Subtropical Chinese Lake Research Articles

Long‐term empirical evidence, early warning signals and multiple drivers of regime shifts in a lake ecosystem

Abstract Catastrophic regime shifts in various ecosystems are increasing with the intensification of anthropogenic pressures. Understanding and predicting critical transitions are thus a key challenge in ecology. Previous studies have mainly focused on single environmental drivers (e.g. eutrophication) and early warning signals (EWSs) prior to population collapse. However, how multiple environmental stressors interact to shape ecological behaviour and whether EWSs were detectable prior to the recovery process in lake ecosystems are largely unknown. We present long‐term empirical evidence of the critical transition and hysteresis with the combined pressures of climate warming, eutrophication and trophic cascade effects by fish stocking in a subtropical Chinese lake in the Yangtze floodplain. The catastrophic regime shifts are cross‐validated by 64‐year multi‐trophic level monitoring data and paleo‐diatom records. We show that EWSs are detectable in both the collapse and recovery trajectories and that including body size information in composite EWSs requires shorter time‐series data and can improve the predictive ability of regime shifts. Although full recovery has not yet been observed, EWSs prior to recovery provide us with the opportunity to take measures for a clear‐water regime. Climate warming and top‐down cascade effects have a negative influence on water clarity by altering lower trophic level abundance and body size, which, in turn, have a negative effect on macrophyte abundance. Furthermore, we identify a shift in the dominant driving forces from bottom‐up to top‐down after regime shifts, decoupling the relationships between nutrients and biological components and thus decreasing the efficiency of nutrient reduction. Synthesis. This study provides new insights into ecological hysteresis under multiple external stressors and improves our understanding of trait‐based early warning signals in both the collapse and recovery processes in natural freshwater ecosystems. For management practice, our work suggests that slowing down climate warming and weakening the fish predation pressure on food webs are necessary to increase the effectiveness of nutrient reduction in the restoration of lakes.

Long-term eutrophication affects vertical changes of Daphnia ephippia in the sediments of a subtropical Chinese lake.

Cladoceran remains (e.g., Daphnia ephippia) in sediments are usually able to accurately reflect the historical succession of cladoceran in lakes. However, mechanisms describing the interaction between density changes of empty ephippia and ephippia containing resting eggs in lake sediments and environmental factors remain unclear. The diversity and vertical changes of Daphnia ephippia in the 30-cm sedimentary layer (equivalent to about 90years) were investigated in a subtropical Chinese lake, Lake Chaohu. The ephippia of D. similoides sinensis, D. pulex, and D. galeata were identified in the lake sediments. The densities and accumulation rates of both empty ephippia and ephippia containing resting eggs of three Daphnia species showed similar patterns. The values in the surface sedimentary layers (after the 1970s) were notably higher than in the bottom sedimentary layers (before the 1970s). The densities and accumulation rates of both empty ephippia and ephippia containing resting eggs of D. similoides sinensis were higher than those of D. pulex and D. galeata. Highly significant relationships (P< 0.001) between the TN contents and the ephippial densities and accumulation rates of D. similoides sinensis and D. galeata. TP contents had also significant correlations with the ephippial densities and accumulation rates of D. similoides sinensis and D. galeata, whereas it was not significant correlations with those of D. pulex (P> 0.05). Our results suggested that long-term eutrophication and cyanobacterial blooms might significantly affect the production of Daphnia ephippia and their vertical distribution in the sediments in eutrophic lakes.

Effect of Temperature and Food on Development and Growth of Daphnia similoides (Cladocera: Daphniidae) from Lake Donghu

ABSTRACT Growth, reproduction, and life history parameters of Daphnia similoides from a subtropical eutrophic Chinese lake (Lake Donghu) were measured at four temperatures (10, 15, 25 and 35°C) and two food concentrations (0.2 and 2 mg carbon L−1 of a green alga) in the laboratory. D. similoides had five juvenile instars and up to 28 adult instars. The number of adult instars was greater at the low food level than at the high food level. The number of adult instars with maximal number of offspring, the cumulative number of offspring per female, development time, and body lengths of females and offspring were higher or longer at the low temperatures than at the high temperatures. Body length of females was significantly correlated with offspring number per adult instar at all combinations of temperature and food. Innate capacities of increase (0.13–1.01) linearly increased with temperature.

