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A three-dimensional spatial perspective was used to determine annual mac- roinvertebrate production in two low-gradient, first-order stream systems on the Coastal Plain physiographic province of Virginia. Production in predominant habitats was weighted by the proportional areal abundance of each habitat to determine production in the channel surface, hyporheic zone, and floodplains of both blackwater streams. Highest production as dry mass in Buzzards Branch, a sand-bottomed stream, was in debris dams (27.4 g/m2); production on the sand sediment was lower (4.9 g/m2) and decreased with depth into the hyporheic zone, primarily as a function of decreasing oxygen concentration. Production on the floodplain varied with elevation, and hence period of inundation; it was 1.1 and 4.3 g/m2 at high and low elevations, respectively. Production, weighted for habitat abundance, was 1.7 g/m2 on the floodplain and 15.8 g/m2 in the channel (10.2 g/m2 in the hyporheic zone; 5.6 g/m2 on the channel surface). Production in the Buzzards Branch stream system, incorporating the channel and floodplain, was 2.4 g/m2, 67% of which was on the floodplain, 21% in the hyporheic zone, and 12% on the channel surface. Production in the Colliers Creek system, with a clay and detritus sediment and no hyporheic zone, was only 2.8 g/m2 in debris dams but was 6.1-6.6 g/m2 on channel sediments and floodplain soils. Production in the stream system was 6.1 g/m2, 95% of which was on the floodplain. Primary consumers composed 72-74% of production in the channels and 80-85% on the floodplains. The predominant functional feeding groups differed between habitats, suggesting a spatial mosaic of functioning within the stream systems. The majority of production in debris dams and on the sediment was by shredders (30-46%) and predators (31-48%). Collector-gatherers composed 58% of production in the Buzzards Branch hy- porheic zone and 56-61% on the floodplains of both streams. This study further emphasizes the patchy nature of lotic systems and that habitat- specific sampling is required to adequately determine invertebrate production and function in streams. It also shows the need to view streams with a three-dimensional spatial per- spective. Both the hyporheic zone and floodplains can contribute a significant proportion of invertebrate production in low-gradient stream systems. |
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