Shallow Hypertrophic Lake Research Articles

Comparison of spatiotemporal burial and contamination of heavy metals in core sediments of two plateau lakes with contrasting environments: implication for anthropogenic-driven processes.

Investigating the impacts of climatic factors and human activities on sedimentary records of heavy metal (HM) contamination in lakes is essential for decision-making in global environmental monitoring and assessment. Spatiotemporal distributions of grain size (GS) and HM (Al, Cr, Mn, Ni, Cu, Zn, As, and Pb) concentrations have been conducted in core sediments that are collected from two adjacent plateau fault-bound lakes in southwest China with contrasting environments, i.e., deep oligotrophic Lake Fuxian (FX) and shallow hypertrophic Lake Xingyun (XY). Results showed that the average value of d50 in FX (4.61 μm) was lower than that in XY (8.35 μm), but the average concentrations of HMs (except Cr and Mn) in XY were higher than those in FX. Heavy metal burial rates (HMBR) were mainly controlled by sediment accumulation rates (SARs) rather than HM concentrations. The correlation coefficients between GS and HM concentrations became strong as the increasing water depths were associated with a stable sedimentary environment. Time-integrated enrichment factors (EF) and source identification of HMs between FX and XY represented that Cr, Ni, and Cu originated from natural sources but Mn, Zn, As, and Pb from anthropogenic sources, respectively. Regardless of FX and XY, the transition times of HMs from natural to anthropogenic sources occurred in the mid-1960s. Comparison of qualification impacts of climatic factors and human-induced factors on increased anthropogenic HMBR by the partial least squares path modeling (PLS-PM) implied that socio-economic activities, such as population density (PD) and gross domestic product (GDP), provided higher contributors to increased anthropogenic HMBR in XY (0.23/0.71) than FX (0.11/0.18). The comparative results of this study provided new insights into environmental monitoring and management of HM contamination for adjacent lakes with contrasting environments.

Functional dynamics of phytoplankton assemblages in hypertrophic lakes: Functional- and species diversity is highly resistant to cyanobacterial blooms

Under increasing pressure of climatic change and anthropogenic eutrophication, water blooms, i.e. the formation of high phytoplankton biomass of a single or a few species, have become more and more frequent in lake ecosystems that is caused mostly by Cyanobacteria. The dynamics of phytoplankton under a cyanobacterial pressure may provide important information about what a competitive advantage of Cyanobacteria have over eukaryotic microalgae. The research, therefore focused on the relationship between cyanobacterial blooms and the accompanying species in hypertrophic shallow lakes where nutrients are in excess. The following hypotheses were selected for testing: i) Cyanobacterial blooms negatively affect the species richness and diversity of phytoplankton assemblages in the studied hypertrophic lakes. ii) Cyanobacterial blooms negatively affect the multi-trait functional diversity of phytoplankton assemblages in the studied hypertrophic lakes. iii) The formation of a cyanobacterial bloom causes a shift in the species and trait composition of phytoplankton assemblages. It was found that the species diversity of non-cyanobacterial assemblages was not affected negatively by increasing density of Cyanobacteria. Cyanobacterial biomass negatively affected the functional richness and functional dispersion of the assemblages, but both relationships were only marginally significant. Some temporal shift was detected in the community weighted mean values of width, colonial ability of accompanying microalgae, and also in flagellatedness and mixotrophic ability. We conclude that Cyanobacterial blooms basically alter dominance relations in the phytoplankton and reduce availability of light in waters. However, it does not necessarily coincide with the elimination of other taxa and reduction their species numbers or diversity. The formerly published data on the decrease in species richness can be likely explained by methodological deficiencies, which are responsible for reduced detectability of species. To better understand the seasonal dynamics and resilience of phytoplankton assemblages in hypertrophic water bodies the development of new methodologies by which diversity and succession of subordinate species can be evaluated is required.

