• Energy Research

On Isla Isabela, Galapagos Archipelago, three so far unexplored lakes were investigated in the caldera of Cerro Azul, one of the most active volcanoes in the world. The lakes face recurrent desiccation and eruption events and showed distinct differences in their water chemistry. Thirty cores from the upper 15 cm of sediment indicate distinct differences in the composition of meiobenthic communities between the lakes. In total, 27 different aquatic metazoan species could be distinguished. Numerically, rotifers dominated in two of the lakes, with mean densities up to 4.56 × 106 individuals m−2 while the third lake was dominated by a gastrotrich of the genus Chaetonotus (0.67 × 106 individuals m−2). The largest lake harboured up to 14.4 × 106 nematodes m−2, which is the highest nematode density thus far reported for a freshwater habitat. The lakes yielded few nematode species (S = 7, N = 887) and calculation of the Shannon–Wiener index (H′) indicated an exceptionally low nematode diversity. The nematode community of one lake was clearly dominated by an undescribed suction-feeding Mesodorylaimus (59.6%), the community of the other lake by the epistrate feeder Achromadora pseudomicoletzkyi (89.3%), whereas the third lake surprisingly contained no nematodes. The benthic nematode biomasses for the two nematode-containing lakes differed by a factor 50. The food webs of the three lakes are presumed to have an exceptionable simply structure.

In this study we investigated the dynamics of meiofaunal and macrofaunal communities in artificial water-filled tree holes. The abundances and, for the first time, biomasses and secondary production rates of these communities were examined. The experimental set-up consisted of 300 brown plastic cups placed in temperate mixed forests and sampled five times over a period of 16 months to determine the impact of (i) seasonal events, (ii) physicochemical parameters, and (iii) food resources on the tree hole metazoans.Metazoan organisms, especially the meiofauna (rotifers and nematodes) occupied nearly all of the cups (> 99%) throughout the year. Between 55% and 99% of the metazoan community was represented by rotifers (max. 557,000 individuals 100 cm-2) and nematodes (max. 58,000 individuals 100 cm-2). Diptera taxa, particularly Dasyhelea sp. (max. 256 individuals 100 cm-2) dominated the macrofaunal community. Macrofauna accounted for the majority of the metazoan biomass, with a mean dry weight of 5,800 μg 100 cm-2 and an annual production rate of 20,400 μg C 100 cm-2, whereas for meiofauna mean biomass and annual production were 100 μg 100 cm-2 and 5,300 μg C 100 cm-2, respectively. The macrofaunal taxa tended to show more fluctuating population dynamic while the meiofaunal dynamic was rather low with partly asynchronous development. Seasonality (average temperature and rain intervals) had a significant impact on both meiofauna and macrofauna. Furthermore, bottom-up control (chlorophyll-a and organic carbon), mainly attributable to algae, was a significant factor that shaped the metazoan communities. In contrast, physicochemical water parameters had no evident influence. 23.7% of organism density distribution was explained by redundancy analysis (RDA) indicating a high dynamic and asynchrony of the systems.

In a three-year field study we determined the seasonal and annual variation of abundance, biomass, species richness, and community structure of freshwater nematodes at a single station in the eutrophic Lake Obersee. The seasonal patterns observed were consistent and varied little between sampling years. Nematode abundance and biomass were highest in summer. Species richness and diversity peaked in winter. With 152 identified species, Obersee is the most species-rich lake ever described. Species number was not constant over years but the main assemblage structure hardly changed. The community was dominated by the same 4 species throughout the study. Colonization was very strong in winter while extinction rates were high in summer. This study shows that the community structure of free-living nematodes depends strongly on the season investigated.

Nature, 572 (7768) ISSN:0028-0836 ISSN:1476-4687

The distribution of Ethmolaimus pratensis (n = 1627 individuals), a common nematode species of Konigssee, was studied throughout 1986. E. pratensis comprised 3.5% of all nematodes in the littoral, 3.1% in the littoriprofundal and 5.5% in the profundal. The species contributed to a considerable portion of nematode biomass in Konigssee with mean annual biomass being highest at 1 m depth (0.14 mg/100 cm 2 ww; 17% of total nematode biomass) and 2 m depth (0.13 mg/100 cm 2 ww; 22% of total nematode biomass). At 190 m depth it was 40% of total nematode biomass. Population density of E. pratensis peaked three times a year at 1 m and 2 m water depth (spring, summer, winter) and twice in deeper regions. There was considerable water depth and time dependent variation in sex ratios of E. pratensis. In littoral and littoriprofundal zones the sex ratio was about 1 : 1, while in the profundal males were very rare. About 48% of E. pratensis individuals were found in the top sediment layer and the percentage increased with increasing water depth. Juveniles and adults of E. pratensis were partially segregated in the sediment column. Morphometric data revealed negligible sexual dimorphism in size, body shape and weight.