Natural Zooplankton Communities Research Articles

Key determinants controlling the seasonal variation of coastal zooplankton communities: A case study along the Yellow Sea

Zooplankton play key top-down and bottom-up regulatory roles in aquatic food webs, and are also ecologically indicative in marine ecosystems. However, there are relatively limited data on the effects of environmental changes on natural zooplankton communities, especially in coastal ecosystems. In the present study, we systematically evaluated the potential effects of various environmental variables, such as temperature, salinity, and nutrients, on the zooplankton communities along the coastal Yellow Sea during spring, summer, and fall. The results showed that the average abundance of zooplankton decreased in general from spring to autumn, but the biomass exhibited a different seasonal variation trend, with the highest in summer and the lowest in fall. Throughout the three seasons, copepods were the most dominant species within the zooplankton communities, followed by Pelagic larvae and Hydromedusae. However, Noctiluca miliaris accounted for a large proportion of zooplankton abundance during spring. Moreover, the correlation analysis was applied to explore the potential effects of environmental factors on the seasonal variation of zooplankton communities. The results showed that chlorophyll a (Chl a) and salinity were significantly correlated with zooplankton abundance and biomass during spring. The implication is that high phytoplankton biomass (expressed as Chl a) and salinity would benefit the growth of zooplankton in spring. During summer and fall, the effects of dissolved inorganic phosphate (DIP) on the zooplankton abundance and biomass showed a significant positive correlation, indicating that zooplankton were better able to tolerate high DIP during summer and fall. Taken together, Chl a, salinity, and DIP may be the key determinants controlling the seasonal dynamics of zooplankton communities in the coastal Yellow Sea.

Morphological defences and defence–cost trade‐offs in Daphnia in response to two co‐occurring invertebrate predators

Abstract Inducible morphological defences are crucial for understanding predator–prey interactions. Such defences have been mostly studied in a single‐predator context, ignoring the fact that prey organisms are often exposed to multiple predators. In deep peri‐Alpine European lakes, the keystone grazer Daphnia coexists with two cladoceran predators, Bythotrephes longimanus and Leptodora kindtii. Up to now, life history and morphological responses of Daphnia to these two predators have not been analysed systematically. We studied the responses of two life history (age at first reproduction, and offspring production) and five morphological traits (body size, body width, head size, spina size, and eye diameter) of eight Daphnia galeata clones to the presence of Bythotrephes and Leptodora in a common garden experiment. We compared each response trait between treatments using linear mixed models, and investigated the covariation between defence traits and demographic costs (neonate production) for the two predators. Our results show that the responses of Daphnia are predator‐ and trait‐specific. Daphnia developed a typical helmet and a larger eye only in the presence of Bythotrephes, not in the presence of Leptodora. In contrast, both predators induced larger body sizes and longer spines. Age at first reproduction was latest and demographic costs were highest in the Bythotrephes treatment, suggesting that the development of a helmet exceeds the costs of spina elongation. The responses of Daphnia clones revealed a defence‐cost trade‐off for helmet formation in the Bythotrephes, and for spina elongation in the Leptodora treatment. Hence, despite Bythotrephes and Leptodora being closely related co‐occurring predators, Daphnia responds with a unique combination of trait changes and defence‐cost trade‐offs to the two predators. The presence of predator‐specific clonal defence‐cost trade‐offs suggests that the presence of these invertebrate predators can drive different evolutionary processes in natural zooplankton communities. Disentangling the evolutionary ecology of phenotypic responses of prey species to co‐occurring predators will require multi‐trait, multi‐clone studies of induced antipredator defences.

Metabarcoding using multiplexed markers increases species detection in complex zooplankton communities.

Metabarcoding combines DNA barcoding with high‐throughput sequencing, often using one genetic marker to understand complex and taxonomically diverse samples. However, species‐level identification depends heavily on the choice of marker and the selected primer pair, often with a trade‐off between successful species amplification and taxonomic resolution. We present a versatile metabarcoding protocol for biomonitoring that involves the use of two barcode markers (COI and 18S) and four primer pairs in a single high‐throughput sequencing run, via sample multiplexing. We validate the protocol using a series of 24 mock zooplanktonic communities incorporating various levels of genetic variation. With the use of a single marker and single primer pair, the highest species recovery was 77%. With all three COI fragments, we detected 62%–83% of species across the mock communities, while the use of the 18S fragment alone resulted in the detection of 73%–75% of species. The species detection level was significantly improved to 89%–93% when both markers were used. Furthermore, multiplexing did not have a negative impact on the proportion of reads assigned to each species and the total number of species detected was similar to when markers were sequenced alone. Overall, our metabarcoding approach utilizing two barcode markers and multiple primer pairs per barcode improved species detection rates over a single marker/primer pair by 14% to 35%, making it an attractive and relatively cost‐effective method for biomonitoring natural zooplankton communities. We strongly recommend combining evolutionary independent markers and, when necessary, multiple primer pairs per marker to increase species detection (i.e., reduce false negatives) in metabarcoding studies.