Tributary Loadings and Their Impacts on Water Quality of Lake Xingyun, a Plateau Lake in Southwest China

Lake Xingyun is a hypertrophic shallow lake on the Yunnan Plateau of China. Its water quality (WQ) has degraded severely during the past three decades with catchment development. To better understand the external nutrient loading impacts on WQ, we measured nutrient concentrations in the main tributaries during January 2010–April 2018 and modelled the monthly volume of all the tributaries for the same period. The results show annual inputs of total nitrogen (TN) had higher variability than total phosphorus (TP). The multi-year average load was 183.8 t/year for TN and 23.3 t/year for TP during 2010–2017. The average TN and TP loads for 2010–2017 were 36.6% higher and 63.8% lower, respectively, compared with observations in 1999. The seasonal patterns of TN and TP external loading showed some similarity, with the highest loading during the wet season and the lowest during the dry season. Loads in spring, summer, autumn, winter, and the wet season (May–October) accounted for 14.2%, 48.8%, 30.3%, 6.7%, and 84.9% of the annual TN load and 14.1%, 49.8%, 28.1%, 8%, and 84.0% of the annual TP load during 2010–2017. In-lake TN and TP concentrations followed a pattern similar to the external loading. The poor correlation between in-lake nutrient concentrations and tributary nutrient inputs at monthly and annual time scales suggests both external loading and internal loading were contributing to the lake eutrophication. Although effective lake restoration will require reducing nutrient losses from catchment agriculture, there may be a need to address a reduction of internal loads through sediment dredging or capping, geochemical engineering, or other effective measures. In addition, the method of producing monthly tributary inflows based on rainfall data in this paper might be useful for estimating runoff at other lakes.

CARD-FISH and prey tracer techniques reveal the role of overlooked flagellate groups as major bacterivores in freshwater hypertrophic shallow lakes.

Heterotrophic nanoflagellates (HNF) and ciliates are major protistan planktonic bacterivores. The term HNF, however, describes a functional guild only and, in contrast to the morphologically distinguishable ciliates, does not reflect the phylogenetic diversity of flagellates in aquatic ecosystems. Associating a function with taxonomic affiliation of key flagellate taxa is currently a major task in microbial ecology. We investigated seasonal changes in the HNF and ciliate community composition as well as taxa-specific bacterivory in four hypertrophic freshwater lakes. Taxa-specific catalyzed reporter deposition-fluorescence in situ hybridization probes assigned taxonomic affiliations to 51%-96% (average ±SD, 75± 14%) of total HNF. Ingestion rates of fluorescently labelled bacteria unveiled that HNF contributed to total protist-induced bacterial mortality rates more (56%) than ciliates (44%). Surprisingly, major HNF bacterivores were aplastidic cryptophytes and their Cry1 lineage, comprising on average 53% and 24% of total HNF abundance and 67% and 21% of total HNF bacterivory respectively. Kinetoplastea were important consumers of bacteria during summer phytoplankton blooms, reaching 38% of total HNF. Katablepharidacea (7.5% of total HNF) comprised mainly omnivores, with changing contributions of bacterivorous and algivorous phylotypes. Our results show that aplastidic cryptophytes, accompanied by small omnivorous ciliate genera Halteria/Pelagohalteria, are the major protistan bacterivores in hypertrophic freshwaters.

Photosynthetic and morphological traits control aquatic plant distribution according to light stress

The eutrophication of aquatic systems resulting from human activities frequently leads to phytoplankton blooms, which decrease water transparency and therefore limit light availability for aquatic plants. However, several submerged macrophytes are able to grow despite the light competition pressure exerted by phytoplankton. Photosynthetic performance and plant morphology may be key traits allowing plant species to tolerate phytoplankton-induced light limitation in eutrophic systems. The aim of this study was to explore how morphological and photosynthetic traits allow submerged macrophytes to withstand the stress induced by decreased light in eutrophic systems. Morphological and photosynthetic traits were measured in 7 species of submerged macrophytes collected from mesotrophic to hypertrophic shallow lakes (Dombes, Ain, France). Then, we determined whether these traits were related to species occurrence in lakes according to depth and water turbidity. This study highlighted 2 strategies potentially resulting in a trade-off between morphological and photosynthetic traits. The first strategy was tolerance to reduced light and was characterized by a higher investment in leaf area and photosynthetic efficiency (initial slope of photosynthetic activity measured at low light intensities), allowing plants to use low amounts of light energy. The second strategy was to avoid the regions of reduced light and was characterized by a greater investment in vertical growth and maximum photosynthesis at high light intensities. The morphological and physiological traits favoured in turbid lakes corresponded to the avoidance strategy with a low compensation point, allowing plants to grow to the photic zone to reach the water areas where light is at a maximum. Small species exhibiting a light stress tolerance strategy may be maintained in turbid lakes subjected to disturbances, offering a window of opportunity for successful recruitment and reproduction.

