Changes in autumn zooplankton in the pelagic zone of Lake Sevan (Armenia) during the increase in fish abundance

The qualitative composition and structure of the autumn zooplankton in the pelagic zone of Lake Sevan in years that were characterized by different abundancies of fish are described. In October 2013, upon the increase in whitefish abundance, the species richness of zooplankton increased insignificantly; the values of the trophic coefficient and the Shannon index, calculated by the number, increased; and the specific number of Rotifera and Cladocera decreased. At the same time, atypical changes were recorded in the development parameters of zooplankton invertebrates. They were expressed as an increase in the total biomass due to Cladocera, among which large Daphnia (Ctenodaphnia) magna Straus occupied the leading position. The possible reasons for the particular changes in zooplankton are discussed.     

Specific features of the zooplankton of the coastal zone of Lake Sevan

Specific features of the Lake Sevan zooplankton development during a rise in water level in July 2007 and a change in the fish density were revealed. Compared to earlier data (from the second half of the 20th century), the pelagial zooplankton quantitatively dominates over the zooplankton of shallows. It is shown that the diversity and abundance of rotifers increase in the coastal zones subject to inundation. The rotifer indicator species of eutrophic and α-mesosaprobic waters contribute to this increase. The stimulating effect of the waste products of a bird colony on the zooplankton of the lake shallows is revealed.     

Limnological variability and pelagic fish abundance (Stolothrissa tanganicae and Lates stappersii) in Lake Tanganyika

The abundance of two main pelagic fish species in Lake Tanganyika (Stolothrissa tanganicae and Lates stappersii) has always been observed to fluctuate considerably at different time scales. The inverse correlation between the abundance of these species has often been interpreted as the consequence of predator−prey relations (avoidance behaviour by the prey). However, currently the two species often appear spatially segregated in the lake, S. tanganicae dominating in the north while L. stappersii is generally abundant in the south where it feeds mostly on shrimps. A fluctuating abundance of the species is nevertheless observed. As these fish species have a major importance for the fisheries, we investigated the limnological variability in relation to the short-term variability of fish catches. The abundance of S. tanganicae was positively correlated to plankton biomass (r = 0.65), while water transparency (r = 0.56), depth of mixed layer (r = −0.70) and oxygenated water appeared important drivers for the abundance of L. stappersii. Alternating “mixing” and “stable” states of the epilimnion related to seasonal and internal waves variability are probably determinant for the short-term variability in abundance of S. tanganicae and L. stappersii. In the framework of this study, remote sensing has shown a potentially interesting application for fisheries research at Lake Tanganyika. We observed a close correspondence between phytoplankton blooms at the time of trade winds changes and increased catches of S. tanganicae in the south. The anti-correlated abundance of S. tanganicae and L. stappersii probably mainly reflects the underlying fluctuating limnological environment. Fisheries studies need to integrate limnological and planktonic monitoring to better understand large and complex ecosystems such as Lake Tanganyika.

Patterns of the quantitative distribution of plankton heterotrophic flagellates in the high-mountain Lake Sevan (Armenia)

Patterns of the quantitative development and spatial distribution of heterotrophic flagellates have been studied in Lake Sevan (Armenia). The abundance and biomass of flagellates is lower in summer than in autumn, but on the whole these parameters correspond to the quantitative parameters in oligotrophic lakes. Differences in the vertical and horizontal distribution of flagellates have been found during summer stratification and autumn homothermy.     

The effects of a colony of the Armenian gull (<Emphasis Type=Italic>Larus armeniacus</Emphasis> Buturlin) on zooplankton in the Lake Sevan littoral zone (Armenia)

The zooplankton of the open and semi-protected parts of Lake Sevan littoral zones that have been affected by the products of vital activity of the colony of the Armenian gull (Larus armeniacus Buturlin) were studied. It was shown that in the open coastal area the number of species is higher as compared to a similar reference site. However, the impact of wave-driven mixing of the water resulted in the lack of significant changes in the number and biomass of invertebrates at the compared observation sites. At the semi-protected part, the number of species, as well as the number and biomass of zooplankton, are significantly higher compared to the reference site. At the nesting area at both types of shoals copepods compose the basis of the zooplankton number and biomass, with Calanoida being part of the dominant complex. At the reference sites of the open shoal Copepoda also prevail; at the semi-protected sites, it is Cladocera that prevail. The basic differences between the effect of the products of the birds’ vital activities and anthropogenic eutrophication on zooplankton were revealed.     

