Maternal consumption of non‐toxic Microcystis by Daphnia magna induces tolerance to toxic Microcystis in offspring

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Effects of the blue-green alga Microcystis aeruginosa on zooplankton competitive relations

Summary Field distribution patterns and laboratory feeding experiments have suggested that blooms of colonial blue-green algae strongly inhibit relatively large-bodied daphnid cladocerans. We conducted laboratory experiments to test the hypothesis that blooms of the colonial blue-green alga Microcystis aeruginosa would shift competitive dominance away from large-bodied daphnid cladocerans toward smaller-bodied cladocerans, copepods, and rotifers. In laboratory competition experiments, increasing the proportion of M. aeruginosa in the algal food supply resulted in a shift from dominance by the relatively largebodied cladoceran Daphnia ambigua to dominace by the copepod Diaptomus reighardi. The small-bodied cladoceran Bosmina longirostris was always numerically heavily dominant over D. ambigua, but its estimated population biomasses were only slightly higher than those of D. ambigua. Daphnia ambigua consistently outcompeted the rotifer Brachionus calyciflorus. Our results demonstrate that blooms of M. aeruginosa can alter zooplankton competitive relations in laboratory experiments, favoring small-bodied cladocerans and copepods at the expense of large-bodied cladocerans. However, contrary to predictions, blooms of M. aeruginosa did not improve the competitive ability of rotifers.     

Combined effects of four Microcystis aeruginosa strains and Scenedesmus obliquus concentrations on population dynamics and resting egg formation of two Daphnia species

We examined the hypothesis that toxic effects of Microcystis aeruginosa to population dynamics, ephippial production, and resting egg formation of Daphnia were restricted by food quality levels. Four unicellular, toxic M. aeruginosa strains with contrasting concentration of microcystin and two Daphnia species were used in this study. The number of offspring at first reproduction, population densities, and maximum population growth rate of two Daphnia species were lower at the highest toxic M. aeruginosa 7820 treatment compared with other treatments under two concentrations of Scenedesmus obliquus. The maximum number of offspring at first reproduction of two Daphnia species appeared in the higher toxic M. aeruginosa 526 strain at the high concentration of S. obliquus. The effects of M. aeruginosa strains, S. obliquus, and their combination on the number of offspring at first reproduction and the maximum population growth rate of two Daphnia species were significant (P < 0.001). Two Daphnia species could not reproduce in the highest toxic M7820 treatment under two concentrations of S. obliquus. They had lower population size and maximum population growth rate in the higher toxic M526 treatment at the low concentration of S. obliquus, but they were higher at the high concentration of S. obliquus. This result suggests that high S. obliquus concentration could relieve the toxicity of M. aeruginosa to Daphnia, and Daphnia could utilize the lower toxic Microcystis as food. The cumulative ephippia numbers of two Daphnia species were more at the high concentration of S. obliquus than those at the low concentration. The percentage of ephippia containing no resting eggs of two Daphnia species was evidently higher at the low concentration of S. obliquus but was lower at the high concentration of S. obliquus. Our results indicated that the cumulative ephippia numbers of Daphnia were population density dependent at the high-level food, and the productions of ephippia in Daphnia were significantly controlled by microcystin concentration at the low-level food.     

Response of Daphnia's Antioxidant System to Spatial Heterogeneity in Cyanobacteria Concentrations in a Lowland Reservoir

