Effects of the endocrine disruptor 4-nonylphenol on the demography of rotifers Plationus patulus and Brachionus havanaensis: a multigenerational study.

The emerging contaminant 4-nonylphenol is an active ingredient in many surfactants. Though 4-nonylphenol has been reported in Mexican waterbodies, no stringent law is available to establish its permissible limits. Most ecotoxicological assays using zooplankton species are based on previously unexposed populations, and multigenerational experiments are rare. Plationus patulus and Brachionus havanaensis are widely distributed species in Mexico and have been used in ecotoxicological assessments. In this work, the median lethal concentration of 4-nonylphenol (LC50, 24 h) for both rotifer species was derived. Based on our acute toxicity data (24 h LC50 of 4-nonylphenol for P. patulus and B. havanaensis were 500 and 250 µg L-1, respectively), three sublethal concentrations (µg L-1) (7.81, 15.82 and 31.25 for B. havanaensis and 15.62, 31.25 and 62.5 for P. patulus) were used in the life table demography tests for two successive generations (F0 and F1) of either rotifer species. Regardless of the concentrations used, 4-nonylphenol had an adverse effect on both rotifer species over two successive generations. P. patulus was more sensitive than B. havanaensis in chronic toxicity tests. F1 generation of both rotifer species was far more adversely affected than the F0 in all toxicant treatments.

Population responses and fatty acid profiles of Brachionus calyciflorus (Rotifera) in relation to different thermal regimes.

Diurnal temperature fluctuations affect ectothermic species more than endothermic taxa. We tested the effect of three fixed temperatures (20, 25 and 30 °C) and a 24 h variable (20-30 °C) on the population growth and fatty acid profiles of the common rotifer Brachionus calyciflorus. Depending on the temperature treatment, the peak population abundances of B. calyciflorus varied from 65 to 80 ind./ml, the lowest being on variable temperature range. The rate of population increase varied from 0.31 to 0.52 per day, highest being at 30 °C. There was a curvilinear relationship between the population density and the egg ratio (number of eggs/female) in all the tested temperature regimes. The egg ratio was higher (>0.6) for treatments involving fixed temperatures, but for variable temperature regime, the egg ratios were lower (<0.5). Temperature also induced changes in the fatty acid content of B. calyciflorus. While the total saturated fatty acids increased, both mono-unsaturated and poly-unsaturated fatty acids decreased with increasing temperature regime. These results have been interpreted in relation to the role of temperature (both intensity and mode of exposure) on the population growth characteristics of rotifers.

Demographic responses of Cladocerans (Cladocera) in relation to different concentrations of humic substances.

Cladocerans are constantly exposed to humic substances in nature, yet the effects of these substances on their survival and reproduction are not well known. Here, the effects of humic substances (20 and 40 mg L-1) (HS) on the life history variables of three common cladocerans, Ceriodaphnia dubia, Moina macrocopa, and Daphnia pulex were evaluated. The results showed that the effect of humic substances on the tested cladocerans is species-specific, affecting either survival, reproduction or both. For M. macrocopa, exposed to HS at a concentration of 40 mg L-1, the average lifespan and the life expectancy at birth were significantly reduced as compared to controls, but for C. dubia and D. pulex these parameters were increased. Gross reproductive rate was unaffected by the HS level for both D. pulex and M. macrocopa, but it was significantly higher for C. dubia. When compared to the corresponding controls, for HS-exposed cladocerans, the rate of population increase was significantly reduced in case of D. pulex while it was stimulated for both C. dubia and M. macrocopa. It appears that humic substances had a slightly stronger influence on survivorship than on reproduction of the tested cladocerans.

Multigenerational effects of triclosan on the demography of Plationus patulus and Brachionus havanaensis (ROTIFERA).