Spatial variability in the abundance and composition of the free-living bacterioplankton community in the pelagic zone of Lake Bourget (France)

Spatial variations in the abundance and diversity of the free-living bacterioplankton community of a large Alpine lake, Lake Bourget (France), were investigated in the pelagic zone by means of two two-dimensional samplings taken in 2003. Lake-water samples were collected in winter during water mixing, and in early summer during stratification. The population abundance in each sample was determined by flow cytometry. Denaturing gradient gel electrophoresis of PCR-amplified 16S rRNA gene fragments from organisms measuring less than 2 mum was used to assess eubacterioplankton community composition. In winter, no obvious differences were observed in either the abundance or the diversity of the bacterial community, on either the horizontal or the vertical scales. The only influence detected was that of river water input, but this was at a very minor scale relative to the surface area of the lake. In early summer, changes were found in the community composition on the vertical scale related to the thermal stratification of the water column. There were also marked differences on the horizontal scale at 15 m depth due to internal waves. The implications of these findings for sampling strategies are very important from the perspective of comparative studies of free-living bacterial community diversity and functioning in large and deep lakes.     

Zooplankton in Lake Sevan during the period of high water level and low fish density

It is shown that, upon the flooding of ephemeral waterbodies in the coastal region and the formation of new warmer sites of the shoals due to the rising water level in Lake Sevan in 2011, species typical for temporary waterbodies and various shoals have been recorded in the zooplankton. Among them, Daphnia (Ctenodaphnia) magna Straus reached mass development, which was favored by the minimal control from “the top” and favorable trophic conditions. Owing to the abovementioned factors, the total lake zooplankton biomass and water transparency increased. The uneven distribution of D. (C.) magna was determined by the density of fish that prefer it as food.     

Changes in relative abundance of zooplankton in northern Lake Victoria,East Africa

The zooplankton community of Lake Victoria was studied between March 1990 and February 1991 with the aim of identifying its constituent groups/species, their distribution, abundance, and long term changes in community structure. Zooplankton samples were taken from four stations using plankton nets (75–300 m mesh size) hauled vertically through the water column. The plankton comprised mainly Crustacea and to a lesser extent early stages of aquatic insects. Cyclopoid copepods, their nauplii and copepodites constituted the most frequent group at all stations. Calanoid copepods, Cladocera and Caridina nilotica (Roux), though widely distributed, contributed a small proportion. Chaoborus larvae and pupae and mites were occasionally present. A comparison of the relative proportions of the main zooplanktonic groups from three sources revealed remarkable changes in community structure since 1931. These changes are discussed with respect to predation, eutrophication and other changes in the food web structure of the lake.     

High-resolution satellite remote sensing of littoral vegetation of Lake Sevan (Armenia) as a basis for monitoring and assessment

Physics-based remote sensing in littoral environments for ecological monitoring and assessment is a challenging task that depends on adequate atmospheric conditions during data acquisition, sensor capabilities and correction of signal disturbances associated with water surface and water column. Airborne hyper-spectral scanners offer higher potential than satellite sensors for wetland monitoring and assessment. However, application in remote areas is often limited by national restrictions, time and high costs compared to satellite data. In this study, we tested the potential of the commercial, high-resolution multi-spectral satellite QuickBird for monitoring littoral zones of Lake Sevan (Armenia). We present a classification procedure that uses a physics-based image processing system (MIP) and GIS tools for calculating spatial metrics. We focused on classification of littoral sediment coverage over three consecutive years (2006–2008) to document changes in vegetation structure associated with a rise in water levels. We describe a spectral unmixing algorithm for basic classification and a supervised algorithm for mapping vegetation types. Atmospheric aerosol retrieval, lake-specific parameterisation and validation of classifications were supported by underwater spectral measurements in the respective seasons. Results revealed accurate classification of submersed aquatic vegetation and sediment structures in the littoral zone, documenting spatial vegetation dynamics induced by water level fluctuations and inter-annual variations in phytoplankton blooms. The data prove the cost-effective applicability of satellite remote-sensing approaches for high-resolution mapping in space and time of lake littoral zones playing a major role in lake ecosystem functioning. Such approaches could be used for monitoring wetlands anywhere in the world.     

