Rotifer Density Research Articles

Genome-wide identification of seven superoxide dismutase genes in the marine rotifer Brachionusrotundiformis and modulated expression and enzymatic activity in response to microplastics and nutritional status.

Microplastics (MPs) pollution has attracted worldwide attention. Superoxide dismutase (SOD) is a sensitive indicator for assessing the toxic effects of MPs in aquatic organisms. However, few studies have been performed to identify all genes encoding SOD in aquatic invertebrates. Especially, effects of MPs on SOD activity and expression in aquatic organisms under starvation or a subsequent refeeding status are unclear. In the present study, all full-length genes encoding SOD were cloned and characterized from the marine rotifer Brachionus rotundiformis, including CuZnSOD1, CuZnSOD2, CuZnSOD3, CuZnSOD4, CuZnSOD5, MnSOD1, and MnSOD2. The CuZnSOD1, CuZnSOD2 and MnSOD2 are homologous to SODs from vertebrates and the other SOD proteins are rotifer-specific according to the results from the phylogenetic tree. The conserved signature sequences and binding sites of Cu2+, Zn2+and Mn2+ were also identified in the seven SOD proteins. Compared with feeding, starvation down-regulated SOD activity and mRNA expression of CuZnSOD2, CuZnSOD4, CuZnSOD5, MnSOD1 and MnSOD2 while refeeding maintained SOD activity comparable to the feeding level and up-regulated CuZnSOD5 and MnSOD2. Intake of MPs by B. rotundiformis was observed by examining fluorescence signals from the fluorescently-labeled microplastics under different nutritional status. Exposure to MPs reduced rotifer density and increased malondialdehyde (MDA) content and SOD activity in the rotifers under the refeeding condition, but did not affect these indicators under the feeding and starvation conditions. However, mRNA expression of some tested genes was responsive to MPs in the fed, starved and refed rotifers. The present study for the first time demonstrated a nutritional status-dependent effect of MPs on oxidative stress response, and provided more sensitive molecular biomarkers for assessing the toxicity of MPs using B. rotundiformis as a model animal.

Responses of plankton community to water environment changes in karst springs in Wuming Basin, Guangxi Province

