Seasonal influence on head capsule deformities in Chironomus zealandicus (Hudson) (Diptera: Chironomidae)

The natural incidence of deformities in the head capsules of Chironomus zealandicus was investigated at four lake sites in the central North Island (New Zealand) in summer (December) of 1994, 1995 and 1996, and winter (June) of 1995 and 1996. Significant differences were observed in deformities between sites and seasons. Individuals from Hamurana Stream, a relatively clean site, had the lowest incidence of deformities. However, there were still significant numbers of deformed chironomids. The incidence of deformity increased in summer in larvae from all sites except Hamurana Stream. No seasonal differences were observed in larvae from Hamurana Stream. There are indications that substrate type, season and genetic factors, as well as sediment chemistry may have collectively contributed to the incidence of head capsule abnormalities in C. zealandicus.     

The impact of acidification on Chironomidae (Diptera) as indicated by subfossil stratification

The impact of acidification on the chironomid profundal fauna was studied by comparing chironomid head capsule deposits from different sediment layers. The 14–15 cm level was regarded as relatively unaffected by acid precipitation and therefore used as a control. The total number of capsules decreased, especially the number of Tanytarsinii capsules, from deeper to more superficial layers. Phaenopsectra and Psectrocladius showed a relative increase towards the surface level of the sediment. Probable causes for these changes are discussed.     

The effect of tubificids and chironomids on particle redistribution of lake sediment

The effects of chironomid larvae,Chironomus plumosus, and tubificid worms,Limnodrilus spp., on particle redistribution in lake sediment were investigated experimentally using pots containing sediments obtained from Lake Suwa, Japan. The chironomids and tubificids increased the water content of surface sediment. The chironomid larvae had no effect on particle size distribution, while tubificids continuously accumulated small particles on the surface sediment through their selective feeding activity. Particles larger than 0.125 mm were buried at a sediment depth of 6 cm. In Lake Suwa, long diatom frustules, large plant debris and blue-green algal flocs were found to accumulate in the deeper layer of the lake sediment inhabited by tubificids at high density.     

Particle transport by benthic invertebrates: its role in egg bank dynamics

The ecological and evolutionary dynamics of zooplankton is in part a function of the numbers and ages of dispausing eggs hatching from aquatic sediments. Successful recruitment from this egg bank must depend upon the eggs being present at or near the sediment surface. Often, however, zooplankton diapausing eggs are found as deep as 15 to 30 cm in the mud. Bioturbation may provide a mechanism for the regular return of buried eggs to the sediment surface. A substantial portion of the population of the copepod, Diaptomus sanguineus, living in Bullhead Pond, a small lake in Rhode Island, USA, is present as diapausing eggs. To study the role of bioturbation in egg-bank dynamics, we introduced polystyrene beads, the same size and specific gravity as copepod eggs, at two depths in large-diameter sediment chambers in the laboratory. Treatments included chambers with natural and reduced densities of benthos. Consistent with other studies, our results show that the joint activities of tubificid oligochaetes and chironomid larvae are responsible for bidirectional (up and down) transport of beads in the top 2 cm of the sediment. We observed no bead movement below this depth. Thus, eggs in the top two centimeters of sediment in this lake are exposed with some regularity to conditions that stimulate hatching at the sediment-water interface. In Bullhead Pond, these eggs have a mean age of 12.2 years (based on ²¹⁰Pb-dating). Eggs buried more deeply will only be returned to the sediment surface by relatively rare, localized disturbances. This return of old eggs to the surface affects ecological and evolutionary dynamics in a complex way.     

Morphology and development of <Emphasis Type="Italic">Pteromalus cerealellae</Emphasis> (Ashmead) (Hymenoptera: Pteromalidae) on <Emphasis Type="Italic">Callosobruchus maculatus</Emphasis> (F.) (Coleoptera: Chrysomelidae)

Pteromalus cerealellae (Ashmead) (Hymenoptera: Pteromalidae) is an ectoparasitoid of several stored-product insect pests. Very little information has been published on its biology and development in host larvae, which typically are concealed within seeds. We documented the development of P. cerealellae within fourth instar larvae of its concealed host, Callosobruchus maculatus (F.) (Coleoptera: Chrysomelidae) infesting cowpea seeds. The preimaginal life stages of the parasitoid were characterized for the first time using morphological structures revealed by microscopic techniques including scanning electron microscopy. Pteromalus cerealellae produces hymenopteriform eggs and larvae. Eggs hatch into 13-segmented first instar larvae with peripneustic condition of spiracles. The larvae have simple, tusk-like mandibles, whereas the mandibles of the pupae and the adults are of the conventional toothed types. Using statistical analyses of the sizes of the larval mandibles and head capsules in conjunction with reliable characters such as the number of exuviae on the body of parasitoid larvae, cuticular folding, and excretion of the meconium, we recorded four larval instars for P. cerealellae. The data showed significant positive correlations between larval mandible lengths and widths of larval head capsules, as well as between mandible lengths and larval instars, suggesting that mandible length is a good predictor of the number of instars in P. cerealellae. Developmental time from egg to adult emergence was ∼12 d for females and ∼11 days for males at 30 ± 1°C, 70 ± 5% r.h. and 12L:12D photoperiod.     

