Potential toxicity of the freshwater Chrysochromulina species C. parva (Prymnesiophyceae)

A mass development of Chrysochromulina parva Lackey — 614 000 cells per ml, associated with fish mortality, is reported from a small Danish lake. None of the analyses performed showed any reason for the fish kill. Even oxygen conditions were favourable. Thus toxins from the Chrysochromulina might be responsible for the fish kill. If true, this would be the first record of toxicity in a freshwater Chrysochromulina.     

Spatial patterns and population dynamics of plant-associated microcrustacea (Cladocera) in an English shallow lake (Little Mere, Cheshire)

Little Mere (Cheshire) is a small (2.7 ha) and shallow (average depth 0.7 m) fertile lake in Cheshire, UK. Nymphaeids cover almost 40 % of its entire surface during the growing season (April to October) and practically all the rest is covered by a mixed community of submerged plants. The lake was intensively sampled for plant-associated Cladocera and zooplankters from April 1998-April 2000. Samples were collected at five sites across the lake, three of them located within lily beds, the other two over submerged plant beds of mixed composition. Specific sampling techniques were developed for floating lily leaves, petioles, submerged plants and water. Significant horizontal differences were identified for most cladoceran species, both open-water and plant-associated, for chydorid periphyton scrapers and for filter-feeders. Daphnia hyalina (L.) and Ceriodaphnia sp were significantly more abundant in lily beds than in more open water in both growing seasons, suggesting lily beds are an effective refuge against fish predation. Size-structure and egg-ratio data support this contention. Egg-ratio models were examined for Daphnia hyalina and Simocephalus vetulus (O.F. Müller), a plant-associated cladoceran. The fertility of S. vetulus in lily beds was generally high throughout growing seasons. The construction of egg-ratio models for this species was hampered by their generally very patchy distributions.     

Weather-driven ecology of planktonic diatoms in Lake Tovel (Trentino,Italy)

The seasonal development of planktonic diatoms in Lake Tovel has been studied from 2002 to 2004 within the SALTO research project. The objective was to investigate the role of weather conditions and lake hydrology in regulating the development of Cyclotella and Fragilaria functional species groups. The different responses of the two species to the environmental variability and their reciprocal relationships were considered. Cyclotella showed a weak relationship with the selected environmental variables and its seasonal development was difficult to interpret, also in relation to its opportunistic behaviour. On the contrary, Fragilaria complex showed a prompt response to environmental variability and its seasonal development appeared to be mainly regulated by hydrological conditions, through nitrate nitrogen and, secondarily, by silica concentrations. Water temperature and thermal stability affected only Fragilaria species, while different nutrient requirements allowed Cyclotella species to grow under conditions of limited resources for Fragilaria. Since Lake Tovel has scarce internal nutrient reserves and its hydrology is strongly affected by precipitation both in winter and summer, diatom phenology seems to be indirectly regulated by weather conditions.     

Decline of a sylvatic focus of Schistosoma mansoni in Guadeloupe (French West Indies) following the competitive displacement of the snail host Biomphalaria glabrata by Ampullaria glauca

Summary Grand Etang lake in Guadeloupe is a focus of intestinal schistosomiasis characterized by the virtual absence of human contact and the presence of a heavily infested rat population. The lake, with a surface area of 0.03 km², is situated in rain forest at an altitude of 450 m. Originally it contained an extensive belt of the floating plant Pistia stratiotes which provided a favourable habitat for Biomphalaria glabrata, the intermediate host snail of Schistosoma mansoni. In 1974 and 1975, a low prevalence (about 3%) of S. mansoni was observed in B. glabrata while a high prevalence (about 60%) was recorded in rats. The rat population was infected with another trematode, Ribeiroia guadeloupensis, which requires two intermediate hosts, B. glabrata and the fish Tilapia mossambica. In 1976, the prosobranch snail, Ampullaria glauca, was accidentally introduced into the lake. By 1980, this snail had spread over half of the lake and had caused marked regression of the P. stratiotes layer. Concomitantly, the population of B. glabrata declined to such a point that in 1985 only a small colony remained in the northern section of the lake. The consequences of these changes are reflected in a rapid decline of S. mansoni in the rat population from 61.3% in 1980 to 34.8% in 1981, 6.5% in 1982, 2.8% in 1983, 11.6% in 1984, and 0% in 1985 and 1986.     

