LETTER TO THE EDITOR

SUMMARY

A two-year limnological survey between 1973 and 1975 showed that the Roodeplaat Dam is a warm monomictic impoundment which has stable thermal stratification during summer. Oxygen supersaturation of the surface waters and anaerobic conditions in the hypolimnion were encountered during summer. Water transparency was seasonal and related to existing phytoplankton populations and allochthonously-introduced clay material. Chemical stratification during summer was prominent and appeared to be caused not only by mineralization processes in the hypolimnion, but also by dilution in the surface layers during summer flooding. Because of the morphometry of the impoundment and the fact that sewage effluents are discharged into the major inflow, considerable horizontal variation in most chemical and biological characteristics was found. On the basis of the nutrient content of the waters and the magnitude of the phytoplankton populations observed, Roodeplaat Dam may be classified as a highly eutrophic water body.     

Carbon sink to source: longitudinal gradients of planktonic P:R ratios in subtropical reservoirs

Spatial patterns of planktonic production and respiration in the surface mixed layer were examined in eight Texas, USA reservoirs to test the hypothesis that P:R ratios are lowest in upreservoir inflow zones and highest in downreservoir open-water zones, as predicted by the heuristic reservoir zonation model. We measured summer planktonic metabolism with light–dark bottles and physical–chemical conditions in epilimnetic water at 85 sites distributed among sixteen longitudinal transects within the eight reservoirs (2 transects per reservoir). Volumetric production and plankton biomass were lowest in the open-water zones and increased upreservoir; however, that pattern was reversed for areal production due to greater photic depths at open-water sites. Volumetric respiration was similar in the three zones; however, corresponding planktonic P:R ratios in the surface mixed layer were significantly lower at open-water sites, which is opposite than hypothesized. Based on linear regressions of production and respiration rates on chlorophyll a, open-water sites were net heterotrophic during the summer regardless of trophic state; whereas inflow and mid-reservoir zone sites were heterotrophic when chlorophyll concentrations were respectively less than 9.5 and 35?mg?m^?3. Although variation among reservoirs was high, five of the eight reservoirs had inflow zones that were net carbon sinks while seven had open-water zones that were carbon sources. Mean (±standard error) carbon flux rates of inflow, mid-reservoir, and open-water zones were ?0.22?±?0.12 (C sink), 0.39?±?0.44 (moderate C source), and 1.33?±?0.50 (strong C source) g C m^?2 day^?1 respectively. Inflow and mid-reservoir zones comprised approximately 45% of the total reservoir area studied. Therefore, omitting their contribution as often done when a single open-water site is sampled may substantially overestimate reservoir carbon flux.     

Species-Specific Effects on Throughfall Kinetic Energy in Subtropical Forest Plantations Are Related to Leaf Traits and Tree Architecture

Soil erosion is a key threat to many ecosystems, especially in subtropical China where high erosion rates occur. While the mechanisms that induce soil erosion on agricultural land are well understood, soil erosion processes in forests have rarely been studied. Throughfall kinetic energy (TKE) is influenced in manifold ways and often determined by the tree’s leaf and architectural traits. We investigated the role of species identity in mono-specific stands on TKE by asking to what extent TKE is species-specific and which leaf and architectural traits account for variation in TKE. We measured TKE of 11 different tree species planted in monocultures in a biodiversity-ecosystem-functioning experiment in subtropical China, using sand-filled splash cups during five natural rainfall events in summer 2013. In addition, 14 leaf and tree architectural traits were measured and linked to TKE. Our results showed that TKE was highly species-specific. Highest TKE was found below Choerospondias axillaris and Sapindus saponaria, while Schima superba showed lowest TKE. These species-specific effects were mediated by leaf habit, leaf area (LA), leaf pinnation, leaf margin, stem diameter at ground level (GD), crown base height (CBH), tree height, number of branches and leaf area index (LAI) as biotic factors and throughfall as abiotic factor. Among these, leaf habit, tree height and LA showed the highest effect sizes on TKE and can be considered as major drivers of TKE. TKE was positively influenced by LA, GD, CBH, tree height, LAI, and throughfall amount while it was negatively influenced by the number of branches. TKE was lower in evergreen, simple leaved and dentate leaved than in deciduous, pinnated or entire leaved species. Our results clearly showed that soil erosion in forest plantations can be mitigated by the appropriate choice of tree species.     