Triclosan is a personal care product widely used in North America, Europe and Asia as antimicrobial ingredient in many consumer chemical products. In Mexico concentrations of triclosan have been reported in aquatic systems. However, there is no law regulating the presence of chemicals such as triclosan, in aquatic systems. The scarce data about this chemical has increased concern among ecotoxicologists regarding possible effects on aquatic organisms. Moreover, multigenerational studies are rarely studied and the results vary depending on the contaminant. Rotifers, are a dominant group of zooplankton, and have been used in aquatic risk assessments of personal care products due to their sensitivity and high reproductive rates. Plationus patulus and Brachionus havanaensis are common rotifers distributed in aquatic ecosystems of Mexico and have been used in ecotoxicological bioassays. In this study, the median lethal concentration (LC50, 24h) of P. patulus and B. havanaensis exposed to triclosan was determined. Based on the LC50, we tested three sublethal concentrations of triclosan to quantify the demographic responses of both rotifers for two successive generations (F0, and F1). The 24h LC50 of triclosan for P. patulus and B. havanaensis were 300 and 500µgL-1 respectively. Despite the concentration, triclosan had an adverse effect on both Plationus patulus and Brachionus havanaensis in both generations exposed. Experiments show that P. patulus was more sensitive than B. havanaensis when exposed to triclosan. When exposed to triclosan the parental generation (F0) of P. patulus was far more affected than F1.

Is Aluminum Innocuous to Zooplankton at pH Below 6?

Aluminum (Al) use has increased greatly during the last two decades, yet little information is available on its toxic effects in relation to pH particularly on zooplankton. In this work, we determined the acute toxicity (LC50) and life table responses for Moina micrura exposed to 0.008, 0.016 and 0.08 mg of Al L-1 at pH of 5, 6 and 7. The age-specific survivorship and reproduction showed a steep decline (80% mortality by the second day) at pH 5, independent of Al level. Both gross and net reproductive rates were significantly lower at pH 6 compared to pH 7, regardless of Al concentration. At pH 7 the rate of population increase of M. micrura was not significantly influenced by the Al level, while at pH 6 it was significantly lower (p < 0.05), suggesting that M. micrura is sensitive to changes in Al under slightly acidic conditions.

Effect of crude extracts of Dolichospermum planctonicum on the demography of Plationus patulus (Rotifera) and Ceriodaphnia cornuta (Cladocera).

In this study we evaluated the toxicological effects of crude extracts of Dolichospermum planctonicum on Plationus patulus (Rotifera) and Ceriodaphnia cornuta (Cladocera). The active compound from the laboratory-cultured cyanobacteria was extracted by lyophilizing it at -70 °C for 48 h, after which the sample was thawed. The cycle was repeated five times, and between each cycle the cells were sonicated to lyse them. Using the extract of Anabaena, experiments of population growth and life table demography of the two zooplankton species were compared in media with and without (controls) the cyanotoxins. In both experiments two toxic concentrations diluted in US EPA medium were used, one in a 5:15 proportion, and the other in a 2.5:17.5. We found that the rotifers were more resistant to the toxicants than the cladocerans. Their population growth rates ranged from 0.11 d(-1) in controls to 0.08 d(-1) in the presence of the crude extracts for P. patulus and from 0.18 in controls to -0.27 in the presence of the crude extracts for C. cornuta. The crude extract also had a significant negative impact on the demographic parameters, adversely affecting reproduction more than survival. The data have been discussed with emphasis on the importance of further research using cyanobacterial crude extracts.

Temperature and age affect the life history characteristics and fatty acid profiles of Moina macrocopa (Cladocera).

Demographic responses and fatty acid profiles of Moina macrocopa were quantified under different temperature regimes (20°C, 25°C and 30°C and diurnally variable 20-30°C) and at fixed ration (10.65µgDWml(-1)) of Chlorella. Highest constant temperature (30°C) reduced the density of M. macrocopa. The cladocerans under the fluctuating temperature regime too had lower population growth (about 50% lower than that at constant 25°C). The survivorship of M. macrocopa was higher at 20°C than that at 25°C and 30°C or at variable temperature regime. Gross and net reproductive rates were higher at 25°C. At 20°C, neonates had the highest proportion (67%) of myristic, palmitic and stearic acids while the adults had the lowest (26%) proportion. For both adults and neonates, palmitoleic, linoleic and linolenic comprised of 15-35% of the total fatty acids. Higher percentage (19%) of linoleic acid was present in adults than neonates (7%). Adults had linolenic acid level which was 3-times higher than in neonates. Linoleic and linolenic fatty acids decreased with increasing temperature for neonates and adults from 20°C to 30°C. The demographic responses and fatty acid profiles of M. macrocopa were discussed in relation to level and mode of temperature exposure.

