Spruce monoculture establishment affects functional traits of soil microarthropod communities

Structure and density of soil microarthropod communities (Oribatida and Collembola) were studied in one natural beech forest and one spruce monoculture planted on a former beech stand in South Bohemia (Czech Republic). The spruce monoculture establishment increased microarthropod densities (93,000 ind. m^?2 in the natural beech forest vs. 400,540 ind. m^?2 in the spruce monoculture for Oribatida; 66,360 ind. m^?2 in the natural beech forest vs. 136,360 ind. m^?2 in the spruce monoculture for Collembola); additionally, it changed greatly the community structure in terms of species composition and functional traits. In the spruce monoculture, groups susceptible to disturbance were suppressed. The oribatid trophic structure changed as well with opportunistic herbifungivorous species increasing in the monoculture at the expense of fungivorous species. Similarly, hemiedaphic collembolans increased in the monoculture at the expense of euedaphic species. We conclude that the “functional approach” seems to be fruitful in revealing soil fauna response to environmental change.     

A population study of epigean and subterranean Potamolithus snails from southeast Brazil (Mollusca: Gastropoda: Hydrobiidae)

Population and reproductive biology of four Potamolithus species, two epigean (P. ribeirensis and Potamolithus sp. 1) and two subterranean (P. troglobius and Potamolithus sp. 2), from the Upper Ribeira Valley, southeast Brazil, were studied, from June 1996 to June 1997, using 1 m² quadrats and counting all individuals found in these quadrats. P. ribeirensis showed the highest variation in population densities (from 0 – after floods – to 467 ind.m^?2), with average ranges from 16.0 to 284.8 ind.m^?2. The densities of the two troglobitic species (average ranges for Potamolithus sp. 2 and P. troglobius, from 17.6 to 58.4 and from 0 to 61.5 ind.m^?2, respectively), higher than those of Potamolithus sp. 1 (average from 0 to 9.3 ind.m^?2), were high for cave organisms. P. ribeirensis and the two troglobitic species showed higher densities in warmer, wetter months, while Potamolithus sp. 1 showed higher densities in cooler, less rainy months. The troglobitic species were non-seasonal, since high percentages of mature females were observed throughout the year. In the epigean species, the reproductive periods showed no detectable pattern: in 1996 P. ribeirensis had a peak in winter and in the next year (1997) in spring and summer. Potamolithus sp. 1 had reproductive peaks in fall and winter.     

Distribution, standing stock, growth, mortality and production of Strongylocentrotus pallidus (Echinodermata: Echinoidea) in the northern Barents Sea

The regular sea urchin, Strongylocentrotus pallidus (G.O. Sars, 1871), is a widespread epibenthic species in high-Arctic waters. However, little is known about its distribution, standing stock, population dynamics and production. In the northern Barents Sea, S. pallidus was recorded on seabed still photographs at 10 out of 11 stations in water depths of 80–360?m. Mean abundances along photographic transects of 150–300 m length ranged between <0.1 and 14.7?ind. m^?2 yielding a grand average of 3.6?ind.?m^?2. The small-scale distribution along the transects was patchy, with densities varying from nil to an overall maximum of 25.5 ind. m^?2, and exhibited a significant relation to the number of stones present. Sea urchin test diameters, measured on scaled photographs, extended from 7 to 90?mm. Median values at single stations varied from 14 to 46?mm, showing a significant inverse relationship to water depth. Biomass, estimated by combining photographic abundances, size frequencies and a size-mass function established with trawled specimens, ranged between <0.1 and 3.0?g ash-free dry mass m^?2, averaging about 1.0?g ash free dry mass m^?2. An analysis of skeletal growth bands in genital plates was carried out with 143 trawled individuals ranging in test diameter (D) from 4 to 48?mm. Assuming these bands to represent annual growth marks, the ages of the specimens analysed ranged between 3 and 42 years. A von Bertalanffy function was fitted to size-at-age data to model individual growth pattern (D_∞?=?102.3?mm, k?=?0.011 year^?1, t₀?=?0.633?year). The annual mortality rate Z of the population in the northern Barents Sea was estimated from a size-converted catch curve to be 0.08 year^?1. Applying the weight-specific growth rate method, the average P/B ratio and the mean annual production of this population were estimated as 0.07 year^?1 and 0.076?g AFDM m^?2 year^?1, respectively. In conclusion, S. pallidus is characterized by slow growth, low mortality, high longevity and low productivity. Because of its relatively high biomass, it is considered to contribute significantly to total benthic standing stock and carbon flux in the study area.     