近年来我国西南岩溶地区一些岩溶泉出现水生生态系统由草型向藻型转变的问题，但水化学监测却显示泉水水质依旧良好.岩溶泉的水质变化与水生生态系统退化不同步，故探寻岩溶泉水环境的变化以及浮游生物群落的响应，对全面掌握和客观评价泉水的健康状况具有重要意义.于2016年7月、2020年7月两个时期对广西武鸣盆地内4个岩溶泉进行了浮游生物调查和水环境监测.结果显示，4年后浮游生物检出种类和数量增加.浮游植物快速繁殖，总丰度由1.71×10⁵ cells/L上升至79.92×10⁵ cells/L，蓝藻门或绿藻门比例上升；浮游动物总丰度由264.28 ind./L降至137.25 ind./L，大型甲壳类浮游动物密度减少，部分泉口轮虫类密度增加.同时，优势种转变为更适应于富营养化环境的耐污种，或耐污种种类和数量上升.浮游生物的多样性与水体营养水平并未完全吻合.理化指标的水质评价结果显示，4个泉的水质仍属于I类或Ⅱ类，但综合营养状态指数显示营养状态均向富营养化方向发展，大部分泉基本已处于中-富营养水平.冗余分析结果显示，水温、pH、亚硝态氮、总磷、硝态氮浓度是影响岩溶泉浮游植物群落结构的主要环境因子.浮游动物群落结构与水温、亚硝态氮、氨氮和叶绿素a浓度显著相关.两个时期浮游生物对主要水环境因子的变化有了一定的响应.建议岩溶泉的水质评价依据理化指标的同时还应关注生物指标，多个指标的相互验证才能更全面的反映水体健康状况.同时，为使泉口以及区域地下水资源的可持续开发利用，建议对重要泉口划分泉域保护区，控制泉口污染物的输入，合理开发和管理地下水资源，对已转变的水生生态系统可考虑尽快进行生态修复.;The shift from a clear macrophyte-dominated state to a turbid algal-dominated state has been occurred in some karst springs of Southwest China in recent years, but hydrochemical monitoring still reveal good conditions. The trend of water quality in karst springs is not synchronous with the degradation of aquatic ecosystem. To understand comprehensively and evaluate objectively the health of the spring water, we discussed how plankton community responds to water environment. Plankton investigation and physicochemical parameters monitoring have been conducted at four karst springs of the Wuming Basin, Guangxi province, in July 2016 and July 2020, respectively. The results showed that the number of species and density of plankton detected have increased 4 years later. Reproduced rapidly, the phytoplankton abundance of those all springs increased from 1.71×10⁵ cells/L to 79.92×10⁵ cells/L. And the total abundance of zooplankton decreased from 264.28 ind./L to 137.25 ind./L. The proportion of Cyanophyta or Chlorophyta in each spring increased, while density of some large crustacean's species reduced. And density of Rotifer has increased in some springs. Furthermore, some dominant species have converted to the species typically found in the eutrophicated environment. The number of species tolerant to pollution has increased also. However, the plankton biodiversity does not fully coincide with trophic levels. The water quality based on physicochemical parameters were ranked as either category I or II, while the comprehensive trophic state index of all springs has raised gradually. And most springs has been polluted in middle -heavy eutrophication level. Result from Redundancy analysis showed that phytoplankton communities were significantly related to water temperature (WT), pH, nitrite nitrogen (NO^-₂-N), total phosphorus and nitrate nitrogen. The main environmental factors affecting the zooplankton community structure were WT, NO^-₂-N, ammonia nitrogen and chlorophyll-a. The plankton has responded to changes of the major water environmental factors. We suggest that water quality evaluation of karst spring should be combined with the physicochemical parameters and biological indicators. Multiple indexes could more comprehensively reflect the health state of water. Lastly, in order to develop groundwater sustainably, it is suggested to divide spring protection areas for important springs, take measures to prevent springs from being polluted, and reasonably manage and develop groundwater resources. Ecological restoration should be conducted for the damaged aquatic ecosystem as soon as possible.

Evaluation of the Effect of Different Locally Available Manures on Planktonic Quality and Quantity

An experiment was conducted in out-door, circular cement tanks (1000 L) with soil base for one year to estimate the plankton productivity and microbial characteristics in relation to seasons and inputs. Effect of three locally available manures - Cow dung (CD), poultry manure (PM) and goat manure (GM), on plank.tonic quality and quantity was evaluated. CD, GM and PM were applied to freshly prepared triplicate tanks at isonitrogenous dose at fortnightly intervals. Water quality, plankton quality and quantity were estimated. The study revealed that irrespective of the treatment, the water quality parameters showed similar pattern during the study period. Though the ammonia levels remained almost similar throughout the study, after initial 3 months, drastic decrease in nitrate levels were noticed in all the tanks owing to higher growth of plank.tonic biomass. Significantly higher phosphate content in water was noticed in poultry manure applied tanks. Plankton biomass was higher in all seasons with poultry manure followed by goat manure and cow dung. Bacillariophyceae (diatoms), Chlorophyceae and Cyanophyceae were higher with CD followed by GM, PM and Control (CO-without inputs). Density of Crustaceans, Rotifers and Euglenophyceae was the highest with PM, while it was similar in CD and GM. Control recorded the least value. Further, there was variation in tanks with respect to density of lactobacillus group of bacteria (on MRS agar plates) with highest count in CD (3.9x104 CFU/ml of bacteria) followed by GM (5.1x103 CFU/ml), PM (5.1x102 CFU/ml) and CO (3.7x102 CFU/ml). The study revealed superiority of poultry manure as a nutrient source for higher production of rotifers and crustaceans at all seasons under the prevailing conditions.

