Functional Diversity among 3 Detritivorous Hyporheic Invertebrates: An Experimental Study in Microcosms

Abstract

Macroinvertebrates living at the sediment-water interface can be classified into different functional groups according to their modes of feeding and/or their bioturbation activities. We compared 3 detritivorous taxa of hyporheic habitats (chironomid larvae, asellid isopods, and tubificids) to test whether they represented distinct functional groups. This hypothesis was tested in experimental conditions using slow filtration gravel-and-sand columns during 20-d experiments. We measured the effects of the 3 taxa on particle redistribution, organic matter processing, nutrient fluxes, and microbial distribution. There were 3 experimental treatments: 1) 50 chironomid larvae (>80% Prodiamesa), 2) 25 asellids (Asellus aquaticus), and 3) 100 tubificids (Tubifex sp.). Although biologically mediated sediment fluxes were weak because of the small proportion of fine sediment in the columns, the 3 species redistributed sediment differently. They also had different effects on microbial activity, and O₂ and nutrient transport into the sediment. Chironomids caused a downward flux of sediment that accumulated as far as 4 cm below the sediment surface. Asellids homogeneized sediments at 1 to 3 cm depth in the column. Tubificids generated a biodiffusive mixing of sediment at the sediment surface together with a vertical transport inside the sediment and increased microbial respiration at all depths. Both asellids and chironomids increased O₂ penetration and decreased microbial activity in the first 5 cm of the sediment. Deeper in the sediment, tubificids and asellids stimulated anaerobic processes, whereas chironomid larvae had little effect on these processes. The initial hypothesis that the 3 taxa could be usefully classified into distinct functional groups was accepted. Their distinct activities in the sediment, demonstrated by specific modes of mixing, produced different effects on sediment properties and microbial activities.