Fluctuation of the zooplankton community in Lake Biwa during the 20th century: a paleolimnological analysis

Detailed zooplankton records from a 26-cm sediment core with a time resolution of approximately 3–10 years were obtained from Lake Biwa, Japan, to examine the historical variations in the zooplankton community during the 20th century. In the sediments, selected zooplankton remains have fluctuated over the years. Daphnia – large zooplankton herbivores – did not occur from 1900 to 1920, and formed a very minor component of the zooplankton community in the following 30 years, while Bosmina – small zooplankton herbivores – were common during this period. In the mid-1960s, however, when eutrophication was noticeable in this lake, Daphnia numbers increased dramatically and became the dominant zooplankton thereafter. In contrast, Difflugia brevicolla and D. biwae, two amoeboid protozoans that live in connection with the lake bottom environment, occurred abundantly until the late 1950s, but gradually decreased after the mid-1960s. In particular, D. biwae, a species peculiar to this lake, was not found in sediment dated after 1980, suggesting its extinction. These results indicate that the zooplankton community structure changed greatly in the 1960s, and suggest that the eutrophication occurring at this time altered the relative strength of top-down and bottom-up forces on the zooplankton community in Lake Biwa.     

Ecological sensitivity of marl lakes to nutrient enrichment: evidence from Hawes Water,UK

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Major changes in trophic dynamics in large, deep sub-alpine Lake Maggiore from 1940s to 2002: a high resolution comparative palaeo-neolimnological study

1. We describe the changes in trophic dynamics in Lake Maggiore from c. 1943 to 2002 using subfossil cladoceran data from a high resolution sediment record, long‐term contemporary data series and historical information. During this period the lake went through a eutrophication phase until 1980 followed by oligotrophication. 2. During the eutrophication period a major increase occurred in the abundance of Chydorus sphaericus, the proportion of planktonic cladocerans and total abundance of cladocerans in the sediment. Since 1980 the abundance declined again and subfossil Eubosmina mucro length and contemporary Daphnia body length increased, most probably as a result of higher abundance of invertebrate predators. 3. Changes in the fish stock composition caused by the introduction of exotic fish during the pre‐eutrophication period and a complete ban on fishing because of Dichloro‐diphenil‐ethanes (DDTs) pollution of the lake (during oligotrophication) could also be detected in the community assemblage and size structure of the sediment zooplankton. 4. We found good correspondence between trophic changes inferred from cladoceran subfossils (community composition, size and predation pressure) and contemporary data, suggesting that sediment samples can be used to infer past development in trophic dynamics, including predation by fish and pelagic invertebrates in lakes with scarce neolimnological data. 5. Furthermore, by combining palaeolimnological cladoceran data rarely obtained from contemporary samples (e.g. benthic and plant‐associated cladocerans, mucro length of bosminids) with contemporary data of organisms poorly represented in the sediment record (e.g. remains of Bythotrephes and fishes) a more complete understanding of changes in trophic dynamics was obtained. 6. The detection in the sediments of meteorological events whose effects on zooplankton had been recorded in the long‐term studies also provided evidence that eutrophication tends to override climate signals. 7. We conclude that a combined palaeo‐neolimnological approach can be a powerful tool for elucidating past changes in the trophic dynamics of lakes and the interaction with climate induced changes, not least when high resolution sediment records are available.     

Information value of chironomid remains in the uppermost sediment layers of a complex lake basin

The subfossil remains of chironomids were analyzed from four short (35–50 cm) sediment cores in eutrophicated Lake Vanajavesi, southern Finland. The chironomid analysis was found to be a useful palaeolimnological technique for indicating the following aspects: (1) The pollution history of the lake during the last 150 years. A succession from an oligotrophic Micropsectra-Monodiamesa community, through eutrophic Chironomus communities, to the disappearance of the chironomid fauna, was detected. (2) The bottom dynamic conditions at the sampling site. (3) The water level changes of the lake. (4) The significance of bioturbation in the core chronology and stratigraphy. The disappearance of the chironomid remains in the cores was correlated with the occurrence of annually laminated sediment.The most important factor disturbing the interpretation of the cores was the redeposition of head capsules by erosion and transportation. Detailed identification of the remains is necessary to avoid the misinterpretations caused by redeposition. Bioturbation reduces the time resolution of the cores. It is necessary to use dating methods which take the effect of bioturbation into account.     