Macroinvertebrate Community Structure in Three‐Gorges Reservoir,China

We investigated the macroinvertebrate community structure in Three‐Gorges Reservoir during a three‐year period after the reservoir became operational. Comparison with data of previous authors obtained before the damming showed that the benthic community changed drastically in the reservoir. Oligochaetes and chironomids dominate the present community. An apparent annual cycle in the benthic community (expressed as richness, density and community type) was found in the second year, lagging one year behind the annual cycle of transparency. After the second year, a Nais–Polypedilum community type, occurring in winter and spring with low inflow discharge and high transparency, was followed by a Limnodrilus community type that occurred in autumn and summer under contrasting conditions. Despite river regulation and damming, it appeared that macroinvertebrates in subtropical canyon‐shaped reservoirs remained influenced by floods. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Repartition spatio-temporelle,biomasse et production des populations zooplanctoniques du lac d'aydat en periode estivale

Lake Aydat is a young eutrophic lake (mean depth 7.4 m, area 6o ha) situated in the french Central Massif. Secondary production was studied during the summer months June and July. In early summer, dissolved oxygen in the deep layers gradually gets depleted, and depletion reaches the 7m level in July. Zooplankton is therefore concentrated in the upper layers: Rotatoria are very abundant and the various species are segregated in space and / or succeed each other in time, thus maximally exploiting the water layers favorable to their development. Cladocera are represented by small species, which sharply increase in numbers at the time of maximum primary production, and cause a dramatic drop in primary production. They are in competition with the Rotatoria. Copepods are the least abundant group, but they were in a reproductive phase at the time of this study.88% of the Zooplankton biomass is accumulated in the Cladocera; the Rotifera represent 9%, and the Copepods 8%. Secondary production is intense, and due almost exclusively to filter-feeders in July. Since the carnivores are scarce in numbers, their maximum production is poor relative to that of the herbivores.The P/B coefficient was extremely high during the two summer months, and is indicative of a process of intense consumption of the resources of the lake during that particular short period of the year.     

The effect of hydropower on fish stocks: comparison between cascade and non-cascade reservoirs

Spatial distributions of fish were studied in two types of reservoirs by means of hydroacoustics during the summer. Different patterns of fish distribution were found in non-cascade (Římov, Želivka) and cascade (Kamyk, Slapy, Štěchovice, Vrané) reservoirs. Maximum biomass and density of fish were observed in the tributary area of the non-cascade reservoir. The biomass declined towards the dam area. Average weight of fish showed the opposite trend—maximum average weight was observed in the dam area and declined towards the tributary area. In the cascade reservoirs fish distribution was found to be more complicated but the maximum biomass and density of fish were observed in the dam areas, whereas the tributary areas were nearly fishless. Poor ecohydrological conditions in the tributaries of cascade reservoirs are likely to be responsible for the low abundance of fish in these areas. This was evident in the example of Vrané reservoir which has two major inflows. Low biomass and density of fish were observed in the cold and low oxygen saturated inflowing cascade Vltava River while higher biomass and density were observed in the warm and well oxygen saturated Sázava River inflow. Average weights of fish followed the same trend as in non-cascade reservoirs—the maximum was in the dam area and it declined towards the tributary area. The vertical distribution of fish is driven by the fully developed stratification of temperature and oxygen during the summer season in non-cascade reservoirs. A significantly lower biomass of fish was observed below the thermocline in vertical surveys in both non-cascade and cascade reservoirs. Summer stratification in cascade reservoirs is weakly developed or is not developed at all due to hypolimnetic releases of cold water and short retention times. High transparency suggests lower primary production in cascade reservoirs. Guest editors: R. L. Welcomme & G. Marmulla Hydropower, Flood Control and Water Abstraction: Implications for Fish and Fisheries     

