Invertebrate distribution on a macroalgae/macrophyte mixed mat in flowing water

Abstract

In flowing waters, aquatic fauna are typically concentrated in the benthos, wood or other fixed substrates. Aquatic plants and filamentous algae may offer refugia for invertebrates within the water column, due to flow attenuation and alimentation. We quantified invertebrate abundance, size distribution and species composition along linear, flow-oriented gradients of mixed Cladophora sp. and Elodea canadensis mats in Crow Creek, northern Montana (USA). Mats were divided into four sections (0-20, 20-40, 40-60 and 60-80 cm, ordered basal to distal) using a customized sampler. Mat biomass, invertebrate richness and abundance were significantly higher in the 20-40 cm section, driven primarily by Cladophora. Meanwhile Elodea biomass was correlated with invertebrate richness in the 60-80 cm section. Most taxa were not individually related to Cladophora or Elodea biomass, however Elodea biomass showed a positive relationship with the amphipod Hyalella azteca in the 0-20 cm section and with cladocerans and copepods (meiofauna) in the 60-80 cm section. Biomass size spectra showed no difference among mat segments, but were prominently skewed towards smaller size classes (meiofauna), implying that mats permit the existence of slow-water meiofauna. Stream epiphytic invertebrate communities may exhibit complex patterns in abundance and composition due to the differential probabilities of colonization, movement, and loss of individuals along a linear flow-oriented substrate, substrate biomass, and inter-specific interactions.