Effects of bioturbation by tube-dwelling chironomid larvae on oxygen uptake and denitrification in eutrophic lake sediments

1. Oxygen uptake and denitrification were determined in two bioturbated sediments from a eutrophic lake in southern Sweden. In laboratory mesocosms, an organic profundal sediment was incubated with Chironomus plumosus L. and a sandy littoral sediment with an organic-rich top layer was incubated with Polypedilum sp. Both species of chironomid are sediment tube-dwelling. 2. Oxygen consumption, expressed per gram of larval dry weight, was enhanced to the same extent by the larvae in both sediments. Measurements of the respiration rate of individual larvae revealed that the respiration per gram dry weight of the smaller Polypedilum sp. was more than three times higher than that of C. plumosus. 3. Denitrification was measured using the ‘nitrogen isotope pairing’ technique. In the organic sediment, denitrification of nitrate from the water phase (dw) and denitrification of nitrate from coupled nitrification (dn) were each correlated with the biomass of C. plumosus. In the sandy sediment, dw was correlated with the biomass of Polypedilum sp., while dn did not show any correlation with Polypedilum sp. 4. Oxygen uptake in the organic sediment was increased by a factor of 2.5, dw 5-fold and dn 2.5-fold at a biomass of 10 g m^–2 dry weight of C. plumosus. The same biomass of Polypedilum sp. in the sandy sediment resulted in a 2-fold stimulation of oxygen uptake and a 3-fold stimulation of dw, while dn was not affected. These differences in stimulation between oxygen uptake and denitrification by the larvae in the sediments suggest that the stimulation pattern cannot be explained by simple extension of the sediment surface. The burrows evidently reduce the distance between the nitrate source in the water column and the denitrifiers in the anoxic zones. 5. This study indicates that bioturbation by macrofauna elements can have a great impact on denitrification in lake sediments, and that different organisms can influence nitrogen turnover in specific ways.     

Habitat and substrate influences on population and production dynamics of a stream caddisfly, Ceraclea ancylus (Leptoceridae)

SUMMARY. Population and production dynamics of Ceraclea ancylus (Vorhies), a leptocerid caddisfly with a univoltine life cycle, five larval instars, and a single cohort population, were examined in Brashears Creek, Kentucky. A systematic sampling approach that included analysis in riffle, pool, and waterwillow bed habitats was used and fourteen sampling transects were taken across Brashears Creek between May 1971 and July 1972. For each of the 70 m² samples taken along a transect, the depth, habitat, and substrate type were recorded. The number of C. ancylus cases and numbers and dry weights of each larval instar were determined. The majority of the C. ancylus population was in the second larval instar in July, the third larval instar in August, the fourth larval instar in September, and the fifth larval instar from October through May. The spring population was 30% of the early instar maximum estimated the previous August. The spatial distribution of C. ancylus reflects a logarithmic pattern with early instar larvae more highly clumped than the late larval instar and pupal populations. Initial standing stock estimates were less than 1% of the standing stock biomass. A production estimate of 9.9 mg/m²·year (dry weight) was determined using the instantaneous growth method, Allen's Curve method, and the Hynes method of estimating secondary production. Turnover ratios (7.0 annual TR, 5.7 cohort TR) were higher than estimates reported as typical for benthic macroinvertebrates because of the low initial to final mean individual weight of the C. ancylus larvae (0.4%). Production estimates calculated for C. ancylus populations in waterwillow bed (4.4 mg/m²·year) and pool (13.7 mg/m²·year) habitats were less than calculated for the riffle population (19.9 mg/m²·year). The annual turnover ratio (10.2) was higher in the pool population than in other habitats because of spring larval migration into the pool from other habitats. A trend of higher production estimates with larger sized substrate particles and increased numbers of particles may reflect the increase in available food sources or attachment sites. The accuracy of production estimates and their applicability to water resource management may be limited by a failure to consider the spatial distributions of the population being sampled and the tendency for sampling procedures to be confined to single habitats.     