Distribution of Gammarus lacustris Sars (Amphipoda, Gammaridae) in Lake Shira (Khakasia, Siberia) and laboratory study of its growth characteristics

Spatial distribution of Gammarus lacustris in Lake Shira and growth of young specimens that feed on lake biota were studied. The amphipods have been shown to inhabit the littoral, sublittoral and the upper aphytal zones of the lake on stony-sandy soil and silted sand. The young and adult individuals stay apart. The young live in submerged or semi-submerged vegetation in the littoral, the adults in the sublittoral and upper aphytal zones. Maximum density of amphipods was encountered in the areas influenced by human activity.The feeding experiments revealed that the lake plankton is a more important food source for G. lacustris than any other food species. The specific growth rate measured was 0.039 d^–1, with a length increment 0.095 mm d^–1.     

The breeding season of tilapia, Oreochromis niloticus L. in Lake Awassa (Ethiopian rift valley)

The breeding season of Oreochromis niloticus in Lake Awassa was studied from fish caught monthly from December 1987 to November 1988 in relation to a number of environmental factors. Fish at various stages of gonad development and spawning were caught in almost all months, but breeding fish were more frequent twice a year: main peak occurred during January–March and a secondary one occurred during July–September. Gonadosomatic index (GSI) values peaked twice in the study year, a major peak being during January–March and a less pronounced one during July–September. Thus, it was concluded that O. niloticus in Lake Awassa breeds intensively in the former and less intensively during the latter period, and that some breeding occurs at any time of the year. Intensive breeding activity during January–March appeared associated with increase in solar radiation and sunshine hours d^–1 whereas that during July–September appeared associated with heavy rainfall. However, intensive breeding in both periods coincided with increase in phytoplankton biomass. Previously, phytoplankton biomass in this lake is shown to increase following increase in nutrients due to mixing- and rainfall-associated changes in the hydrography and hydrology of the lake. Thus, although further studies are required, increase in phytoplankton may be one of the environmental cues to stimulate spawning in O. niloticus in Lake Awassa whereas other factors such as sunshine and rainfall may have indirect roles through their effects on the hydrology and hydrography of the lake ultimately resulting in changes in phytoplankton biomass.     

Encentrum (Parencentrum) walterkostei n. sp., a new dicranophorid rotifer (Rotatoria: Monogononta) from the high alpine zone of the Central Alps (Austria)

A new dicranophorid rotifer, Encentrum (Parencentrum) walterkostei n.sp., is described from an oligotrophic alpine lake. The species was abundant in the psammolittoral of the Unterer Schwarzhornsee, 2543 m above sea level. The main distinguishing taxonomic features of the new species are long, swordshaped toes and only four vitellary nuclei. Its known occurrence in the sandy substrate of only one lake suggests a stenobiontic nature of this apparently oligothermic new species. Taxonomic problems associated with the Encentrum/Dicranophorus group are briefly discussed.     

The late-glacial/holocene Bosmina (Eubosmina) fauna of Lake Constance (Untersee) (F.R.G.): traces of introgressive hybridization

Subfossil Bosmina (Eubosmina) remains were analysed in a sediment core from the Untersee of Lake Constance which covered the Late-Glacial/Holocene period.During the Late-Glacial and the early Holocene the lake was inhabited only by Bosmina longispina Leydig. In the Late Holocene a second species, Bosmina coregoni f. kessleri Uljanin, appeared. In the uppermost sediment layers the morphological gap between the two taxa disappeared, apparently from introgressive hybridization.     

Impacts of bleak (Alburnus alburnus) and roach (Rutilus rutilus) on water quality,sedimentation and internal nutrient loading

Lake Vesijärvi, southern Finland, suffered sewere eutrophication by sewage effluent from the city of Lahti during the 1960's and the early 1970's. The municipal sewage loading was diverted from the lake in 1976 and the lake started to recover. However, in the 1980's blue-green algal blooms increased again and the recovery of the lake faded. Enclosure experiments demonstrated that high roach (Rutilus rutilus) biomass is one of the key factors in the fading recovery of the lake. In this study, the influence of roach and another cyprinid fish species (bleak, Alburnus alburnus) to planktonic algal productivity and biomass in Lake Vesijärvi was examined. Enclosure experiments in the field showed the impacts of planktivorous bleak on water quality; in an enclosure with a density of 1 fish m^–2 average daily algal production (1370 mg C m^–2) and chlorophyll-a concentration (50–90 µg 1^–1) were more than twice that in an enclosure without fish. Laboratory experiments showed that the availability of planktonic food affects the foraging behaviour of roach and consequently the internal nutrient loading from the sediment into the water. Roach caused the highest phosphorus loading and turbidity when there was no zooplanktonic food available in the water. The possible interactions between planktivorous and omnivorous fish species are discussed.     