Effect of malathion on the demography of Daphnia pulex Leydig and Diaphanosoma birgei Korinek (Cladocera).

Malathion is a common pesticide used to control insects in agricultural, domestic and industrial sectors in different parts of the world. In this work we evaluated the effects of sublethal concentrations of malathion on the survivorship and reproductive variables of two cladoceran species Daphnia pulex and Diaphanosoma birgei using standard life table demography method. Based on preliminary tests, we selected four sublethal concentrations of malathion for each cladoceran species. For D. pulex, the malathion concentrations were 0. 225, 0.45, 0.9 and 1.8ng I(-1) and for D. birgei, these were 0.0281, 0.0562, 0.1125, 0.225ng I(-1). Our results showed that in general, Daphnia pulex was less sensitive than Diaphanosoma birgei to malathion. The average lifespan of Daphnia pulex in controls was about 19 days while under similar conditions, that of D. birgei was about 21 days. For either cladoceran species, increased pesticide concentration resulted in decreased survival; this was more evident in the treatment containing the highest concentration of malathion (0.225 ng l(-1)) for D. birgei. Fecundity of D. pulex and D. birgei also decreased with increase in the concentration of malathion. For a given cladoceran species, compared to controls, the gross reproductive, net reproductive rates and the population growth rate significantly decreased due to malathion. Compared to the survivorship variables (age-specific survival, life expectancy and average lifespan), the reproductive parameters (gross reproductive rate, net reproductive rate and the rate of population increase) of the two cladoceran species were decreased by the pesticide.

Feeding and filtration rates of zooplankton (rotifers and cladocerans) fed toxic cyanobacterium (Microcystis aeruginosa).

Microcystis aeruginosa is generally dominant in many Mexican freshwater ecosystems interacting with zooplankton species. Hence, feeding and filtration rates were quantified for three cladoceran (Daphnia pulex, Moina micrura and Ceriodaphnia dubia) and three rotifer species (Brachionus calyciflorus, Brachionus rubens and Plationus patulus) using sonicated M. aeruginosa alone or mixed with Scenedesmus acutus in different proportions (25, 50 and 75%, based on cell density), offering a combined initial density of 100,000 cells·ml(-1). All the three cladoceran species ingested M. aeruginosa (100-300 cells ind(-1) min(-1)) when fed exclusively with cyanobacterium. When green alga offered as exclusive diet, the number of cells ingested by the tested cladocerans varied from 80 to 400 cells ind(-1) min(-1). Compared to cladocerans, rotifers in general consumed much lower quantity (< 200 cells ind(-1) min(-1)) of M. aeruginosa and S. acutus. The filtration rate for Daphnia pulex was inversely related to the proportion of green alga in the diet. For other tested cladocerans, no such clear trend was evident. In mixed treatments containing M. aeruginosa, the filtration rate of Daphnia was highest (about 220 µl ind(-1) min(-1)) when the medium contained 75% of S. acutus. Among the rotifer species, P. patulus filtered highest volume (100 µl ind(-1) min(-1) from mixed diets containing higher proportions (50 or 75%) of M. aeruginosa. Thus, there were species-specific differences in the filtration and feeding rates of zooplankton when offered mixed diets of green algae and toxic cyanobacteria. These probably explain the coexistence of different zooplankton species in Microcystis-dominant waterbodies.

Effect of paracetamol and diclofenac on population growth of Plationus patulus and Moina macrocopa.