Soil microarthropods and macrofauna in monsoon tropical forests of Cat Tien and Bi Dup-Nui Ba National Parks,southern Vietnam

The abundance, biomass, vertical distribution, and taxonomic composition of soil invertebrates (springtails, macrofauna, and termites) were studied in forest formations differing in edaphic and climatic conditions: lowland forests dominated by Lagerstroemia spp. or Dipterocarpus spp. in the Cat Tien National Park and in a mountain pine (Pinus kesiya) forest on the Da Lat Plateau, southern Vietnam. In the lowland forests, springtails had a relatively low density (10000–12000 ind./m²), but their diversity was high (41–43 species in each forest). The density of large soil invertebrates (without ants and termites) reached 500–700 ind./m² at a biomass of approximately 30 g/m² (with earthworms accounting for up to 230 ind./m² and 19–28 g/m²). Among termites, species of the genera Macrotermes and Odontotermes were dominant. Their total biomass in some areas exceeded 15–20 g/m². In the mountain pine forest, the total biomass of soil macrofauna was approximately 11 g/m², the abundance and diversity of springtails were low (7500 ind./m², 28 species), and wood-destroying species of the genera Schedorhinotermes sp. and Coptotermes sp. dominated among termites.     

Epiphytism in a subtidal natural bed of Gracilaria gracilis of southwestern Atlantic coast (Chubut, Argentina)

The diversity of epiphytes, their temporal abundance variation and the anatomical structure of host–epiphyte interfaces were studied in the agarophyte Gracilaria gracilis from a natural bed in Bahía Bustamante, Chubut Province, Argentina. Twenty-nine algal species were recorded as epiphytes during 2 years of monthly sampling (March 2006–February 2008). Total epiphyte density ranged between 0.037 ind. cm^?2 (November 2006) and 39.37 ind. cm^?2 (April 2007), with higher density values throughout the second sampling year. Ceramiales species were the most abundant epiphytes. The density of Ceramium rubrum ranged from 0.09 ind. cm^?2 (52 % of the total amount) in September 2006 to 17.4 ind. cm^?2 (44.18 % of the total amount) in April 2007. Epiphyte infection was more dependent on spore recruitment and sporeling development, especially on thalli derived from fragmentation, than on seasonal environmental variations. The different infections were analysed, taking into account the epiphytic attachment strength and invasiveness and the degree of damage inflicted on the host. Calothrix confervicola was one of the most abundant species. This epiphyte, weakly attached to the host surface, generated no host tissue damage. In contrast, C. rubrum, Polysiphonia abscissa and other Ceramiales were the species that caused more damage to the host because their rhizoids penetrated the cortical portion of the host thallus, sometimes reaching the medullary tissue. Some generalisations and characterisations of the different epiphyte groups in relation to their consequences to Gracilaria spp. are presented.     

Earthworm Communities in the Pineapple (Ananus comosus) and Mixed Fruit Plantations of West Tripura, India

Present studies on the community characteristics of earthworms revealed the occurrence of 11 species of earthworms in the pineapple (Ananus comosus) and 14 species in the mixed fruit plantations of west Tripura (India). While 9 species of earthworms namely Drawida assamensis, Drawida papillifer papillifer, Drawida nepalensis, Kanchuria sp., Metaphire houlleti, Eutyphoeus gigas, Eutyphoeus scutarius, Eutyphoeus comillahnus and Pontoscolex corethrurus are of common occurrence to both the pineapple and the mixed fruit plantations, two and five earthworm species namely Kanchuria sumerianus, Eutyphoeus sp. and Metaphire posthuma, Perionyx excavatus, Lampito mauritii, Amynthus alexandri, Eutyphoeus gammiei are restricted to the pineapple and the mixed fruit plantations respectively. Earthworms were found mostly within 15 cm depth of soils having temperature 25–25.8 °C, moisture 18.8–22.4 %, water holding capacity 26–31.7 % and organic matter content 2.4–4.0 %. Mean earthworm densities (158 ind. m^?2) was significantly higher (p < 0.01, t = 9.67) and biomass (36.67 g m^?2) significantly lower (p < 0.01, t = ?5.98) in the pineapple plantation than the mixed fruit plantation (density 93 ind. m^?2, biomass 56 g m^?2). High density value of earthworms in pineapple plantation is linked with dominance of D. assamensis and high biomass value in mixed fruit plantation was due to the higher relative abundance of larger species like E. gigas, E. scutarius, E. comillahnus and E. gammiei. Compared to the mixed fruit plantation, significantly (p < 0.05) higher index of dominance, lower index of diversity, species richness index and species evenness were recorded in the pineapple plantation.     