Many species and clones of Daphnia inhabit ecosystems with permanent algal blooms, and they can develop tolerance to cyanobacterial toxins. In the current study, we examined the spatial differences in the response of Daphnia longispina to the toxic Microcystis aeruginosa in a lowland eutrophic dam reservoir between June (before blooms) and September (during blooms). The reservoir showed a distinct spatial pattern in cyanobacteria abundance resulting from the wind direction: the station closest to the dam was characterised by persistently high Microcystis biomass, whereas the upstream stations had a significantly lower biomass of Microcystis. Microcystin concentrations were closely correlated with the cyanobacteria abundance (r = 0.93). The density of daphniids did not differ among the stations. The main objective of this study was to investigate how the distribution of toxic Microcystis blooms affects the antioxidant system of Daphnia. We examined catalase (CAT) activity, the level of the low molecular weight antioxidant glutathione (GSH), glutathione S-transferase (GST) activity and oxidative stress parameters, such as lipid peroxidation (LPO). We found that the higher the abundance (and toxicity) of the cyanobacteria, the lower the values of the antioxidant parameters. The CAT activity and LPO level were always significantly lower at the station with the highest M. aeruginosa biomass, which indicated the low oxidative stress of D. longispina at the site with the potentially high toxic thread. However, the low concentration of GSH and the highest activity of GST indicated the occurrence of detoxification processes at this site. These results demonstrate that daphniids that have coexisted with a high biomass of toxic cyanobacteria have effective mechanisms that protect them against the toxic effects of microcystins. We also conclude that Daphnia''s resistance capacity to Microcystis toxins may differ within an ecosystem, depending on the bloom''s spatial distribution.     

Eutrophication mediates rapid clonal evolution in Daphnia pulicaria

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Daphnia vertical distribution and the presence of toxic cyanobacteria

Daphnia can alter its vertical position in the water column in response to chemical cues from predators. In this study we tested the hypothesis that a Daphnia pulex clone with little evolutionary exposure to cyanobacteria would move away from patches of cyanobacteria (Anabaena affinis and A. flos-aquae) which contain potent endotoxins. Daphnia was censused at 2 h intervals for 6 h in laboratory columns in which there was a steep vertical gradient of cyanobacteria. Data were analyzed by repeated-measures ANOVA. Control (no Anabaena) and experimental columns showed no significant differences in Daphnia distributions. Daphnia in experimental columns frequently moved into areas with dense concentrations of Anabaena and stayed there for long periods of time. Our results show that this D. pulex clone does not exhibit a rapid (within 6 h) avoidance response to toxic Anabaena.     

Foraging strategies and growth inhibition in five daphnids feeding on mixtures of a toxic cyanobacterium and a green alga

1. Laboratory experiments were used to study the feeding, growth and reproduction of five daphnids in mixtures of a toxic cyanobacterium, Microcystis aeruginosa, and a green alga, Scenedesmus acutus. The mixtures included 0%, 20%, 50%, 80% and 100% Microcystis with a total food concentration of 0.5 mg C L^??1 in each treatment. The feeding rate was measured after 1 and 24 h of acclimatization to the mixtures. 2. Toxic Microcystis inhibited feeding in all the species, but they exhibited an unexpected diversity and complexity in the pattern of feeding inhibition. Daphnia magna exhibited the strongest inhibition of feeding after 1 h of exposure to toxic food, but had substantially recovered after 24 h in the same mixtures. This pattern of inhibition and recovery may balance the benefits of reduced ingestion of toxin with the disadvantage of a reduced energy intake. 3. All five daphnids grew quickly in the Scenedesmus control, whereas growth and reproduction declined with an increasing proportion of the toxic alga in the diet. Daphnia pulicaria showed the least inhibition of growth and reproduction, D. pulex showed the strongest inhibition and the three remaining species exhibited intermediate sensitivity. 4. Estimates of gross growth efficiency (GGE; growth/ingestion) provided a means for discriminating between the effects of feeding inhibition and direct toxicity on zooplankton growth. Daphnia pulex exhibited a sharp decline in GGE, suggesting that growth inhibition was a result of both feeding inhibition and direct toxicity. In contrast, D. magna exhibited a nearly constant GGE, indicating that feeding inhibition accounted for its decline in growth. However, two Daphnia species (i.e. D. pulicaria and D. galeata) exhibited improbable increases in GGE with toxic cyanobacteria, suggesting that their feeding rates were underestimated. Growth assays with sensitive and resistant zooplankton species are proposed for testing the potential impacts toxic cyanobacteria in lakes.     