Bioaccumulation of the hepatotoxic microcystins in various organs of a freshwater snail from a subtropical Chinese Lake, Taihu Lake, with dense toxic Microcystis blooms

In this paper, we describe the seasonal dynamics of three common microcystins (MCs; MC-RR, MC-YR, and MC-LR) in the whole body, hepatopancreas, intestine, gonad, foot, remaining tissue, and offspring of a freshwater snail, Bellamya aeruginosa, from Gonghu Bay of Lake Taihu, China, where dense toxic Microcystis blooms occur in the warm seasons. Microcystins were determined by liquid chromatography electrospray ionization mass spectrum. Microcystin (MC-RR + MC-YR + MC-LR) content of the offspring and gonad showed high positive correlation, indicating that microcystins could transfer from adult females to their young with physiological connection. This study is the first to report the presence of microcystins in the offspring of the adult snail. The majority of the toxins were present in the intestine (53.6%) and hepatopancreas (29.9%), whereas other tissues contained only 16.5%. If intestines are excluded, up to 64.3% of the toxin burden was allocated in the hepatopancreas. The microcystin content in the intestine, hepatopancreas, and gonad were correlated with the biomass of Microcystis and intracellular and extracellular toxins. Of the analyzed foot samples, 18.2% were above the tolerable daily microcystin intake recommended by the World Health Organization (WHO) for human consumption. This result indicates that public health warnings regarding human ingestion of snails from Taihu Lake are warranted. In addition, further studies are needed to evaluate the occurrence by Microcystis in relation to spatial and temporal changes in water quality.

Development of the microbial communities in lake donghu in relation to water quality

There is increasing recognition that protozoa is very useful in monitoring and evaluating water ecological healthy and quality. In order to study the relationship between structure and function of protozoan communities and water qualities, six sampling stations were set on Lake Donghu, a hypereutrophic subtropical Chinese lake. Microbial communities and protists sampling from the six stations was conducted by PFU (Polyurethane foam unit) method. Species number (S), diversity index (DI), percentage of phytomastigophra, community pollution value (CPV), community similarity and heterophy index (HI) were mensurated. The measured indicators of water quality included total phosphorus (TP), dissolved oxygen (DO), Chemical oxygen demand (COD), NH(4)(+), NO(2)(-) and NO(3)(-). Every month water samples from stations I, II, III, IV were chemically analyzed for a whole year, Among the chemically analyzed stations, station I was the most heavily polluted, station II was the next, stations III and IV had similar pollution degrees. The variable tendencies of COD, TP, NH(3), NO(2)(-), NO(3)(-), and DO during the year was approximately coincident among the six stations. Analysis from the community parameters showed that the pollution of station 0 was much more serious than others, and station V was the most slight. Of the community parameters, CPV and HI were sensitive in reflecting the variables of the water quality. Community similarity index was also sensitive in dividing water qualities and the water quality status of different stations could be correctly classified by the cluster analysis. DI could reflect the tendency of water quality gradient, species number and percentage of Phytomastigophora was not obvious in indicating the water quality gradient.

Accumulation of microcystins in various organs of the freshwater snail Sinotaia histrica and three fishes in a temperate lake, the eutrophic Lake Suwa, Japan

So far, there has been only one study to examine microcystin (MC) contents in various organs of snails in a subtropical Chinese lake. In this study, tissue distribution and seasonal dynamics of MC-RR and -LR were investigated in various organs of a freshwater snail ( Sinotaia histrica) in a temperate eutrophic lake, Lake Suwa, Japan. Accumulation of microcystins in some fish was also investigated. There was marked temporal variation in the MC content of various organs of the snail. The digestive tract had the highest MC content (mean 9.03 μg g −1 DW and range 3.74–23.2 μg g −1 DW), followed by the gonad (mean 6.90 μg g −1 DW and range 0.07–22.7 μg g −1 DW) and hepatopancreas (mean 5.38 μg g −1 DW and range 1.08–8.79 μg g −1 DW), whereas the foot had the least (mean 2.48 μg g −1 DW and range 0.04–4.45 μg g −1 DW). The disappearance of MC-LR in the hepatopancreas indicated that S. histrica is able to depurate MC-LR efficiently. MC-RR was detected in the muscle of three species of fish, with the highest content in Carassius auratus (79.4 μg kg −1 BW). Because of substantial MC accumulation in these edible aquatic animals in Lake Suwa, it is recommended that regular monitoring of MCs should be undertaken in both cyanobacteria and aquatic animals.