Benthic infaunal community variability on the northern Svalbard shelf

Marine benthic communities are effective indicators of environmental change. Yet in the Arctic, there are few empirical tests of how sustained climatic change may influence community structure. Northern Svalbard is influenced by both warm Atlantic and cold Arctic water masses, providing an opportunity to assess potential effects of long-term environmental changes by examining spatial variation in community structure. We examined benthic macroinfaunal communities and sediment pigments under Atlantic and Arctic water masses on the northern shelf and fjords of Svalbard. We report on infaunal biomass, abundance, species composition, and diversity at 10 stations spanning 79°–81°N and ranging in depth from 200 to 500?m. Benthic biomass averaged 128?g?WW?m^?2 (48–253?g?WW?m^?2), mean density was 3,635?ind.?m^?2 (780–7,660?ind.?m^?2), and species richness varied from 45 to 136?taxa?stn.^?1. Abundance-based community structure clustered stations in groups related to water mass characteristics, with Atlantic and Arctic shelf stations being well distinguished from each other. Dominant taxa were different in Atlantic- and Arctic-influenced locations. Faunal biomass was highest in the Atlantic-influenced fjords, followed by Arctic fjords and Arctic shelf stations, with Atlantic shelf stations having the lowest biomass. Species richness and diversity were inversely related to biomass. Patterns in faunal biomass were strongly correlated with sedimentary pigments (R ²?=?0.74 for chl a and R ²?=?0.77 for phaeopigments), with large differences in sedimentary pigment concentration among stations. These relationships suggest that benthic fauna on the northern Svalbard shelf are food limited and dependent on predictable, albeit episodic, delivery of organic matter from the water column.     

Population structure, growth and production of Thoracophelia furcifera (Polychaeta: Opheliidae) on a sandy beach in Southern Brazil

This study investigates aspects of the life history of the polychaete Thoracophelia furcifera on a sandy beach in southern Brazil. Two fixed transects perpendicular to the shoreline in the intertidal zone were sampled fortnightly from May 2008 to April 2009 at low tide. Five T. furcifera samples were collected along each transect and sediment temperature and the salinity of interstitial water were recorded. The material was washed over 0.5- and 0.088-mm sieves, and the width of setiger 8 of each specimen was measured. A total of 5,870 organisms were examined and the estimated parameters of the von Bertalanffy growth curve were L _∞ 3.60?mm (Wd8S), K 0.63?year^?1, C 0.3 and WP 0.97 (Rn 0.132). Life span was 2.6?years, instantaneous mortality rate Z was 3.8?year^?1 and the growth index φ′ 0.91. Mean density ranged from 644.44?±?191.77 to 2,783.33?±?453.64 ind m^?2 and mean biomass ranged from 2.52?±?0.55 to 9.52?±?1.83?g?m^?2. Recruitment occurred from April to July and ovigerous females were found from June to November. Annual secondary production was 6.582?g?m^?2?year^?1, mean biomass was 5.638?g?m^?2 and turnover rate was 1.167. The high values for density, secondary production and biomass suggest that T. furcifera constitute an important food source. These features of T. furcifera’ life strategy demonstrate the significant role this species plays in ecosystem dynamics.     

Floristic composition,structure and regeneration status of Masha forest,south‐west Ethiopia

The natural forest of Masha was studied to determine the species composition and population structure of woody species in the area. A total of 130 plant species belonging 61 families were recorded, in which family Acanthaceae was represented by 11 species followed by Rubiaceae (10) and Asteraceae (9). The composition and population structure of woody species, with diameter at breast height (DBH) >2.5 cm and height >2 m, and their regeneration were assessed in 65 sample plots of 400 m² each. The density for trees and shrubs with DBH >2.5 cm was 1681 ind.ha^?1, and the basal area of the study area was 142.6 m² ha^?1. Four community types were recognized. This forest is of international and national conservation importance given its high biodiversity. Managing the remaining forest would be an appropriate option for sustainable management of biodiversity.     