Phytoplankton succession in the euphotic zone of Lake Corlo (Northern Italy) during summer stratification

Abstract

Lake Corlo is a dammed reservoir for hydro-electrical purposes in Northern Italy. Weekly samplings were made from May to August 1985 in the euphotic layer. Two major categories, flagellates and diatoms, alternatively dominated the algal community. Flagellates prevailed with stratified conditions. The strong influence of hydraulic conditions on phytoplakton succession is emphasized.     

Energy budget of Oligochaeta and its relationship with the primary production of Lake Sevan, Armenia

From 1928 to 1991 the following oligochaete energy budget quantitative values: biomass (B), production (P), respiration (R), assimilation (A), ration (C) changed 8 to 12 times. With increasing depth the ratios of energy budget decreased: P/B ratio from 3.4 to 0.1, R/B ratio from 4.5 to 0.5, net production efficiency from 43 to 18%. A relationship was revealed between oligochaete biomass and the primary production (PP) of the lake. There is a delay in the response in oligochaete biomass to primary production. In Lake Sevan the delay is 2 years in the littoral, 4 years in the sublittoral and 11 years in the profundal zone. The closest correlation was revealed between oligochaete energy budget quantitative values and the values of primary production of the preceding 10 years, which enables a prediction of the quantitative indices of the community of Oligochaeta. The values of energy budget ratios depend on temperature and oxygen regimes but not on the trophic status of the reservoir and were comparatively stable during the observed period.     

Zooplankton abundance in the pelagic region of Lake Kasumigaura (Japan): monthly data since 1980

Microscopic crustaceans (cladocerans and copepods) and rotifers are the principal zooplankton components of the pelagic food webs in lakes. They play important ecological roles, functioning as essential links between primary producers and planktivorous fish. Individuals zooplankton are important nodes of matter flow in pelagic ecosystems. The zooplankton community structure can provide useful indicators of top–down processes, such as the magnitude or strength of planktivorous predators and the extent of zooplankton grazing. Here we report the abundance of zooplankton taxa (crustaceans and rotifers) that were recorded monthly, from January 1980 to September 2015, at two stations on Lake Kasumigaura, a shallow eutrophic lake that is the second largest lake in Japan. The data include information on 12 copepod species (taxa), 20 cladoceran species (taxa), 40 rotifer species (taxa), and the opossum shrimp (Neomysis intermedia). In the 1980s, the plankton of the lake were characterized by cyanobacterial blooms and the co-dominance of Bosmina and Diaphanosoma in the summer. In addition, the two cladoceran genera, Daphnia galeata and Chydorus sphaericus were often prominent. The plankton profile changed dramatically in the middle (1997–2004) of the present long-term monitoring period, when cyanobacteria disappeared and diatoms became dominant even in the summer; concurrently, only the Diaphanosoma cladocerans were evident. However, in the past 10 years, cyanobacterial blooms, the co-dominance of Bosmina and Diaphanosoma, and D. galeata have re-emerged. Zooplankton monitoring forms part of the Lake Kasumigaura Long-Term Environmental Monitoring Program, which has been conducted by the National Institute for Environmental Studies (NIES) since 1977. Data on other planktonic components (phytoplankton and the elements of microbial food webs) noted during monitoring and on primary production were published in Takamura and Nakagawa (Ecol Res 27:839 2012a, Ecol Res 27:837 2012b, Ecol Res 31:287 2016). Lake Kasumigaura is a core site of the Japan Long-term Ecological Research Network, a member of the International Long-term Ecological Research Network. Our quantitative dataset spanning several decades is unique in terms of the work on lakes and the plankton therein, and is freely available. The dataset has been used in ecological and environmental programs, as well as in studies on lake management.     

On the chironomid (Diptera,Chironomidae) fauna in Lake Sevan

An annotated list of the chironomid species collected in Lake Sevan in the first 10 days of October 2006 and 2007 is presented. Chironomid species were identified by the aquatic stages of their development (larvae and pupae) and imagoes. The list includes 26 species, among which nine species were recorded in the lake for the first time. Altogether, 53 species of chironomids have been recorded in Lake Sevan.     

