Nodularin concentrations in Baltic Sea zooplankton and fish during a cyanobacterial bloom

Toxic cyanobacterial blooms, dominated by Nodularia spumigena, are a recurrent phenomenon in the Baltic Sea during late summer. Nodularin, a potent hepatotoxin, has been previously observed to accumulate on different trophic levels, in zooplankton, mysid shrimps, fish as well as benthic organisms, even in waterfowl. While the largest concentrations of nodularin have been measured from the benthic organisms and the food web originating from them, the concentrations in the pelagic organisms are not negligible. The observations on concentrations in zooplankton and planktivorous fish are sporadic, however. A field study in the Gulf of Finland, northern Baltic Sea, was conducted during cyanobacterial bloom season where zooplankton (copepod Eurytemora affinis, cladoceran Pleopsis polyphemoides) and fish (herring, sprat, three-spined stickleback) samples for toxin analyses were collected from the same sampling areas, concurrently with phytoplankton community samples. N. spumigena was most abundant in the eastern Gulf of Finland. In this same sampling area, cladoceran P. polyphemoides contained more nodularin than in the other areas, suggesting that this species has a low capacity to avoid cyanobacterial exposure when the abundance of cyanobacterial filaments is high. In copepod E. affinis nodularin concentrations were high in all of the sampling areas, irrespective of the N. spumigena cell numbers. Furthermore, nodularin concentrations in herring samples were highest in the eastern Gulf of Finland. Three-spined stickleback contained the highest concentrations of nodularin of all the three fish species included in this study, probably because it prefers upper water layers where also the risk of nodularin accumulation in zooplankton is the highest. No linear relationship was found between N. spumigena abundance and nodularin concentration in zooplankton and fish, but in the eastern area where the most dense surface-floating bloom was observed, the nodularin concentrations in zooplankton were high. The maximum concentrations in zooplankton and fish samples in this study were higher than measured before, suggesting that the temporal variation of nodularin concentrations in pelagic communities can be large, and vary from negligible to potentially harmful.     

No simple sum: seasonal variation in tannin phenotypes and leaf-miners in hybrid oaks

Parent-hybrid asymmetries in the biosynthesis of defense chemistry are believed to affect the distribution of herbivorous insects within plant hybrid zones. While tannins are often produced as complex, dynamic mixtures, the ecological effects of biological and ontogenetic variation in tannin metabolism within hybrid zones are poorly understood. Here, we examine correlations between the seasonal variation of absolute and relative concentrations of individual ellagitannins and total proanthocyanidins in the Quercus gambelii × Q. grisea hybrid complex and a community of leaf-mining Lepidoptera (Gracillariidae, Heliozelidae, Nepticulidae, Tischeriidae), and examine the interactive effects between tannins on leaf-miner abundance. We found strong seasonal variation in oak tannin phenotypes, but two general phenotypes are characteristic of oaks of the Q. gambelii × Q. grisea complex, based upon biosynthetic variation in high molecular weight ellagitannins. Leaf-miner community structure was significantly correlated with the dominant tannin phenotypes in the Q. gambelii × Q. grisea hybrid zone in mid-summer (July, August). Additionally, leaf-miner density was significantly negatively correlated with the ratio of vescalagin-derivatives and total proanthocyanidins in August and September. Overall, hybridization affects biological and ontogenetic variation in ellagitannin and proanthocyanidin biosynthesis within the Q. gambelii × Q. grisea hybrid complex and this variation correlates with herbivore distribution and abundance. Future studies of phenolic chemistry in plant-herbivore interactions in hybrid zones should include a more rigorous quantification of tannin phenotypes as complex, temporally dynamic mixtures.     