Mesocyclops dissimilis n. sp. from Lake Biwa,Japan (Copepoda,Cyclopoida)

The status of the Mesocyclops from lake Biwa is reexamined and a new species, Mesocyclops dissimilis n. sp., distinct from M. thermocyclopoides Harada, 1931, is described.     

Contribution from nitrogen fixation (acetylene reduction) to the nitrogen budget of Lake Tohopekaliga (Florida)

Nitrogen fixation, as assayed by the acetylene reduction technique, provided 44% of the input of nitrogen to a lake in central Florida (Lake Tohopekaliga) during 1984. Ninety-four percent of the lake total fixation was found in the water column and associated with Anabaena spp. The lake-wide average nitrogen fixation rate of 5.7 g N/m²-yr amounted to a mass loading of 497 metric tons of nitrogen for the year, and is one of the highest nitrogen fixation rates reported.     

Reproductive biology of Rutilus rubilio (Bonaparte, 1837) in Lake Trichonis

The reproductive biology of R. rubilio in lake Trichonis was examined. Absolute fecundity depended on size, age and condition of the female spawners but these relationships were less clear in the case of relative fecundity and egg size. The adaptive significance of the reproductive process of R. rubilio in the oligotrophic conditions of the lake is discussed.     

Horizontal distribution of the plankton rotifers Keratella cochlearis (Bory de St Vincent) and Polyarthra vulgaris (Carlin) in a small eutrophic lake

The plankton rotifers Keratella cochlearis and Polyarthra vulgaris were sampled at 10 cm below the surface at different distances from two dissimilar shores and in the centre of a small eutrophic lake. Light and depth were measured at each sampling point. In each case the numbers of rotifers per liter increased with distance from the shore. There was a significant correlation between the numbers for the two species for the two shores, but none in the centre. In the case of one shore there was a strong correlation between rotifer numbers and supra-surface ambient light.     

Tropodiaptomus zambeziensis,T. bhangazii and T. capriviensis,three new species of Tropodiaptomus (Copepods,Calanoida) from southern Africa

Three new species of Tropodiaptomus from southern Africa are described, Tropodiaptomus zambeziensis, T. bhangazii, and T. capriviensis. Type localities are a rice paddy on the Zambezi river delta, a coastal lake in Zululand, and temporary pools in Bushmanland and the eastern Caprivi in Namibia. Speciation of Tropodiaptomus in the warm inland waters of southern Africa is much more extensive than was formerly realised.     

The seasonal cycle of Thermocyclops crassus (Fischer, 1853) (Copepoda: Cyclopoida) in a shallow,eutrophic lake

The seasonal cycle of Thermocyclops crassus was studied from 1985 to 1987, in the Gronne, a shallow, productive lake. T. crassus was present from late April to early October, while water temperature was above 10 °C. It produced three generations per year. Population peak was usually reached by the second generation, in August. Abundance was positively correlated with water temperature. Females carried 18.3 to 32.3 eggs on average. Reproduction rates were highest in July and August. Sex ratio was low, as females generally outnumbered males. Between October and April copepodites 4 went in diapause, predominantly in the deepest part of the lake and 0–4 cm deep in the bottom mud. Variations in body size were low across the year. T. crassus coexisted with the cyclopids, Cyclops vicinus and Mesocyclops leuckarti. In 1985 and 1986, abundances of M. leuckarti were low, while those of T. crassus and C. vicinus were high. In 1987 lower water temperatures, caused by cold weather, resulted in a marked decrease of the population density of T. crassus, while the abundance of M. leuckarti increased, and density of C. vicinus remained high. Comparison of the egg duration times of T. crassus, C. vicinus and M. leuckarti showed that at 15 °C and below T. crassus may be outcompeted by its comparatively longer egg development times. However, experiments in limnocorrals showed that T. crassus has an advantage over C. vicinus when fish predation is high.Department of Biology III, University of Ulm     

Plankton community of a polyhumic lake with and without Daphnia longispina (Cladocera)

The impact of Daphnia longispina (Cladocera) on the plankton food web was studied in a polyhumic lake where this species comprised almost all zooplankton biomass. Plastic enclosures (volume 7 m³) were inserted into the lake retaining the initial water stratification except that in one enclosure zooplankton was removed. After the removal of Daphniaa rotifer, Keratella cochlearis, ciliates and heterotrophic nanoflagellates increased markedly and the density and biomass of bacteria decreased. Edible algal species, Cryptomonas rostratiformisand three small chrysophytes,Ochromonas, Pedinella and Spinifermonas, took advantage of the removal of Daphnia, while more grazing-resistant species declined. In spite of the changes in the species composition of phytoplankton, the removal of Daphnia did not affect the biomass, primary production or respiration of plankton. The results implied that the density of heterotrophic flagellates and ciliates was controlled by Daphnia, but in its absence the former took its role as the bacterial grazers.     