The impact of calcium decline on population growth rates of crustacean zooplankton in Canadian Shield lakes

AbstractWithin the last decade, calcium decline has emerged as a stressor for many soft water lakes. A legacy of long‐term acid deposition and logging, calcium decline has been implicated in the loss of large herbivores, such as Daphnia, and changes in community structure as taxa with presumed low calcium demand also increase. Although lake surveys, paleolimnological studies, and laboratory experiments have provided considerable evidence that declining Daphnia abundances are associated with low calcium concentration, causal relationships for zooplankton response to calcium decline have not been assessed using field experiments with natural zooplankton communities. We conducted a six‐week field experiment under low food conditions typical of Canadian Shield lakes, using four calcium concentrations (0.6 mg/L, 1.0 mg/L, 1.4 mg/L, and 2.4 mg/L) and zooplankton communities originating from eight lakes along a gradient from 1.78 mg Ca/L to 24.8 mg Ca/L, to examine the effect of calcium on growth rates and reproduction of individual and groups of taxa. Population growth rates for daphniids, Bosmina spp., and three cyclopoids declined as calcium concentration decreased. At low calcium, growth rates of Daphnia pulex, Daphnia catawba, and Mesocyclops edax were negative indicating declining populations. There was no effect of calcium on reproduction of the most common cladocerans. Although we detected some variation in growth rates amongst source lakes, zooplankton communities did not differ in their response to low calcium. As calcium concentrations continue to decline in soft water lakes, reduced zooplankton growth rates may result in shifts in zooplankton community structure and overall declines in total zooplankton production.

Zooplankton as a potential vector for white band disease transmission in the endangered coral, Acropora cervicornis.

Coral diseases are a leading factor contributing to the global decline of coral reefs, and yet mechanisms of disease transmission remain poorly understood. This study tested whether zooplankton can act as a vector for white band disease (WBD) in Acropora cervicornis. Natural zooplankton communities were collected from a coral reef in Bocas del Toro, Panama. Half of the zooplankton were treated with antibiotics for 24 h after which the antibiotic-treated and non-antibiotic-treated zooplankton were incubated with either seawater or tissue homogenates from corals exhibiting WBD-like symptoms. A total of 15 of the 30 asymptomatic A. cervicornis colonies exposed to zooplankton incubated in disease homogenate in tank-based experiments showed signs of WBD, regardless of prior antibiotic incubation. These results indicate that in our experimental conditions zooplankton were a vector for coral disease after exposure to disease-causing pathogens. Given the importance of heterotrophy on zooplankton to coral nutrition, this potential mode of disease transmission warrants further investigation.

Interactions between bivalves and zooplankton: competition or intraguild predation? Implications for biomanipulation in subtropical shallow lakes

Zooplankton and bivalves are the main consumers of phytoplankton in shallow lakes; however, knowledge regarding trophic interactions between them is scarce. Competition for resources appears to be an obvious direct interaction, but the scheme may be more complex. Bivalves can consume all or only part of the zooplankton, constituting an intraguild predation module. In the present study we investigated the interaction between bivalves and zooplankton and its effects on phytoplankton grazing and community structure using an experimental approach in an aquarium. Two bivalve species were considered, namely Diplodon parallelopipedon (native) and Corbicula fluminea (non-native), in addition to a natural zooplankton community dominated by small-sized zooplankton. The findings indicate that phytoplankton consumption by the zooplankton is substantially less than that by the bivalves. Under the experimental conditions, the bivalves actively consumed small-sized zooplankton (rotifers and nauplii), but no consumption of medium-sized individuals (cladocerans and copepods) was observed. The differential consumption may have implications on the size and structure of zooplankton, favouring an average larger-sized community. Thus, phytoplankton may be negatively affected both directly by consumption and indirectly because of the resulting structure of the zooplankton community. The main results of the study are also discussed from the perspective of biomanipulation techniques in tropical and subtropical regions.