Seasonal development of scyphozoan medusae and the predatory impact of Aurelia aurita on the zooplankton community in the Bornholm Basin (central Baltic Sea)

The annual cycle of abundance and distribution of the scyphozoan medusae Aurelia aurita and Cyanea capillata was studied in the Bornholm Basin (central Baltic Sea) in 2002. Seasonal changes in prey composition and predatory impact were investigated by analyzing stomach contents. A. aurita occurred from July to November, with a maximum mean abundance of 2.3 ind. per 100 m³ in August, whereas C. capillata was caught in much smaller numbers from July to September. No ephyrae of either species were found; therefore, advection of medusae from the western Baltic Sea is assumed. From July to October, ~80% of A. aurita medusae was distributed in the upper 20 m above the thermocline, whereas C. capillata occurred only in the halocline below 45 m. A. aurita did not migrate vertically and fed mainly on the most abundant cladoceran species Bosmina coregoni maritima. Further prey organisms were the cladocerans Evadne nordmanni and Podon spp., mollusk larvae and copepods. Copepod nauplii and copepodite stages I–III were not eaten by the medusae, neither were fish eggs and larvae used as prey. Based on mean medusa and zooplankton abundance from the upper 20 m, the predatory impact was very low. In August, when mean abundance of A. aurita was highest, only 0.1% of the copepod and 0.5% of the cladoceran standing stock were eaten per day. However, in regions with higher medusa or lower zooplankton abundance, up to 7.9% of the cladoceran standing stock was consumed per day. Hence, A. aurita did not regulate the zooplankton community in the Bornholm Basin, and fish larvae did not suffer from competition with and predation by the medusae.Communicated by O. Kinne, Oldendorf/Luhe     

Monthly changes in the abundance and biomass of zooplankton and water quality parameters in Kukkarahalli Lake of Mysore, India

Zooplankton abundance and distribution are of ecological importance, as they are very sensitive to change, therefore zooplankton make ideal indicators of aquatic ecosystem. This study carried out on the abundance of rotifer, cladoceran, cyclopoid-copepod and ostracod zooplankton groups and biomass of total zooplankton were studied every month for one year. It is interesting and noteworthy to note that Calanoid and Harpacticoid zooplankton groups and free carbon dioxide were completely absent in all the four sampling sites throughout the study year. About 53% of the variation in the abundance of Cladoceran, 55% of variation in the Cyclopoid-copepod, 39% of variation in the ostracod and 53% of variation in the abundance of total zooplankton were mainly due to pH. Interestingly, negative relationship was found between the total zooplankton and concentration of phosphate as in this lake 67% decrease in wet biomass was mainly because of phosphate, where as 47% of dry biomass of total zooplankton was positively correlated with conductivity.     

Iron interference on matrix-assisted laser desorption/ionisation time-of-flight mass spectra of condensed tannins

Matrix-assisted laser desorption/ionisation time-of-flight mass spectrometry (MALDI-TOF MS) was used to reveal the influence of iron on the detectability of condensed tannins extracted from Picea abies. Iron chloride mixed with condensed tannins suppresses the detection of tannins by MALDI-TOF MS in a dose-dependent manner. Total suppression is observed at concentrations of 250 and 500 mg FeCl₃ g^–1 of sample. At lower concentrations, spectrum quality is still greatly reduced, but short chain tannins are detected. At 100 mg FeCl₃ g^–1 of sample, iron increased the intensity of the tannin galloyl ester peaks above those of the corresponding tannin polymers. This points to an antagonistic effect of the galloyl ester moiety on iron complexation with condensed tannins and, thus, their reduced susceptibility to iron interference during MALDI-TOF MS.     