Comparison of spatiotemporal carbon, nitrogen, and phosphorus burial in two plateau lacustrine sediments: implication for N and P control.

The long-term accumulation, burial and release of nutrients, such as carbon (C), nitrogen (N), and phosphorus (P) in lacustrine sediments are responsible for the global lake eutrophication. Interpretation of the spatiotemporal sedimentary record of nutrients (C, N, and P) in contrasting trophic level of lakes is helpful for understanding the evolutionary process of water eutrophication. Based on the radiochronology of 210Pbex and 137Cs, a comparative study of spatial and temporal concentrations, burial of total organic carbon (TOC), total nitrogen (TN), and total phosphorus (TP), the sources of organic matter were conducted using sediment cores from two plateau lakes Dianchi (DC) and Fuxian (FX) of SW China. Results showed that concentrations and burial of C, N, and P in sediments of DC, a shallow hypertrophic lake with the maximum depth of 5.8 m, were both higher than those in FX, an oligotrophic deep lake with the maximum depth of 155.0 m. For both lakes the molar ratio of TOC/TN increased in the sediments moving from north to south. The values of TOC/TN molar ratios increased over time in DC and were higher than in FX. The extremely high values of TOC/TN appeared in the central and southern parts of FX, indicating the impacts of accumulation effect and sediment focusing in the deeper region and indirect supplement from the Lake Xingyun (XY), an adjoining lake connected with FX via the Gehe River. Time-integrated sources identification in DC indicated the contribution of allochthonous sources was dominant over the past few decades, which contributed to the increased trophic level of the lake. The comparison of relationships of carbon accumulation rates (CAR), nitrogen accumulation rates (NAR), and phosphorous accumulation rates (PAR), the ratios of N/P and the utilizations of N and P fertilizer between DC and FX implied that both of N and P inputs should be limited for reducing the trophic level, but N control was predominant in comparison with P for both lakes. The results indicated that caution is required in plateau lakes to limit transition from oligotrophic to eutrophic in these lakes.

ОСОБЕННОСТИ СТРУКТУРЫ И ДИНАМИКИ ЗООПЛАНКТОНА ОЗЕРА НЕРО (ЯРОСЛАВСКАЯ ОБЛАСТЬ) В ПЕРИОД ПОТЕПЛЕНИЯ КЛИМАТА

Data on the abundance, biomass, and composition of the dominant complex of zooplankton in the shallow hypertrophic lake Nero (Yaroslavl region) from 2007 to 2017 are present. The spatiotemporal distribution of zooplankton is analyze. The high zooplankton species richness is characteristic of Lake Nero (>100 species in the general list and up to 22–27 species in the sample). 2007 to 2011, a low amount of zooplankton was noted (<37.9 thous. ind./m3 and 0.15 g/m3), with a relatively high proportion of copepods (up to 83%). Since 2012, the abundance of planktonic animals has increased (313 thous. ind./m3 and 1.20 g/m3). In 2007–2017 the composition of dominants remained close to that in previous years; small crustaceans (Bosmina longirostris, Chydorus sphaericus, Mesocyclops leuckarti), rotifers (genera Asplanchna, Brachionus, Keratella, Trichocerca and Anuraeopsis) dominated the zooplankton community. Rotifer-dominated communities (with a high abundance of thermophilic Brachionus diversicornis) formed during abnormally warm summers. Two species groups are determined: the first dominates in spring, early summer, and autumn, and the second in the second half of summer. The high abundance of zooplankton (>5 mil. ind./m3 and 10 g/m3) is in the thickets of macrophytes, which is 8–9 times higher than in the open littoral and pelagic zones. In the western littoral zone, the low abundance of zooplankton (<1 g/m3) may be associated with local pollution, high concentration of chlorophyll a, high phytoplankton biomass, represented by low food cyanobacteria. The center part and the adjacent open littoral zone differed in composition and abundance of dominant species from the southern bays occupied by macrophytes. The trophic status of the lake ecosystem in terms of zooplankton is hypertrophic since the 1980s. The abundance, composition and structure of the community in the long-term aspect are stable with interannual fluctuations common for a high-trophic shallow lake.