Instar distribution and biomass of Chironomidae larvae in Lago El Junco, Isla San Cristobal, the Galápagos

The distribution, abundance and standing crop biomass of chironomid larvae were determined at one-meter depth intervals along three radial transects. Samples were collected by coring soft sediments while diving. Three genera were found in the lake: Chironomus sp. (collector-filtering larvae), Ablabesmyia sp. (predatory larvae) and Goeldichironomus sp. (collector-filtering larvae). Standing crop densities of chironomids, averaged over the entire lake, varied from 30,594 larvae/m² to 11,428 larvae/m² at different depths. No statistically significant zonation in density was found for the two most common taxa, Chironomus sp. (87.8% of specimens) and Ablabesmyia sp. (9.0%), however the deepest zones (>4 m) had the lowest estimated densities. Significant differences in standing crop biomass were detected, with the 6 m depth having greatest biomass. The increase in standing crop biomass was a function of (1) lower frequency of first instars of Chironomus sp. and Ablabesmyia sp. at 6 m (2) higher average larval biomass of both species at 6 m and (3) very significant increase in average biomass of fourth instars of Chironomus sp. at 6 m compared to fourth instars at shallower depths. These results indicate that the lentic chironomids of this isolated oceanic habitat consist of a small number of species that are ecological generalists and tolerant of low oxygen concentrations.     

Production and Ecology of Benthic Chironomid Larvae (Diptera) in Lake Hayes,New Zealand,a Warm-monomictic Eutrophic Lake

The distribution and abundance of larval chironomids in Lake Hayes were studied from December 1973 to March 1975. The mean annual production of the two dominant species, Chironomus zealandicus and Chironomus sp. a, was 29.2 g m⁻² dry weight which is approximately 4.3 % of the average annual phytoplankton production in the lake. A high annual P/B ratio of 18.5 is consistent with the multivoltine life cycle of C. zealandicus. Larval chironomid production in the second summer when Anabaena blooms were absent was only one quarter of that in the first summer and is consistent with the hypothesis that the production of benthic chironomids in Lake Hayes is closely linked to that of the phytoplankton through the sedimentation of autochthonous organic matter.     

Factors influencing the species composition,distribution and abundance of benthic invertebrates in the profundal zone of a eutrophic northern lake

Factors influencing the species composition, distribution and abundance of benthic invertebrates were determined in a eutrophic subarctic lake from April 1978 to April 1979. Collections were made at five stations located at depths of 4 to 13 m. The largest populations of up to 5 × 10³ animals m^–2 were found in the deepest part of the lake. of the 24 species recorded in this area, the chironomidsProcladius denticulatus, Dicrotendipes modestus, Chironomus decorus andGlyptotendipes barbipes were most common. The strong development of benthos in the profundal zone was attributed to a consistently large supply of food and warm (4 °C) winter temperatures on bottom. Slightly smaller populations (up to 4 × 10³ animals, m^–2), composed of 19–23 species, occurred in shallower water, a reflection of lower (1.5 °C) winter temperatures. In the anoxic northern part of the lake, only 4–8 species were found in low numbers (400–1 000 animals m^–2). This was likely due to low (<5% saturation) oxygen levels in water and high organic content (18.5%) of the sediments.     

Species composition and assemblage structure of chironomid larvae (Diptera: Chironomidae) attaching to the artificial substrates in a Japanese temperate basin,in relation to the longitudinal gradient

The relative importance of natural and anthropogenic factors, especially topographic type, riparian canopy, altitude, temperature and bank protection, on larval chironomid assemblage was investigated in a Japanese basin. To focus on the macro-scale factors, a concrete block, as an artificial substrate, was used for chironomid collection so that sampling regime may be identical among the sites. Partial CCA using sampling month as a covariable revealed that topographic type, riparian canopy coverage, water temperature and altitude were the main factors influencing species distribution. Stempellinella tamaseptima, Polypedilum tamanigrum and five Rheotanytarsus species showed positive, whereas five Cricotopus species showed negative associations with canopy coverage. Some traditional longitudinal zonations of species were still shown. Chironomus flaviplumus and Chironomus yoshimatsui were merely associated with lower reaches. Stepwise multiple regressions of the assemblage indices on the environmental variables were applied. Bank protection and depth showed negative correlations with Shannon diversity H′. Both topographic type and depth showed negative correlations with Pielou equitability J. Topographic type (lower reach) and specific conductance showed positive, while bank protection showed a negative correlation with abundance. Species richness was not explained by any variables. As a whole, topographic type was the most directly related factor to chironomid assemblages.     

