Genus-specific and total secondary production of a nonbiting midge assemblage (Chironomidae) were estimated along a hyporheic gradient (surface to 40–50-cm depth) in 2 contrasting stream types and 3 habitat types in north-central Alabama to assess the extent of the variation within a river basin. Total production (surface hyporheic) was much higher in a gravel–cobble habitat (68 g m−2 y−1) of a Valley and Ridge stream than in either the sandy habitat (8 g m−2 y−1) or mud–silt habitat (12 g m−2 y−1) of a Coastal Plain stream. A strong seasonal pattern of high production in spring was apparent in the Valley and Ridge stream and differed from the more uniform seasonal production of the Coastal Plain stream. In the Valley and Ridge stream, 37% of production (25 g m−2 y−1) occurred in the hyporheic zone (below 10-cm depth). In the sandy habitat of the Coastal Plain stream, 42% of production (3.2 g m−2 y−1) occurred at depths below 10 cm. In both streams, the dominant hyporheic taxa were generally the same as the dominant taxa in the surface (0–10 cm) zone. In contrast, only ∼5% of total chironomid secondary production was found below 10-cm depth in the mud–silt habitat of the Coastal Plain stream (0.6 g m−2 y−1). These data suggest that the contribution of chironomids to stream ecological processes can be substantially underestimated without considering the hyporheic distributions of their production in certain habitats. Production may be a direct reflection of other functional variables, such as respiration, nutrient turnover, and decomposition. Furthermore, these deeper sediments may serve as an important refuge from natural or human disturbance at the benthic surface.
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Vol. 31 • No. 1