Metabolic activities in flocculent surface sediments and underlying sandy littoral sediments1

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Abstract
Parts of the sandy littoral sediment of a Precambrian shield lake (Lake 302S) were intermittently covered with a layer of flocculent organic‐rich material 1–20 mm thick. Sandy sediments with flocculent surface sediment had higher rates of respiration (31–105%) and photosynthesis (37– 224%) than those without. Densities of invertebrate macrofauna were 10 times higher in the flocculent surface sediment than in the underlying sandy sediment, accounting for 17 vs. 5% of the total respiration. Microprofiles of oxygen concentration showed that almost all of the dissolved oxygen in the overlying water was consumed as it diffused through 5–7 mm of the flocculent surface sediment, so that the underlying sandy sediment was almost completely anaerobic in the presence of flocculent surface material. The presence of flocculent surface sediment also decreased the depth of penetration of sulfate into the sandy sediment since the zone of sulfate reduction was located closer to the sediment‐water interface. In this experimentally acidified lake, neutralization of H2SO4 via sulfate reduction was more rapid (2.04 × 10−1 mmol H+ m−2 d−1) in the presence of surface flocculent sediment than in its absence (1.57 × 10−1 mmol H+ m−2 d−1).