The ecology of Lake Nakuru (Kenya)

Summary Consumer biomass and spatial distribution in the equatorial alkaline-saline Lake Nakuru were studied from 1972–1976. These data will provide the basis for estimating feeding and production rates and for quantifying energy flow at the consumer level. Two of the main consumers, the Lesser Flamingo (Phoeniconaias minor) and the Soda Tilapia (Sarotherodon alcalicus grahami), were covered by earlier papers. The biomass of the only planktonic crustacean, the copepod Lovenula (=Paradiaptomus) africana was very high (1.5 gDW (dry weight)·m^-3, mean in 1972/73) in comparison with other tropical lakes. Lovenula was absent in 1974 and 1975, and at very low levels (0.1–0.4 gDW·m^-3) in part of 1976. Among the rotifers Brachionus dimidiatus dominated in 1972/73 (0.2 gDW· m^-3), but was outnumbered by B. plicatilis throughout most of 1974 to 1976 (mean total rotifer biomass 1.4 gDW· m^-3, peak densities 7 gDW·m^-3); during high salinity periods (>20) Hexarthra jenkinae occurred in very low numbers. For short periods rotifers can be the dominant species of L. Nakuru. Aquatic heteroptera (four species) played a minor role: they contributed 0.4% to total consumer biomass in 1972/73; in 1974–1976 the lake had no aquatic heteroptera. Benthic biomass (0.4 gDW·m^-2) was within the range of other tropical lakes, it consisted almost exclusively of Leptochironomus deribae. Bird counts of the twelve most important species are given for the years 1972–1974: Pelecanus onocrotalus accounts for 90% of the biomass (0.44 gDW·m^-3, mean 1972/73) with peak densities of almost 20,000 birds.-The consumer organisms covered by this and the two preceding papers represent >99% of L. Nakuru's consumer biomass. Population dynamics of various consumer species are discussed.     

Progress of eutrophication and change of chironomid fauna in Lake Yamanakako,Japan

Benthic macroinvertebrate distribution was studied on April 9, 1994, and March 2, 2003, in Lake Yamanakako. The average density of the benthic community for the entire lake was 3168ind.m^–2, comprising principally oligochaetes (41.0%) and chironomids (59.0%) in 1994. In 2003, the benthic community for the entire lake was 1847ind.m^–2, principally consisting of oligochaetes (69.9%) and chironomids (30.1%). In 1994, the larval density of Propsilocerus akamusi was 3.5 times that of Chironomus nipponensis and in 2003 the figure was 5.7 times. However, the larval biomass of P. akamusi was 2.1 times greater than that of C. nipponensis in 1994 and 2.8 times greater in 2003. The larval density of Tanypodinae decreased drastically, by about 12-fold, from 1994 to 2003. P. akamusi larvae were particularly abundant at the lake center in 1994, but they inhabited the entire lake bottom in 2003. P. akamusi density was closely related to water depth and ignition loss. C. nipponensis larvae also showed the widest distribution pattern in 2003, whereas their larvae had inhabited the northeastern parts and the lake center in 1994. Recently, the number of C. nipponensis larvae in Lake Yamanakako is tending to decrease, whereas that of P. akamusi larvae is increasing, suggesting ongoing eutrophication.     

The structure of the plankton community of the Öregrundsgrepen (southwest Bothnian Sea)

Taxonomic composition and variations in density and biomass of the plankton community in the Öregrundsgrepen, a shallow coastal area, were investigated from June 1972 to November 1973. The phytoplankton biomass was large in spring but small during the rest of the year. The spring bloom was dominated by diatoms and dinoflagellates, especially byThalassiosira spp. which were also important during other seasons. Small forms, such asCryptomonas spp.,Rhodomonas spp. and monads, dominated during summer. Blue-green algae were never of any major importance. During the summer, the trophogenic layer exceeded 10 m in thickness. The metazoan fauna was of lower diversity than the plankton flora. The dominating species, the copepodsAcartia bifilosa andEurytemora affinis, constituted on the average 83% of the standing crop. The low salinities, 5–6 S, were regarded as the principal pertinent limiting factor. The metazoan fauna reached large biomass values from July to October. The protozoan fauna (in the case of ciliates), obtained biomass maxima during the spring bloom. It is suggested that the Öregrundsgrepen represents an area of elevated productivity within a region of low overall production, presumably due to local upwelling. From June 1972 to May 1973, the average biomasses were: phytoplankton 0.464 g C m^–2, ciliates 0.040 g C m^–2, copepod nauplii 0.010 g C m^–2, micro-rotifers 0.004 g C m^–2, and mesozooplankton (larger than 0.2 mm) 0.312 g C m^–2. It is estimated that about than 60% of the phytoplankton production is consumed by the microzooplankton (<0.2 mm).     