Possible spreading of toxic Alexandrium tamarense blooms on the Chukchi Sea shelf with the inflow of Pacific summer water due to climatic warming

A high abundance of resting cysts of the toxic dinoflagellate Alexandrium tamarense was recently reported in the vast continental shelf of the Chukchi Sea in the Arctic Ocean, suggesting that the species is widespread in the shelf. Nevertheless, little is known about the occurrence of A. tamarense vegetative cells in the water column of the arctic. Sea ice reduction and the inflow of Pacific summer water (PSW) through the Bering Strait have recently increased owing to warming in the shelf. To determine the spatial and temporal distributions of A. tamarense in the Chukchi Sea shelf and their relationship to the inflow of PSW, field samplings were conducted in the Chukchi Sea and north Bering Sea shelves three times during the summer of 2013 from July to October. Vegetative cells of A. tamarense was detected in both shelves at all sampling periods with a maximum density of 3.55 × 10³ cells L⁻¹. This species was also observed at the station at 73°N, indicating the northernmost record of this species to date. The center of the A. tamarense distribution was between the north Bering and south Chukchi Sea shelf during the first collection period, and spread to the north Chukchi Sea shelf during the second and third collection periods. The species occurrences were mainly observed at stations affected by the PSW, especially Bering shelf water. Water structure of PSW was characterized by warmer surface and bottom water temperatures, and increased temperatures may have promoted the cell growth and cyst germination of A. tamarense. Therefore, it is suggested that an increase in the PSW inflow owing to warming promotes toxic A. tamarense occurrences on the Chukchi Sea shelf.     

Seasonal inter‐calibration between acoustic and multi‐mesh gillnets sampling for fish biomass assessment in reservoirs

This paper presents the results of an inter‐calibration between acoustic and gillnets sampling in two North African reservoirs according to seasons. Gillnets with multi‐mesh were designed for sampling fish in lakes while acoustic surveys were performed with a split beam Simrad EK60 echosounder. Sampling events were carried out during summer (September 2015), autumn (December 2015), winter (March 2016) and spring (June 2016) in two Tunisian reservoirs (Kasseb and Siliana) with different depths and shapes. Gillnet catches showed a high proportion of barbell whatever the seasons in Kasseb Reservoir, while at Siliana Reservoir, significant seasonal changes in relative abundances have been evidenced. The highest fish biomass of the entire water column was observed in winter daytime (103 kg/ha) in Kasseb Reservoir and in summer daytime (283 kg/ha) in Siliana Reservoir. Average biomass observed in autumn (December) for the two reservoirs were lower than the other seasons. During spring, density daytime values in Kasseb were higher than during nighttime while it was the opposite in summer and autumn. Fish densities detected in Kasseb Reservoir in vertical beaming was higher than those detected in horizontal beaming (p < .05). The longitudinal distribution of fish in the reservoirs showed that there is no clear trend in fish densities according to strata. High numbers of fish were detected in deep strata and big fish were located in the surface water near the dam of Kasseb Reservoir. A significant linear correlation was showed between acoustic density/NPUE and acoustic biomass/BPUE but the perfect correlation with the 1:1 fit was showed only between acoustic biomass/BPUE.     

Effect of Tillage and Water Management on the Population Behaviour of Root-Knot Nematodes Meloidogyne Triticoryzae in Rice Crop

The effects of puddling and water regimes on hydraulic conductivity (cm/day) of soil and on bulk density (mg/m 3 ) during rice culture, soil physical characteristics of the experimental field population densities of plant-parasitic nematodes have been studied. Puddling reduced the bulk density of soil and decreased the hydraulic conductivity in the upper layers but not in the deeper layers of soil aeration was reduced due to high moisture levels retained in the puddled soil. Population density of M. triticoryzae declined in puddled soil. The invasion of the roots by the second-generation infective juveniles was reduced. The population density of the root-knot nematodes was higher in the non-puddled soil especially in unsubmerged condition compared to puddled and submerged soil. However, if the seedlings were already infected before transplanting and submergence, the nematode could survive well and reproduce within the aerenchyma of the root.     