Effect of Different Diets on the Growth Rate of the Rotifer, Brachionus plicatilis under 40 ppt Salinity Stress

Aquaculture is important business some fish farms use underground water that may has high salinity. The aim of this study is to evaluate the effect of eight different types of diets on enhancing the growth rate of the rotifer, Brachionus plicatilis lives in 40 ppt saline water in 2020. The diets used in the study are T1 (Nannochloropsis oculata), T2 (Tetraselmis suecica), T3 (N. oculata + T. suecica), T4 (N. oculata + yeast; Saccharomyces cerevisiae), T5 (T. suecica + yeast), T6 (S. parkle), T7 (S. Parkle + yeast) and T8 (yeast). The results revealed that the rotifer fed for S. parkle showed the highest growth population (818 Indv. /mL) with a highly significant difference (p < 0.05). On the other hand, rotifers fed the yeast (S. cerevisiae) showed the lowest population growth (355 Indv. /mL). The outcome of the current study indicates that S. parkle commercial diet is very appropriate for the stock culture of B. plicatilis due to its ability to increase the growth rate of the rotifer in a short time under the current conditions. S. Parkle feeding led to an increase in the densities of rotifers in high salinities.

A comparative study on the density and seasonal variations of rotifer and copepod fauna in a pond of Tripura, India with a note on water quality parameters

A Comparative study was carried out in a pond of Tripura to observe the density and seasonal succession of rotifer and copepod fauna and the observation recorded 26 species of rotifer and 6 species of copepod fauna. Percentage composition of different families as well as percentage composition of each species in the recorded total rotifer and copepod fauna was also noted. Rotifer fauna showed maximum density in the winter and minimum density during summer. On the contrary, copepod fauna showed maximum density in the summer and minimum density in monsoon season. Rotifer fauna showed significant positive correlation with water temperature, free carbon dioxide, total alkalinity, nitrate-nitrogen, phosphate phosphorus and negative correlation with transparency. On the contrary, copepod fauna showed positive correlation with water temperature and pH and negative correlation with dissolved oxygen, total alkalinity and nitrate nitrogen. The observation infers that the densities of rotifer and copepod fauna varies with seasons which might be the dynamic nature of the aquatic ecosystem and might also be due to optimal condition in the water quality parameters of the studied pond ecosystem.

Effects of rotifer densities on larval performance of spotted scat (Scatophagus argus)

This study is aimed to determine the optimal rotifer feeding density for larviculture of spotted scat fish (Scatophagus argus). The experiment was randomly set up in triplicate with 5 treatments including 5, 10, 15, 20 and 25 rotifers/mL. The fish larvae with the initial body length and depth at 2,343 ± 110 µm and 812 ± 85µm, respectively, were stocked in 250-L tanks at 10 larva/L of density and 25‰ of salinity. After 21 days of rearing, the results showed that the larval growth were at 243 – 330 µm/day of DLG and 5.50 – 6.70 %/day of SGRL; 131 –204 µm/day of DDG and 7.20 – 8.93 %/day of SGRD, but no significant difference in larval growth performance was recorded among treatments (p>0.05). However, the highest survival rate of fish larvae (35.4-48.1%) was recorded in the treatments fed 5 and 10 rotifers/mL and was significantly higher than other treatments (p<0.05). The data demonstrated that spotted scat larvae fed effectively across as wide range of rotifer densities and the range at 5-10 rotifers/mL were suggested for the husbandry practices.

The effects of live prey and greenwater on the early larval rearing of orchid dottyback Pseudochromis fridmani