Distribution and production of chironomids (Diptera: Chironomidae) in a shallow, alkaline lake (Neusiedler See, Austria)

From July 1990 to July 1991 the benthic community of the open water zone of Neusiedler See, one of the largest shallow lakes in central Europe, was studied with special reference to the chironomids. Only 16 spp. of chironomids inhabited the sediment of the open water zone. The numerically dominant species were Tanypus punctipennis, Procladius cf. choreus, Microchironomus tener and Cladotanytarsus gr. mancus. Most invertebrates showed a distinct horizontal distribution. Species richness and abundance were highest on muddy and organically rich substrates near the reed belt. Chironomid densities in this area reached 54,000 ind m^–2 and biomass was 2.0 g dw m^–2. The two tanypod species accounted for more than 90% of the standing stock of the macrozoobenthos near the reed belt. The sediment of the open lake and of the eastern part of Neusiedler See was composed of compact clay and sand as a result of the erosion of fine material due to strong waves and currents. Individual densities in these areas were much lower. Production of the numerically dominant species T. punctipennis was estimated using the increment-summation method, whereas production of the remaining species was estimated using an empirically derived multiple regression. Mean annual production of chironomids exceeded 6 g dw m^–2 yr^–1 near the reed belt, but it reached only 0.55 g dw m^–2 yr^–1 in the open lake. These values are rather low compared with other lakes and can be explained by unfavourable sediment conditions due to wave action and by physiological stress due to the water chemistry.     

The ecology of Lake Nakuru (Kenya)

Summary Abiotic factors, standing crop and photosynthetic production were studied in the equatorial alkaline-saline closed-basin Lake Nakuru (cond. 10,000–160,000 S). Meteorological conditions and abiotic factors offer suppositions for a high primary productivity: mean solar radiation is 450–550 kerg·cm^-2·s^-1, with little seasonal variation, regular winds circulate the lake every day and nutrient concentrations are usually high (>100 g P–PO₄·l^-1). Oxygen concentrations near sediments were <1 gO₂·m^-3 for at least 6 h·d^-1 in 1972/73, resulting in a release of 45 mg P–PO₄·m^-2·d^-1. Attenuation coefficients vary from 3.6–16.5 according to algal densities and mean depth from 0–400 cm. Algal biomass was 200 g·m^-3 (d.w.) in 1972/73, due to a lasting Spirulina platensis bloom (98.5% of algal biomass). In 1974 algal biomass suddenly dropped to 50 g·m^-3 (d.w.). Spirulina and several consumer organisms almost vanished, but coccoid cyanobacteria, Anabaenopsis and diatoms increased. Several causes for this change in ecosystem structure are discussed. The use of the light/dark bottle method to measure photosynthetic production in eutrophic alkaline lakes is discussed and relevant experiments were done. Oxygen tensions of 2–35 gO₂·m^-3 do not influence primary production rates. Net photosynthetic rates (mgO₂·m^-3·h^-1; photosynthetic quotient=1.18) reached 12–17.7 in 1972/73 and 2–3 in 1974, but vertically integrated rates were only 1–1.4 in 1972/73 and 0.8 in 1974, and daily net photosynthetic rates (gO₂·m^-3·24 h^-1) 3.5 in 1972/73 and 1 in 1974. 50% of areal rates were produced within the 10 most productive cm of the depth profile. The disproportion between high algal standing crops and relatively low production rates is due to self-shading of the algae, reducing the euphotic zone to 35 cm in 1972/73 and 77 cm in 1974. Efficiency of light utilization is 0.4–2%, varying with time of day and phytoplankton density. In situ efficiencies show an inverse relationship to light intensities. Photosynthetic rates of L. Nakuru remain within the range of other African lakes (0.1–3 gO₂·m^-2·h^-1). The relation of O₂ produced/Chl a of the euphotic zone is 50% lower then in tropical African freshwater lakes and conforms to lakes of temperate regions.     

The life history and production of Chaoborus punctipennis (Diptera: Chaoboridae) in Lake Norman,North Carolina,U.S.A.