Drivers of Macrophyte and Diatom Diversity in a Shallow Hypertrophic Lake

We studied macrophyte and diatom assemblages and a range of environmental factors in the large hypertrophic Dehtář fishpond (Southern Bohemia, Czech Republic) over the course of several growing seasons. The spatial diversity of the environment was considered when collecting diatoms and water samples in three distinct parts of the fishpond, where automatic sensor stations continually measuring basic factors were established. Macrophytes were mapped in 30 segments of the fishpond littoral altogether. High species richness and spatiotemporal variability were found in assemblages of these groups of autotrophs. Water level fluctuations, caused by the interaction of fish farming management and climatic extremes, were identified as one of the most important factors shaping the structure and species composition of diatom and macrophyte assemblages. The distance of the sampling sites from large inflows reflected well the spatial variability within the fishpond, with important differences in duration of bottom drainage and exposure to disturbances in different parts of the fishpond. Disturbances caused by intensive wave action are most probably a crucial factor allowing the coexistence of species with different nutrient requirements under the hypertrophic conditions of the Dehtář fishpond. Due to a range of variables tested and climatic extremes encountered, our study may be considered as a basis for predictive model constructions in similar hypertrophic water bodies under a progressing climate change.

РАСТИТЕЛЬНЫЙ ПОКРОВ МЕЛКОВОДНОГО ГИПЕРТРОФНОГО ОЗЕРА В РАЗЛИЧНЫХ ГИДРОЛОГИЧЕСКИХ УСЛОВИЯХ

Comparison of the distribution of aquatic vegetation in shallow hypertrophic Lake Nero (Yaroslavl oblast, Russia) in different hydrological conditions – during low water phase in 2002–2004 and water level rise in 2017 is presented. The total area covered by aquatic vegetation increased from 14.8 km2 in 2000–2004 to 15.8 km 2 in 2017. Correspondingly, the lake area covered by macrophytes showed its increase from 27% to 29%. The observed increase in the total plant coverage was mainly driven by helophytes which increased in the coverage area from 8.4 km2 to 11.2 km2. The coverage area of submerged vegetation during this period decreased from 5.4 km2 to 3.6 km2. Potamogeton perfoliatus showed the highest reduction in the coverage area by factor of 2.6 in the central part of the lake. In contrast, the coverage area of Ceratophyllum demersum and Myriophyllum verticillatum did not change mostly in the shallow, southern part of the lake. The analysis of literature and our data revealed that increase in water level due to building a dam on the River Veksa in the 1980s and impact of climatic factors had a negative influence on submerged vegetation. Thus, the high water level is one of the mechanisms stabilising the ecosystem of the Lake Nero in the turbid, phytoplankton-dominated state.

Covariation patterns of phytoplankton and bacterioplankton in hypertrophic shallow lakes.