The effects of the cyanobacterium Microcystis aeruginosa, the cyanobacterial hepatotoxin microcystin–LR, and ammonia on growth rate and ionic regulation of brown trout

Brown trout were exposed for 63 days to five treatments: a control; the purified cyanobacterial hepatotoxin microcystin—LR (MC—LR) (41—57 μg MC—LR 1^?1); lysed toxic Microcystis aeruginosa cells (41–68 μg MC—LR 1^?1 and 288 μg chlorophyll a 1^?1); lysed non—toxic M. aeruginosa cells (non—MC—LR containing and 288 μg chlorophyll a 1^?1); ammonia (65–325 μg NH₃ 1^?1). All treatments produced significantly reduced growth compared to controls (P<0·05, Fisher test). Exposure to ammonia resulted weight loss over the first 7 days followed by weight increase, though at a significantly lower level than in the other treatments. First exposed to lysed toxic M. aeruginosa cells grew less than those exposed to lysed non—toxic cyanobacteria or purified MC—LR. Sodium influx rates after 63 days exposure to purified MC—LR, lysed toxic M. aeruginosa cells, or ammonia showed a significant increase compared to control fish or those exposed to lysed non—toxic M. aeruginosa cells. There were no significant differences in Na⁺ efflux or net Na⁺ uptake rates between treatments. Significant increases in body Na⁺ and Cl^— were seen in fish exposed to lysed toxic M. aeruginosa cells or ammonia. Only fish exposed to ammonia showed a significant increase in body ammonia. Short—term exposure, over 4 h, to lysed toxic cells, non—toxic cells or purified MC—LR resulted in insignificant changes in Na⁺ flux rates compared to controls although there was a significant net Na⁺ loss in fish exposed to ammonia. Chronic exposure of fish to toxic cyanobacterial blooms may result in ionic imbalance and reduced growth.     

Context dependency of infectious disease: the cyanobacterium Microcystis aeruginosa decreases white bacterial disease in Daphnia magna

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Effects of Microcystis aeruginosa on interference competition between Daphnia pulex and Keratella cochlearis

Laboratory experiments tested the hypothesis that a toxic strain of Microcystis aeruginosa decreases the ability of Daphnia pulex to interfere with Keratella cochlearis. To test a variety of conditions, juvenile and adult Daphnia were exposed to the cyanobacterium for different time periods prior to, and during the experiments. Adult Daphnia not only suppressed rotifers over successive two-day intervals, but also had a significant impact within a 24-hour period. However, the presence of Microcystis (5 × 105 cells ml^–1) decreased the Daphnia effect in both experiments. Although juvenile Daphnia also significantly suppressed Keratella population growth, the presence of Microcystis (10⁵ and 5 × 10⁵ cells ml^–1) caused a significant reduction in daphniid body size and decreased the ability of both nonacclimated and acclimated daphniids to suppress rotifers. Keratella inhalation and mortality are positively correlated with filtering rates and body size of Daphnia. Therefore, the feeding rates and size structure of a Daphnia population will determine its potential to interfere with vulnerable rotifers. In all experiments the presence of Microcystis significantly decreased the ability of Daphnia to interfere with this rotifer despite the fact that Keratella was also inhibited. In the field this effect might be augmented if Microcystis colonies are more easily ingested by cladocerans than by the rotifers.     

Phytoplankton composition modifies predator-driven life history evolution in Daphnia

Organisms experience competing selective pressures, which can obscure the mechanisms driving evolution. Daphnia ambigua is found in lakes where a predator, the alewife (Alosa pseudoharengus) either does (anadromous) or does not (landlocked) migrate between marine and freshwater. We previously reported an association between alewife variation and life history evolution in Daphnia. However, differences in alewife migration indirectly influence phytoplankton composition for Daphnia. In ‘anadromous lakes’, Daphnia are present in the spring and experience abundant high-quality green algae. Intense predation by young-of-the-year anadromous alewife quickly eliminates these Daphnia populations by early summer. Daphnia from ‘landlocked lakes’ and lakes without alewife (‘no alewife lakes’) are present during the spring and summer and are more likely to experience high concentrations of sub-optimal cyanobacteria during the summer. To explore links between predation, resources, and prey evolution, we reared third-generation laboratory-born Daphnia from all lake types on increasing cyanobacteria concentrations. We observed several significant ‘lake type × resource’ interactions whereby the differences among lake types depended upon cyanobacteria concentrations. Daphnia from anadromous lakes developed faster, were larger at maturation, produced more offspring, and had higher intrinsic rates of increase in the absence of cyanobacteria. Such trends disappeared or reversed as cyanobacteria concentration was increased because Daphnia from anadromous lakes were more strongly influenced by the presence of cyanobacteria. Our results argue that alewife migration and phytoplankton composition both play a role in Daphnia evolution.     