Zooplankton grazing on natural algae and bacteria under hypertrophic conditions

Zooplankton grazing on natural algae and bacteria under hypertrophic conditions The grazing characteristics of the natural zooplankton community in the shallow hypertrophic lagoon Albufera de Valencia (Spain) were compared in two periods of the annual cycle: (1) at the onset of spring, after a period of increased water flow through the lagoon that reduces phytoplankton density and cyanobacterial prevalence, and (2) in mid-spring, after the spring phytoplankton bloom and under conditions of cyanobacterial dominance. Clearance, ingestion and assimilation rates were measured by labelling natural seston with 14 C-bicarbonate and 3 H-thymidine/leucine. The clearance rates (CRs) of Daphnia magna, the dominant species at the onset of spring, were low, very likely as a consequence of the high abundance of filamentous cyanobacteria in the lagoon. The CRs were nearly equal for phytoplankton and bacteria, corroborating the unselective feeding of Daphnia. Bacteria supplied ≈40 % of the planktoniccarbon ingested byD. magna, and this carbon was assimilated with the same efficiencyas that provided by phytoplankton.This suggests a strong coupling betweenthe microbial loop and the classic grazer trophic web at the onset of spring in the lagoon and, consequently, an important contribution of the microbial loop to energy transfer to higher trophic levels (fish). The CRs of Bosmina longirostris, the dominant species in mid-spring, were also low probably because the food concentrations were well above the incipient limiting level. The CRs were higher for 1-15 µm algae than for <1 µm bacteria, indicating positive selective feeding towards larger cells. Algae sized 3-15 µm provided most of the planktonic carbon ingested by B. longirostris. Similarly, regarding the total mid-spring zooplankton community, phytoplankton contributed 90 % of the planktonic carbon ingested. This suggests that during the spring phytoplankton bloom when cyanobacteria dominate, the energy transferred to zooplankton via bacteria constitutes a smaller fraction than during periods of lower phytoplanktondensity. Finally, our results indicate that zooplankton consumed a very small proportion of phytoplankton and bacterioplankton production during the two studied periods, which highlights the reduced ability of zooplankton to control phytoplankton growth in hypertrophic systems.

A potential barrier to the spread of the invasive cladoceran Cercopagis pengoi (Ostroumov 1891) in the Northern Baltic Sea

The spread of the invasive cladoceran Cercopagis pengoi has been well documented in southern areas of the Baltic Sea, however, little research on this invasive species (nor the zooplankton community) has focused on the Gulf of Bothnia (Bothnian Sea and Bay). We analysed data collected over a 12–13 year period at two main stations, one coastal and one offshore, to examine the occurrence of C. pengoi, invasion dynamics, effects on natural zooplankton communities and associated environmental factors. Nine other stations in the Gulf of Bothnia were also examined and the contribution to three-spined stickleback (Gasterosteus aculeatus) diet was quantified. The zooplankton community response apparently differed between coastal and offshore stations with Bosmina, Eurytemora, and Acartia populations being influenced during peak abundances of C. pengoi. It appears that the native zooplankton community has some resilience, returning to its prior structure outside of peak invasion periods. C. pengoi, where present, contributed significantly to stickleback diet. We explored possible barriers for C. pengoi in the Bothnian Bay, suggesting that the low productive Bothnian Bay ecosystem may be incapable of supporting such a predator. This highlights the need for further studies, especially in the light of global climate change.

Unreliable quantitation of species abundance based on high-throughput sequencing data of zooplankton communities

High-throughput sequencing (HTS) is rapidly becoming a popular and robust tool to characterize biodiversity of complex communities, especially for those dominated by microscopic species such as zooplankton. The popular use of HTS-based methods has prompted a possible method of inferring relative species abundance from sequencing data. However, these methods remain largely untested in many communities as to whether sequence data can reliably quantify relative species abundance. Here we tested the relationship between species abundance and sequence abundance in zooplankton using 2 methods: (1) spiking known amounts of indicator species into existing zooplankton communities, and (2) comparing results obtained from parallel replicates for the same natural zooplankton communities. Although we detected a general trend that low-abundance species usually corresponded to low-abundance sequence reads, further sta- tistical analyses revealed that sequencing data could not reliably quantify relative species abun- dance, even for the same indicator species spiked into different zooplankton communities. The distribution of sequence reads statistically varied even between parallel replicates of the same natural zooplankton communities. Our study reveals that sequence abundance may generally qualitatively reflect species abundance as the general trend between these 2 variables exists; however, extra caution is required when using HTS-based approaches to make quantitative infer- ences regarding zooplankton communities.

Responses of cyanobacteria to herbivorous zooplankton across predator regimes: who mows the bloom?

