ZOOBENTHOS ECOLOGY OF THINGVALLAVATN - VERTICAL-DISTRIBUTION, ABUNDANCE, POPULATION-DYNAMICS AND PRODUCTION

Macrozoobenthos of the medium oligotrophic Thingvallavatn (64-degrees-N, 21-degrees-W) is composed of ca. 60 taxa dominated by 24 Chironomidae and 16 Oligochaeta species. Abundance, life cycle, biomass and production were estimated for the 42 dominant taxa. The vertical distribution delimits five communities: (1) the surf zone community from 0 to 2 m, (2) the upper stony littoral community from 2 to 6 m, (3) the lower stony littoral community from 6 to 10 m, (4) the Nitella zone community from 10 to 20 m, and (5) the profundal zone community from 20 to 114 m. Abundance declined from ca. 20000 ind m-2 in the three upper zones to ca. 3500 ind m-2 in the Nitella - and profundal zones, and averaged ca. 5900 ind m-2 on a lakewide basis. By numbers, Tubifex tubifex (37%), Enchytraeidae (14%) and Lymnaea pereger (9%) dominated the average fauna. Life history varied from probably a 7-yr life cycle (Spirosperma ferox) to multivoltinism with a dominance of uni- and bivoltine species. Annual P/BBAR ratio averaged 2.4 with a range from 3.6 in the surf zone to 1.4 in the profundal zone. Annual production varied from 1.2 to 1.4 g AFDW m-2 in the profundal zone and the surf zone, respectively. L. pereger contributed 26%, T. tubiflex 16% and Pseudodiamesa nivosa 12% of total average production, which, on a lakewide basis, was 3.4 g AFDW or 78 kJ m-2 yr-1. Thingvallavatn thus belongs at the lower end of measured lake zoobenthic productions, which range from 10 kJ m-2 yr-1 in Arctic lakes to ca. 1600 kJ m-2 yr-1 in highly eutrophic shallow lakes.