Above- and belowground dynamics of plant community succession following abandonment of farmland on the Loess Plateau, China

Aboveground and belowground changes during vegetation restoration and vegetation successions need to be characterized in relation to their individual responses to changes in soil resources. We examined above- and belowground vegetation characteristics, soil moisture, and nutrient status at the end of the growing season in 2006 in plots with vegetation succession ages of 2, 4, 6, and 8 years (two replicates each) that had been established on abandoned cropland, where potatoes had been grown for 3 years, using hoe and plow cultivation, immediately prior to vegetation clearance and subsequent natural plant colonization. A plant community comprising pioneer species [e.g., Artemisia capillaries, (subshrub)] was characterized by low levels of species richness (7.5?±?1.4 species m^?2), plant density (35.7?±?4.2 stems m^?2), fine root length density (940.1?±?90.1 m m^?2), and root area density (2.3?±?0.3 m² m^?2) that increased rapidly with time. Aboveground and belowground characteristics of both A. capillaries and the later successional species, Stipa bungeana (C3 perennial grass), increased in the first 6 years, but in the following 2 years A. capillaries declined while S. bungeana thrived. Thus, the fine root length density of A. capillaries, 812.4 m m^?2 after 2 years, changed by a factor of 1.7, 2.0, and 0.4 in the 4th, 6th, and 8th years, whereas that of S. bungeana changed from 278.4 m m^?2, after 4 years, and by 1.7 and 23.3 times in the 6th and 8th years, respectively. Secondary vegetation succession resulted in reduced soil moisture contents. Soil available P and N mainly influenced aboveground characteristics, while soil moisture mainly influenced belowground characteristics. However, soil moisture had no significant affect on S. bungeana belowground characteristics at the population level in this semiarid region.     

Light induction of nonphotochemical quenching,CO2 fixation,and photoinhibition in woody and fern species adapted to different light regimes

We aimed to find out relations among nonphotochemical quenching (NPQ), gross photosynthetic rate (P _G), and photoinhibition during photosynthetic light induction in three woody species (one pioneer tree and two understory shrubs) and four ferns adapted to different light regimes. Pot-grown plants received 100% and/or 10% sunlight according to their light-adaptation capabilities. After at least four months of light acclimation, CO₂ exchange and chlorophyll fluorescence were measured simultaneously in the laboratory. We found that during light induction the formation and relaxation of the transient NPQ was closely related to light intensity, light-adaption capability of species, and P _G. NPQ with all treatments increased rapidly within the first 1–2 min of the light induction. Thereafter, only species with high P _G and electron transport rate (ETR), i.e., one pioneer tree and one mild shade-adapted fern, showed NPQ relaxing rapidly to a low steady-state level within 6–8 min under PPFD of 100 μmol(photon) m^?2 s^?1 and ambient CO₂ concentration. Leaves with low P _Gand ETR, regardless of species characteristics or inhibition by low CO₂ concentration, showed slow or none NPQ relaxation up to 20 min after the start of low light induction. In contrast, NPQ increased slowly to a steady state (one pioneer tree) or it did not reach the steady state (the others) from 2 to 30 min under PPFD of 2,000 μmol m^?2 s^?1. Under high excess of light energy, species adapted to or plants acclimated to high light exhibited high NPQ at the initial 1 or 2 min, and showed low photoinhibition after 30 min of light induction. The value of fastest-developing NPQ can be quickly and easily obtained and might be useful for physiological studies.     

Effects of light,temperature and salinity on the growth rate of harmful marine diatoms,Chaetoceros convolutus and C. concavicornis that kill netpen salmon

Chaetoceros convolutus and C. concavicornis have been implicated in the death of salmon in netpens in the Pacific Northwest by damaging the salmon's gills. To better understand how environmental factors affect the distribution of these two species, the interacting effects of light, temperature and salinity on growth rate were examined by growing these species under a range of temperatures (4–18 °C), light (10–175 μmol photon m⁻² s⁻¹) and salinities (10–30‰). For C. convolutus, the growth rate showed a hyperbolic relationship with irradiance at 8, 14 and 18 °C and light saturation occurred at 9, 14 and 20 μmol photon m^t s⁻¹ respectively. At 4 °C for C. convolutus and 8 °C for C. concavicornis, cells grew at μ_max, even at the lowest irradiances tested (10 μmol photon m⁻² s⁻¹). For C. convolutus, the amount of light required to saturate growth rate increased with temperature in an approximately linear fashion. The Q₁₀ was 1.88, calculated by averaging over both species. C. concavicornis was the more euryhaline species growing at salinities as low as 17.5‰, while C. convolutus grew only at 25‰ and above.     