Adaptation strategies of the corallimorpharian <Emphasis Type="Italic">Rhodactis rhodostoma</Emphasis> to irradiance and temperature

Corallimorpharians may dominate some habitats on coral reefs and compete with stony corals for access to light, yet little is known concerning their photosynthetic traits. At Eilat in the northern Red Sea, we observed that the abundance of individuals of the corallimorpharian Rhodactis rhodostoma decreased significantly with depth on the reef slope. Field and laboratory experiments revealed that they employ several mechanisms of photoadaptation to high irradiance on the shallow reef flat. Their endosymbiotic microalgae (zooxanthellae) varied significantly in both abundance and chlorophyll content with level of irradiance. Use of a diving pulse amplitude modulated fluorometer revealed that the zooxanthellae of R. rhodostoma effectively disperse excess light energy by expressing significantly higher values of non-photochemical quenching and maximum excitation pressure on photosystem II when experimentally exposed to high light (HL) versus low light (LL). Host corallimorpharian tissues mediated this response by shielding the algal symbionts from high irradiance. The endoderm of host tentacles thickened significantly and microalgal cells were located further from the mesoglea in HL than in LL. The clades of zooxanthellae hosted by the corallimorpharians also varied with depth. In shallow water, all sampled individuals hosted clade C zooxanthellae, while in deep water the majority hosted clade D. The photosynthetic output of individuals of R. rhodostoma was less affected by HL than was that of a stony coral examined. When exposed to both high temperature (HT) and HL, individuals of R. rhodostoma reduced their maximum quantum yield, but not when exposed to HL at low temperature (LT). In contrast, colonies of the scleractinian coral Favia favus reduced their photosynthetic output when exposed to HL in both temperature regimes. After 2 weeks of HT stress, R. rhodostoma polyps appeared to bleach completely but re-established their zooxanthella populations upon return to ambient temperature. We conclude that mechanisms of photoadaptation to high irradiance employed by both the endosymbiotic zooxanthellae and host corallimorpharians may explain in part the abundance of R. rhodostoma on some shallow reef flats. The ability to survive for weeks at HT while bleached also may allow corallimorpharians to repopulate shallow reef areas where scleractinians have been killed by thermal stress. B. Kuguru and G. Winters contributed equally to this work.     

Shifting abundance of the ctenophore Mnemiopsis leidyi and the implications for larval bivalve mortality

Along the mid-Atlantic coast of the US, the ctenophore Mnemiopsis leidyi (Agassiz) appears to be increasing in abundance and undergoing shifts in its historical seasonal distribution. We provide new data on shifting ctenophore abundance in Long Island estuaries and its implications for top-down control of the plankton community. Peak mean biovolume estimates of M. leidyi in Long Island estuaries in 2006 revealed ctenophore abundance values that were a factor of two to five times greater than previous studies conducted two decades ago. Furthermore, peak M. leidyi densities in 2006 occurred 2–3 months earlier than previously documented, suggesting a shift in the seasonal maxima of M. leidyi. Application of daily ingestion rates to zooplankton abundance indicates that, at its highest densities M. leidyi can remove an overall average of 20–89% per day of bivalve veligers and other zooplankton taxa, including adult copepods, nauplii, and tintinnids. Increasing ctenophore abundance, especially during a period when they were not historically abundant (i.e., June) may have significant consequences for species which spawn at this time. For example, current populations of M. leidyi represent a major source of larval mortality for bivalves which may inhibit recovery of shellfish populations and reinforce their low abundance state in Long Island estuaries.     

Modeling the refuge effect of submerged macrophytes in ecological dynamics of shallow lakes: A new model of fish functional response

Submerged macrophytes often provide refuge for zooplankton from fish predation in temperate and subtropical shallow lakes. However, since the relationship between submerged macrophyte abundance and its refuge effect has not been well established, the refuge effect is difficult to be simulated. In this paper, we constructed mathematical models to describe the refuge effect of submerged macrophytes on fish foraging activities and ecological dynamics of shallow lakes based on the previous studies. We clarified the underlying behavioral mechanisms of the observed functional responses through analyses of the fish foraging behavior, extracted the affected variables related to the refuge effect, formulized the relationship between the affected variables and submerged vegetation density, and determined parameter values with a compensative procedure. Calibration and validation results indicated that the new functional response model was successful to simulate the refuge effect on interfering with fish foraging behavior. Moreover, the model was cooperated into a minimal ecological model for shallow lakes. Modeling results showed that the model was able to simulate the refuge effect in ecological dynamics, and made the ecological model produce significantly different results from those with the existing functional response models.     

