which made up most of the invertebrates at the reference site.  Gammarids are sensitive to high levels of nitrogen and phosphorus, which explains their low numbers at the impacted locations.  The results suggest that the rate of litter breakdown, but not microbial activity, is a useful index of agricultural impact at the catchment level, but they also highlight the key role played by one dominant group (gammarids) as opposed to the invertebrate or shredder communities as a whole.

Reference:  Piscart, C., Genoel, R., Doledec, S., Chauvet, E. & Marmonier, P.  2009.  Effects of intense agricultural practices on heterotrophic processes in streams.  Environmental Pollution 157, 1011–1018.