Density dependent growth in adult brown frogs Rana arvalis and Rana temporaria – A field experiment

In species with complex life cycles, density regulation can operate on any of the stages. In frogs there are almost no studies of density effects on the performance of adult frogs in the terrestrial habitat. We therefore studied the effect of summer density on the growth rate of adult frogs during four years. Four 30 by 30 m plots in a moist meadow were used. In early summer, when settled after post-breeding migration, frogs (Rana arvalis and Rana temporaria that have a very similar ecology and potentially compete) were enclosed by erecting a fence around the plots. Frogs were captured, measured, marked and partly relocated to create two high density and two low density plots. In early autumn the frogs were again captured and their individual summer growth determined. Growth effects were evaluated in relation to two density measures: density by design (high/low manipulation), and actual (numerical) density. R. arvalis in plots with low density by design grew faster than those in high density plots. No such effect was found for R. temporaria. For none of the species was growth related to actual summer density, determined by the Lincoln index and including the density manipulation. The result suggests that R. arvalis initially settled according to an ideal free distribution and that density had a regulatory effect (mediated through growth). The fact that there were no density effects on R. temporaria (and a significant difference in its response to that of R. arvalis) suggests it is a superior competitor to R. arvalis during the terrestrial phase. There were no density effects on frog condition index, suggesting that the growth rate modifications may actually be an adaptive trait of R. arvalis. The study demonstrates that density regulation may be dependent on resources in frogs' summer habitat.     

Chironomus species (Diptera: Chironomidae) in the profundal benthos of Spanish reservoirs and lakes: factors affecting distribution patterns

• 1 The taxonomy, distribution and habitat preferences of Chironomus larvae from profundal benthos of reservoirs and natural lakes in Spain are presented. The 114 water bodies studied were medium to large reservoirs and lakes throughout Spain, and were visited on at least two occasions (summer and winter). Out of the natural lakes investigated, Chironomus larvae were found only in one karstic lake (Banyoles), and to date, these have not been found in the high mountain lakes of the Spanish Pyrenees or Sierra Nevada, or in Spain's largest oligotrophic lake (Sanabria).
• 2 Multiple regression with environmental data from reservoirs showed that the density of Chironomus larvae was inversely correlated with water depth, temperature and sulphide content of the bottom water, but positively correlated with alkalinity and particulate organic nitrogen (PON) content of bottom water. Thus, the density of Chironomus larvae was higher in shallow reservoirs from the eastern part of Spain (calcareous water and higher PON), but markedly lower in southern reservoirs (higher temperature and higher sulphide).
• 3 According to morphological and cytological characteristics, three species of this genus have been identified in the samples examined: C. bernensis, C. plumosus and C. nuditarsis. Reasons for the absence of other widespread European species are discussed.
• 4 Using canonical correspondence analysis to relate species and environmental variables, C. bernensis showed a significant preference for reservoirs with higher oxygen concentrations in bottom water and lower total reactive phosphorus (TRP) content at the surface than those reservoirs where C. plumosus‐type were found. This suggests that the smaller C. bernensis prefers mesotrophic reservoirs and the larger C. plumosus‐type (C. plumosus + C. nuditarsis) prefers eutrophic reservoirs. Taking into account the TRP/depth index for those samples collected at depths below the thermocline, and thus, where oxygen can be a limiting factor, C. bernensis and C. nuditarsis were present in reservoirs in which the index was low (< 1:170 for C. bernensis and >1:800 for C. nuditarsis), while C. plumosus thrived in reservoirs with higher index values (<1:20). These differences may reflect the preference of C. bernensis and C. nuditarsis for detrital material, while C. plumosus prefers phytoplankton. These observations are in agreement with the depth distribution of each species: C. plumosus was restricted to reservoirs less than 46 m deep, whereas the other two species (i.e. C. bernensis and C. nuditarsis) were found up to depths of 65 and 80 m, respectively.
• 5 Data from a transect (5–20 m deep) in Lake Banyoles are consistent with the general pattern described in reservoirs. In this lake, a high density of C. bernensis was recorded from littoral and sublittoral areas with abundant oxygen (above or close to the thermocline), whereas C. plumosus was scarce in several parts of the lake, especially in oxygen‐deficient areas during the summer. This karstic, oligotrophic lake has a low TRP/depth index, and therefore, we expected C. bernensis to be dominant in the littoral, as is the case. Nevertheless, the severe depletion of oxygen caused by meromixis, and the high hydrogen sulphide concentration benefit C. plumosus in deeper zones. Chironomus nuditarsis was scarce in Lake Banyoles and only found at considerable depths.