Captive breeding of marine ornamental fish is considered as a sustainable alternative to the current practice of fishing the wild stocks to supply the marine aquarium trade. However, efficient larviculture remains the biggest bottleneck, as many marine ornamentals suffered total mortality or only have extremely low survival at early larval stage. This study investigated the optimal conditions for the rearing of early larvae of a valuable ornamental fish, the orchid dottyback Pseudochromis fridmani, in terms of density and combination of live prey, as well as the application of the “greenwater” technique. In the first experiment, larvae were reared in clearwater and fed rotifers at 2, 5, 15, and 40 mL−1. Although there was no significant difference in larval survival among treatments on 8 days post-hatching (DPH), the larvae fed higher rotifer densities (15 and 40 rotifers mL−1) had significantly better growth. In the second experiment, by 8 DPH, a significant interaction between rearing condition (clearwater vs. greenwater) and live prey combination (rotifers only vs co-feed rotifers and copepods) on larval survival was detected, and the mean survival of larvae co-fed rotifers and copepods in clearwater (18%) was significantly lower than the other treatments (45–64%). Moreover, larvae reared in greenwater, or co-fed rotifers and copepods, grew significantly larger than those reared in clearwater, or fed rotifers only, respectively. In the third experiment, the larval survival was higher cell density of Nannochloropsis paste (NAN) used to make greenwater (0–3.40 × 106 NAN cells mL−1). The larval survival of the highest algae density treatment was the highest on 8 DPH (72 ± 8%), and it was significantly higher than those reared in the treatments of 1.13 × 106 or 0 NAN cells mL−1, but not significantly different from that of 2.27 × 106 cells mL−1 treatment. Overall, the results of this study suggest that co-feeding early P. fridmani larvae with rotifers and copepods, with the complementary use of greenwater comprising of ≥ 2.27 × 106 NAN cells mL−1, can enhance larval survival, growth and development. However, when the algal cell density is below this threshold, using such a co-feeding strategy may compromise the survival of P. fridmani larvae. The results of this study highlighted the importance of optimizing the application of live prey and greenwater for improving the survival of marine fish larvae.

Rotifers stimulate the specific uptake rate in lotic phototrophic biofilms

Abstract Stream biofilms can improve water quality (e.g. excessive nitrogen loads) because their microorganisms such as microphytobenthos and bacteria, are involved in self‐depuration processes in the biofilm. In marine sediments, meiofauna can influence primary productivity of phototrophic biofilms, stimulate bacterial growth, and exert a significant role on nitrogen cycling. We hypothesised that in rivers biofilms, increased meiofauna abundance in combination with nutrient enrichment can lead to improve biofilm nutrient uptake ability. River biofilms (from the Garonne River, France) were incubated in flumes supplied with natural river water. The uptake rates of the biofilms as well as the response of microorganisms were studied under different conditions of meiofaunal density and concentration over a 10‐day experiment. Rotifers dominated the meiofauna community in all biofilms. Under high nitrate conditions, the biofilm uptake rate was significantly higher in meiofauna‐enriched than non‐enriched biofilms over the first 2 days. This suggests that under high nitrate concentration conditions, high densities of rotifers enhanced the nitrogen uptake capacity of phototrophic biofilm despite a lower biomass. Densities of rotifers and bacteria in biofilms were positively correlated under enriched conditions suggesting that these two communities interacted over the experimental period. Signature pigment concentrations show that, under low concentrations of , biomasses of diatoms and green microalgae were higher in meiofaunal‐enriched than non‐enriched biofilms, indicating that rotifer activity favoured biomass accumulation of these algae. This study demonstrates that meiofauna inhabiting biofilms may contribute to the limitation of nitrogen loads in stream water and supports the thesis that the potential interactions between faunal groups and microbial communities of biofilms merits further investigation to improve our understanding of processes which regulate interactions between biofilms and the overlying water in rivers.

Snow algae blooms are beneficial for microinvertebrates assemblages (Tardigrada and Rotifera) on seasonal snow patches in Japan

Although studies on snow algae and macroinvertebrates have been frequently conducted on snow patches, only few surveys have been focused on microinvertebrates which reach high biomass and play various trophic roles in other cold habitats. The aims of this study were (1) to search for microinvertebrates in seasonal surface snow patches located on the slope of Mt. Gassan, in northern Japan, and (2) to identify factors determining their distribution associated with snow algal blooms of various colorations (orange, green, and golden-brown) collected from the same sampling site over two seasons (2018, 2019). Microscopic observation revealed presence of two major groups of microinvertebrates: Tardigrada and Rotifera. They were concentrated in green snow colored by blooms of Chloromonas sp. in comparison to orange or golden-brown snow and only a few were found in white snow. Mean body length of tardigrades increased throughout the melt season, their intestine content was green and they laid eggs on colored snow. These results suggest that tardigrades preferentially grew and reproduced on green snow patches. Population densities of tardigrades, rotifers and concentration of chlorophyll a were significantly correlated. Our study indicates that green snow patches in temperate mountainous forests constitute important and unique low-temperature ecosystems for microinvertebrates. Snow covered by algae is an unrecognized novel habitats for tardigrades and rotifers.

