Vertical distribution and abundance of invertebrates within the sandy substrate of a low‐gradient headwater stream

SUMMARY. 1 The vertical distribution of invertebrates (>0.053 mm) was studied in a sandy-bottomed, first-order stream on the Coastal Plain of Virginia, U.S.A. Invertebrate species composition, abundance and bio-mass were determined monthly over one year at sediment depth intervals of 0–1, 1–5, 5–15,15–30 and 30–40 cm. 2. The subsurface community was numerically dominated by species of Chironomidae, Nematoda and Crustacea, while much of the biomass was due to early instars of several species of Trichoptera. 3. Invertebrate density and biomass decreased significantly with depth in the substrate (ANOVA; P<0.05). Annual mean density decreased from 1,346,844 individuals m−3 at the surface to 13,578 individuals m−3 at 15–30 cm. Annual mean biomass decreased from 66.30 g m−3 at the surface to 0.44 g m−3 at 15–30 cm. 4. Dissolved oxygen decreased markedly from the surface to the 5 cm depth in the substrate, anaerobic conditions often occurring below 10 cm. Density and biomass both showed a significant positive relationship with dissolved oxygen concentration (Linear regression; P<0.05). 5. Physical forces were important in structuring the subsurface invertebrate community. Besides low dissolved oxygen concentration, sediment scouring resulting from storm discharge dramatically reduced density and biomass

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