     

Investigations on fecundity of Bythotrephes longimanus in Lake Lucerne (Switzerland) and on Niche Segregation of Leptodora kindti and Bythotrephes longimanus in Swiss lakes

In Lake Lucerne, Switzerland, the predaceous cladocerans Leptodora kindti and Bythotrephes longimanus segregate along spatial and temporal dimensions. In spring (April–May/June), Bythotrephes longimanus occurs below 0–20 m, while Leptodora is absent. In summer and early autumn (July–September/October), when Leptodora dominates during daytime in the 0–20 m depth, Bythotrephes longimanus also lives in deeper zones. Food competition and fish predation pressure may be the cause of differences in ecology of Leptodora and Bythotrephes acquired during evolution. Due to its transparency and tolerance of higher temperature, Leptodora could avoid fish predation and, therefore, competes with Bythotrephes longimanus successfully. In addition, the differences between the two species may account for the spatial and temporal niche segregation in oligotrophic Swiss Lakes. But spatial niche segregation is less important in mesotrophic lakes with high prey density than in oligotrophic lakes with low prey density. In small, eutrophic lakes importance of temporal niche segregation also decreases, and Bythotrephes is seldom or not present. The preference of Bythotrephes to live in deeper water to avoid fish predation during summer may be the cause of its difficulties to establish itself in small and eutrophic lakes with high prey densities, where the hypolimnion is missing or anoxic.In the spring, Bythotrephes exhibits r-strategy (smaller body size and a higher fecundity), the female is already fertile after the first molt. In the summer, a K-strategy prevails (larger body length and lower fecundity than in the spring), and female Bythotrephes are fertile only after the second molt. Shortage of prey (biomass of Bosmina and Daphniadecreased after June especially in the surface layers) and the maximum fish predation pressure in summer may change the life strategy of Bythotrephes: while fecundity decreases from generation to generation, body length increases. Enhanced prey densities (e.g. during mesotrophic conditions in L. Lucerne) lead to larger individuals in summer and autumn.     

Vertical distribution of pelagial zooplankton in a middle-sized dimictic valley reservoir

Our observations indicate the vertical distribution of zooplankton and its seasonal changes in Dubník II reservoir (Slovakia) are determined mainly by the thermal regime of the reservoir, by transparency, and by fish and invertebrate predation. During periods of circulation, zooplankton vertical distribution in the whole water column was more homogeneous, whilst during summer temperature stratification zooplankton concentrated in the epilimnion — rotifers in higher layers than crustaceans. During summer stagnation a steep thermal gradient occurred at the boundary of the epi-and hypolimnion and low temperature and low dissolved oxygen in hypolimnion offered a refuge for Chaoborus flavicans larvae against fish, enabling coexistence of vertebrate and invertebrate predation. This evidence supports our previous findings concerning dominance of rotifers in zooplankton and representation of crustaceans by small-bodied species in the study reservoir. Steep thermal gradient and the presence of Chaoborus larvae caused very low zooplankton abundance in the lower part of the water column and a reduction of cladocerans refuges against fish to layers of thermocline or closely under thermocline where Daphnia cucullata and Daphnia parvula were found. Our previous assumptions about the high density of zooplanktivorous fish in Dubník II reservoir are supported by the fact that these small cladocerans are represented by smaller individuals in the upper layers and bigger individuals in deeper layers.     

