Distribution of benthic nematodes in the littoriprofundal and profundal of an oligotrophic lake (Königssee, National Park Berchtesgaden, FRG)

Abstract

Although the nematodes are the most abundant and species richest organisms in the littoral and profundal of the benthos of lakes, little is known about their distribution and species composition. In this study, the spatial and temporal distribution of the nematode fauna of an alpine, oligotrophic lake was studied over a one-year period. In the littoriprofundal and profundal of Konigssee, nematodes comprise respectively 59 % and 75 % of the meiobenthic organisms, with a mean abundance of 578 individuals/100 cm 2 in the littoriprofundal, and of 375 individuals/100 cm 2 in the profundal. Differences in abundance between the depths examined, however, were considerable in the littoriprofundal (15 m, 20 m and 30 m) but not in the profundal (60 m, 120 m and 190 m). Seasonal variation in abundance was significant at all depths, with the exception of 190 m water depth. Minimum nematode abundances were recorded in winter or spring and maximum values in the summer or autumn. The highest abundances were recorded at 15 m and the lowest at 190 m water depth. The fresh weight biomass of nematodes ranged from 0.03 mg/100 cm 2 to 1.2mg/100 cm 2 depending on the season and water depth. Monhystera paludicola and Eumonhystera filiformis were the abundant of the 71 and 60 species found respectively in the littoriprofundal and profundal. Monhystera paludicola was found predominantly in 15 m, 20 m, 120 m and 190 m, whereas the abundance of Eumonhystera filiformis was very similar at all depths. About 70-84 % of the nematodes were found in the upper 2 cm of the sediment and overall 95 % of all individuals in the first five centimetres. Nematodes occurred deeper in the sediment at 15 m water depth where the proportion of nematodes in the top 2 cm layer was only 62 % and about 90 % in the first 5 cm. The remainder was found between 5 cm and 20 cm sediment depth. The proportion of nematodes in the top sediment layer was highest in summer/autumn. Deposit-feeders were the most common feeding-type, followed by epistrate feeders. There was also a remarkable proportion of omnivores and predators, especially at 30 m and 60m water depth. The study shows that the structure of freeliving nematodes of an oligotrophic lake is also in the deeper regions very dependent on seasonal, bathymetric and vertical distribution.