Life history and secondary production of Electrogena ujhelyii (Ephemeroptera: Heptageniidae) in a small forest stream in West Carpathians

In this study we focused on life history parameters of Electrogena ujhelyii. We studied life cycle, nymphal growth pattern, biomass and secondary production in a small, calcareous stream in Malé Karpaty Mts (West Carpathians). The life cycle was univoltine with a large range in the size of nymphs during most samples dates. Mean annual density was 40 ind. m^?2 and the annual secondary production reached 183 mg DW m^?2 y^?1. There are no published data available on the secondary production values of this species, therefore these are the first published data.     

Spatial structure of a subtidal macrobenthic community in the Bay of Veys (western Bay of Seine, English Channel)

The spatial distribution of the muddy fine sand community from the Bay of Veys (western English Channel) were investigated during spring and autumn 1997. A grid of 55 and 54 sites was sampled in March and October, respectively, using two replicates per site of a Hamon grab (0.25 m²) for macrofauna collection and an additional one for sediment analysis. A total of 172 species were sampled with a dominance of polychaetes, followed by crustaceans and bivalves. The species richness and abundance show low temporal changes despite higher values in October than in March. In March, the mean abundance was 165 ind. 0.5 m⁻²; in October, the mean abundance was 212 ind. 0.5 m⁻². Four assemblages from the Abra alba-Pectinaria koreni community were identified corresponding to a bathymetric and sedimentary gradient from muddy fine sands with high levels of fine particles in shallow water to fine sands in deeper water. The discussion focuses on factors prevailing on the spatial structure of sandy communities in the English Channel.     

Ecological investigations of the zooplankton community of Balsfjorden,northern Norway: Population structure and breeding biology of the chaetognath Sagitta elegans Verrill

The population dynamics of the chaetognath Sagitta elegans Verrill has been followed in Balsfjorden in 1976 and 1977. Seasonal variation in abundance, length-frequency distribution, growth in total length, and maturity stages are presented and discussed in relation to changes in hydrography.An annual generation of S. elegans was found, with a protracted and more or less continuous breeding season from May until October during 1977. The 1976 year-class consisted of two distinct length groups, both of which participated in the 1977 spawning. This spawning gave rise to possibly four sub-populations during 1977. The variation in numbers of sub-populations produced during the spawning season in 1976 and 1977 is discussed in relation to the hydrographical conditions in Balsfjorden. From November 1976 to March 1977 the abundance of S. elegans varied between 1 and 8 ind. · m^?3. The lowest value was recorded in May (0.9 ind. · m^?3). From September to December 1977 the population abundance was ≈2 ind. · m^?3.     

Feeding efficiency of Chaoborus flavicans (Insecta, Diptera) under turbulent conditions

Turbulence can affect predator–prey interactions. The effect of turbulence on the feeding efficiency of an ambush predator was tested with laboratory experiments. The experiments were conducted in 100-L aquaria in which ten individuals of fourth instar Chaoborus flavicans larvae were placed as predators. Two prey densities (3 and 10 ind. of Daphnia pulex L^?1) and two durations (30 and 120 min) were tested in a nonturbulent treatment and five different turbulence levels [average root-mean-square (RMS) velocities ranging from 0 to 7.3 cm s^?1, corresponding dissipation rates from 7.2 × 10^?7 to 1.3 × 10^?3 m² s^?3]. We hypothesized that the feeding rate of C. flavicans would be enhanced by turbulence due to increasing encounter rates up to a turbulence level above which a disturbance in post-encounter processes would lead to reduced feeding efficiency. However, the results showed no significant increase in the feeding rate of C. flavicans at intermediate turbulence. At high turbulence we found the expected significant negative response in the feeding rate of Chaoborus larvae. The feeding rate declined below the rates at nonturbulent and intermediate turbulence conditions as the average RMS velocity exceeded 3.1 cm s^?1 (dissipation rate 9.9 × 10^?5 m² s^?3, respectively).     

Time-resolved fluorescence depolarization measurements of F-actin binding to myosin subfragments-1 bearing different alkali light chains.