Two non-steroidal anti-inflammatory commonly used in Mexico are paracetamol and diclofenac. These drugs reach natural waterbodies transport through the hydrological areas of agriculture and effluent treatment plants and can affect plankton. In this study, we quantified the chronic effects of paracetamol and sodium diclofenac on the population dynamics of the rotifer Plationus patulus and the cladoceran Moina macrocopa under laboratory conditions. Both these species of zooplankton are common in freshwaters and are sensitive to chemicals in the medium. Our results showed that densities of both P. patulus and M. macrocopa decreased with increasing levels of both the analgesic drugs. Both zooplankton species did not survive beyond when paracetamol was applied at 32 mg I(-1) in the medium. Diclofenac in general had more adverse effect than paracetamol for either zooplankton species. P. patulus was more sensitive than M. macrocopa to both analgesic drugs. When diclofenac was present in the medium at > or = 12.5 mg I(-1), rotifer reproduction was inhibited, while the tested cladocerans continued to grow but to lower densities compared to control. The rate of population increase (r) per day of P. patulus and M. macrocopa was significantly and inversely related to the concentration of paracetamol and diclofenac in the medium. However, the relationship between r and drug concentration differed depending on the zooplankton species and drug. In controls, the r of P. patulus was 0.18 d(-1), for M. macrocopa under similar conditions, it was slightly lower (0.16 d(-1)). The rvalues of both zooplankton populations became negative (-0.10 to -0.15 d(-1)) when exposed to paracetamol at 32 mg l(-1) or diclofenac at 25 mg I(-1).

Synergistic effects of microplastics and cyanotoxins on the demography of the rotifer Brachionus calyciflorus Pallas.

Plastic contamination in aquatic ecosystems all around the world is on the rise. Microplastics (MPs) interfere with the ecological functions of different aquatic animals, including zooplankton. Other contaminants can also be transported by microplastics, resulting in higher toxicity. Here, we evaluated the effect of microplastics alone and in combination with cyanotoxins (CT) on the demographic and feeding responses of the freshwater rotifer Brachionus calyciflorus. The microplastics used were polystyrene spheres of 30 µm diameter. The cyanobacteria were obtained from a sample collected from the Valle de Bravo reservoir in Central Mexico, and the secondary metabolites were obtained after 5 cycles of freezing, thawing and sonication. We conducted acute (LC50, 24h bioassay) and chronic toxicity tests involving population growth, life table experiments, and feeding experiments. The LC50 value using microplastics was 21.91 mg/L, and for the cyanobacterial crude extract it was 2.56 µg/L, but the combination of both resulted in a higher adverse effect (16.64 mg/L) due to the microplastics. Rotifer survival, reproduction, and feeding decreased in the presence of either MPs or CT but the adverse effect was more in the presence of both contaminants. We found that the adverse effects of MPs and CT on the test rotifer species were enhanced when exposed to both contaminants simultaneously.

Effect of microplastics on the demography of Brachionus calyciflorus Pallas (Rotifera) over successive generations.

Microplastics (MPs) are becoming increasingly common in freshwater ecosystems. Rotifers may involuntarily ingest MPs through non-selective filtration, thus causing non-real satiety, leading to adverse effects on their demography. Here, we evaluated the acute and chronic effects of MPs (30 µm) on two successive generations of Brachionus calyciflorus s.s. at 20 and 25 °C. Demographic variables and ingestion rates were compared in the absence and presence of microplastics (550 spheres mL-1). For the life table experiments, cohorts of twenty neonates were introduced into 20 mL medium with four replicates per treatment. Ingestion rates of the rotifer were tested at both test temperatures in the absence and presence of the microplastics in 20 mL containers with 1 ind. mL-1 per container. The rotifer population growth rates ranged from 0.55 to 1.05 d-1 depending on the treatment. MPs decreased the fecundity by 20-24 %. The average lifespan of rotifers exposed to MPs in the F0 generation was lower than that in the F1 generation at both temperatures, indicating that pre-exposure of mothers may offer some protection to the offspring. The population growth rate was higher at 25 °C. The ingestion rates were significantly lower in the presence of the microplastics. Our observations indicated that this strain of B. calyciflorus was better adapted to 20 °C; microplastics and higher test temperature resulted in a pronounced changes in the demographic variables of the rotifer.

Morphological, morphometrical and molecular (CO1 and ITS) analysis of the rotifer Asplanchna brightwellii from selected freshwater bodies in Central Mexico.