Changes of the serotonergic system activity in fish tissues during an increase of water salinity

Changes of environment parameters produce essential effects on the serotonergic system activity in the body tissues. This paper describes study of the serotonergic system activity in various tissues of half-migratory fish - the Caspian roach Rutilus rutilus caspicus and the bream Abramis brama orientalis - caught in the fresh and brackish water. Using the method of indirect solid-phase ELISA test, the serotonergic system activity was assessed by determining in the studied fish tissues of the amount of serotonin-modulating anticonsolidation protein (SMAP) that is in the linear correlation with serotonin level. A significant change of the SMAP level in the Caspian roach and bream brain was found under effect on increased water salinity. Analysis of the SMAP amount in the bream liver did not reveal any differences in the animals caught in the brackish and fresh water. At the same time, in the bream gill there was a small rise of the SMAP content in the fish caught in the brackish water. The revealed in this experiment elevation of the SMAP amount in the Caspian roach and bream brain reflecting a corresponding increase of the serotonergic system activity indicates involvement of adaptive readjustments in the animal body.     

Changes of the serotonergic system activity in fish tissues during an increase of water salinity

Changes of environment parameters produce essential effects on the serotonergic system activity in the body tissues. This paper describes study of the serotonergic system activity in various tissues of half-migratory fish—the Caspian roach Rutilus rutilus caspicus and the bream Abramis brama orientalis—caught in the fresh and brackish water. Using the method of indirect solid-phase ELISA test, the serotonergic system activity was assessed by determining in the studied fish tissues of the amount of serotonin-modulating anticonsolidation protein (SMAP) that is in the linear correlation with serotonin level. A significant change of the SMAP level in the Caspian roach and bream brain was found under effect on increased water salinity. Analysis of the SMAP amount in the bream liver did not reveal any differences in the animals caught in the brackish and fresh water. At the same time, in the bream gill there was a small rise of the SMAP content in the fish caught in the brackish water. The revealed in this experiment elevation of the SMAP amount in the Caspian roach and bream brain reflecting a corresponding increase of the serotonergic system activity indicates involvement of adaptive readjustments in the animal body.     

Distribution and abundance of zooplankton populations in Crater Lake,Oregon

The zooplankton assemblages in Crater Lake exhibited consistency in species richness and general taxonomic composition, but varied in density and biomass during the period between 1988 and 2000. Collectively, the assemblages included 2 cladoceran taxa and 10 rotifer taxa (excluding rare taxa). Vertical habitat partitioning of the water column to a depth of 200 m was observed for most species with similar food habits and/or feeding mechanisms. No congeneric replacement was observed. The dominant species in the assemblages were variable, switching primarily between periods of dominance of Polyarthra-Keratella cochlearis and Daphnia. The unexpected occurrence and dominance of Asplanchna in 1991 and 1992 resulted in a major change in this typical temporal shift between Polyarthra-K. cochlearis and Daphnia. Following a collapse of the zooplankton biomass in 1993 that was probably caused by predation from Asplanchna, Kellicottia dominated the zooplankton assemblage biomass between 1994 and 1997. The decline in biomass of Kellicottia by 1998 coincided with a dramatic increase in Daphnia biomass. When Daphnia biomass declined by 2000, Keratella biomass increased again. Thus, by 1998 the assemblage returned to the typical shift between Keratella-Polyarthra and Daphnia. Although these observations provided considerable insight about the interannual variability of the zooplankton assemblages in Crater Lake, little was discovered about mechanisms behind the variability. When abundant, kokanee salmon may have played an important role in the disappearance of Daphnia in 1990 and 2000 either through predation, inducing diapause, or both. Electronic supplementary material Electronic supplementary material is available for this article at and accessible for authorised users.     