Studies on the Food Web Structure of Lake Donghu Using Stable Carbon and Nitrogen Isotope Ratios

ABSTRACT Food web structure was studied by using carbon and nitrogen isotope ratios in a hypereutrophic subtropical Chinese lake, Lake Donghu. High external nutrient loading and the presence of abundant detritus from submersed macrophytes were responsible for the high sediment δ13N and δ13C, respectively. 13C was significantly higher in submersed macrophytes than in other macrophytes. The similar δ13C values in phytoplankton, zooplankton, zoobenthos, and planktivorous fish indicate that phytoplankton was the major food source for the consumers. By using a δ15N mass balance model, we estimate that the contributions of zooplankton to the diet of silver carp and bighead carp were 54% and 74%, respectively, which is in agreement with previous microscopic observations on intestinal contents of these fishes.

Effect of Food and Temperature on the Growth and Development of Moina irrasa (Cladocera: Moinidae)

ABSTRACT Body length, instar duration, fecundity, and survival rate of Moina irrasa from a subtropical Chinese lake were studied at three food concentrations (4, 8, and 40 mg/L, wet weight) and six temperatures (10, 15, 20, 25, 30, and 35°C) in the laboratory. Body length tended to decrease with increase of temperature, while the trend was reversed as food concentration rose. M. irrasa had three juvenile instars, except there were four at 10°C, and the number of adult instars showed great variation (3–15). Water temperature and food concentration both affected the duration time of adult instars. The largest broods were from the third to sixth adult instars, depending on food and temperature, and the mean highest number of offspring per brood was 56 at 25°C. A significant relationship between body length and brood size appeared at high (40 mg/L) and medium (8 mg/L) food concentrations, while there was no significant relationship at low food concentration except at 25°C. The intrinsic rate of population increase ranged between 0.104 and 1.825 ind./day.

The Impact of Silver Carp (Hypophthalmichthys molitrix) on the Rotifer Community in a Eutrophic Subtropical Chinese Lake

ABSTRACT From 29 April to 25 June in 1999, an enclosure experiment was conducted in Lake Donghu to assess the impact of planktivorous silver carp on the planktonic rotifer community. We set up four treatments with silver carp biomass at 0, 116, 176, and 316 g m−2. Total rotifer density was significantly higher in the no-fish enclosure than in fish-present enclosures. Fish predation on the rotifers alleviated zooplankton competition and resulted in dominance of small zooplankton species (Anureaopsis fissa, Trichocerca pusilla and Moina micrura) in fish-present enclosures. However, some relatively larger species (Polyarthra vulgaris, Brachionus angularis, Brachionus calyciflorus, and Asplanchna spp.) showed higher densities in the no-fish enclosure than in fish-present enclosures.

Enhancement of dissolved phosphorus release from sediment to lake water by Microcystis blooms—an enclosure experiment in a hyper-eutrophic, subtropical Chinese lake

To clarify the possible influence of Microcystis blooms on the exchange of phosphorus (P) between sediment and lake water, an enclosure experiment was conducted in the hypereutrophic subtropical Lake Donghu during July–September 2000. Eight enclosures were used: six received sediment while two were sediment-free. In mid-August, Microcystis blooms developed in all the enclosures. There was a persistent coincidence between the occurrence of Microcystis blooms and the increase of both total P (TP) and soluble reactive P (SRP) concentrations in the water of the enclosures with sediments. In sediment-free enclosures, TP and SRP concentrations remained rather stable throughout the experiment, in spite of the appearance of Microcystis blooms. The results indicate that Microcystis blooms induced massive release of P from the sediment, perhaps mediated by high pH caused by intense algal photosynthesis, and/or depressed concentrations of nitrate nitrogen (NO 3-N).