The aim of this work was to assess the temporal patterns in the community composition of phytoplankton (PCC) and bacterioplankton (BCC) in two interconnected and hypertrophic Pampean shallow lakes in Argentina. Factors shaping their community dynamics and community temporal covariations were also analysed. We performed 4 years of seasonal samplings (2012-2016) and communities were studied by the Utermöhl approach (PCC) and Illumina MiSeq sequencing (BCC). We found marked seasonal variations in both communities and inter-annual variations with decreasing microbial community similarities during the study. We also observed covariation in community-level dynamics among PCC and BCC within and between shallow lakes. The within-lake covariations remained positive and significant, while controlling for the effects of intrinsic (environmental) and extrinsic (temporal and meteorological) factors, suggesting a community coupling mediated by intrinsic biotic interactions. Algal-bacterial associations between different taxa of phytoplankton and bacterioplankton within each lake were also found. PCC was mainly explained by pure regional extrinsic (17-21%) and intrinsic environmental (8-9%) factors, while BCC was explained by environmental (8-10%) and biotic interactions with phytoplankton (7-8%). Our results reveal that the influence of extrinsic regional factors can be channeled to bacterioplankton through both environmental (i.e. water temperature) and phytoplankton effects.

Long-term responses of epiphytic midges (Diptera, Chironomidae) to emergent macrophytes removal and P concentrations in a shallow hypertrophic lake ecosystem

Chironomid larvae are used as indicators of environmental changes in neolimnological and paleolimnological research. In the present study, we evaluated the responses of epiphytic chironomids to changes in environmental conditions over a long time scale. We intended to decipher whether changes in the trophic status of a lake (hypertrophic-eutrophic) would affect the taxonomic structure of epiphytic chironomids by influencing their food availability (structure of periphytic algae) and whether the responses of chironomids are taxa specific. In a shallow hypertrophic lake ecosystem, epiphytic chironomids associated with the emergent macrophyte Phragmites australis were studied from 2001 to 2018. In the autumn of 2006, emergent macrophyte removal led to an improved water transparency and reduced phytoplankton biomass. Epiphytic chironomids responded clearly to the shift from hypertrophic to eutrophic conditions. Under hypertrophic conditions larvae of detritivorous Cricotopus sp. (gr. sylvestris) and filter-feeder larvae of Glyptotendipes sp. prevailed. After macrophyte removal, we observed high relative abundances of Endochironomus albipennis and Paratanytarsus austriacus, which are classified as grazers, utilise periphytic algae (mainly diatoms) as a food source. The results indicated that the density of P. australis, relative abundances of cyanobacteria and diatoms in periphyton communities, Secchi disc depth, and periphytic and planktonic Chl-a are significant determinants of the diversity of epiphytic chironomids. The results can provide a reliable reference to the neolimnological and paleominological studies on chironomid responses to multiple environmental stressors in shallow lakes.

Causes of Daphnia midsummer decline in two deep meromictic subalpine lakes

Abstract Daphnia are key organisms in pelagic food webs, acting as a food resource for fish and predatory zooplankton and regulating phytoplankton through grazing. Its population dynamic follows regular seasonal patterns, with spring peaks followed by summer population declines (midsummer declines, MSDs). Midsummer declines show high inter‐annual variation, which has been attributed to different causes. However, the mechanisms controlling the MSD remain poorly understood, especially in deep stratified lakes. We tried to disentangle the factors causing Daphnia MSDs in Lake Lugano and Lake Iseo (in Switzerland and Italy), two deep peri‐alpine lakes with similar trophic status and vertical mixing dynamics, characterised by phosphorus accumulation in the hypolimnion and variable mixing during late‐winter turnovers. Specifically, we assessed the effects of three different hypothetical pathways according to which: (1) winter air temperature controls MSDs by influencing mixing depth during turnovers and epilimnetic phosphorus replenishment; (2) vernal air temperature influences MSD by accelerating the timing of spring population peak; and (3) summer temperature influences MSDs by increasing fish predation. We assessed the relative strength of these pathways using structural equation modelling on long‐term datasets for the two lakes (29 years for Lake Lugano and 19 years for Lake Iseo). Between the hypothesised pathways, the one driven by winter air temperature (through P replenishment) influenced Daphnia abundance in spring in both lakes, but the effects propagated to summer Daphnia abundance only in Lake Lugano. Additionally, summer Daphnia abundance was influenced by the summer air temperature through a positive (although weak) effect. By comparison, vernal air temperature had no detectable effects on summer Daphnia abundance. The results revealed marked differences between the meromictic study lakes and the shallow hypertrophic water bodies that were the focus of previous research on Daphnia MSD, and also between the two study lakes. The influence of epilimnetic P replenishment on the summer Daphnia abundance in Lake Lugano, which was recovering from past eutrophication, may have reflected the greater susceptibility of deep, stratified lakes to P depletion after spring compared to shallow hypertrophic lakes or reservoirs. This effect might not have been detected in Lake Iseo because P was more consistently depleted during the study period (i.e. variance in the predictor was too low to detect an effect). This study highlighted the complexity of the effects of climate variability on Daphnia MSD in deep lakes, showing that the responses can differ even between two neighbouring lakes with similar vertical mixing dynamics and trophic status. At the same time, the results suggest that future increases in winter air temperature, caused by global warming, may cause critically low densities of Daphnia during spring and summer and compromise the ability of zooplankton to control phytoplankton biomass.