Temporal variation of rotifer community in three climatic zones and their relationship with environmental factors

Comparative analyses of rotifer community structure in freshwater lakes among different climatic zones are limited. In this study, six freshwater lakes located in warm temperate, subtropical and tropical zones were selected to investigate the annual dynamics of rotifer communities and their major driving factors. The results indicated that rotifer community structure did not exhibit significant seasonal variations. However, significant differences were observed among climatic zones. The species number and total density of rotifers were significantly higher in subtropical lakes than in lakes from other zones (p < .05). Both multidimensional scaling and detrended correspondence analysis revealed significant separation of tropical samplings from warm temperate and subtropical samplings, which was mainly caused by the higher temperature and lower chlorophyll a content in the tropical lakes. Further analyses indicated that rotifer community structure was mainly affected by water temperature, trophic indices and pH in the warm temperate zone but by chlorophyll a content and Secchi depth in subtropical lakes. In addition, only the Simpson index was significantly associated with Secchi depth and pH in tropical lakes. Overall, these results indicated great differences in annual dynamics of rotifer communities and their driving factors among warm temperate, subtropical and tropical zones.

Changing Distributions of Zooplankton Communities in a Coastal Lagoon in Response to Rainfall Seasonality

Choi, J.-Y.; Kim, J.-C., and Kim, S.-K, 2020. Changing distributions of zooplankton communities in a coastal lagoon in response to rainfall seasonality. In: Jung, H.-S.; Lee, S.; Ryu, J.-H., and Cui, T. (eds.), Advances in Geospatial Research of Coastal Environments. Journal of Coastal Research, Special Issue No. 102, pp. 69-74. Coconut Creek (Florida), ISSN 0749-0208.Coastal lagoons are unique and complex ecosystems which exhibit very high primary and secondary production rate, and their productivity is underpinned by plankton communities. Empirical studies suggest that summer rainfall determines the spatial and temporal pattern of not only physicochemical variables but also zooplankton communities in coastal regions. It was hypothesized that drier or wetter than average summer rainfall years would change the environmental variables (e.g., salinity, dissolve oxygen, and nutrients) in coastal lagoons, thereby disrupting the spring growth trend of zooplankton communities. Long-term (2014–2017) monitoring data (rainfall, environmental variables, and zooplankton) were divided into two groups: Rainy and Dry years, corresponding to years with an annual rainfall that was higher or lower than the total annual average, respectively. The results showed that summer and autumn densities of zooplankton fell sharply in Rainy years but increased steadily in Dry years. The highest density of rotifers was mainly observed in sites adjacent to the inflow of tributary streams, while copepods were abundant near the outlet to the ocean. The differing spatial distribution of rotifers and copepods is attributed to the salinity gradient in the study site, and changed with freshwater inflows. Based on these findings, it is suggested that summer rainfall variations play an important role in driving the spatial and temporal distribution of zooplankton.

Minimum rotifer density for best growth, survival and nutritional status of Brazilian sardine larvae, Sardinella brasiliensis

Live feed accounts for an important share of hatcheries costs and until now no study has examined how to reduce these costs in Brazilian sardine aquaculture. Therefore, this study verified and defined the minimum rotifer density in the first feeding of Brazilian sardine larvae that results in the best growth, survival, rotifer clearance and nutritional status (Lipase activity and RNA:DNA ratio). After initial exogenous feeding, six groups in triplicate were fed at 0.1, 1, 5, 10, 15 and 20 rotifer mL−1 during eight days. No residual rotifers were observed in the low-density treatments after the second day, which reduced by half larvae survival at 0.1 rotifer mL−1 (20 ± 2%) and mean growth by more than 20% when larvae were fed below 1 rotifer mL−1 (7.7 ± 0.1 mm length). On the other hand, densities above 5 rotifer mL−1 did not increase the RNA:DNA ratio (6.02–8.68) and decreased digestion efficiency at 15 and 20 rotifer mL−1, since lipase activity dropped by 30% to 3.79 U mg−1 of protein. Therefore, based on segmented regression, the breaking point that obtained maximum growth with the minimum requirement for rotifer ranged from 2.66 to 5.79 mL−1, or 266 to 579 rotifer larvae−1 per day.

