Metabolic activities in flocculent surface sediments and underlying sandy littoral sediments’

Parts of the sandy littoral sediment of a Precambrian shield lake (Lake 302s) were intermittently covered with a layer of flocculent organic-rich material l-20 mm thick. Sandy sediments with flocculent surface sediment had higher rates of respiration (31-105%) and photosynthesis (37224%) 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 H,SO, via sulfate reduction was more rapid (2.04 x 10-l mmol H+ m-2 d-l) in the presence of surface flocculent sediment than in its absence (1.57 x 10-l mmol H+ m-2 d-l). One commonly observes patches of highly organic flocculent material on the surface of the sandy littoral sediments of lakes. This flocculent surface sediment is composed of material produced in situ, material transported from the underlying sediment, and material of allochthonous and autochthonous origin that has sedimented from the trophogenic layer. The quantity and distribution of the flocculent material is related to rates of resuspension and bottom transport (Bloesch 1982; Gons and van Keulen 1983), lake morphometry (Hakanson 198 l), and activities of bottom-dwelling animals. Seasonal quantitative and qualitative changes in seston also affect its rate of accumulation (Molongoski and Klug 1980) as does the distribution of algae on the sediment surface (Kingston et al. 1983; HappeyWood and Priddle 1984). In contrast to consolidated sediments and cyanobacterial mats, this material is as much part of the water as of the sediment because it is easily resuspended, mixed, and transported. This flocculent surface sediment is normally found on the surface of the more consolidated sandy littoral sediments. The living community growing on littoral sediment, either in the flocculent surface material or on the sand, is known as the epipelic community (Round 1973; Gons 1982) and is composed of benthic algae, bacteria, fungi, viruses, and zoobenthos. Previous studies of metabolic activity in sediments have not distinguished between the flocculent surface sediment and the underlying sediment (Wetzel 1964; Hargrave 1969; Graneli 1978). The relative contributions of the organisms in these two sites to total metabolic activity in sediment are 1 This work was supported by Canadian Department thus unknown; we report here experiments of Fisheries and Oceans, a grant from the Subfaculteit Biologie of the University of Amsterdam, and Canadesigned to obtain this information. These dian NSERC grant A267 1. experiments also provided data on how the 2 Current address: Limnologisch Instituut, Rijkintermittent presence of a layer of flocculent straatweg 6, 363 1 AC Nieuwersluis, Nederland. surface sediment affected the chemical and