We evaluated different strains of the rotifer Asplanchna brightwellii collected from central Mexico using morphology, morphometry and molecular tools (CO1 and ITS). Three distinct clonal populations from each of the 3 regions (Mexico City, State of Mexico and State of Guerrero) were established under laboratory conditions. For a given waterbody, morphometric comparisons within the populations of A. brightwellii showed almost stable measurements of trophi and with no statistically significant differences among them (p > 0.05). However, asplanchnid body length and width as well as the cyst diameter varied significantly depending on the waterbody from which A. brightwellii was collected. The smallest adults (about 700 microm) were from Valerio Trujano lake (Guerrero State) samples while the largest were from Xochimilco lake. Similar tendencies were reflected in the diameter of resting eggs. In addition, morphologically the cysts of A. brightwellii from the three waterbodies showed slightly different pattern. The number of globular structures on the surface of cysts was smaller for Valerio Trujano strain, while these were larger and less numerous for both Xochimilco and Zumpango strains. The ITS region tree displayed two groups Xochimilco and Valerio Trujano -Zumpango, this analysis did not reflect the morphological grouping; on the contrary the CO1 gene tree separated the populations according to morphological clusters and location (Xochimilco, Valerio Trujano and Zumpango lakes). When the tree was built using the combination of both ITS and CO1 sequences, the phylogenetic relationships observed on CO1 gene were consistent; but showed differences with the relationships observed on ITS region tree (only two groups).

Experimental Studies on Zooplankton-Toxic Cyanobacteria Interactions: A Review.

Cyanobacterial blooms have been recognized as a problem in fresh water for about 150 years. Over the past 50 years, experimental studies on the subject have gained importance considering the increasing need to control toxic cyanobacterial blooms. This article presents information on the different lines of research that have been undertaken on zooplankton-cyanobacteria interactions over the past 50 years. These include information on filtering/ingestion rates and phytoplankton preferences of small and large rotifers, cladocerans, and copepods; growth rates of zooplankton on cyanobacterial diets; feeding rates of other freshwater invertebrates on cyanobacteria; role of zooplankton in top-down biomanipulation efforts; effect of cyanotoxins on zooplankton; bioaccumulation of cyanotoxins; and physical and chemical control of cyanobacterial blooms. We also highlight measures that have led to successful lake management and improvement of water quality in selected waterbodies.

Impact of chromium and aluminium pollution on the diversity of zooplankton: a case study in the Chimaliapan wetland (Ramsar site) (Lerma basin, Mexico).

Biological monitoring and the use of biotic indices are important in evaluating the health of aquatic systems. However, zooplankton are rarely included in biomonitoring protocols. We conducted a one-year study (March 2008-February 2009) at two sites from the Chimaliapan wetland, with concentrations of aluminium (Al) and chromium (Cr) above and within the permissible limits, respectively. Metals in the sediment and water were analyzed from three locations per site every two months. In addition to analyses of the abundance and diversity of rotifers, cladocerans and copepods, we sampled 11 physicochemical variables in the water and six from the sediments. The metal concentration in the polluted site (significantly above the permissible limits) ranged between 7266-8174 mg Kg(-1) of Al and 14.6-18.3 mg Kg(-1) of Cr. We found 92 species of rotifers, cladocerans and copepods. The Brillouin index for both sites ranged from 3.9-5.4, the Shannon-Wiener index from 4.2-5.5 while the Brachionus-Trichocerca ratio ranged between 1.0 and 1.7. The Wetland Zooplankton Index was significantly different among the sites; 2.63 at site 1 and 2.13 at site 2. The saprobic index was 3.2 for both sites. Data analyses using multifactorial techniques suggested that zooplankton can be used to evaluate the impact of the metals aforementioned, since these organisms are generally more sensitive than other groups and also have a high ecological relevance.

Demography of zooplankton (Anuraeopsis fissa, Brachionus rubens and Moina macrocopa) fed Chlorella vulgaris and Scenedesmus acutus cultured on different media.

