Shallow Subtropical Lake Research Articles

Understanding trophic structure variation in fish assemblages of subtropical shallow lakes: Combined effects of ecosystem size, productivity, and disturbance

Global environmental change has altered biotic interactions in ecosystems. Trophic structure is a major characteristic of ecosystems, and understanding the factors determining trophic structure is thus important for predicting the response of ecological dynamics and ecosystem services to future environmental change. Different kinds of drivers, including ecosystem size, productivity, and disturbance, have been proposed to explain trophic structure variation in various ecosystems. Much remains unknown about how these drivers operate in determining trophic structure and its underlying mechanisms, particularly in shallow lakes. Here, we evaluated the relative importance of ecosystem size, productivity and disturbance in influencing the trophic structure of fish assemblages in 30 subtropical shallow lakes. We determined that (1) ecosystem size, productivity and disturbance had no effect on food chain length; (2) the mean trophic level increased with increasing ecosystem size and productivity, with productivity contributing the most to explaining the variance in mean trophic level; and (3) ecosystem size, productivity, and disturbance, alone or combined, affected the proportion of species at specific trophic levels. These results indicate the potential for systematic discrepancies in the drivers of trophic structure among different systems and highlight that focusing on food chain length alone may not be sufficient in reflecting the variation in trophic status. Thus, changes in the mean trophic level and specific trophic-level guilds must be considered in future lake management practices.

Intraguild predation dampens trophic cascades in shallow aquatic mesocosms in the subtropics: Implications for lake restoration by biomanipulation

Abstract Intraguild predation (IGP), defined as killing and eating among potential competitors, is commonly observed in shallow lakes and is predicted to dampen trophic cascades and affect ecosystem properties (e.g. phytoplankton biomass or primary production). We tested this hypothesis by manipulating the density of two common lake predators, the small‐sized fish Pelteobagrus fulvidraco (the intraguild predator) and the shrimp Exopalaemon modestus (the intraguild prey), in outdoor mesocosms containing natural phytoplankton and zooplankton communities. In single predator treatments, both predators induced a strong trophic cascade, as evidenced by extinction of the key herbivore, Daphnia pulex, and increasing phytoplankton biomass and chlorophyll a concentration. When the two predators were added together, however, the strength of collective predator effects on zooplankton and the cascading effects on phytoplankton growth were weaker than the sum of the individual predator effects. We attributed this antagonism among predators to intraguild predation as demonstrated by the lower shrimp catch per unit effort in the presence of fish. Our study suggests that common predators in subtropical shallow lakes may act antagonistically and thereby dampen trophic cascades in food webs. Thus, when implementing biomanipulation efforts to suppress algal growth via enhancing zooplankton herbivory, both the quantity and diversity of lake predator assemblies should be considered.

The changing characteristics of phytoplankton community and biomass in subtropical shallow lakes: Coupling effects of land use patterns and lake morphology

The community composition and biomass of phytoplankton in shallow lakes are impacted by many environmental factors including water quality physicochemical parameters, land use in the watershed, and lake morphology. However, few studies have simultaneously evaluated the relative importance of these factors on the effect of community composition and biomass of phytoplankton. The relative importance of the water quality physicochemical parameters (water temperature [WT], total nitrogen [TN], total phosphorus [TP], pH, dissolved oxygen [DO], electrical conductivity [EC], turbidity and Secchi depth [SD]), land use (built-up land, farmland, waters, forest, grassland, and unused land) in the watershed, and lake morphology (area and depth) on the composition and biomass of phytoplankton communities were assessed in 29 subtropical shallow lakes in Wuhan, China, during different seasons from December 2017 to November 2018. The results showed that phytoplankton in all 29 lakes was mainly composed of Cyanophyta, Chlorophyta, and Bacillariophyta. Phytoplankton abundance was highest in summer and lowest in winter. We analyzed the relative importance of the three groups of variables to the community composition of the phytoplankton by variance decomposition. The results showed that the three groups of environmental variables had the highest explanation rate (> 80%) for the composition of the phytoplankton community in summer and autumn, and the explanation rates in spring and winter were 42.1% and 39.8%, respectively. The water quality physicochemical parameters were the most important variables affecting the composition of phytoplankton communities, followed by land use in the watershed. Through generalized additive model and structural equation model analysis, we found that the land use and lake morphology had minimal direct impact on the Chl-a and cell density of phytoplankton, mainly by altering the TN, TP, turbidity, SD, DO, and EC, which indirectly affected phytoplankton. WT and nutrients were still the main predictors of phytoplankton abundance. Built-up land was the main source of nitrogen and phosphorus in lakes. Correlation analysis found that forest and grassland had positive impacts on reducing lake nitrogen and phosphorus contents. This showed that increasing grassland and forest in the watershed could reduce the pollutants entering the lake. Our findings will contribute to water quality management and pollution control for subtropical shallow lakes.

