| Summary: | In chalk streams, Gammarus pulex is an abundant macroinvertebrate shredder that egests cohesive, cylindrical faecal pellets. The size and mass of faecal pellets influences their transport and utilisation in aquatic ecosystems, and are determined both by body size and diet in Gammarus. The number of Gammarus faecal pellets in sediments in the River Chess (a chalk stream and my field study site) varied over time, with the highest numbers found in Autumn and the lowest numbers found in Summer. Faecal pellet numbers did not vary spatially between different habitat types within the river. Gammarus faecal pellets are bound together by exopolymer substances, and also initially by a peritrophic membrane. Their breakdown is complex and involves changes to the size of organic matter particles within the pellet (which become larger over time), and is primarily mediated by bacteria internal to the pellets themselves. Although changes occur to the pellet during breakdown, the overall structure of the pellet is maintained for long periods, and Gammarus faecal pellets are relatively stable structures which have the potential to store organic matter in the environment. Organic materials in Gammarus faecal pellets are utilised by bacteria and other micro organisms as a substrate, bacterial numbers peaking soon after pellet egestion, and microbial respiration rates of faecal pellets are maintained at relatively high rates. Invertebrates also consume Gammarus faecal pellets, either to gain nutrition from the microbial communities there, or directly from organic matter within the pellets. Gammarus faecal pellets are likely to be an important ecological component of chalk streams, transforming, retaining and recycling organic matter (especially allochthonous materials) so that it can be utilised by other organisms in the ecosystem.
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