Enhancement of Moina micrura by the filterfeeding silver and bighead carps in a subtropical Chinese lake

The changes of cladoceran zooplankton from 1980 to 1996 were studied in a hypereutrophic subtropical Chinese lake, Lake Donghu, and an enclosure experiment was conducted to examine the possible role of the increased fish production in the enhancement of Moina micrura in the lake after mid-1980s. During the 1980s, the most striking event of the cladoceran community in the lake was that dominance of Daphnia was replaced by Moina following a steady increase in the production of planktivorous fish. This replacement was a direct result of increased fish predation, since our enclosure experiment indicates that Moina are less vulnerable to fish predation than Daphnia, and that increase in fish-stocking rate favors the development of M. micrura. The stronger resistance of M. micrura to fish predation may be attributed to its smaller body size and higher intrinsic growth rate than the daphnids. The present study has a strong parallel with the responses of zooplankton community to predators observed in many temperate lakes, and perhaps the only real difference is that in our lake the small rapidly growing cladoceran is Moina, rather than Bosmina or some other typical temperate take species. In the present study, the strong fish predation caused a shift from Daphnia to small zooplankton but not a corresponding increase in phytoplankton, which is in sharp contrast to what is expected with the classic qtrophic cascadeq process.

Long-term (1956–1999) dynamics of phosphorus in a shallow, subtropical Chinese lake with the possible effects of cyanobacterial blooms

This paper describes the long-term dynamics of phosphorus concentrations in both the lake water and the sediment in a subtropical Chinese lake, Lake Donghu. The total phosphorus (TP) concentration in the lake water experienced an upward trend from the 1950s, and peaked in 1983/1984, but declined obviously afterwards. From the 1950s to the1990s, TP content in the upper 10 cm sediment of the lake increased steadily from 0.307 to 1.68 mg P g DW −1 at Station I and from 0.151 to 0.89 mg P g DW −1 at Station II, respectively. The TP increase in the lake water before mid-1980s was mainly attributed to the massive input of sewage P. The outbreak of cyanobacterial blooms coincided with the peaks of TP and Orthophosphate (PO 4-P) in the water in mid-1980s, and the maximum TP of the water reached as high as 1.349 mg/l at Station I and 0.757 mg/l at Station II (in 1984), respectively. The declines of TP and PO 4-P in the water after mid-1980s was coincident with the disappearance of cyanobacterial bloom.

Long‐term changes of planktonic rotifers in a subtropical Chinese lake dominated by filter‐feeding fishes

1. The long‐term changes (1956–1998) in density and species composition of planktonic rotifers were studied at two sampling stations (I, II) of Lake Donghu, a shallow eutrophic Chinese Lake densely stocked with filter‐feeding fishes. Annual average densities of rotifers increased with an increase in fish yield and eutrophication, whilst species number decreased from 82 in 1962–1963 to 62 in 1994–1998.2. During 1962–98, some species such as Anuraeopsis fissa, Polyarthra spp. (including P. dolichoptera &amp; P. vulgaris), Trichocerca pusilla and Synchaeta oblonga increased their percentage in abundance remarkably, whilst the proportion of Keratella cochlearis decreased at two relatively eutrophic stations from 19 to 4.2% at Station I and from 30 to 3.2% at Station II.3. The high rmax of A. fissa probably made it more successful than other rotifers under high predation pressure by planktivorous fish. The decrease in the K. cochlearis population might be attributed partly to predation by Cyclops vicinus.4. Small rotifers were less vulnerable to fish predation than large‐sized cladocerans. Decreases in cladocerans coincided with increases in rotifers, suggesting that the indirect effect of fish predation on cladocerans might have partly contributed to the population development of rotifers in Lake Donghu during recent decades.5. We also conducted surveys (1994–1998) of seasonal dynamics of rotifers at four sampling stations (I–IV) which have varied in trophic status after fragmentation of the lake in the 1960s. A total of 75 species were identified at the four stations. Both densities and biomass of rotifers were considerably higher in the two more eutrophic stations than in the two less eutrophic stations. This indicates that the population increase of rotifers at Stations I and II during recent decades might be partly attributed to eutrophication of the lake water.