Daphnia lumholtzi and Daphnia ambigua: population comparisons of an exotica and a native cladoceran in Lake Okeechobee, Florida

The population dynamics of an exotic cladoceran (Daphnia lumholtzi Sars) and a native cladoceran (Daphnia ambigua) were studied over a 12 month period in subtropical Lake Okeechobee, Florida (USA), to quantify the extent of invasion of the exotic species and compare ecological niches. Daphnia lumholtzi accounted for up to 70% of the Daphnia assemblage during the summer months (June-August), while D.ambigua accounted for up to 97% of the Daphnia assemblage from fall to spring (October-April). The densities of the two species were inversely corelated. The exotic species was most concentrated in the shallower, warmer, north and south ends of the lake during the summer. It also was present, but at much lower densities, in the central lake region during the fall. The native species displayed a ubiquitous distribution throughout the lake during spring and winter, but was concentrated in the deeper, cooler, central region during the summer. Relationships of the two species with environmental conditions indicate that water column temperature might affect the seasonal and spatial distribution of the two Daphnia species. The results also indicate that D.lumholtzi may be filling a 'vacant' seasonal or spatial niche when conditions are unfavorable for D.ambigua.      

The effects of Daphnia on nutrient stoichiometry and filamentous cyanobacteria: a mesocosm experiment in a eutrophic lake

1. Stoichiometric theory predicts that the nitrogen : phosphorus (N : P) ratio of recycled nutrients should increase when P‐rich zooplankton such as Daphnia become dominant. We used an enclosure study to test the hypothesis that an increased biomass of Daphnia will increase the relative availability of N versus P sufficiently to decrease the abundance of filamentous cyanobacteria. The experiment was conducted in artificially enriched Lake 227 (L227) in the Experimental Lakes Area (ELA), north‐western Ontario, Canada. Previous studies in L227 have shown that the dominance of filamentous, N‐fixing cyanobacteria is strongly affected by changes in the relative loading rates of N and P. 2. We used a 2 × 2 factorial design with the addition or absence of D. pulicaria and high or low relative loading rates of N and P (+NH₄, –NH₄) in small enclosures as treatment variables. If Daphnia can strongly affect filamentous cyanobacteria by altering N and P availability, these impacts should be greatest with low external N : P loading rates. The phytoplankton community of L227 was predominantly composed of filamentous Aphanizomenon spp. at the start of the experiment. 3. Daphnia strongly reduced filamentous cyanobacterial density in all enclosures to which they were added. The addition of NH₄ had only a small impact on algal community composition. Hence, we conclude that Daphnia did not cause reductions in cyanobacteria by altering the N : P ratio of available nutrients. 4. Despite the lack of evidence that Daphnia affected filamentous cyanobacteria by altering the relative availability of N and P, we found changes in nutrient cycling consistent with other aspects of stoichiometric theory. In the presence of Daphnia, total P in the water column decreased because of an increase in P sedimentation. In contrast to P, a decrease in suspended particulate N was offset by an increase in dissolved N (especially NH₄). Hence, dissolved and total N : P ratios in the water column increased with Daphnia as a result of differences in the fate of suspended particulate N versus P. There was minimal accumulation and storage of P in Daphnia biomass in the enclosures. 5. Our experiment demonstrated that Daphnia can strongly limit filamentous cyanobacterial abundance and affect the biogeochemical cycling of nutrients. In our study, changes in nutrient cycling were apparently insufficient to cause the changes in phytoplankton community composition that we observed. Daphnia therefore limited filamentous cyanobacteria by other mechanisms.     