Aerobic anoxygenic phototrophs are highly abundant in hypertrophic and polyhumic waters.

Aerobic anoxygenic phototrophs (AAPs) are a group of photoheterotrophic bacteria common in natural waters. Here, AAP abundance and contribution to total bacterial abundance and biomass were investigated to test whether the trophic status of a lake or content of coloured dissolved organic matter (CDOM) play a role in determining AAP distribution and abundance in shallow inland lakes, with special focus on hypertrophic and polyhumic waters. Twenty-six different shallow lakes in Hungary were monitored. AAP abundance and biomass were determined by epifluorescence microscopy. The lakes exhibit a broad range of CDOM (2-7000 mg Pt L-1) and phytoplankton biomass (2-1200 μg L-1 chlorophyll a concentration). Very high AAP abundance (up to 3 × 107 cells mL-1) was observed in polyhumic and hypertrophic shallow lakes. AAP abundance was influenced by phytoplankton biomass and CDOM content, and these effects were interrelated. As determined, 40 μg L-1 chlorophyll a and 52 mg Pt L-1 CDOM are threshold levels above which these effects have a synergistic relationship. Hence, the observed high AAP abundance in some soda pans is a consequence of combined hypertrophy and high CDOM content. AAP contribution was influenced by total suspended solids (TSS) content: the success of AAP cells could be explained by high TSS levels, which might be explained by the decrease of their selective grazing control.

Raphidiopsis mediterranea (Nostocales) exhibits a flexible growth strategy under light and nutrient fluctuations in contrast to Planktothrix agardhii (Oscillatoriales)

Raphidiopsis mediterranea is a freshwater cyanobacterium that forms toxic blooms in eutrophic water bodies. Factors controlling its proliferation have not been explored in detail. We investigated R. mediterranea autecology by (i) analyzing its dynamics in a hypertrophic shallow lake dominated by Planktothrix agardhii (Oscillatoriales) and its relationship with environmental factors; and (ii) studying the effect of light intensity and phosphate availability on R. mediterranea isolates growing in mono or in co-cultures with P. agardhii. The redundancy analysis demonstrated that water temperature, light, and phosphate concentrations were important driving factors for the seasonal succession of the two species. When grown together with P. agardhii, R. mediterranea growth was strongly promoted under the highest light intensity treatment. On the other hand, in monoalgal cultures under phosphorus starvation, both strains exhibited a significant increase in total alkaline phosphatase activity, and changes in the expression of homologs to phoA-like and phoD genes (members of the Pho regulon). However, R. mediterranea showed higher phosphatase activity than P. agardhii, suggesting greater tolerance to phosphate limitation. Taken together, we conclude that physiological features of R. mediterranea play an important role in the coexistence with P. agardhii under environmental changes.