Influence of Thermal Stratification on Seasonal Net Ecosystem Production and Dissolved Inorganic Carbon in a Shallow Subtropical Lake

AbstractThermal stratification is a critical physical process controlling carbon (C) flux from lakes into the atmosphere. In general, vertical water temperature profiles in shallow subtropical lakes can vary significantly because typhoons frequently induce vertical mixing across the entire lake due to strong winds and rapid flushing from river inputs. Since C fluxes are driven by dissolved inorganic C (DIC), it is necessary to understand stratification's influence on DIC dynamics in shallow subtropical lakes. Therefore, we aimed to clarify the impact of stratification on DIC in Yuan‐Yang Lake, a typical shallow subtropical mountain lake, by developing a net ecosystem production (NEP) model. We measured the vertical profile of water temperature and DIC once or twice a month from July 2004 to December 2017. We applied a three‐dimensional hydrological model to estimate residence time and NEP, which revealed that large amounts of DIC are stored in the lower layer from spring to summer due to the suppression of vertical mixing by stratification. In autumn and winter, the lake was well mixed, and DIC was evenly distributed in the water column due to vertical mixing. This was confirmed by vertical DIC profiles.

Southern coastal subtropical shallow lakes skin temperature driven by climatic and non-climatic factors.

Subtropical coastal shallow lakes (SCSL) are sensitive ecosystems. The lake-skin-water temperature (LSWT) is an average lake temperature proxy and responds to changes in surroundings, affecting biological and physical lake processes. In this study, M*D11A1 products are used to develop daytime and nighttime LSWT time series for 20 SCSL in South America. The influence of climatic (air temperature, surface net solar radiation, wind speed, and wind direction) and non-climatic (latitude, lake area, perimeter, width, length, and morphology) factors are evaluated from 2001 to 2017. Pearson's coefficients (ρ) and auto- and cross-correlations are used to establish the relation between LWST and the selected factors. We identify that the dynamic of LSWT is sensitive to geomorphological factors (latitude and lake width) throughout the year, especially in summer. In winter, the LSTW regime is mainly affected by wind direction (ρ = -0.66, p value < 0.01). Linear models are fitted to the temperature series to check the trend changes in the inflection points and the warming or cooling trend for LSWT. Considering the complete series, the maximum warming rate of LSWT is 0.25 °C per decade (°C/dec). The analysis of the identified sub-periods reveals that warming and cooling can occur (significantly) in shorter periods. The average trends within sub-periods for skin temperature-daytime (± 0.0105 °C/dec), skin temperature-nighttime (0.0041 °C/dec), and air temperature (- s0.006 °C/dec; 0.007 °C/dec) are estimated. Our approach has the potential to be applied in future studies due to the expansion of knowledge about the behavior of SCSL and the understanding of the current and potential effects of climate change in association with physical and geomorphological traits.

Vertical patterns of leaf physiology and biofilm characteristics for submerged macrophytes in a shallow subtropical lake

Little is known about vertical patterns of leaf characteristics for submerged macrophytes in freshwater ecosystems. Here, after sampling Ceratophyllum demersum and Hydrilla verticillata in deep (3.8 m) and shallow areas (1.0 m) in a shallow subtropical lake, we cut the individuals into segments along the vertical direction, and measured leaf biofilm and physiology characteristics. In the deep area, leaf pigment concentrations showed declining trends with an increasing water depth, but the enzymatic specific activity of peroxidase (POD-ESA) was precisely the opposite. Moreover, the amount of attached biofilm matter and epiphyte biomass were greatest on leaves of upper middle segments, and lowest on bottom segments. In the shallow area, bottom leaves of C. demersum had more attached biofilm matter and epiphyte biomass, but the opposite was true for H. verticillata. Additionally, leaf chlorophyll-a concentration was higher on the bottom segments for H. verticillata, and C. demersum had a higher POD-ESA in bottom leaves. Our analysis found that vertical pattern of leaf physiology was closely related to light intensity, and biofilms played an auxiliary role. Our study highlighted vertical patterns of leaf biofilm and physiology for submerged macrophytes, and suggested that such patterns are generally species-specific, especially in shallow areas.