A study of the life history and production of Chaoborus punctipennis (Say) in Lake Norman, North Carolina, U.S.A. was conducted from February 1978 through January 1979. Four sublittoral (～8 m) and two profundal (～30 m) locations were sampled. Larvae and pupae were collected with a modified Petersen grab and a plankton net, and adults were collected with emergence and light traps. Based on larval, pupal, and adult collections, there appear to be two generations per year — an overwintering spring generation and a summer generation. Annual dry weight standing stock biomass, dry weight production, and P/ B ratio were estimated from each sampling location and depth zone. Production was estimated by the size-frequency method. Standing stock biomass (30.9 mg · m^-2) and production (170.8 mg · m^-2) were highest in the profundal zone. In the sublittoral zone, standing stock biomass and production were 4.7 mg · m^-2, and 29.6 mg · m^-2, respectively. Annual P/ B ratios in the profundal and sublittoral zones were 5.5 and 6.3, respectively.     

The second investigation of Lake Puma Yum Co located in the Southern Tibetan Plateau,China

Limnological features of Lake Puma Yum Co, a typical alpine lake located at the altitude of 5030 m in the Tibetan Plateau, China, are described based on the findings of the second expedition in September 2004. The lake surface is about 280km² and maximal depth is 65 m. Total inflow just after the rainy season was estimated to be about 860 000m³ day⁻¹, and the lake water was drained from a newly excavated channel at a rate of 960 000m³ day⁻¹. We may have to expect undesirable lowering of the water level by this new drainage, especially in the dry season. Thermocline developed from 20 to 30 m depth, and the euphotic zone reached the 50 m depth. Dissolved oxygen in surface water was supersaturated as in productive lakes, although there were no large point and nonpoint sources of nutrient in the catchment. Vertical distribution of phytoplankton biomass and primary production suggested the presence of photoinhibition. What should be noted about the flora and fauna is that a Chara zone and a shell zone were distributed at about 30 or 40 m of depth.     

Production and ecology of eelgrass (Zostera marinal L.) in the Grevelingen estuary,the Netherlands,before and after the closure

The Grevelingen estuary was cut off from the North Sea and from the influences of the river Rhine by a dam in 1971, and became a stagnant salt-water lake. Production and ecology ofZostera marina L. were studied in 1968 and in 1973–1975, both through standing stock estimations, biomass increases in permanent quadrats, and correlation of distribution patterns with ecological factors. After the closure of the estuary the intertidal eelgrass population extended downwards to 5 m below lake level, probably owing to the increased transparency of the water; the area occupied, and the density of the eelgrass beds increased strongly. Eelgrass annual overground production, based on doubled maximum standing crop values in July–August, was estimated at 50 g C/m² in 1968, 121 g C/m² in 1973 and 91 g C/m² in 1975 inZostera beds, and 4 g C/m² in 1968, 18 g C/m² in 1973 and 23 g C/m² in 1975 for the entire Grevelingen area. A minimum estimate of net production inZostera beds at a depth of 0.50–0.75 m, based on short term changes in biomass in 2 permanent quadrats in 1974 and 1975, was 40.5 g C/m²/yr for overground parts and 12.7 g C/m²/yr for underground parts. Horizontal distribution of celgrass is not primarily limited by grainsize distribution, but more by exposure to wave action and currents. On account of irradiance reduction light is a limiting factor in the vertical distribution of the eelgrass population in Lake Grevelingen. Communication no. 146 of the Delta Institute for Hydrobiological Research, Yerseke, The Netherlands.     

Annual production by a stream brook charr population and by its principal invertebrate food

Synopsis Annual production by the brook charr (Salvelinus fontinalis) population in Valley Creek, Minnesota, over the five-year period 1968–1972, was closely correlated to production by its main invertebrate food, Gammarus pseudolimnaeus, which had been reduced by siltation. Annual production was 163–191 kg ha^–1 (wet weight) in 1968–1969, respectively, and then dropped to a mininum of 79 kg ha^–1 in 1971, the year most seriously affected by decreased invertebrate production and siltation; year-class strength, standing stock, and total year-class (cohort) production followed approximately the same pattern. Total year-class production was highest for the 1968–1969 year classes at 211–178 kg ha^–1, respectively, and lowest for the 1971 year class at 76 kg ha^–1. Annual P/B ratios ranged from 1.0 to 1.9; cohort P/B ratios, for the 1968–1972 year classes, ranged from 5.6 to 7.2.Paper No. 11,384, Scientific Journal Series, Minnesota Agricultural Experiment Station, St. Paul, Minnesota 55108, U.S.A.     