Microbial food-web components in two hypertrophic human-impacted Pampean shallow lakes: interactive effects of environmental, hydrological, and temporal drivers

Few studies have intensively assessed the dynamic of all planktonic components and their microbial communities in hypertrophic shallow lakes. The aim of this work was to study the potential drivers shaping the microbial food-web components (heterotrophic bacteria and strictly phototrophic microplankton). Thus, we studied the monthly abundances and functional groups of the different planktonic food-web components (heterotrophic bacteria, picocyanobacteria, picoeukaryotes, heterotrophic flagellates, ciliates, phytoplankton, zooplankton) in two interconnected and hypertrophic Pampean shallow lakes (Gomez and Carpincho) during dry–wet periods (27-month study). We hypothesized that temporal (intra and inter-annual) factors exert a major role in shaping the microbial food-web components in both lakes. Both shallow lakes showed similar dynamic in the environmental variables, that followed inter-annual and seasonal variations. In Gomez, the variation of microbial components was mainly explained by a combination of environmental, predation, and temporal factors (38.2%), whereas in Carpincho by pure temporal drivers (31.8%). Microbial and predator components were significantly different between dry and wet periods. The connection and closeness between both lakes seem not to play a major role in the factors driving the microbial component abundances. These lakes are strongly influenced by temporal factors, which regulate not only the microbial components, but also the physical, chemical, and biological variables.

Ecological restoration and factors regulating phytoplankton community in a hypertrophic shallow lake, Lake Taihu, China

A restoration program for the control of cyanobacterial blooms and the re-establishment of submerged macrophytes was conducted in Meiliang Bay of Lake Taihu since 2003. The effect of this ecological projects on plankton community and water quality, and factors regulating phytoplankton community were investigated in 2005. In general, some improvements of water quality occurred in the ecological restoration region, especially in the region of restoring aquatic macrophytes, where we detected significant reduction of nutrients. However, it seems the abundance of phytoplankton cannot be effectively control by the present ecological engineering. The phytoplankton abundance was high in the target restoration zone. Results of CCA and correlation analysis indicate that the phytoplankton community was mainly controlled by physico-chemical factors. Cyanobacteria species were positively related with pH, temperature, TP and TSS, while negatively related with TN, TN/TP and conductivity. The most discriminant variable was TN/TP, which explained 15% of the total variance of phytoplankton. However, TN was more important for the fluctuation of TN/TP than TP. It suggested that TN may be the ultimate factor controlling the phytoplankton community in Lake Taihu. Variation partitioning analysis showed that the pure contribution of crustacean was low for the variation of phytoplankton, suggesting that top-down control by crustacean zooplankton was weak in Lake Taihu. In general, this study suggested the reduction of nutrient load should be more important than top-down control using zooplankton for the ecosystem restoration in Lake Taihu.

The response of a shallow hypertrophic lake to innovative restoration measures – Uzarzewskie Lake case study

The aim of nitrate treatment as a restoration method is to bring about a reduction of the high potential reactivity of sediments by the oxidation of biodegradable organic matter. As a consequence, oxygen uptake by sediments is reduced, sulphate reduction is lowered, hydrogen sulphide formation diminished and iron present in sediments binds phosphorus (P) effectively. This approach was applied in Uzarzewskie Lake, where innovative nitrate treatment of sediments was proposed through the diversion of subsurface waters to the lake hypolimnion. Nitrate-rich and oxygenated water from two tributaries was directly delivered to the hypolimnion through pipelines in order to increase redox potential and eliminate sediments as a P source. Research on selected physico-chemical and biological water characteristics was conducted in the years 2005–2015 to determine the results of restoration measures.P concentrations in lake waters were reduced both in surface water layers and the hypolimnion, hence epilimnetic chlorophyll-a content was diminished as well. Statistically significant changes in phytoplankton community structure were noted as the biomass of cyanobacteria had decreased in 2010 and 2013–2014 in comparison to 2006, while cryptophytes biomass increased in 2011–2013 in relation to 2007. Unfortunately, planktivorous fish domination in fish fauna exert a distinct pressure on zooplankton, thus Rotifera dominated and Cladocera were rarely noted. Additionally, the morphometric lake features (steep slopes, small surface surrounded by forest) create unfavorable conditions for submerged macrophyte development, and hinder the effectiveness of the restoration. Long-term studies revealed that the reconstruction of the phytoplankton community was not permanent, hence further restoration activity is required, with the inclusion of new techniques.

Phenotypic plasticity in freshwater picocyanobacteria.