Spatial variations in diffusive methane fluxes and the role of eutrophication in a subtropical shallow lake

Shallow lakes account for most of the diffusive CH4 emissions from global lakes, and they also suffer from eutrophication worldwide. Determining the effect of eutrophication on diffusive CH4 fluxes is fundamental to understanding CH4 emissions in shallow lakes. This study aimed to investigate the spatial variations in diffusive CH4 fluxes and explore the role of eutrophication in Lake Chaohu, a large and shallow eutrophic lake in the lower reaches of the Yangtze River. A one-year field observation was carried out to examine CH4 concentrations in the sediment and water and the diffusive fluxes of CH4 across the sediment-water interface (Fs-w) and water-air interface (Fw-a). Both Fs-w (0.306–1.56 mmol m−2 d−1) and Fw-a (0.097–0.529 mmol m−2 d−1) were upward and showed significant spatial heterogeneity and were significantly positively correlated. Parameters related to eutrophication had significant positive relationships with Fw-a, and the total phosphorus distribution in the water explained the greatest proportion of the spatial variation in Fw-a. Distance to shore and water depth were inversely correlated with Fw-a and modified the effects of eutrophication. Overall, the results provide direct evidence of the key role of eutrophication in shaping the spatial distribution of diffusive CH4 fluxes and a scientific basis for predicting changes in CH4 emissions with future eutrophication changes in shallow lakes in subtropical zones.

Drivers assessment of zooplankton grazing on phytoplankton under different scenarios of fish predation and turbidity in an in situ mesocosm experiment

Zooplankton play a key role in energy transfer within lake food webs, but we have a poor knowledge concerning their role as phytoplankton grazers in shallow subtropical lakes. In this study, we aimed to determine how zooplankton grazing upon phytoplankton is altered in different scenarios of fish predation and turbidity, and we explored the relevance of grazing compared to other environmental variables, to explain phytoplankton biomass changes. A mesocosm experiment was conducted by including the following treatments: fish, turbidity, fish + turbidity, and a control (without fish or varying turbidity). The experiment lasted 21 days, and samples were taken four times. Zooplankton grazing was only effective for the microphagous group upon Cryptophyceae, while large Chlorophyceae and small pennate Bacillariophyceae biomass were benefited in the presence of copepods and cladocerans, being negatively affected by depletions in nitrogen availability. In the turbidity treatment, a reduction in phytoplankton biomass was obtained, artificially increasing zooplankton grazing on phytoplankton, while fish presence inhibited grazing of adult copepods and cladocerans. The other groups of phytoplankton were only influenced by the environment. This experiment suggests that phytoplankton biomass variations would be more affected by the environment than by zooplankton grazing in shallow lakes from the Parana River.

Winter Climate Shapes Spring Phytoplankton Development in Non‐Ice‐Covered Lakes: Subtropical Lake Taihu as an Example

AbstractWinter warming plays a vital role in spring phytoplankton community succession in temperate lakes due to variation of ice cover duration in winter. How winter conditions affect spring phytoplankton in subtropical lakes without winter ice cover is, however, largely unknown. In this study, covering 26 years, we elucidated the effects of both climate conditions and nutrient levels in winter on the phytoplankton community (expressed as Morpho‐Functional Groups) in spring in Lake Taihu, a large shallow subtropical lake in China. During this period, wind speed declined significantly in both winter and spring from 1992 to 2017 and the sunshine hours increased slightly from 2000 to 2017. The biomass of eight dominant phytoplankton groups showed increasing trends from 1992 to 2017, while it declined for one group, which mainly included filamentous green algae such as Ulothrix sp. Nonmetric multidimensional scaling (NMDS) results indicated that wind speed and sunshine duration in winter and spring were the most important factors affecting the spring phytoplankton community in Lake Taihu. Also, partial least squares path modeling (PLS‐PM) suggested that the spring phytoplankton community was strongly affected by winter conditions. Our study indicated that the “climate memory” effects on phytoplankton, resulting from winter and acting on the following spring, not only occur in north temperate winter ice‐covered shallow lakes but also in subtropical lakes without ice‐covered in winter.