Secondary production of crustacean zooplankton and biomass of major rotifer species in Lake Bosumtwi/Bosomtwe,Ghana, West Africa

Studies have shown a strong linkage between zooplankton and fisheries' potential in tropical lakes. High zooplankton production provides the basis for fish production, but knowledge of zooplankton production dynamics in African lakes is extremely limited. Crustacean zooplankton production and the biomass of dominant rotifers in Lake Bosumtwi were assessed over a 2‐year period. The crustaceans comprised an endemic and extremely abundant cyclopoid copepod, Mesocyclops bosumtwii and the cladoceran Moina micrura. Mean standing stock of the crustaceans was 429 mg dw m^?3, whilst annual production averaged 2.1 g dw m^?3 y^?1. Production doubled from 1.4 g dw m^?3 y^?1 in 2005 to 2.8 g dw m^?3 y^?1 in 2006. Copepods accounted for 98.5% of crustacean production. The biomass of the dominant rotifers Brachionus calyciflorus and Hexarthra intermedia was less than 1% of total zooplankton biomass. Daily turnover rate and turnover time of the crustaceans was 0.19 day^?1 and 6.2 days respectively. Crustacean production yielded no statistical relationship with phytoplankton biomass. Production was well within the range of tropical lakes. Peak crustacean production synchronized maximum rainfall, lake mixing and phytoplankton production. Most importantly, no one year's set of dynamics can be used to characterize zooplankton production in the lake.     

Phytoplankton productivity in findley lake

Findley Lake is a dimictic, oligotrophic, subalpine lake located in the western Cascade Mountains, Washington. The lake is snow covered for most of the year so that the growing season was 3.5 months in 1971 and 4.5 months in 1972. Rapid melt of the lake's snow cover in summer allowed the sudden development of a phytoplankton productivity maximum (as measured by the ¹⁴C tracer method) of 86 mg m⁻² hr⁻¹ and a peak of 48 mg chlorophyll a per m¹ within two weeks of surface clearing in 1972, followed by a rapid decline of productivity and biomass. Annual production (between 10 October, 1971 and 21 October, 1972) was 36 g/m² in the 27.5 m water column. Autotrophic carbon assimilation during the snow-covered period was insignificant. The total production for the lake in 1972 was 530 kg carbon. The concentration of available nitrogen (NO₂ + NO₃ + NH₃ as N) at 15 m ranged from 12 to 76 mg/m³ while PO₄-P ranged from 0.5 to 8.3 mg/m³. In vitro nutrient enrichment experiments with natural phytoplankton communities from the lake indicated that while N and P together were growth limiting, P alone produced a growth response while N alone did not. Contributions to production from net-, nanno-, and ultraplankton were determined by fractional filtration of ¹⁴C-labeled phytoplankton samples. The nannoplankton, dominated by diatoms, accounted for 58% to 94% of productivity.     

Seasonal variation in phytoplankton primary production in relation to light and nutrients in Lake Awasa,Ethiopia

The biomass and primary production of phytoplankton in Lake Awasa, Ethiopia was measured over a 14 month period, November 1983 to March 1985. The lake had a mean phytoplankton biomass of 34 mg chl a m^–3 (n = 14). The seasonal variation in phytoplankton biomass of the euphotic zone (mg chl a m^–2 h^–1) was muted with a CV (standard deviation/mean) of 31%. The vertical distribution of photosynthetic activity was of a typical pattern for phytoplankton with light inhibition on all but overcast days. The maximum specific rates of photosynthesis or photosynthetic capacity (Ø_max) for the lake approached 19 mg O₂ (mg chl a)^–1 h^–1, with high values during periods of low phytoplankton biomass. Areal rates of photosynthesis ranged between 0.30 to 0.73 g O₂ m^–2 h^–1 and 3.3 to 7.8 g O₂ m^–2 d^–1. The efficiency of utilisation of PhAR incident on the lake surface varied from 2.4 to 4.1 mmol E^–1 with the highest efficiency observed corresponding to the lowest surface radiation. Calculated on a caloric basis, the efficiency ranged between 1.7 and 2.9%. The temporal pattern of primary production by phytoplankton showed limited variability (CV = 21 %).     