Impacts of Filter-Feeding Fishes on the Long-Term Changes of Crustacean Zooplankton in a Eutrophic Subtropical Chinese Lake

ABSTRACT Long-term changes in the crustacean zooplankton community (calanoid and cyclopoid copepods and cladocerans) were studied in Lake Donghu, a shallow and eutrophic Chinese lake. This lake had been earlier stocked with two pump Alter-feeding fishes, silver carp (Hypopthalmichthys molitrix) and bighead carp (Aristichthys nobilis). During the 1950s and the mid- 1980s, the ratio of copepods to cladocerans was relatively stable but showed a general increase thereafter. From the early-1980s to the 1990s. calanoid/cyclopoid ratios decreased obviously. In the 1990s. Cyclops vicinus, Diaphanosoma brachyurum, and Moina micrura were dominant; the abundance of C. vicinus and M. micrura increased significantly; and D. brachyurum showed a substantial decrease. The study shows that under extremely high pressure of fish predation, the species which could recover rapidly from fish predation would be the most likely to survive and increase their numbers.

Long-term changes of Copepoda community (1957-1996) in a subtropical Chinese lake stocked densely with planktivorous filter-feeding silver and bighead carp

In contrast to the relatively well documented impact of particulate-feeding fish on zooplankton communities, little attention has been devoted to the impact of filter-feeding fish. Filter-feeding silver and bighead carp are the most intensively cultured fish species in Asia and comprise much of the production of Chinese aquaculture. However, little information is known about the impact of either fish on the zooplankton community. Long-term changes in the Copepoda community (1957-1996) were studied at two sampling stations of a subtropical Chinese lake (Lake Donghu) dominated by silver and bighead carp. For both calanoids and cyclopoids, the littoral station (I) was much more resource profitable than the pelagic station (II). There has been a tremendous increase in the annual fish catch over the past 30 years due to the increased stocking with fingerlings of the two carp species. There was a notably higher fish density at Station I than at Station II. Cyclopoid abundance was notably higher at Station I than at Station II during the 1950s to the 1980s, while the reverse became true in the 1990s. This is probably because when fish abundance increased to an extremely high level, the impact of fish predation on the cyclopoids became more important than that of food resources at the littoral station. At both stations, cyclopoid abundance was relatively low in spite of the presence of abundant prey. Similarly, calanoid density did not differ significantly between the two stations in the 1950s and 1960s, but was significantly lower at Station I than at Station II during the 1980s and 1990s. Such changes are attributed to the gradient of fish predation between the stations and an increasing predation pressure by the fish. The increased fish predation also correlated with a shift in summer-dominant calanoids from larger species to smaller ones. In conclusion, the predaceous cyclopoids are affected by fish predation to a much lesser extent than the herbivorous calanoids, and therefore increased predation by filter-feeding fish results in a definite increase in the cyclopoid/calanoid ratio. Predation by filter-feeding fish has been a driving force in shaping the copepod community structure of Lake Donghu during the past decades.

Gut contents of silver carp, Hypophthalmichthys molitrix, and the disruption of a centric diatom, Cyclotella, on passage through the esophagus and intestine

The digestibility of algae by stomachless filter-feeding fish has been debated for decades. Results from gut contents and digestive enzyme analysis suggest poor utilization, while the measurement of food assimilation using radiolabeled isotope techniques indicate reasonable assimilation efficiency. Phytoplankton in the gut contents of the planktivorous filter-feeding silver carp were studied during March–May. The fish were cultured in a large net cage in a shallow, hypertrophic subtropical Chinese lake, Lake Donghu. In terms of biomass, the dominant phytoplankton in the fore-gut contents were Cyclotella (average 77.8%, range 69.7–93.5%) and Cryptomonas (average 9.57%, range 0–20.4%). The Ivlev's electivity index E of silver carp was much higher for Cyclotella (1.54) than for Cryptomonas (0.56). The majority of the phytoplankton found in the intestines of silver carp were 8–20 μm, but they were also able to collect particles as small as 4.5–10 μm, smaller than their filtering net meshes, suggesting that the secretion of mucus may play an important role in collecting such small particles. We conclude that disruption of cell walls is principally by the pharyngeal teeth, explaining previous contradictory conclusions.