Response of the rotifer community to human-induced changes in the trophic state of a reservoir

Abstract Human disturbance and nutrient runoff lead to water pollution, particularly in downstream waters and reservoirs. We hypothesized that increased human activity in summer would affect the trophic state of downstream reservoirs, affecting the interannual species composition of rotifers. We used long-term data for the Unmun Reservoir in South Korea (2009–2015), which is increasingly affected by human activity. The interannual variation of nitrogen and phosphorus levels was higher in summer and autumn, resulting in eutrophication. This led to a change in species composition of rotifers. Anuraeopsis fissa, Brachionus calyciflorus and Trichocerca gracilis were abundant in the most eutrophic state, while high densities of Ascomorpha ovalis and Ploesoma hudsoni were observed when nutrient concentrations were lower. The trophic state changes in the Unmun Reservoir were largely attributed to summer human activity in tributary streams. Our study location is typical of the stream network in South Korea and we assume that similar trophic state changes in reservoirs will be common. Changes in the density and species diversity of rotifers due to eutrophication indicate the need for active management and conservation, including the restriction of human activity around streams.

Importance of the freshwater rotifer Brachionus angularis for improved survival rate of early life‐history stages of pangasius catfish, Pangasianodon hypophthalmus

A study on the use of freshwater rotifers (Brachionus angularis) as initial live feed for pangasius catfish (Pangasianodon hypophthalmus) larvae was undertaken to assess whether this initial feed improved P. hypophthalmus survival during the first 20 days of rearing. This study was comprised of three trials. The first trial contained three treatments and assessed the effectiveness of freshwater rotifers as initial feed compared to homemade feed and water flea (Moina macrocopa). In the second trial, P. hypophthalmus larvae were fed with (a) 100% freshwater rotifers, (b) 100% water fleas and (c) a combination of freshwater rotifers and water fleas (freshwater rotifers fed during first 3 days and then water fleas fed for remainder of experiment). In the third trial, the fish larvae were fed 3 densities of freshwater rotifers and then water fleas (3, 5 and 7 individuals per ml) to determine the most suitable feeding density for P. hypophthalmus larvae. Mouth opening was measured at days 0, 3, 6, 9, 12, 15, 18 and 20, whereas growth and survival were assessed at the end of each trial. These experiments demonstrated that P. hypophthalmus larvae: (a) exhibited a significant increase in survival rate when fed B. angularis rotifers or Moina micrura water fleas compared to homemade feed, (b) exhibited a significant increase in growth and survival rates when fed a combination of freshwater rotifers (first 3 days) and then water fleas (remainder) compared to feeding either freshwater rotifers or water fleas only, and (c) preferred a density of 5 to 7 individual freshwater rotifers and then water fleas per ml.

Zooplankton communities in Lake Nasser, Egypt, under the current flood regime, before the construction of Grand Ethiopian Renaissance Dam (GERD)

This study focuses on changes in zooplankton composition and abundance in Lake Nasser during different flood periods before the construction of the Grand Ethiopian Renaissance Dam (GERD). Field surveys were carried out in August 2016 (flood period), December 2016 (post-flood period), and May 2017 (pre-flood period). Fifteen sampling sites representing five sectors were selected to cover the lake whole area. The results revealed a highly significant difference (p ≤ 0.016) in the density of all identified zooplankton groups among different flood seasons and lake locations. The zooplankton standing crop during the flood was higher, by a factor of approximately 3.7, than that recorded in post-flood and pre-flood periods. Zooplankton was dominated by medium-sized Copepoda, two cyclopoids (Thermocyclops neglectus and Mesocyclops ogunnus) and one calanoid (Thermodiaptomus galebi). The euplanktonic rotifer Keratella cochlearis dominated, during post-flood and pre-flood periods, constituting approximately 83% and 77% of total rotifer densities, respectively. Keratella tropica occurred in greatest density during flood made up 52.6% of total rotifers. Diaphanosoma mongolianum was the main cladoceran during flood and post-flood periods, whereas Ceriodaphnia dubia densities were highest for pre-flood periods. This study found that flood regime affect zooplankton composition and density in Lake Nasser.