El papel del banco de propágulos en el restablecimiento de macrófitos después de la gestión para la recuperación de un lago somero en el sur de Brasil

At eutrophic shallow subtropical lakes, free-floating macrophytes can occupy the entire water surface. The removal of this vegetation favors the recolonization of submerged macrophytes from the propagule bank. Our study presents the response of the propagule bank of a shallow subtropical lake after management to reduce its hypertrophy. We sampled lake sediment in March 2016 at 15 points along three transects to investigate the richness and density of the propagule bank by emergence in the laboratory. We monitored water trophic conditions and restoration of vegetation in the field for 7 months. In the laboratory, 1382 seedlings of eight species emerged. Stuckenia pectinata and Chara zeylanica dominated the propagule bank. We identified the species S. pectinata, C. zeylanica and Ludwigia peploides in the field. The propagule bank was homogeneous, without difference (P&gt;0.05) between lake regions and transects. Sørensen’s similarity was 36% between the field and laboratory. When the submerged macrophytes grew in the field, there was a reduction of chlorophylla, the water remained clear, and the trophic state index was reduced from hypereutrophic to eutrophic. We concluded that the bank of propagules in shallow lakes is essential for the rapid reestablishment of submerged macrophytes after recovery strategies. These organisms help improve water quality.

Effects of Diversity, Coverage and Biomass of Submerged Macrophytes on Nutrient Concentrations, Water Clarity and Phytoplankton Biomass in Two Restored Shallow Lakes

Transplantation of submerged macrophytes to restore shallow lakes has been used as an effective measure to maintain a clear water state. Water quality is highly correlated with submerged macrophytes community, however, the relationships between water quality and the diversity, coverage and biomass of submerged macrophytes are, so far, not yet well studied. We analyzed the correlations of nutrient concentrations, water clarity and phytoplankton biomass with the metrics of submerged macrophytes community in two Chinese restored shallow subtropical lakes, Lake Wuli (Wuli-E, 5 ha) and Lake Qinhu (Qin-E, 8 ha). A similar biomass of submerged macrophytes was transplanted into each lake, while both the species richness and coverage of macrophytes in Qin-E were lower than Wuli-E. After a 1–2-year restoration, the diversity almost had no change, but the biomass density and coverage decreased in Wuli-E. As for Qin-E, the coverage of submerged macrophytes increased but biomass density and diversity decreased. The dominance of canopy-forming submerged macrophyte species Myriophyllum spicatum was observed in Qin-E and less meadow-forming biomass and species was observed than that in Wuli-E. Moreover, it was also observed that Wuli-E had a better water quality than that of Qin-E after transplantation. Path analysis results showed that macrophyte coverage and the diversity related to meadow-forming species (e.g., Vallisneria spinulosa) had strong effects on enhancing clarity and reducing nutrient concentrations. But the high biomass density accompanied by the canopy-forming species like M. spicatum was unfavorable for controlling nutrients. Our results provide important insight into the different roles that macrophyte diversity, biomass and coverage play in improving water clarity and controlling nutrient concentrations. This new knowledge will be instrumental in implementing more effective lake restoration, especially using macrophyte transplantation as a restoration tool in warm shallow lakes.

Density versus biomass responses of zooplankton to environmental variability in a subtropical shallow lake

ABSTRACT Density and biomass are commonly used in ecological studies of zooplankton. However, these metrics are not functionally equivalent and provide different information regarding community structure. Differences in the response of zooplankton, as estimated through both density and biomass in relation to environmental variability, were investigated in a large subtropical lake. Results based on 2 years of seasonal sampling at 9 points in Lake Mangueira (southern Brazil) showed a large difference in the pattern of dominance of zooplankton between the density and biomass metrics. Ciliates composed >58% of zooplankton density (rotifers 29.4%) but only 16.1% of zooplankton biomass. Analyzing the composition of zooplankton based only on biomass might therefore underestimate the importance of microzooplankton components, which are an important trophic link in pelagic food webs of subtropical systems. The differences between the 2 metrics in relation to abiotic variables were assessed through redundancy analysis. Seasonality was the main factor driving the variability of zooplankton, and density proved to be the best metric to show the seasonal variation of zooplankton taxa and taxonomic groups in relation to environmental conditions. Density was also the most appropriate metric for analyzing annual cycles and the seasonal succession of species. However, the biomass metric was significantly related to production-related parameters (pH and chlorophyll a) and was essential to better understand zooplankton grazing relationships and the trophic structure of the system.