Crowding‐induced changes in growth,reproduction and morphology of Daphnia

• 1 Daphnia may reach high population densities seasonally, or in patches, in lakes. To test the effects of chemicals released by high daphniid densities on their life‐history traits, nine species of Daphnia, D. magna, D. pulicaria, D. pulex, D. hyalina, D. galeata, D. laevis, D. lumholtzi, D. ambigua and D. cucullata, were grown in water from crowded Daphnia cultures in a flow‐through system in the presence of abundant food.
• 2 Water from Daphnia at 85 L^‐1 depressed growth rate, and lowered body size and clutch at first reproduction of six species of small‐bodied Daphnia (adult body length < 1.8 mm), but had no significant effects on larger species. Two clones of D. pulex differed in their growth rate in response to crowding, indicating that response patterns may vary within species.
• 3 Chemicals released by crowded D. magna reduced tail spine length in D. lumholtzi and D. cucullata by 37% and 11%, respectively, and induced changes in carapace morphology in D. lumholtzi and D. ambigua.
• 4 Chemicals released by crowded conspecifics may provide an additional, density‐dependent mechanism of population regulation; when large species of Daphnia coexist at a high population density with small species, these chemicals may reinforce the competitive advantage of large species.

     

NATURAL SELECTION FOR GRAZER RESISTANCE TO TOXIC CYANOBACTERIA: EVOLUTION OF PHENOTYPIC PLASTICITY?

Abstract We studied the selection response of the freshwater grazing zooplankter, Daphnia galeata, to increased abundance of cyanobacteria in its environment. Cyanobacteria are a poor‐quality and often toxic food. Distinct genotypes of D. galeata were hatched from diapausing eggs extracted from three time horizons in the sediments of Lake Constance, Europe, covering the period 1962 to 1997, a time of change in both the prevalence of planktonic cyanobacteria and levels of phosphorus pollution. We assessed whether the grazers evolved to become more resistant to dietary cyanobacteria by exposing genetically distinct clones to two diets, one composed only of the nutritious green alga, Scenedesmus obliquus (good food), and the other a mixture of S. obliquus and the toxic cyanobacterium Microcystis aeruginosa (poor food). Genotype performance was measured as the specific rate of weight gain from neonate to maturity (g_j). We evaluated evolutionary change in the Daphnia population using an analysis of reaction norms based on relative (log‐transformed) changes in g_j. Log(g_j) is a measure of the proportional effect of dietary cyanobacteria on other fitness components of the Daphnia phenotype. For comparison, we also analyze absolute (i.e., nontransformed) changes in g_j and discuss the interpretations of the two approaches. Statistical results using a general linear model demonstrate a significant effect of genotype (showing differences in g_j among genotypes), a significant genotype X food‐type interaction (showing differences in phenotypic plasticity among genotypes), and, in the case of log‐transformed data, a significant sediment‐genotype‐age X food‐type interaction. The latter shows that phenotypic plasticity evolved over the period studied. Two constraints act on response to selection in the D. galeata‐Lake Constance system. First, g_j on a diet containing poor food is highly correlated with g_j on a diet of good food, thus evolving resistance also meant evolving an increase in g_j on both diets. Second, because genotypes with a high g_j also grow to a large adult body size, which in turn increases Daphnia vulnerability to fish predation, we suggest that selection only acted to favor genotypes possessing a high potential g_j after cyanobacteria became prevalent. The presence of cyanobacteria depressed realized g_j and led to animals of small adult body size even if their genotypes had the potential for high g_j and large size. With realized g_j reduced, genotypes with an inherently high value could be selected even in the presence of predatory fish. The joint action of selection by dietary cyanobacteria and vulnerability to fish predation provides an explanation for the observed evolution of resistance to poor food through reduced phenotypic plasticity.     