THE LIMNOLOGY OF SOME SOUTH AFRICAN IMPOUNDMENTS I. THE PHYSICO-CHEMICAL LIMNOLOGY OF HARTBEESPOORT DAM

SUMMARY

Hartbeespoort Dam is a eutrophic, warm monomictic lake with overturn occurring in April. The lake is stratified for about six months of the year and in late summer the stratification appears stable. Light penetration in the dam depends both on the inflow of turbid water as well as on the magnitude of the algal population. In summer the surface waters are often super-saturated with oxygen, whilst the hype limnion is anaerobic. The concentrations of nitrogen compounds, phosphate, Na, K, Ca, CI and SO₄ are higher in waters entering the dam via the Crocodile River. Nitrogen compounds arc the primary growth-limiting nutrients for algal growth and phosphate is the secondary growth-limiting nutrient. Phosphate concentrations in the water have increased approximately a hundredfold since 1928 and NO₃ ?N concentrations about threefold. The pH as well as the mineral content of the dam have also increased over the last 50 years.     

广东省6座水库及其入库河流底栖动物调查与综合评价

2010年8月对广东省6座水库及其入库河流的大型底栖无脊椎动物群落进行采样调查,并分别利用Goodnight-Whitley指数和BPI指数对水库及入库河流的水质进行评估。结果表明,水库共检出底栖动物13种,优势种为霍甫水丝蚓(Limnodrilus hoffmeisteri)、苏氏尾鳃蚓(Branchiura sowerbyi)和八斑多足摇蚊(Polypedilum masudai),种类丰富度变动范围为2～5种,平均密度为277ind/m²,变动范围160～480ind/m²。入库河流的种类丰富度和密度均高于水库,共检出底栖动物28种,优势种为苏氏尾鳃蚓、霍甫水丝蚓、花翅前突摇蚊(Procladius choreus)和河蚬(Corbicula fluminea),种类丰富度变动范围为2～7种,底栖动物平均密度为382ind/m²,变动范围192～816ind/m²。水质评价结果表明,6座水库的水质状况总体良好,但入库河流均受到不同程度的污染,污染较为严重的黄岗河和黄羌河,其水库汤溪水库和公平水库也产生了污染。     

Modeling the potential distribution of the predator of honey bees,Palarus latifrons,in the Arabian deserts using Maxent and GIS

ObjectivesThe species Palarus latifrons (bee pirates) has been recorded in Saudi Arabia as an invasive species. This pest can destroy honey bee colonies under certain conditions. The origin of this species in Africa and it has a good ability to adapt to desert conditions. Studies on this species are very few but its current distribution in the Arabian deserts is mainly in the Arabian Gulf countries. This study presents maps for the possible expansion of this species to invade other desert areas in the Arabian countries’ under current and near-future conditions (2030).MethodsThis pest is a solitary insect with high activity during summer. It is hypothesized that summer conditions and especially temperature are the limiting factor for its distribution in the deserts. The analysis depended on generating maps based on temperatures during summer and based on two bioclimatic factors. Maxent and the geographical information system (GIS) were used to perform the analysis.Results and conclusionsAll maps showed the high ability of this pest to spread in the Gulf countries. In North Africa: south Egypt and Libya, and some parts of Algeria showed suitability for Palarus. The invasion of this pest towards North Africa can happen mostly due to trading activities with Gulf countries especially materials containing soil. Continues monitoring for the activity of Palarus in the risk areas is highly advised.     