A small omnivorous bitterling fish (Acheilognathus macropterus) facilitates dominance of cyanobacteria, rotifers and Limnodrilus in an outdoor mesocosm experiment.

Small omnivorous fish often dominate in subtropical shallow lakes, and they may affect the community structure of aquatic organisms on at least two trophic levels. However, in the study of aquatic food webs in subtropical lakes, most ecologists have focused on the effects of large-sized omnivorous species (e.g. common carp), studies of small-sized species being scarce. We conducted a mesocosm experiment with two treatments (fish presence and absence) to examine the effects of a small-sized omnivore, bitterling (Acheilognathus macropterus), on phytoplankton, zooplankton and benthic macroinvertebrates. Our results showed that bitterling presence significantly increased the chlorophyll a concentration and biomass of phytoplankton, which became dominated by cyanobacteria (mainly Aphanizomenon spp.) that accounted for >99% of both total phytoplankton abundance and biomass. Both the abundance and biomass of zooplankton were also higher in the fish-present treatment, but small rotifers became dominant, and the zooplankton:phytoplankton biomass ratio decreased, indicating less grazing on phytoplankton. Moreover, both the abundance and biomass of benthic macroinvertebrates (tubificids) were higher in the bitterling-present treatment than in the controls, which is opposite to the situation found when omni-benthivorous fish (e.g. crucian carp) dominate. Higher biomass of tubificids may, in turn, result in higher sediment nutrient release. Our study suggests that A. macropterus, and maybe also other bitterling species, can alter both pelagic and benthic assemblages via both top-down and bottom-up control effects and lead to more turbid water in eutrophic lakes. Thus, more attention should be paid to these small omnivorous species in the restoration and management of shallow subtropical lakes.

Energetic Asymmetry Connected with Energy Flow Changes in Response to Eutrophication: A Study of Multiple Fish Species in Subtropical Shallow Lakes

Energy flow is a central characteristic in all ecosystems, and it has attracted considerable scientific attention due to its significant effects on the stability of food webs. Lake ecosystems that undergo regime shifts (clear water phase, phytoplankton dominated changed into turbid water, macrophytes dominated or vice versa) are characterized by a series of transformation in trophic structure. Although previous studies have mainly focused on the causes and consequences of regime shifts in shallow lakes, studies about responses of energy flow changes to regime shifts is far from complete. In this paper, we estimated trophic position and benthivory (i.e. degree of benthivory) of seventeen fish species from seven shallow lakes. Our data show that the trophic position and benthivory of fish species in clear water phase are significantly higher than in turbid water. This finding might help spark some ideas for subtropical lake eutrophication treatment.

The Role of Top-Down and Bottom-Up Control for Phytoplankton in a Subtropical Shallow Eutrophic Lake: Evidence Based on Long-Term Monitoring and Modeling

Lake ecosystems are exposed to a range of anthropogenic pressures, particularly eutrophication, and in some cases also stocking and/or overfishing of top-predator fish species, all factors that have implications for the food web structure and which could lead to dominance of nuisance cyanobacteria. Restoration of degraded lakes demands insight into the relative role of top-down for bottom-up regulating forces. While knowledge about these forces in temperate lakes is extensive, comparatively little is known of their role in subtropical lakes where the importance of herbivorous and benthic feeding fish is higher. Here, we analyzed a long-term monitoring data set on subtropical, shallow Lake Taihu, China, and applied random forests regression to examine how phytoplankton was related to environmental variables and biotic assemblages. Our results indicate that the biomass and density of phytoplankton increased with increasing biomass of benthivorous and zooplanktivorous fish and decreased with increases in ammonium concentrations, the nitrogen to phosphorus ratio, and zooplankton biomass, while the response to climate fluctuations and changes in the biomass of piscivores was weak. Effects of higher trophic levels explained as much of the variance in phytoplankton biomass as did nutrients and climatic factors. Moreover, the remarkably reduced ratio of zooplankton to phytoplankton biomass and the decline in cladoceran individual biomass emphasized the increasing importance of top-down control in regulating the phytoplankton following extensive stocking. Our findings offer insight into how fish management may be combined with catchment-level restoration measures to conserve and enhance water quality.