摇蚊幼虫对底泥中氮、磷释放作用的研究

本文研究了摇蚊幼虫对底泥中氮、磷释放的影响,初步探讨了摇蚊在湖泊富营养化过程中的生态作用。结果表明,摇蚊幼虫能明显促进底泥中氮、磷的释放,而释放到水层中的氮、磷又容易被藻类吸收利用,从而促进藻类生长。这种生态效应与水体营养循环和富营养化的发生及发展过程间存在着重要的关系。     

The influence of the diel activity pattern of the larvae of Sericostoma personatum (Kirby & Spence) (Trichoptera) on organic matter distribution in stream-bed sediments — a laboratory study

Diel patterns in mobility and feeding behaviour of the larvae of the stream-dwelling trichopteran Sericostoma personatum larvae were investigated. Larvae fed at night on coarse particulate organic matter (CPOM) at the sediment surface. In the daytime they rested a few cm below the sediment surface, during which time their defaecation activity effected a release of fine particulate organic matter (FPOM) into the sediment. The amount of faeces (mean particle size = 0.1 ± 0.044 mm, x ± SD, n = 500) introduced into the sediment by the larvae, evaluated in two experiments, was 0.4–0.56 mg day^–1. This amount did not differ significantly from the organic input resulting from bacterial activity (0.36–0.64 mg day^–1). The presence of S. personatum larvae increased the sediment organic content by 42.9 mg (75.8 %) and 59.8 mg (185.6%) AFDW per 16 cm³ sediment over a 90-day period, as compared with control systems containing no larvae.     

Matching thirty years of ecosystem monitoring with a high resolution microfossil record

1. Monitoring of the ecosystem of Lake Mývatn, Iceland, since 1975 has revealed extreme fluctuations in important food web components, such as chironomids and cladocerans, with amplitudes of several orders of magnitude and a period of 5–8 years. This study uses sediment cores from the lake to examine if the food web fluctuations appear in the microfossil record of the sediment. 2. Dating was achieved by means of a combination of ¹³⁷Cs and volcanic tephra and was fine‐tuned by wiggle‐matching of chironomid microfossil and monitoring data. 3. Cladocera exuviae and chironomid egg capsules in the uppermost 34 cm of sediment were compared with the monitoring record that consisted of 30 years of window trap catches of flying chironomids and a 16‐year record of chydorid Cladocera caught in activity traps. 4. The observed chironomid and cladoceran population fluctuations were reflected in the sediment record of chironomid eggs and of the exuviae of three of seven cladocerans: Alonella nana, Alona rectangula and Eurycercus lamellatus, which also had the most extreme fluctuations in the monitoring data (3–4 orders of magnitude). Chydorus sphaericus, and to some extent Alona quadrangularis and Acroperus harpae, showed regular fluctuations in the core that the monitoring did not reveal. Density of subfossil chironomid eggs correlated positively with that of larval head capsules but not with other microfossils. 5. This study shows a reasonably good correspondence between the fossil records of chironomids and cladocerans on the one hand and biomonitoring data on the other. The results pave the way for an extension of the food web history to much earlier time intervals of the ecosystem, allowing the study of long‐term variation in the food web dynamics, including the impact of climatic variation and other external forcing. The results also indicate the usefulness of chironomid egg capsules in palaeolimnological studies.     

Long- and short-term Changes of the Macrophyte Vegetation in Strongly Stratified Hypertrophic Lake Verevi

The aim of study was to bring out changes in the macrophyte vegetation, caused by eutrophication, short-term lowering of the water level and the following restoration of equilibrium in L. Verevi. Also biomass and N and P content of shoots of main submergent species were studied in 1999–2001, to follow the temporal and specific differences. Due to strong eutrophication, the type of the lake changed from a Myriophyllum-Potamogeton-Charophyta lake to a Ceratophyllum-Lemna trisulca lake in 1984–1988, obviously owing to the formation of loose organic-rich sediment. Water lowering by 0.7 m during summer months of 1998 facilitated mineralization of sediments, as a consequence of which a mass development of Ranunculus circinatus and a temporary increase in the abundance and biomass of other nutrient-demanding species took place during following years. Our data suggest differences in nutrient supply and release of submerged species and the need for more species-related approach to this group. The problem of nutrient supply of unrooted plants at the time of stratification arises. Regarding the increase of biomass of Ceratophyllum demersum in second half of summer, we suppose that one part of nutrients for this growth may derive from freshly decayed filamentous algae or vascular plants.     