Effects of tank shapes and aerations on survival, growth and swim bladder inflation of red seabream Pagrus major larvae

This study aimed to seek for the optimal condition for small-scale larviculture of red seabream Pagrus major larvae. We examined the effects of tank shapes and aerations, which were assumed to influence the larval survival, growth and swim bladder inflation of P. major larvae. Seawater (50-l) was filled into three cylindrical (CT: 1.7 × 103 cm2 water surface area, 30 cm deep) and six rectangular (RT: 1.8 × 103 cm2 water surface area, 28 cm deep) tanks. One air stone with 100 ml/min aeration rate was set at the bottom center of three CT and RT (RT1AS), and two air stones with 50 ml/min aeration rate were set at the half bottom center of three RT (RT2AS). Five hundred eggs were distributed into each experimental tank. Rotifers were fed to larvae at 10 individuals/mL on 3 days post hatching (dph) and their distribution in tanks were measured. Survival rate at 14 dph in CT (54.7 ± 11.0 %) and RT1AS (55.3 ± 6.0 %) were significantly higher than that in RT2AS (29.6 ± 9.3 %, p < 0.05). However, the growth of larvae was not significantly different between tank shapes and aerators. Swimbladder inflation rates were not different between tank shapes and aerations, however, CT (58.9±28.3 %) showed lower trend (RT1AS 94.4±6.9 %, RT2AS 92.2±10.7 %). Rotifer distribution in tanks was higher at tank bottom (p < 0.05). Low flow regions were observed along the side walls of the tanks and bottom areas in CT and RT1AS due to a single-pair vortex system and formed at the center (between air stones) and from the air stone to the tank walls in RT2AS due to two single-pair vortex systems. These low-flow areas were coincided with higher rotifer distribution areas at the tank bottom indicating that measuring rotifer density can estimate the flow in a tank. We recommend the rectangular tank with one air stone system for red seabream larvae.

A lake-wide approach for large lake zooplankton monitoring: Results from the 2006–2016 Lake Superior Cooperative Science and Monitoring Initiative surveys

Whole-lake surveys of Lake Superior were completed during late summer in 2006, 2011, and 2016 to assess lower food web conditions under the Cooperative Science and Monitoring Initiative (CSMI). These surveys used a spatially stratified probability approach based on depth to assess food web conditions within different depth zones. We evaluated differences in crustacean zooplankton biomass, rotifer density, and the community structure of both groups in nearshore (<30 m), midshore (30–100 m), and offshore (>100 m) depth zones and investigated changes in these parameters within zones over time. Although nearshore crustacean biomasses and rotifer densities were highly variable, the depth zones differed from each other based on these parameters and should be considered separately. Crustacean biomass, community structure, and vertical position were consistent over time across depth zones. The differences that did occur were within the range of known annual variability. Total rotifer densities were lower in 2016 than in 2006 in all depth zones, but the genera that contributed to the lower values were not the same across zones. Further studies are needed to know whether these differences reflect annual variability or long-term trends. Finally, we show how the depth zones used in this study can facilitate comparisons between monitoring programs. This is important because most zooplankton studies are limited to certain depth zones and changes in zooplankton parameters may not occur uniformly across zones. The high variability in nearshore zooplankton parameters suggests that additional research may be needed to effectively track changes there.