Tolerance of marine calanoid resting eggs: Effects of freezing,desiccation and Rotenone exposure - a field and laboratory study

Tolerance of calanoid resting eggs from bottom sediment in a manipulated marine enclosure (Svartatjønn, western Norway) to freezing, desiccation and exposure to the insecticide Rotenone was examined in 1989 and 1990, both in situ and in the laboratory. Egg abundances in the sediment were compared before and after draining followed by Rotenone treatment. The drained parts of the basin were exposed to both drying and freezing. A significant decrease in hatching success was found in the drained parts during the study (p<0.05,t-test) and in the deepest parts, which were exposed to Rotenone. No significant difference was found in an intermediate zone around the new water level established by the draining. Both drying (at 20°C) and freezing (at –15°C) caused total mortality of the eggs in laboratory experiments. Laboratory experiments also showed that Rotenone concentrations higher than 0.5 ppm resulted in significantly reduced hatching rates, indicating that resting eggs of calanoids tolerate 20 to 100 times higher Rotenone concentrations than do later ontogenetic stages. The operational procedure (i.e. draining and Rotenone treatment) employed in enclosed ponds used for marine fish fry production will, within a 1-yr cycle, reduce the abundance of resting eggs and therefore the potential for self-recruitment of the zooplankton stocks.     

Differential responses in American (<Emphasis Type="Italic">Castanea dentata</Emphasis> Marshall) and Chinese (<Emphasis Type="Italic">C. mollissima</Emphasis> Blume) chestnut (Falales: Fagaceae) to foliar application of jasmonic acid

Summary.  Tannins are plant defense compounds that exhibit antibiotic (e.g. toxic) and antixenotic (e.g. repellent) effects against an array of plant pests. They are broadly divided into two major groups, hydrolysable tannins (gallotannins and ellagitannins) and proanthocyanidins, each with an undetermined number of compounds. We investigated constitutive levels of hydrolysable tannins and proanthocyanidins in the leaves and stems of American (Castanea dentata Marshall) (Fagales: Fagaceae) and Chinese (C. mollissima Blume) chestnut. American chestnut contained more proanthocyanidins in leaves and stems than Chinese chestnut, but Chinese chestnut contained more foliar hydrolysable tannins. Regardless of these differences, gypsy moth (Lymantria dispar L, Lepidoptera: Lymantriidae) performance did not differ when fed American and Chinese chestnuts. We also investigated the effects of jasmonic acid (JA) treatment on differential tannin induction in American and Chinese chestnut leaves and stems. JA treatment increased proanthocyanidins in American chestnut stems and hydrolysable tannins in both tissue types of American chestnut, but did not influence tannin concentrations in Chinese chestnut leaves or stems. HPLC-ESI-MS analysis of pooled samples suggested that hydrolysable tannins in each tissue were qualitatively comprised primarily of ellagitannins, and JA generally increased the number of hydrolysable tannins that could be detected by ESI-MS. Third, we investigated the performance of gypsy moths on JA treated and untreated American and Chinese chestnut. Caterpillar relative growth was not influenced by JA treatment on Chinese chestnut, but decreased in response to JA application on American chestnut. Our results indicate that JA-dependent defenses differ between these chestnut species. This study improves our understanding of ecologically important differences in tannin induction and herbivore susceptibility in Castanea, and has important implications in efforts for American chestnut restoration and commercial chestnut production.     