Picocyanobacteria can occur as single-cell (Pcy) or as colonies (CPcy). Published evidence suggests that some Pcy strains have the capability to aggregate under certain culture conditions, however this has not been demonstrated to occur in natural environments. We investigated whether the Pcy and CPcy belong to the same species (i.e. phylotype), and the factors that determine their morphological and genetic variability in a hypertrophic shallow lake dominated by picocyanobacteria. Six main different morphologies and >30 phylotypes were observed. All sequences retrieved belonged to the 'Anathece + Cyanobium' clade (Synechococcales) that are known to have the capability of aggregation/disaggregation. The temporal variation of picocyanobacteria morphotype composition was weakly correlated with the DGGE temporal pattern, and could be explained by the composition of the zooplankton assemblage. Laboratory experiments confirmed that the small cladoceran Bosmina favoured the dominance of CPcy, i.e. Cyanodictyon doubled the size of the colonies when present, most likely through the aggregation of single-cell picocyanobacteria into colonies. Flow cytometry cell sorting and 16S rRNA + ITS sequencing of the Pcy and CPcy cytometrically-defined populations revealed that some phylotypes could be found in both sorted populations, suggesting phenotypic plasticity in which various Synechococcales phylotypes could be found in situ either as single-cells or as colonies.

Distribution of planktonic crustaceans and its diurnal changes in a hypertrophic shallow lake: does the switch from turbid-water state to clear-water state matters?

The paper presents results of post hoc analysis of diurnal changes in the crustacean distribution along the horizontal transect in the shallow hypertrophic lake, which undergone a switch from turbid-water, phytoplankton-dominated state (chl-a : 257.8 ± 100.2 µg dm−3 , Secchi depth: 0.17 ± 0.02 m) to clear-water plant-dominated state (chl-a : 26.8 ± 4.8 µg dm−3 , Secchi depth: 1.03 ± 0.29 m). Changes in crustacean biomass were observed in two consecutive years during two July days (every sixth hour, starting with the noon) in three sampling sites (emergent macrophytes, submerged macrophytes and center), situated along the horizontal transect between the lake shore and the center of the lake. Analysis revealed that: (a) in turbid-water conditions crustaceans aggregated at night-time near the water surface, both in the littoral zone and the lake center; (b) in clear-water state a nocturnal increase in crustacean biomass was noted only in submerged macrophyte site, but concerned only cladocerans. In conclusion, we hypothesized that switch from phytoplankton-dominated to plant-dominated state effects in change of predator-avoidance strategy showed by planktonic crustaceans.

Is Recovery of Large-Bodied Zooplankton after Nutrient Loading Reduction Hampered by Climate Warming? A Long-Term Study of Shallow Hypertrophic Lake Søbygaard, Denmark

Nutrient fluctuations and climate warming can synergistically affect trophic dynamics in lakes, resulting in enhanced symptoms of eutrophication, thereby potentially counteracting restoration measures. We performed a long-term study (23 years) of zooplankton in Danish Lake Søbygaard, which is in recovery after nutrient loading reduction, but now faces the effects of climate warming. We hypothesized that the recovery of large-bodied zooplankton after nutrient loading reduction would be hampered by climate warming through indirect effects on fish size structure. We found a shift in macrozooplankton from initial dominance of Daphnia spp. towards Bosmina spp. as well as a decline in the body size of copepods and an increase in the abundance of nauplii. These changes coincided with the increase in small sized fish as a result of rising water temperature. Despite a reduction in body size, the total biomass of cladocerans increased coinciding with a diminished fish catch per unit effort (CPUE), and likely then an overall reduction in the predation on zooplankton. A cascading effect to phytoplankton was evidenced by enhanced zooplankton:phytoplankton and cladoceran:phytoplankton ratios and a decrease in Chl-a:TP and Chl-a:TN ratios. Our results indicate that climate warming, through changes in the size structure of fish community, has major effects on zooplankton size structure. In Lake Søbygaard, the decline in zooplankton size did not prevent, but modulated, the positive cascading effect on phytoplankton through an expected diminished fish CPUE related to nutrient loading reduction.