Changes in fossil chironomid remains along a depth gradient: evidence for common faunal thresholds within lakes

Many environmental variables that are important for the development of chironomid larvae (such as water temperature, oxygen availability, and food quantity) are related to water depth, and a statistically strong relationship between chironomid distribution and water depth is therefore expected. This study focuses on the distribution of fossil chironomids in seven shallow lakes and one deep lake from the Plymouth Aquifer (Massachusetts, USA) and aims to assess the influence of water depth on chironomid assemblages within a lake. Multiple samples were taken per lake in order to study the distribution of fossil chironomid head capsules within a lake. Within each lake, the chironomid assemblages are diverse and the changes that are seen in the assemblages are strongly related to changes in water depth. Several thresholds (i.e., where species turnover abruptly changes) are identified in the assemblages, and most lakes show abrupt changes at about 1–2 and 5–7 m water depth. In the deep lake, changes also occur at 9.6 and 15 m depth. The distribution of many individual taxa is significantly correlated to water depth, and we show that the identification of different taxa within the genus Tanytarsus is important because different morphotypes show different responses to water depth. We conclude that the chironomid fauna is sensitive to changes in lake level, indicating that fossil chironomid assemblages can be used as a tool for quantitative reconstruction of lake level changes.     

Seasonal Vertical Migration and Aestivation of <Emphasis Type="Italic">Rhyacodrilus hiemalis</Emphasis> (Tubificidae,Clitellata) in the Sediment of Lake Biwa,Japan

The vertical distribution of the tubificid worm Rhyacodrilus hiemalis Ohtaka, the numerically dominant species of oligochaete in the littoral of Lake Biwa, was studied with special reference to seasonal vertical migration in the lake sediment. Monthly collections of lake sediment cores were made using PVC tubes. Core sections of sample sediments ranged from 76 to 117 cm. The vertical distribution of the worms showed no diurnal variation; therefore diel vertical migration was not evident. Seasonal downward migration started in April, and upward migration started in October. From December to March, almost all worms remained in the near-surface sediment layer (surface to 30 cm deep), while from July to September almost all worms remained deeper than 30 cm. However, few individuals migrated deeper than 90 cm. No discontinuous layers were found in grain size composition, water content, loss on ignition, particulate carbon, nitrogen or phosphorus. In deep sediment there was no free oxygen, as evidenced by negative ORP values. For 4 months in summer, R. hiemalis aestivated, probably utilizing anaerobic respiration. It appeared that R. hiemalis moved deeper in the sediment in response to sediment temperature, because sediment temperatures in the deep layers seemed to converge at around 20–21 °C in the summer months. The life history traits of seasonal vertical migration and summer aestivation perhaps arose as an adaptation to the climatic conditions accompanying the geographical origin of R. hiemalis, and they also serve to minimize predation risk during summer when most invertebrate predatory fishes are active.     

Harnessing the potential of the multi‐indicator palaeoecological approach: an assessment of the nature and causes of ecological change in a eutrophic shallow lake

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Limnological effects of peatland drainage and fertilization as reflected in the varved sediment of a deep lake

Stratigraphy of diatoms and chemistry in the surface sediment deposited at 35 m depth in Lake Polvijärvi was studied. The existence of annual laminations or varves in the sediment allowed a precise dating of the profile. Diatoms were analysed in 0.5 cm sequences; from 0 to 16.0 cm continuously and then intermittently every fourth 0.5 cm down to 44.0 cm. Sediment chemistry (loss-on-ignition, C, N, Fe, Mn, Mg, P, chlorophyll and carotenoids) was analysed from sediment surface down to 10.5 cm of altogether 33 subsamples, each containing 1–3 varves, and spanning the period 1921–1980. From 4.5 cm depth upwards the diatom concentration strongly increases, and the plankton diatom succession from Tabellaria flocculosa through Asterionella formosa to Melosira ambigua and Fragilaria crotonensis reflects a marked eutrophication of the lake. This algal succession occurs in pace with an increase in sediment accumulation rate and changes in sediment chemistry, which indicate increased allochthonous inputs and enhanced algal production in the lake. The change of the lake ecosystem is contemporaneous with extensive peatland draining and fertilizing that was carried out on its watershed during the past two decades. Existing chemical data from a number of lakes situated within the drainage area prove that at present the treated peatlands are the main source of nutrient loading of Lake Polvijärvi. A former period with indications of slightly increased productivity of the lake was dated by varve counting to AD 1690–1910 (35–12 cm). This period (characterised by Asterionella formosa) may coincide with that of the slash-and-burn cultivation in the area.     