Cyanospira rippkae and Cyanospira capsulata (gen. nov. and spp. nov.): new filamentous heterocystous cyanobacteria from Magadi lake (Kenya)

New filamentous heterocystous cyanobacteria were isolated from the alkaline soda lake Magadi in Kenya. The characteristics of the isolates are summarized and their taxonomic position discussed.Uniform attributes of the strains, grouped in two types, Mag II 702 and Mag I 504 are the following: helical structure of the trichomes, immotility, gas vacuolation, obligate autotrophy, nitrogen fixation under aerobic conditions, and closely similar fatty acid composition, including the uncommon cis-vaccenic acid.For these organisms the assignment to a new genus named Cyanospira is proposed with the species C. rippkae and C. capsulata, separated on the basis of structural, chemical and mean DNA-base composition. Type strains 702 and 504 will be deposited at the ATCC and PCC.     

Respiratory and dissimilatory nitrate-reducing communities from an extreme saline alkaline soil of the former lake Texcoco (Mexico)

The diversity of the dissimilatory and respiratory nitrate-reducing communities was studied in two soils of the former lake Texcoco (Mexico). Genes encoding the membrane-bound nitrate reductase (narG) and the periplasmic nitrate reductase (napA) were used as functional markers. To investigate bacterial communities containing napA and narG in saline alkaline soils of the former lake Texcoco, libraries of the two sites were constructed (soil T3 with pH 11 and electrolytic conductivity in saturated extract (EC_SE) 160 dS m⁻¹ and soil T1 with pH 8.5 and EC_SE 0.8 dS m⁻¹). Phylogenetic analysis of napA sequences separated the clone families into two main groups: dependent or independent of NapB. Most of napA sequences from site T1 were grouped in the NapB-dependent clade, meanwhile most of the napA sequences from the extreme soil T3 were affiliated to the NapB-independent group. For both sites, partial narG sequences were associated with representatives of the Proteobacteria, Firmicutes and Actinobacteria phyla, but the proportions of the clones were different. Our results support the concept of a specific and complex nitrate-reducing community for each soil of the former lake Texcoco.     

The effect of agriculture on the primary production in Lake Beskie (Poland) as recorded in the stratigraphy of fossil pigments

Analysis of fossil pigments deposited in the bottom sediments of Lake Beskie, was used to assess changes in the primary productivity during the past years. Three characteristic periods of lake development were distinguished. These periods correspond with a transformation in the lake's catchment area induced by the development of agriculture. A first period was characterized by intensive inflow of allochthonous matter into the lake, due to agriculture in the catchment area, favouring soil erosion. This erosion and the subsequent increase in mineral fertilization resulted in decrease of sorption ability of the soil. This in turn led to increased leaching of nutrients into the lake which resulted in increased primary production and hypolimnetic anoxia. These high oxygen deficits were characterized by a development of photosynthetic bacteria of the genus Chlorobium, and an intensification of the lake's enrichment, mainly with phosphorus. In a final period organic fertilizers (manure) were used in the catchment area. A noticeable improvement of sorption ability of the soil occurred, migration of nutrients to the lake was inhibited, and primary productivity decreased.     

An early meromictic stage in Lobsigensee (Switzerland) as evidenced by ostracods and Chaoborus

Lobsigensee is a small kettle on the western Swiss Plateau in a tectonic depression of the tertiary Molasse. At present its maximum depth is 2.7 m. Analysis of ostracods and other organisms (Chaoborus) from a 13 m core indicates that the lake became meromictic during the Bølling and returned again to a holomictic stage during the Boreal when the lake was approximately 10 m deep. The meromictic period is characterized by the disappearance of ostracods — especially of Cytherissa lacustris and Limnocythere sanctipatricii — and the appearance of Chaoborus, whereas the onset of the second holomictic stage is marked by the recolonization of the profundal zone by ostracods not present before in the lake (Metacypris cordata, Darwinula stevensoni). A very similar case of fossil meromixis has been found in Kleinsee in Carinthia.