Some reflections about the structure of the pelagic zone of the brackish Lake Grevelingen (SW-Netherlands)

Summary The seasonal succession of the plankton in the marine brackish Lake Grevelingen, a closed sea arm in the S.W.-Netherlands, comprises the initial stagessensu Margalef and is characterized by predominantly small phytoplankton (flagellates, diatoms) and zooplankton (rotifers, tintinnids, copepods), maintaining relatively high levels of production from early spring (February) to late summer (September). The structure of the plankton in the course of seasonal succession is in agreement with the concepts of Margalef.Simplification of the pelagic food web in Lake Grevelingen has occurred as a consequence of the elimination of the tides. Some examples are given in relation to the composition of the phyto- and zooplankton and of its significance. The occurrence of rotifer-dominated zooplankton blooms in early spring is emphasized.Closed sea arms such as Lake Grevelingen, showing the same morphometry as the previous tidal estuary, contain extended shallow areas which influence strongly the pelagic zone. The abundance in the zooplankton of larval stages of several littoral-benthic species demonstrate these influences clearly. The shallows of the lake, occupied by eelgrass beds (Zostera marina) in summer, influence the pelagic zone in several ways: large quantities of detritus are given off after the growing season, sheltered habitats are supplied for small pelagic animals, and eelgrass leaves represent a substrate for epifauna species.Contribution no. 168 of the Delta Institute for Hydrobiological Research.     

Changes in the blood parameters of an air-breathing fish during different respiratory conditions.

Some of the blood parameters recorded in an air-breathing eel, Amphipnous cuchia under normal respiratory condition during non-breeding period (September-April) are haemoglobin (Hb) concentration 19.26%, haematocrit value 56.16%, RBC number 1.71 million/mm3, RBC size 18.86 X 9.70 mum, mean corpuscular haemoglobin (MCH) 113.4 ng, mean corpuscular haemoglobin concentration (MCHC) 34.2%, blood sugar 77 mg% and ascorbic acid 0.435 mg%. The higher concentration of haemoglobin (19.26%) appears to be related to its obligatory air breathing habit and habitat in a water of low oxygen content. Though a definite trend of increase in the haemoglobin and haematocrit concentration with an increase in the body weight of the fish was lacking, variations were clearly marked related to intrinsic activity of the fish connected with different respiratory conditions. Asphyxiation in a submerged but continuous flow of water (liter/h) for 5 1/2 h resulted in an increase in the above-mentioned parameters to an appreciable extent. These increases were 0.23 million/mm3 in the number of erythrocytes, 6.16% in haemoglobin concentration, 10% in haematocrit value, 20% in blood sugar and 35% in ascorbic acid content. The mean corpuscular haemoglobin showed a decline of 6.2%. Exclusive aerial breathing for 5 1/2 h also caused 7.4% increase in haemoglobin concentration, 9.4% in haematocrit value, 0.14 million/mm3 in RBC number, 20% in blood sugar level, 9% in ascorbic acid content but almost no change in mean corpuscular haemoglobin. The average surface area for diffusion of gases appeared to have reduced by 6.8 mum2 per RBC.     

Spatial and seasonal variation of the zooplankton in the coastal zone and main khors of Lake Nasser (Egypt)

A prominent feature of Lake Nasser is the presence of khors (dendritic side extensions). We studied the zooplankton of the larger khors and coastal zone of the main lake in 1996 and 1997, and found an assemblage of rotifers, cladocerans and copepods that was partly tropical, partly temperate, at relatively high biomasses. Spatial differences were weak, but the upstream khors (Toushka and Korosko) were consistently richer than the downstream khors (El-Ramla and Kalabsha), with a rather sudden transition around km 150 at El Madiq. Summer standing crops were higher than those in winter by a factor 2–3. The zooplankton of the littoral of the main channel showed the same spatial pattern as that in the khors, being more abundant in spring (average 82300 ind m^–3) than in autumn (average 72700 ind m^–3). Zooplankton dry weight increased from 4 g m^–2 at khor El-Ramla to 7 g m^–2 at khor Toushka. These rather high values had low variation. The number of species, diversity and evenness all showed a high degree of similarity among the khors and in the littoral of the main lake. The lake fish fauna is poor, lacking a pelagic planktivore. The predominance of medium-sized Copepoda (one calanoid, two cyclopoids) in the zooplankton suggests that fish predation is moderate. This is confirmed by the persistence of two Daphnia species at low abundance, although rather strong variations in time suggest that Cladocera suffer from summer predation by invertebrates (copepods) as well as vertebrates (mostly larval fish). Because the zooplankton is underutilised by higher trophic levels, we suggest to assess the benefits of introducing a pelagic zooplanktivorous fish.