Shredder fecal pellets as stores of allochthonous organic matter in streams

The freshwater amphipod Gammarus pulex produces cohesive, cylindrical fecal pellets that are found in large numbers in the hyporheic sediments of UK chalk streams. We investigated the rate of fecal pellet accretion (deposition + in situ egestion) in chalk streams using benthic fine particulate organic matter traps. We also assessed how long pellets remain intact by observing them over time in the laboratory at 3 different temperatures (4, 10, and 20 degrees C). Traps were sampled seasonally (winter, spring, summer, and autumn), and their vegetation cover (marginal Nasturtium, mid-channel Ranunculus beds, or no vegetation) was recorded to see if accretion rate was affected by the presence of in-stream macrophytes. Accretion rate was strongly seasonal (10 to 20x higher in autumn than in winter, spring, and summer), but was not affected by vegetation cover. Laboratory experiments showed that fecal pellets remained intact for 81 d at 4 degrees C and 10 degrees C, whereas they began to break apart after 40 d at 20 degrees C. Chalk streams remain at similar to 10 degrees C most of the year. Thus, pellets transferred to hyporheic sediments during autumn remain intact over winter and into spring. In autumn, Gammarus feed on allochthonous C in the form of dead leaves and this C is transferred to hyporheic sediments where it is stored, providing a substrate for microorganisms and a food resource for detritivorous invertebrates at other times of the year.