Global patterns of epipelagic gelatinous zooplankton biomass

There is concern that overfishing may lead to a proliferation of jellyfish through a process known as fishing down the food web. However, there has been no global synthesis of patterns of gelatinous zooplankton biomass (GZB), an important first step in determining any future trends. A meta-analysis of epipelagic-GZB patterns was undertaken, encompassing 58 locations on a global scale, and spanning the years 1967–2009. Epipelagic-GZB decreased strongly with increasing total water column depth (r ² = 0.543, p < 0.001, n = 58): in shallow (<50 m) coastal waters, epipelagic-GZB was typically 742× the levels in deep ocean (>2,000 m) sites. However, the ratio of GZB to primary productivity showed high values across a range of depths, i.e. this measure of the relative abundance of gelatinous zooplankton did not co-vary with depth.     

Fish reintroductions reveal smooth transitions between lake community states

Whether communities respond smoothly or discontinuously to changing environmental conditions has important consequences for the preservation and restoration of ecosystems. Theory shows that communities may exhibit a variety of responses to environmental change, including abrupt transitions due to the existence of alternate states. However, there have been few opportunities to look for such transitions in nature. Here, we examine the impact of a two-orders-of-magnitude decrease and then increase in planktivore abundance in Wintergreen lake (Michigan, USA), caused by the extinction and reintroduction of two dominant fish species (largemouth bass, Micropterus salmoides, and bluegill, Lepomis macrochirus). Over a 16 + yr period of slow change from high planktivory to low planktivory back to high planktivory, the zooplankton community changed smoothly and predictably between states. In years of low planktivory, the zooplankton assemblage was dominated by a single, large, cladoceran species, Daphnia pulicaria, whereas in years of high planktivory, D. pulicaria disappeared and was replaced by a suite of small-bodied cladocerans. We quantified the multivariate change in zooplankton community dissimilarity and found that community state smoothly tracked changes in planktivore density in both a forward and backward direction. Thus, there was little evidence of discontinuity in this system where transitions are strongly driven by planktivory.     

Habitat specialization and the exploitation of allochthonous carbon by zooplankton

The significance of spatial subsidies depends on consumer resource interactions in the recipient habitat. Lakes are subsidized by terrestrial carbon sources, but the pathways of allochthonous carbon through lake food webs are complex and not well understood. Zooplankton vertically partition resources within stratified lakes in response to life history trade-offs that are governed by predators, the quantity and quality of food, and abiotic conditions (e.g., UV, temperature, and viscosity). We measured habitat specialization of zooplankton in an oligotrophic lake where allochthonous and autochthonous resources varied with depth. During stratification, the quantity and quality of zooplankton food was highest in the hypolimnion. We used a yearlong time series of the delta13C of zooplankton and particulate organic matter (POM) to determine which zooplankton species exploited hypolimnetic rather than epilimnetic resources. Because the delta13C of POM decreased with depth, we used the delta13C of zooplankton to detect inter- and intraspecific variation in habitat selection. We incubated Daphnia pulex at discrete depths in the water column to confirm that the delta13C of zooplankton can indicate habitat specialization. Zooplankton that specialized in the epilimnion relied more on allochthonous carbon sources than those that specialized in the hypolimnion. Therefore, the fate of allochthonous carbon subsidies to lakes depends on spatially explicit consumer-resource interactions.     

Effect of the allelochemicals from the macrophyte Egeria densa on the competitive interactions of pelagic and littoral cladocerans

Macrophytes release allelochemicals, which affect pelagic cladocerans such as Daphnia. Using population growth experiments, we analysed the effects of allelochemicals from the macrophyte Egeria densa on the interaction between Daphnia mendotae and three littoral cladocerans (Diaphanosoma birgei, Macrothrix triserialis and Simocephalus mixtus). We found that allelochemicals from E. densa increased the abundance of all the tested cladocerans in spite of the presence of a competitor. This effect was stronger (nearly three to four times higher than in controls) for D. birgei and M. triserialis in the absence of D. mendotae. Independent of the presence of allelochemicals, S. mixtus, but not D. birgei and M. triserialis, reduced the abundance of D. mendotae as compared to controls. The rate of population increase (r) per day was significantly elevated due to the presence of Egeria’s allelochemicals (from 0.07–0.16?d^?1 without allelochemicals against 0.12–0.24?d^?1 with allelochemicals). In our competition experiments, the rates of population increase of the cladocerans were lower than those in single-species cultures when cultured in the absence of allelochemicals. However, in the presence of allelochemicals this trend was not consistent. Thus, the positive effects of Egeria’s allelochemicals over the cladoceran populations may enhance the grazing pressure on phytoplankton.     