Leaf anatomical and photosynthetic acclimation to cool temperature and high light in two winter versus two summer annuals

Acclimation of foliar features to cool temperature and high light was characterized in winter (Spinacia oleracea L. cv. Giant Nobel; Arabidopsis thaliana (L.) Heynhold Col‐0 and ecotypes from Sweden and Italy) versus summer (Helianthus annuus L. cv. Soraya; Cucurbita pepo L. cv. Italian Zucchini Romanesco) annuals. Significant relationships existed among leaf dry mass per area, photosynthesis, leaf thickness and palisade mesophyll thickness. While the acclimatory response of the summer annuals to cool temperature and/or high light levels was limited, the winter annuals increased the number of palisade cell layers, ranging from two layers under moderate light and warm temperature to between four and five layers under cool temperature and high light. A significant relationship was also found between palisade tissue thickness and either cross‐sectional area or number of phloem cells (each normalized by vein density) in minor veins among all four species and growth regimes. The two winter annuals, but not the summer annuals, thus exhibited acclimatory adjustments of minor vein phloem to cool temperature and/or high light, with more numerous and larger phloem cells and a higher maximal photosynthesis rate. The upregulation of photosynthesis in winter annuals in response to low growth temperature may thus depend on not only (1) a greater volume of photosynthesizing palisade tissue but also (2) leaf veins containing additional phloem cells and presumably capable of exporting a greater volume of sugars from the leaves to the rest of the plant.     

Population recovery of the sea urchin Lytechinus variegatus in a seagrass flat (Araruama Lagoon,Brazil): the role of recruitment in a disturbed environment

Summary

The size, structure, and density of a population of Lytechinus variegatus were recorded from a shallow seagrass bed (Halodule wrightii) at 40-day intervals from April 1994 to April 1995. A local mass mortality was observed in May 1994. The density declined from 0.57±0.10 to 0.06±0.05 ind.m^?2, mean size declined from 37.6±0.49 to 16.2±2.14 mm and maximum size from 75 to 40 mm. Ten months after mortality, the maximum and mean sizes had returned to initialvalues, but density had not (0.10±0.03 ind.m^?2). A second, smaller mass mortality occurred in April 1995. High values of gonad index were recorded before mortality (9.40±5.97) and after population recovery (11.96±5.74). Recruitment was continuous throughout the year, but reached a peak from August to October. This can he related to the patterns of upwelling in the region. Mass mortality is probably an annual event in this population that can be correlated with the occurrence of low tides during daytime in the fell. The population can recover during the summer when low tides occur during the night. The massive input of recruits prevents extinction of the population during the season of high mortality. Population recovery depends on the magnitude of recruitment.     

Long-term changes in the community of planktonic crustaceans in Lake Glubokoe in relation to predation and competition

Studies on the planktonic Crustacea in Lake Glubokoe (Moscow region) from 1897 to 1982 have been reviewed. After 1965, when most of the swamp water inflow was diverted from the lake, certain populations have penetrated into deeper layers, while the average overlap in the community has dropped. The summer dominant Mesocyclops leuckarti disappeared and Chydorus sphaericus appeared in mass quantities; this development was not associated with biomass changes of blue-greens. The Cladocera/Calanoida ratio and the relative abundance of certain species showed no considerable changes. The effect of Chaoborus, Cyclops and Leptodora in the 70's on the crustacean populations was insignificant in spite of the increasing abundance of these genera after 1965. Analysis of the dynamics of the clutch size and the density of lake populations in the 1970's and 1980's revealed food limitation and competition in herbivorous Cladocera. In laboratory experiments with lake water they always competed, though the intensity of their interaction varied depending on the species combinations.     

Seasonal changes and population dynamics of the ctenophore Mnemiopsis leidyi after its first year of invasion in the Kiel Fjord, Western Baltic Sea

We analyzed the seasonal variations of the ctenophore Mnemiopsis leidyi weekly collected since its first record in the western Baltic Sea in October 2006. The distribution pattern together with the seasonal dynamics and population outbreaks in late summer 2007 indicate recent successfully establishment of M. leidyi in this area. Seasonal changes showed two periods of high reproductive activity characterized by a population structure dominated by small size classes, followed by an increase of larger ones. These results further revealed that the bulk of the population remains in deep layers during the periods of low population density, whereas it appeared situated in upper layers during the proliferation of the species. We further emphasized the strength of the population outbreaks, which can reach abundances >10-fold higher in time periods shorter than a week. The predatory impact this species may have in pelagic ecosystems warns on the importance of its recent range of expansion.