Invertebrates and microbiota associated with aquatic macrophyte degradation in a shallow lake in southern Brazil

Aquatic macrophytes are the main producers of organic matter in shallow aquatic ecosystems. They are also food sources for many herbivores. When macrophytes die, they enter the debris chain, are conditioned by microbial action and colonized by benthic invertebrates which remobilize nutrients from their biomass. In subtropical aquatic systems, the participation of shredder invertebrates has been questioned, highlighting the participation of fungi and bacteria in the degradation of organic matter. This study evaluated the degradation of two submerged aquatic macrophytes, Mayaca fluviatilis and Stuckenia pectinata, determining the quality of debris and microbiota and invertebrate trophic group density throughout the degradation process. Our results indicated that plants with lower polyphenol concentrations had higher degradation speeds. The shredders invertebrates had reduced abundance in both macrophytes, emphasizing the importance of bacteria and fungi in the nutrient cycling process in subtropical shallow lakes.

Phytoplankton structure is more influenced by nutrient enrichment than by temperature increase: an experimental approach upon the global changes in a shallow subtropical lake

portuguesA mudanca climatica e considerada uma das maiores ameacas aos ecossistemas aquaticos em todo o mundo. Entre as mudancas esperadas, incluindo a dinâmica do fitoplâncton, o crescimento de cianobacterias esta entre os mais citados. O objetivo deste estudo foi avaliar os efeitos isolados e combinados do aumento de temperatura e nutrientes na estrutura da comunidade fitoplanctonica em um grande lago raso subtropical oligo-mesotrofico (Lagoa Mangueira, sul do Brasil). O estudo foi realizado em microcosmos para simular possiveis cenarios de mudanca climaticas, sozinhos e em combinacao com enriquecimento de nutrientes (N e P), a partir de amostras de inverno e verao. Em ambas estacoes, a biomassa total do fitoplâncton, a riqueza e diversidade de especies nao variaram significativamente sob a manipulacao isolada da temperatura. O aumento da temperatura elevou significativamente a biomassa de cianobacterias em experimentos de inverno. No verao, a biomassa total do fitoplâncton, a riqueza de especies, Bacillariophyceae e Cyanobacteria aumentaram significativamente sob o enriquecimento de N e P. A interacao entre aumento de temperatura e adicao de nutrientes nao promoveu efeito significativo sobre os atributos fitoplanctonicos estudados. O aumento isolado da temperatura nao foi forte o suficiente para promover mudancas estruturais na comunidade fitoplanctonica. Entretanto, a adicao de nutrientes (N e P) demonstrou influenciar mais significativamente na maioria dos atributos do fitoplâncton, embora nao tenha sido observado o recrutamento de cianobacterias formadoras de floracoes. Nesse sentido, mais estudos enfatizando a resiliencia dos ecossistemas aquaticos quanto as mudancas climaticas podem contribuir para a conservacao e manejo desses ambientes. EnglishClimate change is considered one of the greatest threats to aquatic ecosystems around the globe. Among the expected changes, including the dynamics of phytoplankton, the growth of cyanobacteria is often cited. This study evaluated the individual and combined effects of increased temperature and nutrients on phytoplankton community structure in a large, oligo-mesotrophic subtropical shallow lake (Mangueira Lake, southern Brazil). The study was conducted in microcosms to simulate possible scenarios of climate change, alone and in combination with nutrient enrichment (N and P); water samples were collected in winter and summer. Phytoplankton total biomass, species richness and diversity did not vary significantly with manipulation of temperature alone, either in winter or in summer. Only cyanobacteria biomass increased significantly with an increase in temperature, in the winter experiments. In summer, phytoplankton total biomass, species richness, Bacillariophyceae and Cyanobacteria increased significantly with enrichment of N and P. The interaction between temperature increase and nutrient addition did not significantly affect phytoplankton attributes. Temperature increase alone was not sufficient to cause structural changes in the phytoplankton community. Instead, nutrient addition (N and P) proved to be the most significant influence on most phytoplankton attributes, even though turnover of bloom-forming cyanobacteria was not observed. Further studies on the resilience of aquatic ecosystems to climate changes may contribute to the conservation and management of these environments

Effects of environmental change on subfossil Cladocera in the subtropical shallow freshwater East Taihu Lake, China