The effects of seston food quality on planktonic food web patterns

In planktonic food webs, the conversion rate of plant material to herbivore biomass is determined by a variety of factors such as seston biochemical/elemental composition, phytoplankton cell morphology, and colony architecture. Despite the overwhelming heterogeneity characterizing the plant–animal interface, plankton population models usually misrepresent the food quality constraints imposed on zooplankton growth. In this study, we reformulate the zooplankton grazing term to include seston food quality effects on zooplankton assimilation efficiency and examine its ramifications on system stability. Using different phytoplankton parameterizations with regards to growth strategies, light requirements, sinking rates, and food quality, we examined the dynamics induced in planktonic systems under varying zooplankton mortality/fish predation, light conditions, nutrient availability, and detritus food quality levels. In general, our analysis suggests that high food quality tends to stabilize the planktonic systems, whereas unforced oscillations (limit cycles) emerge with lower seston food quality. For a given phytoplankton specification and resource availability, the amplitude of the plankton oscillations is primarily modulated from zooplankton mortality and secondarily from the nutritional quality of the alternative food source (i.e., detritus). When the phytoplankton community is parameterized as a cyanobacterium-like species, conditions of high nutrient availability combined with high zooplankton mortality led to phytoplankton biomass accumulation, whereas a diatom-like parameterization resulted in relatively low phytoplankton to zooplankton biomass ratios highlighting the notion that high phytoplankton food quality allows the zooplankton community to sustain relatively high biomass and to suppress phytoplankton biomass to low levels. During nutrient and light enrichment conditions, both phytoplankton and detritus food quality determine the extent of the limit cycle region, whereas high algal food quality increases system resilience by shifting the oscillatory region towards lower light attenuation levels. Detritus food quality seems to regulate the amplitude of the dynamic oscillations following enrichment, when algal food quality is low. These results highlight the profitability of the alternative food sources for the grazer as an important predictor for the dynamic behavior of primary producer–grazer interactions in nature.     

Biogeography of bacterioplankton in lakes and streams of an Arctic tundra catchment

Bacterioplankton community composition was compared across 10 lakes and 14 streams within the catchment of Toolik Lake, a tundra lake in Arctic Alaska, during seven surveys conducted over three years using denaturing gradient gel electrophoresis (DGGE) of PCR-amplified rDNA. Bacterioplankton communities in streams draining tundra were very different than those in streams draining lakes. Communities in streams draining lakes were similar to communities in lakes. In a connected series of lakes and streams, the stream communities changed with distance from the upstream lake and with changes in water chemistry, suggesting inoculation and dilution with bacteria from soil waters or hyporheic zones. In the same system, lakes shared similar bacterioplankton communities (78% similar) that shifted gradually down the catchment. In contrast, unconnected lakes contained somewhat different communities (67% similar). We found evidence that dispersal influences bacterioplankton communities via advection and dilution (mass effects) in streams, and via inoculation and subsequent growth in lakes. The spatial pattern of bacterioplankton community composition was strongly influenced by interactions among soil water, stream, and lake environments. Our results reveal large differences in lake-specific and stream-specific bacterial community composition over restricted spatial scales (<10 km) and suggest that geographic distance and connectivity influence the distribution of bacterioplankton communities across a landscape.     