Factors governing the spatial and temporal distribution of Chironomid larvae in the Maarsseveen lakes with special emphasis on the role of oxygen conditions

Distribution patterns of the larvae of Chironomidae are compared in three water systems in The Netherlands, which vary in trophic state and oxygen regimes. The life cycles and flying periods of some dominant chironomid species in two of the investigated lakes, Lakes Maarsseveen I and II, are determined by comparing data on the seasonal variations in larval densities with existing literature on Chironomidae in the Maarsseveen lakes. In the oligo-mesotrophic Lake Maarsseveen I (LM I), hypoxic or anoxic conditions in the hypolimnion are observed only at the end of the stratification period. A clear zonation of the chironomid fauna is present in this lake. The littoral zone is dominated byCladotanytarsus gr.mancus andStictochironomus sticticus, the littoriprofundal zone byTanytarsus bathophilus, and the profundal zone byChironomus anthracinus. In comparison with the other species in LM I,T. bathophilus larvae show the most variable distribution patterns over time. Larvae are found in all depths from July to September, but disappear from the hypolimnion as soon as oxygen conditions deteriorate. In the eutrophic Lake Maarsseveen II (LM II), oxygen depletion of the hypolimnion starts immediately after the onset of the thermal stratification in June, and continues until autumnal turnover in November. In this lake, the chironomid community consists primarily ofS. sticticus andCl. gr.mancus, and is confined to the narrow littoral zone. No chironomid larvae are found in the deeper parts of the lake. The eutrophic Lake Gijster in the Brabantse Biesbosch is a deep, man-made reservoir, that is artificially destratified during the summer. In this lakeTanytarsus bathophilus is found in the profundal sediments, whereas almost noChironomus is found in this zone. It is concluded that oxygen conditions existing in the deeper regions of the investigated lakes in large part determine the occurrence and distribution of chironomid species. The distribution ofT. bathophilus is limited by unfavorable oxygen conditions and not by the trophic state of the lake. These findings are part of a thesis (HEINIS, 1993).     

Life cycle and production of Chironomidae (Diptera) from Lake Banyoles (NE Spain)

1. The life cycles and annual production of the eight most abundant species of chironomids (Prodadius cf. choreus, Tanypus punctipennis, Chironomus bernensis, Chironomus gr. plumosus, Cladopelma virescens, Microchironomus tencr, Tanytarsus gr. lestagei, and Cladotanytarsus atridorsum) were studied from sublittoral and profundal samples taken monthly in Lake Banyoles during 1987 at five sampling stations (depths ranging from 5 to 20 m). 2. The number of generations per year deduced from instar-frequency data varied from one to four, depending on the species, lake basin and depth. Annual temperature range, dissolved oxygen in the stratified period and presence of sulphide are the key factors that may explain the differences in the number of generations. 3. Production estimates were calculated using the size-frequency (SF) method corrected for the number of generations (SFG), and the increment-summation method (IS) when cohorts could be clearly deduced. 4. Production calculated with the SFG method gave results which were comparable with those of the IS method using smoothed-survivorship curves in the three species for which the use of the IS method was possible (C. virescens, M. tener and C. atridorsum). Using these methods production was estimated to range from 23–70 mg AFDW (ash-free dry weight) m^?2 yr^?1 at 12 m to 74–275 mg AFDW m^?2 yr^?1 in the sublittoral zone of the lake (5-m depth). 5. Calculation of production for the other five species using the size-frequency method with the number of generations (SFG) deduced from monthly instar-frequency data gave values ranging from 12 mg AFDW m^?2 yr^?1 (Chironomus bernensis at 20 m depth) to 3.75 g AFDW m^?2 yr^?1(Prodadius cf. choreus at 12 m). 6. Total chironomid production (with the SFG method) varied from 0.8 to 5 g AFDW m^?2 yr^?1 in the profundal and sublittoral, respectively. At each sampling station two species groups accounted for most of the production: Prodadius cf. choreus and Chironomus spp. Annual production/biomass ratio (P/B) varied from very high values for Prodadius (between 11 and 27, as four generations completed each year) to very low values for Chironomus gr. plumosus (2.20), which completed only one generation each year. The annual production of P. cf. choreus in Lake Banyoles is higher than any reported in the literature due to the completion of four generations and to the high densities of this species.