Long-term variation of zooplankton communities in a large, heterogenous lake: Implications for future environmental change scenarios

In recent decades, freshwater ecosystems have been threatened worldwide by multiple simultaneous stressors, including eutrophication, climate change and competing demands for water sources. However, understanding of the long-term variation of zooplankton communities remains limited because long-term observations are lacking. Here, using a long-term (19 year) monitoring dataset, we demonstrate the spatio-temporal variation of zooplankton communities in Lake Taihu, a large, shallow, heterogenous lake in China. With the development of eutrophication, the abundance and biomass of zooplankton first increased from 1998 to 2004, and then exhibited a decreasing trend thereafter. Specifically, the population of rotifer dramatically declined after 2001, while the abundance of copepod and cladoceran showed an increasing trend even though their biomass decreased significantly after 2008. The dominance of small cladocerans (Bosmina coregoni and Ceriodaphnia cornuta) and copepod (Limnothora sinensis) significantly increased with decreasing rotifer density after 2014. Moreover, the zooplankton community structure exhibited heterogenous spatial population dynamics. Cladoceran and rotifer were predominant in cyanobacteria-dominated regions, while a higher proportion of copepod were found in macrophyte-dominated regions. Analyses revealed that zooplankton communities were strongly affected by climate warming and nutrients. These results reinforce previous work demonstrating that the development of eutrophication and climate warming could change the structure of zooplankton community and increase the dominance of small-bodied crustacean. Our findings address the recognized gap in understanding the variation of the zooplankton community in Lake Taihu, and provide an opportunity to evaluate ongoing changes in the zooplankton community related to future environmental change scenarios.

Responses of Rotifer Community to Microhabitat Changes Caused by Summer-Concentrated Rainfall in a Shallow Reservoir, South Korea

Empirical studies suggest that the structural heterogeneity of aquatic ecosystem microhabitat is determined by the diversity and abundance of macrophytes. However, excessive accumulation of free-floating macrophytes on the water surface can reduce the biomass of submerged macrophytes, resulting in a relatively simplified habitat structure. We hypothesized that heavy summer rainfall disrupts the growth of free-floating macrophytes covering much of the Jangcheok Reservoir’s water surface, thereby resulting in a more complex habitat structure by allowing development of a more diverse of macrophytic community. We divided long-term (2008–2017) monitoring data (rainfall, macrophytes, and rotifers) into two groups: Rainy and Dry years, corresponding to years with annual rainfall higher and lower than the total annual average, respectively. We found that summer densities of rotifers fell sharply in Rainy years, but increased continuously in Dry years. This trend resulted in greater autumn densities in Rainy relative to Dry years, which we attributed to changes in habitat related to differential macrophyte development. Moderate disturbance of the water surface caused by high summer rainfall can promote growth of submerged macrophytes by creating large areas of open water and therefore a more complex autumnal microhabitat structure, resulting in seasonal variations in rotifer community structures and populations. Moreover, a highly complex microhabitat structure restricts foraging activity of fish (i.e., Lepomis macrochirus) that prey on rotifers. Based on these findings, we suggest that summer-concentrated rainfall plays an important role in supporting the density and species diversity of rotifers.

Population growth of Brachionus calyciflorus affected by deltamethrin and imidacloprid insecticides

Rotifers due to their relatively short lifespan, high fecundity and high rate of population increase are ideal for chronic toxicity tests. The aim of this research was determine the lethal concentrations (LC50 24 h) of deltamethrin and imidaclopride and their impacts on reproduction and growth of the freshwater rotifer, Brachionus calyciflorus. Experiments carried out statically based on standard method (O.E.C.D.). Based on LC50 levels of pesticides, different concentration treatment groups designated and rotifer population responses in the five different concentrations of deltamethrin (0.00, 0.05, 0.10, 0.21 and 0.53 mgl-1) and imidacloprid (0.00, 6.22, 12.45, 24.91 and 62.27 mg l-1) with three replicates per each concentration during ten days were studied. The LC50 24h of deltamethrin and imidacloprid for freshwater rotifer determined as 1.06 mg l-1 and 124.54 mg l-1, respectively. The density of rotifers in all treatment groups of pesticides compare to the control group at tenth day decreased significantly (p<0.05). The ovigerous females to nonovigerous females ratio (OF/NOF) and mictic females to amictic females ratio (mic/amic) significantly affected (p<0.05) in all concentrations for both insecticides. This study suggested that B. calyciflorus are severely sensitive to deltamethrin rather than to imidacloprid pesticide.