Terrestrial, benthic, and pelagic resource use in lakes: results from a three-isotope Bayesian mixing model

Fluxes of organic matter across habitat boundaries are common in food webs. These fluxes may strongly influence community dynamics, depending on the extent to which they are used by consumers. Yet understanding of basal resource use by consumers is limited, because describing trophic pathways in complex food webs is difficult. We quantified resource use for zooplankton, zoobenthos, and fishes in four low-productivity lakes, using a Bayesian mixing model and measurements of hydrogen, carbon, and nitrogen stable isotope ratios. Multiple sources of uncertainty were explicitly incorporated into the model. As a result, posterior estimates of resource use were often broad distributions; nevertheless, clear patterns were evident. Zooplankton relied on terrestrial and pelagic primary production, while zoobenthos and fishes relied on terrestrial and benthic primary production. Across all consumer groups terrestrial reliance tended to be higher, and benthic reliance lower, in lakes where light penetration was low due to inputs of terrestrial dissolved organic carbon. These results support and refine an emerging consensus that terrestrial and benthic support of lake food webs can be substantial, and they imply that changes in the relative availability of basal resources drive the strength of cross-habitat trophic connections.     

The interplay between environmental conditions and allee effects during the recovery of stressed zooplankton communities

Many important ecological phenomena depend on the success or failure of small introduced populations. Several factors are thought to influence the fate of small populations, including resource and habitat availability, dispersal levels, interspecific interactions, mate limitation, and demographic stochasticity. Recent field studies suggest that Allee effects resulting from mate limitation can prevent the reestablishment of sexual zooplankton species following a disturbance. In this study, we explore the interplay between Allee effects and local environmental conditions in determining the population growth and establishment of two acid-sensitive zooplankton species that have been impacted by regional anthropogenic acidification. We conducted a factorial design field experiment to test the impact of pH and initial organism densities on the per capita population growth (r) of the sexual copepod Epischura lacustris and the seasonally parthenogenetic cladoceran Daphnia mendotae. In addition, we conducted computer simulations using r values obtained from our experiments to determine the probability of extinction for small populations of acid-sensitive colonists that are in the process of colonizing recovering lakes. The results of our field experiment demonstrated that local environmental conditions can moderate the impacts of Allee effects for E. lacustris: Populations introduced at low densities had a significantly lower r at pH 6 than at pH 7. In contrast, r did not differ between pH 6 and 7 environments when E. lacustris populations were introduced at high densities. D. mendotae was affected by pH levels, but not by initial organism densities. Results from our population growth simulations indicated that E. lacustris populations introduced at low densities to pH 6 conditions had a higher probability of extinction than those introduced at low densities to a pH 7 environment. Our study indicates that environmental conditions and mate limitation can interact to determine the fate of small populations of sexually reproducing zooplankton species. If a more rapid recovery of acid-damaged zooplankton communities is desired, augmentation of dispersal levels may be needed during the early phases of pH recovery in order to increase the probability of establishment for mate-limited zooplankton species.     

Acute toxicity of roundup to the nontarget organism Hyalella curvispina. Laboratory and field study

Glyphosate is the most used pesticide in Argentina. Hyalella curvispina is a widely distributed and commonly abundant component of the invertebrate assemblages in shallow waters of southern South America. The aim of this study was to assess the acute toxicity of the increasingly common Roundup Full II®, commercial formulation of the herbicide glyphosate (66.2% active ingredient), to H. curvispina in laboratory and field assessments. The mean estimated 48-h LC₅₀ of Roundup was 9.9 ± 1.7 mg L^?1. In a field experiment Roundup was applied to soybean plots. Simulated rain was generated the following day by means of irrigation sprinkler equipment. H. curvispina was exposed to runoff water and soy leaves. No mortality was observed. It is suggested that Roundup crop applications represent a low risk of acute toxicity to H. curvispina adults inhabiting water bodies adjacent to crop fields.