Experiments were designed to test for functional differences which might shed light on the differences in actin-activated ATPase activities recently reported for myosin subfragments-1 bearing different light chains. By using the method of A. G. Weeds and R. S. Taylor (1975, Nature (London)257, 54), two types of subfragment-1 (S-1) from myosin of rabbit fast skeletal muscle were prepared: (S-1)·A1 and (S-1)·A2 bearing, respectively, the alkali-1 and alkali-2 light chains. (In agreement with the findings of these investigators, actin enhanced the ATPase activity of (S-1)·A1 more than that of (S-1)·A2 at lower actin concentrations.) Through use of time-resolved fluorescence depolarization techniques, the affinity constants for the binding of the two types of S-1 to F-actin in the absence of ATP were found to be very similar: 3.4 ± 0.3 × 10⁶m^?1 (N = 10) for (S-1)·A1 and 3.9 ± 0.2 × 10⁶m^?1 (N = 7) for (S-1)·A2 of one preparation, and 6.4 ± 0.2 × 10⁶m^?1 (N = 6) for (S-1)·A1 and 7.7 ± 0.5 × 10⁶m^?1 (N = 12) for (S-1)·A2 of another preparation (pH 7.0, 25 °C, 0.28 m KCl, 1.5 mm MgCl₂, 0.5 mm ethylene glycol bis (β-aminoethyl ether) N,N′-tetracetic acid, 10 mm imidazole, and 0.1 mmN-tris (hydroxymethyl) methyl-2-aminoethane sulfonate). The affinity constants for the two species of S-1 and actin also have a similar dependence on ionic strength and are not affected by addition of 0.6 mm CaCl₂ to the above solution. The CaATPase (or the CaITPase) activities of the two species of S-1 show the same pH dependence.     

Population dynamics and production of the small copepod Oithona spp. in a subarctic fjord of West Greenland

The small cyclopoid copepod Oithona is widely occurring in polar areas; however, knowledge of its biology and ecology is very limited. Here, we investigate the population dynamics, vertical distribution, and reproductive characteristics of Oithona spp. from late winter to summer, in a subarctic fjord of West Greenland. During winter–early spring, the abundance of Oithona spp. was low (1.8 × 10³ ind. m^?2) and the population was mainly composed of late copepodites and adults, whereas in summer, abundance peaked and younger stages dominated (1.1 × 10⁶ ind. m^?2). In general, all stages of Oithona spp. remained in the upper 100 m, with nauplii exhibiting a shallower distribution. Although no general seasonal migration was found, a deeper distribution of the adult females in winter was observed. The mean clutch size of Oithona spp. varied from 16 to 30 eggs per female, peaking in summer. Egg production rates (EPR) were low in winter–early spring (0.13 ± 0.03 eggs female^?1 day^?1) and reached maximum values in summer (1.6 ± 0.45 eggs female^?1 day^?1). EPR of Oithona spp. showed a significantly positive relationship with both temperature and protozooplankton biomass, and the development of the population seemed to be appreciably affected by temperature. Oithona spp. remained active throughout the study, stressing the key importance of these small copepods in high-latitude ecosystems, especially in periods when larger copepods are not present in the surface layer.     

On the distribution patterns and population dynamics of sublittoral and profundal oligochaeta fauna from Lake Banyoles (Catalonia,NE Spain)

Oligochaeta assemblages from Lake Banyoles were composed of one species of Naididae and eight species of Tubificidae, Potamothrix heuscheri (8–1350 ind. m^–2) being the most abundant and widely distributed species, followed by Limnodrilus hoffmeisteri (25–858 ind. m^–2) and Psammoryctides barbatus (12–792 ind. m^–2). The remaining species were Potamothrix hammoniensis, Potamothrix bavaricus, Limnodrilus claparedeanus, Branchiura sowerbyi, Aulodrilus pigueti and Dero digitata, all of which occurred at lower densities and with restricted distributions.Regardless of sampling depth, maximum densities of oligochaetes were found in winter (up to 5,142 ind. m^–2 in December) and secondarily in June. Immature Tubificidae with hair setae and mature P. heuscheri were present and dominant throughout the year. Cestode parasites were recorded infesting L. hoffmeisteri and P. heuscheri populations.Oligochaeta densities increased with depth, but low oxygen concentrations (less than 1 mg l^–1) during a long period (4 months) in some basins of the lake acted as a key factor in reducing the density of worms and the species richness to one species, Potamothrix heuscheri.