Plasmid distribution among unicellular and filamentous toxic cyanobacteria

Plasmid content of 5 hepatotoxin and 2 neurotoxin producing cyanobacterial strains were analyzed. Among the hepatotoxin-producing strains, Microcystis aeruginosa PCC7820, M. aeruginosa M228 and M. aeruginosa UV027 were found to carry plasmids, whereas other hepatotoxin and neurotoxin producing strains did not harbor any plasmids. Correlations were sought between toxicity and the presence of plasmids in toxic cyanobacteria as a function of age. Aged cultures of M. aeruginosa PCC7820 exhibited toxicity and harbored plasmids. In other cyanobacterial strains, plasmids were not detected. The data add to and support the current understanding that plasmids are probably not involved in toxin production in cyanobacteria.Author for Correspondence     

Seasonal dependence and functional implications of macrophyte–phytoplankton allelopathic interactions

Invasive plant species such as Ludwigia hexapetala might have a competitive advantage if they produce allelopathically active compounds against primary producers. Both phytoplankton and plant community structure may be affected due to different, species‐specific sensitivity to allelochemicals. Moreover, such allelopathic interactions could vary over the year depending on (i) the plant's phenological stage and (ii) the abilities of the native macrophytes to suppress—or the non‐native macrophytes to stimulate—the non‐native macrophyte population. We tested the allelopathic effects of aqueous leaf extracts of L. hexapetala on the photosynthetic activity of three target phytoplankton strains (Scenedesmus communis, a toxic Microcystis aeruginosa strain and a non‐toxic Microcystis aeruginosa strain) over three seasons of development (spring, summer and autumn). We also tested seasonal allelopathic effects of aqueous leaf extracts of both L. hexapetala (i.e. the non‐native invasive species) and the native Mentha aquatica on L. hexapetala seed germination. Finally, we identified three main secondary compounds present in the aqueous leaf extracts of L. hexapetala and we tested each individual compound on the phytoplankton's photosynthetic activity and on L. hexapetala seed germination. We observed marked seasonal and species‐specific patterns of L. hexapetala allelopathy on phytoplankton. The photosynthetic activities of S. communis and the toxic M. aeruginosa strain were stimulated by L. hexapetala aqueous leaf extracts in autumn and spring, respectively, whereas the non‐toxic M. aeruginosa strain was strongly inhibited in these two seasons. In summer, photosynthesis of all phytoplankton strains was inhibited. The germination rate of L. hexapetala seeds was stimulated by both L. hexapetala and M. aquatica aqueous leaf extracts, especially in summer, concomitant with the strong negative effects observed on the three phytoplankton strains. Three flavonoid glycosides (myricitrin, prunin and quercitrin) were identified as the main secondary compounds present in the L. hexapetala aqueous leaf extracts. The photosynthetic activity of S. communis was slightly stimulated by the three compounds. The photosynthetic activity of the toxic M. aeruginosa strain was stimulated by myricitrin and quercitrin, whereas that of the non‐toxic M. aeruginosa strain was inhibited by prunin. Finally, the germination rate and the germination velocity of L. hexapetala seeds were stimulated by myricitrin and prunin. These findings suggest that L. hexapetala could favour the photosynthetic activity of toxic cyanobacteria in spring and reduce their photosynthetic activity in summer, potentially leading to drastic changes in the phytoplankton communities and therewith ecological functioning of invaded ponds. Moreover, the stimulation of its seed germination could give a strong competitive advantage to L. hexapetala, thus promoting its invasiveness.     

Effects of Daphnia exudates and sodium octyl sulphates on filament morphology and cell wall thickness of Aphanizomenon gracile (Nostocales), Cylindrospermopsis raciborskii (Nostocales) and Planktothrix agardhii (Oscillatoriales)