星云湖硅藻群落响应近现代人类活动与气候变化的过程

随着人类活动的增强与全球气候变暖的持续,近年来云南湖泊的生态系统功能持续退化,而目前对云南湖泊生态系统的研究还主要集中于单一环境压力的生态效应。以星云湖为研究对象,通过沉积物记录与现代监测资料,识别在湖泊富营养化、气候变化以及人类强烈干扰下硅藻群落结构响应的过程,并甄别驱动群落变化的主要环境压力及其强度。结果显示随着湖泊生产力水平(如沉积物叶绿素a浓度)的增加,硅藻物种组成发生了明显的变化,主成分分析表明了水体富营养化是驱动群落变化的主要环境因子(r=-0.63,P0.001)。简约模型与方差分解的结果表明近200年来(钻孔长度38cm),湖泊营养水平和水动力是驱动星云湖硅藻群落变化的主要环境因子,分别解释了群落变化的18.8%和2.9%;而1951年以后,湖泊营养水平和温度分别解释了硅藻群落结构变化的31.4%和26.8%。研究结果表明了硅藻群落长期变化的主控因子是湖泊营养水平,而人类活动及气候变化等可以通过改变湖泊水动力及湖水温度来驱动硅藻群落的演替,同时抚仙湖-星云湖的连通性也对硅藻群落的演替产生了一定影响。     

Denitrification Potential in Lake Sediment Increases Across a Gradient of Catchment Agriculture

Intensification of catchment agriculture has increased nutrient loads and accelerated eutrophication in some lakes, often resulting in episodic harmful algal blooms or prolonged periods of anoxia. The influence of catchment agriculture on lake sediment denitrification capacity as a nitrogen (N) removal mechanism in lakes is largely unknown, particularly in contrast to research on denitrification in agricultural streams and rivers. We measured denitrification enzyme activity (DEA) to assess sediment denitrification potential in seven monomictic and three polymictic lakes that range in the proportion of agriculture in the catchment from 3 to 96% to determine if there is a link between agricultural land use in the lake catchment and sediment denitrification potential. We collected sediment cores for DEA measurements over 3 weeks in austral spring 2008 (October–November). Lake Okaro, with 96% catchment agriculture, had approximately 15 times higher DEA than Lake Tikitapu, with 3% catchment agriculture (232.2 ± 55.9 vs. 15.9 ± 4.5 μg N gAFDM⁻¹ h⁻¹, respectively). Additionally, sediment denitrification potential increased with the proportion of catchment in agriculture (R ² = 0.85, P < 0.001). Our data suggest that lakes retain a high capacity to remove excess N via denitrification under increasing N loads from higher proportions of catchment agriculture. However, evidence from the literature suggests that despite a high capacity for denitrification and longer water residence times, lakes with high N loads will still remove a smaller proportion of their N load. Lakes have a denitrification potential that reflects the condition of the lake catchment, but more measurements of in situ denitrification rates across lake catchments is necessary to determine if this capacity translates to high N removal rates.     

Degraded Softwater Lakes: Possibilities for Restoration

In the Netherlands, the characteristic flora of shallow softwater lakes has declined rapidly as a consequence of eutrophication, alkalization and acidification. The sediment of most lakes has become nutrient rich and anaerobic. We expected that, if a vital seed bank was still present, restoration of the original water quality and sediment conditions would lead to the return of softwater macrophytes. The restoration of 15 degraded, shallow, softwater lakes in the Netherlands was monitored from 1983 to 1998. In eutrophied as well as in acidified lakes, removal of accumulated organic matter from the sediment and shores was followed by rapid recolonization of softwater macrophytes present in the seedbank. After isolation from alkaline water and subsequent mud removal, this recovery was also observed in alkalized lakes. Further development of softwater vegetation correlated strongly with the water quality. When renewed eutrophication was successfully prevented, softwater macrophytes could expand. However, in acidified lakes, Juncus bulbosus and Sphagnum species became dominant after restoration. Liming of an acidified lake was followed by re‐acidification within 3 years. Recolonization by softwater macrophytes was inhibited by high turbidity of the water column and spreading of large helophytes on the shore. As an alternative, controlled inlet of alkaline, nutrient‐poor groundwater was studied in a few lakes. The pH of those lakes increased, the carbon and nitrogen availability decreased and softwater macrophytes returned. Successful restoration has contributed considerably to maintaining biodiversity in softwater lakes in the Netherlands.