East Taihu Lake (the south-eastern portion of Taihu Lake in China) suffers from eutrophication. The study of lacustrine subfossil zooplankton assemblages can be used to reconstruct the long-term evolution of lake water environments. In particular, the analysis of subfossil Cladocera can indicate historical changes in aquaticplants, water level, trophic states, and human disturbance in lakes. In this study, we correlated downcores changes in cladoceran assemblages with several geochemical proxies to identify the response of Cladocera to environmental change in East Taihu Lake over the past one hundred years. Redundancy analysis (RDA) and Pearson correlation analysis identified significant correlations between total phosphorus (TP) and Bosmina spp. (total), B. (E.) longispina, and Chydorus cf. sphaericus (correlation coefficients: −0.720 (P < 0.01), −0.646, and 0.667 (P < 0.05), respectively). We observed decreasing abundances of subfossil Bosminidae and increases in littoral cladoceran species and TP after the 1960s, coinciding with the introduction of land reclamation for fish and crab farming. These anthropogenic pressures exacerbated eutrophication, which led to the rapid growth of submerged vegetation and subsequently altered the zooplankton assemblage.

Phytoplankton functional groups succession and their driving factors in a shallow subtropical lake

The succession of phytoplankton functional groups and environmental variables were investigated from January to December 2018 in a shallow subtropical lake, East Taihu Lake, located in the Yangtze Delta of China. Altogether, 125 genera, 21 functional groups, and 7 predominant functional groups, D (Fragilaria acus, Synedra acus, and Nitzschia spp.), P (Fragilaria capucina and Aulacoseira granulate), MP (Oscillatoria spp., Diploneis ovalis, Gomphonema angustatum, Nitzschia palea, and Achnanthes spp.), Y (Cryptomonas ovata, Cryptomonas erosa, Chroomonas acuta, and Ceratium spp.), H2 (Anabaena spp.), M (Microcystis spp.), and W1 (Euglenoids spp.), were identified, and these predominant functional groups exhibited strong seasonal variations. Groups Y and W1 dominated from winter to spring when temperature and light were low. The external pollution load and high levels of nutrients entered the lake in June, allowing group M to become dominant, even ultimately triggering Microcystis blooming. Meanwhile, the abundance of aquatic plants in summer, combined with high levels of transparency and light availability, resulted in Group M being replaced by Group P and then Groups M and H2 rebounding in autumn. The redundancy analysis revealed that water temperature, nutrients, and light availability (Zeu/Zmix) were the key factors promoting phytoplankton succession. The ecological assessment results demonstrated a good ecological status based on the assemblage index (Q average = 3.0). This work emphasizes the fact that temperature, nutrients, and light were the driving factors for phytoplankton dynamics, and aquatic plants and external pollution were important external factors for the succession of phytoplankton.

Effects of juvenile crucian carp (Carassius carassius) removal on submerged macrophyte growth-implications for subtropical shallow lake restoration.

Rapid expansion of juvenile fish after biomanipulation can delay the successful restoration of submerged macrophytes, leading to a turbid water status in subtropical shallow lakes. Aimed to test the effects of direct removal of juvenile crucian carp (Carassius carassius) on water quality and growth of two submerged macrophytes Vallisneria natans and Hydrilla verticillate, a short-term outdoor mesocosm experiment was conducted in the Taihu Laboratory for Lake Ecosystem Research (TLLER). The results indicated that the concentrations of TN, TP, suspended solids, and chlorophyll a decreased significantly with increasing removal density of juvenile crucian carp, thus resulting in a clear status of the water. Additionally, the mean relative growth rate of V. natans and H. verticillata in the low- and high-density removal treatments were higher than that in the controls. Moreover, the removal of juvenile crucian carp also significantly increased the stem length of V. natans, but no significant effect on that of H. verticillata. Meanwhile, the total number of V. natans and H. verticillata in the low- and high-density removal treatments was higher than that in the controls, but all of H. verticillata were lower than the initials. Our results indicated that removing juvenile crucian carp could improve the water quality, increasing relative growth rate, height, and reproduction of V. natans, and improving the survival rate of H. verticillata. The promotion of fish removal on the V. natans growth was greater than H. verticillata. The results also implied that it was necessary to continuously remove the juvenile benthivorous fish several times for restoring the submerged macrophytes in shallow lakes.