Generally zooplankton growth is often limited by the quality of their algal diet. A cheaper common practice in aquaculture, is to culture algae with fertilizers; however, the demography of zooplankton when fed these algae has not yet been evaluated. We studied the population growth and life table demography of the rotifers Anuraeopsis fissa and Brachionus rubens, and the cladoceran Moina macrocopa. For this, the algae Scenedesmus acutus or Chlorella vulgaris were cultured on defined (Bold's basal) medium or the commercial liquid fertilizer (Bayfolan). Experiments were conducted at one algal concentration 1.0 x 10(6) cells/mL of C. vulgaris or its equivalent dry weight of 0.5 x 10(6) cells/mL of S. acutus. The population dynamics were tested at 23 +/- 1 degrees C in 100 mL transparent jars, each with 50mL of the test medium, with an initial density of 0.5indiv/mL, for a total of 48 test jars (3 zooplankton 2 algal species x 2 culture media x 4 replicates). For the life table experiments with M. macrocopa, we introduced 10 neonates (<24h old) into each test jar containing the specific algal type and concentration. For the rotifer experiments, we set 5mL tubes with one neonate each and 10 replicates for each algal species and culture medium. We found that the average rotifer life span was not influenced by the diet, but for M. macrocopa fed S. acutus cultured in Bold's medium, the average lifespan was significantly lower than with the other diets. The gross and net reproductive rates of A. fissa (ranging from 18-36 offspring per female) were significantly higher for C vulgaris cultured in Bold medium. Regardless of the culture medium, Chlorella resulted in significantly higher gross and net reproductive rates for B. rubens than S. acutus diets. The reproductive rates of M. macrocopa were significantly higher in all the tested diets except when fed with S. acutus in Bold medium. The population increase rate, derived from growth experiments of A. fissa and B. rubens, ranged from 0.1-0.25/d and were significantly higher on C vulgaris cultured in liquid fertilizer as compared to the other diets. The growth rates of M. macrocopa ranged from 0.1 to 0.38/d, and were highest with diets of C. vulgaris cultured in Bold medium and S. acutus cultured in fertilizer. Thus, regardless of the culture medium used, the growth rates of the evaluated zooplankton species were higher with Chlorella than with Scenedesmus. The peak population density was highest (2 800ind/mL) for A. fissa fed Chlorella that was cultured on liquid fertilizers, while B. rubens and M. macrocopa had peak abundances of 480 and 12ind/mL, respectively under similar conditions.

Elaphoidella grandidieri (Harpacticoida: Copepoda): demographic characteristics and possible use as live prey in aquaculture.

In freshwater ecosystems, rotifers and cladocerans are ideal prey for fish larvae whereas copepods, due to their purported low growth rate and predatory tendency, are not. We recently isolated the parthenogenetic Elaphoidella grandidieri (Gueme et Richard, 1893) a benthic freshwater harpacticoid, from a fish farm in the State of Morelos, central Mexico and tested its potential as a live prey organism for larval vertebrates. Population growth and life table demography experiments were conducted, in 100 ml recipients with 50 ml of test medium on a diet of Scenedesmus acutus at a density of 1.0 X 10(6) cell ml(-1); the former on live algae alone while the latter on live algae as well as detritus. We also conducted experiments to document the prey preference for this copepod by the larval Ameca splendens (Pisces: Goodeidae) and Ambystoma mexicanum (Amphibia: Ambystomatidae), fed the rotifer Plationus patulus, the ostracod Heterocypris incongruens, and the cladocerans Moina macrocopa and Daphnia pulex. Elaphoidella grandidieri is relatively easy to maintain under laboratory conditions, reaching densities (copepodites and adults) of more than 10,000 l(-1). The generation time ranged between 30-45 days, depending on the diet. The net reproductive rate was as high as 60 nauplii female(-1) day (1). Population growth rates ranged between 0.03 and 0.11 d(-1), live algae being the superior diet compared to detritus. Both predators showed no preference for E. grandidieri, but in the absence of alternate prey they consumed 80% of the harpacticoids offered. The data have been discussed in relation to the potential of E. grandidierias live food for aquaculture.

Studies on comparative population growth of some species of the rotifer Lecane (Rotifera).

We compared the population growth patterns of 5 species of the rotifer genus Lecane [(L. quadridentata (Ehrenberg, 1830), L. comuta (Muller, 1786), L. papuana (Murray, 1913), L. unguitata (Fadeev, 1925) and L. pyriformis (Daday, 1905)] ranging in adult average body size from 30 to 140 microm. All species were cultured under laboratory conditions for 25-30 days using the green alga Scenedesmus acutus as the exclusive diet, at a density of 1.0 x 10(6) cells ml(-1) at 24 degrees C. Regardless of the species, lecanids reached their peak population densities after 4 weeks. Peak population densities ranged from 15 to 320 ind. ml(-1), depending on body size. There was an inverse curvilinear relation between body lengths and peak population abundances (densities) of the Lecane species. Egg ratios (eggs per female) for the tested species were < 0.6 during the exponential phase but declined to 0.1 (or lower) as the population density increased. The rates of population increase for the lecanids were in general lower(0.10 to 0.21 day (-1)) than other well-studied rotifer species including members of Brachionidae.