Grazing is recognized as one of the selective factors shaping the morphology and physiology of cyanobacteria. A recent study has shown that the filamentous cyanobacterium Aphanizomenon gracile strain SAG 31.79 thickened in the presence of Daphnia (Cladocera) and its exudates. The aims of our study were: (1) to determine whether this type of response to Daphnia cues is common for other strains of A. gracile, and other species of filamentous cyanobacteria, (2) to test whether the response is due to nutrients recycled by Daphnia, or kairomone induced, and (3) whether it is related to toxin production. Prior to the experiment, cyanobacterial strains were inspected using chromatographic methods for the presence of two toxins, cylindrospermopsin (CYN) and three homologues of microcystin (MC-RR, MC-YR, MC-LR). HPLC analyses showed that all strains were free of cylindrospermopsin, whereas microcystins were detected only in one strain (Planktothrix agardhii). We then tested whether Daphnia exudates can cause thickening of cyanobacterial filaments, which would suggest the morphological changes in cyanobacterial filaments are caused by recycled nutrients. Cyanobacteria were also exposed to sodium octyl sulphate (a commercially available Daphnia kairomone). Transmission electron microscopy (TEM) was used to check whether Daphnia exudates and sodium octyl sulphate trigger thickening of cyanobacterial cell walls, which would be a defence mechanism against grazing. The TEM analysis revealed no significant effect of either Daphnia exudates or kairomone (sodium octyl sulphate) on the cell wall thickness of cyanobacteria. However, our study showed that Daphnia exudates triggered filament thickening in nostocalean cyanobacteria, while filaments of the oscillatorialean strain P. agardhii did not show this response. It was also demonstrated that sodium octyl sulphate alone can also cause filament thickening, which suggests that this might be a specific defence response to the presence of grazers.     

Development of tolerance against toxic cyanobacteria in Daphnia

We tested whether previous exposure to a toxic strain of cyanobacteria (Microcystis) affects survival, growth, and reproduction of a common herbivore, Daphnia magna. Samples from three natural populations of D. magna were each divided into two parts; one part was fed a mixture of toxic Microcystis and the non-toxic green alga Scenedesmus whereas the other part was fed only Scenedesmus. After four weeks, we compared the ability of these two populations to withstand the toxic Microcystis by assessing survivorship, growth, and reproduction. We found that the ability of D. magna to cope successfully with toxic Microcystis is improved if the animals have experienced previous exposure to toxic Microcystis. This suggests that the toxin may less affect the D. magna populations that are repeatedly exposed to toxic cyanobacteria in their natural habitat than populations lacking prior exposure. Since the ability to tolerate toxins is manifested in both improved survival and larger size of the animals, it may have considerable impact on zooplankton community composition in fresh-waters.     

Effects of suspended clay on Daphnia body growth and fitness

1. Suspended sediments often reduce the abundance, fitness, and feeding rates of planktonic cladocerans. This paper examines the effect of suspended clay particles on the body growth and life-history parameters of Daphnia ambigua. Cohorts were exposed to zero or 50mgl^?1 suspended clay (particle size <2μm). Individuals exposed to clay had significantly lower body lengths at a given age than individuals in the control cohort. Parameters for the von Bertalanffy body growth equation were calculated for each individual; the asymptotic maximum body length (l_max) was reduced, but the growth rate constant (k) was unaffected, by the presence of suspended clay. Body length at reproductive maturity was lower in the presence of suspended clay; this may be an adaptive response and is similar to the response of cladocerans to limiting food concentrations. 2. Age-specific survivorship and fecundity were both reduced, and the ages of maturity and first reproduction were both increased, in the presence of suspended clay. The overall effect of these demographic changes was a 70% decline in the net reproductive rate (R_o), a measure of fitness. 3. Suspended clay affected the relationship between body length and fecundity. Animals of a given length produced fewer eggs in the presence of suspended clay. In addition, there was a significant interaction in the effects of body length and clay on brood size: the slope of the regression line of brood size as a function of body length was lower in the presence of clay. This interaction has also been observed in limiting food concentrations, and may be caused by reductions in body length at maturity in low food or high clay environments. 4. The similarity between the effects of limiting food concentrations and suspended clay on Daphnia body growth, survivorship, fecundity, and brood size makes sense, given previous observations showing that suspended clay reduces the feeding rate of D. ambigua by up to 70% due to mechanical interference with feeding behaviours.