Seasonal variations in zooplankton abundances in the Iturbide reservoir (Isidro Fabela, State of Mexico, Mexico).

This studywas undertaken to quantify the seasonal variations of zooplankton (rotifers, cladocerans and copepods) and selected physico-chemical variables (temperature, pH, conductivity, Secchi disc transparency, dissolved oxygen, ammonia, nitrate and phosphate concentrations) in the Iturbide dam. Monthly zooplankton samples (50 l filtered through 50 microm mesh, in duplicates from each of the 4 stations) were collected from February 2008 to January 2009. Simultaneously physico-chemical variables were measured. The zooplankton samples were fixed in 4% formalin in the field. In general, the temperature ranged from 9 to 16 degrees C, rarely exceeding 20 degrees C. Secchi transparency was nearly 100% since the reservoir was shallow (< 2 m) even during the rainy seasons. Dissolved oxygen was generally high, 13-18 mg l(-1). Nitrate levels (10 to 170 microg l(-1)) were low while phosphates were relatively high (9 to 35 microg l(-1)). The Iturbide reservoir was dominated by rotifer species. We encountered in all, 55 taxa of rotifers, 9 cladocerans and 2 copepods. The rotifer families Trichocercidae and Notommatidae had the highest number of species (7 each) followed by Colurellidae and Lecanidae (6 and 5 species, respectively). Trichocerca elongata, Ascomorpha ovalis, K. americana, K. cochlearis, Lepadella patella and Pompholyx sulcata were the dominant rotifers during the study period. On an annual average, rotifer density ranged between 50-200 ind.(-1). Among crustaceans Chydorus brevilabris and Macrothrix triserialis were most abundant. The maximal density of these cladocerans was about 50 ind. l(-1). Copepods were much lower in numbers (< 20 ind. l(-1)). In general the density of zooplankton was higher during summer months (April to July) than during winter. Shannon-Wiener diversity index varied from 1.0 to 4.3 depending on the site and the sampling period. Based on the data of Secchi transparency and nutrient concentrations, the Iturbide reservoir appeared to be mesotrophic.

Somatic and population growth responses of Ceriodaphnia dubia and Daphnia pulex (Cladocera) to changes in food (Chlorella vulgaris) level and temperature.

We quantified the combined effects of three food levels (0.01 x 10(6), 0.1 x 10(6) and 1.0 x 10(6) cells ml(-1)) and three temperature levels (15, 20 and 25 degrees C) on the somatic and the clonal population growth of two cladocerans Ceriodaphnia dubia and Daphnia pulex for 24 to 42 days. The somatic growth data showed that both the cladoceran species began to increase their body size within 3 days and remained stable beyond a week. Food density had a significant effect on the body size of C. dubia while for D. pulex, food level and temperature as well as their interaction had significant effect on the body size. The population growth of both the cladoceran species was affected by temperature. At 15 degrees C, C. dubia showed little growth at the highest food density, while at low food level, there was some increase in the population abundances, especially after a long lag phase of about 2 weeks. At 15 degrees C and under the lowest tested food level, the population of D. pulex did not increase beyond the inoculation density. Optimal population growth of D. pulex was observed at intermediate temperature and at 1.0 x 10(6) cells ml(-1) of food level while for C. dubia 25 degrees C and the highest food level yielded highest growth. At low food level and low temperature, the rate of population increase (r) of C. dubia was much lower than that of D. pulex at comparable conditions. On the hand, at higher food levels and temperature regimes, the r of D. pulex was much higher than that of C.dubia. Regardless of the food levels and temperature, the r of C. dubia varied from 0.03 to 0.07 while that of D. pulex varied from 0.01 to 0.08 day(1).