Summary The massive growth of large, toxic cyanobacteria in eutrophic waters has traditionally been explained by their evolution of defences to herbivorous zooplankton. These conclusions come mostly from studies using the large‐bodied cladoceran Daphnia as a grazer model. In contrast, very little is known about the effects of other zooplankters such as copepods and small cladocerans that, unlike Daphnia, often coexist with cyanobacterial blooms and under high levels of fish predation. We performed a series of grazing experiments during a cyanobacterial bloom in a eutrophic lake, subject to decreasing predation on zooplankton (removal of cyprinid fish). We also used long‐term observational data to analyse the response of cyanobacteria to changes in abundance, body size and biomass of the naturally, coexisting zooplankton community across different predator regimes. A natural grazer community, dominated by selective herbivores like calanoid and cyclopoid copepods, positively affected cyanobacterial growth in early summer at low cyanobacterial densities. However, cyclopoid copepods and small cladocerans suppressed the growth of Anabaena, Microcystis and Planktothrix species under bloom conditions in late summer, with the exception of the highly toxic Microcystis botrys. Long‐term observational data support the results from the grazing experiments regarding the potential capacity of the natural zooplankton community to suppress cyanobacterial growth, especially of the dominant taxon Microcystis (including the most toxic species, M. botrys). In particular, these results suggest that direct grazing may contribute significantly to the observed 72% decrease in Microcystis biomass through a 111% increase in abundance of cyclopoid copepods, and an 18% increase in body size of cyclopoid copepods and a 31% increase in body size of the generalist feeder Daphnia, during the biomanipulation. Together, these results suggest that, within the complex interactions brought about by trophic cascades, direct grazing by the coexisting zooplankton community is a strong driver regulating cyanobacterial growth in eutrophic lakes and that this can display contrasting effects, both seasonally and under different levels of fish predation. Importantly, we show that herbivory may be enhanced along a gradient of decreasing fish predation by the combined action of copepods (selective herbivores) and Daphnia (generalist herbivores).

Linking cascading effects of fish predation and zooplankton grazing to reduced cyanobacterial biomass and toxin levels following biomanipulation.

Eutrophication has been one of the largest environmental problems in aquatic ecosystems during the past decades, leading to dense, and often toxic, cyanobacterial blooms. In a way to counteract these problems many lakes have been subject to restoration through biomanipulation. Here we combine 13 years of monitoring data with experimental assessment of grazing efficiency of a naturally occurring zooplankton community and a, from a human perspective, desired community of large Daphnia to assess the effects of an altered trophic cascade associated with biomanipulation. Lake monitoring data show that the relative proportion of Daphnia spp. grazers in June has increased following years of biomanipulation and that this increase coincides with a drop in cyanobacterial biomass and lowered microcystin concentrations compared to before the biomanipulation. In June, the proportion of Daphnia spp. (on a biomass basis) went from around 3% in 2005 (the first year of biomanipulation) up to around 58% in 2012. During months when the proportion of Daphnia spp. remained unchanged (July and August) no effect on lower trophic levels was observed. Our field grazing experiment revealed that Daphnia were more efficient in controlling the standing biomass of cyanobacteria, as grazing by the natural zooplankton community never even compensated for the algal growth during the experiment and sometimes even promoted cyanobacterial growth. Furthermore, although the total cyanobacterial toxin levels remained unaffected by both grazer communities in the experimental study, the Daphnia dominated community promoted the transfer of toxins to the extracellular, dissolved phase, likely through feeding on cyanobacteria. Our results show that biomanipulation by fish removal is a useful tool for lake management, leading to a top-down mediated trophic cascade, through alterations in the grazer community, to reduced cyanobacterial biomass and lowered cyanobacterial toxin levels. This improved water quality enhances both the ecological and societal value of lakes as units for ecosystem services.

Combined effects of turbulence and different predation regimes on zooplankton in highly colored water-implications for environmental change in lakes.

In aquatic ecosystems, predation is affected both by turbulence and visibility, but the combined effects are poorly known. Both factors are changing in lakes in the Northern Hemisphere; the average levels of turbulence are predicted to increase due to increasing wind activities, while water transparency is decreasing, e.g., due to variations in precipitation, and sediment resuspension. We explored experimentally how turbulence influenced the effects of planktivorous fish and invertebrate predators on zooplankton when it was combined with low visibility caused by high levels of water color. The study was conducted as a factorial design in 24 outdoor ponds, using the natural zooplankton community as a prey population. Perch and roach were used as vertebrate predators and Chaoborus flavicans larvae as invertebrate predators. In addition to calm conditions, the turbulent dissipation rate used in the experiments was 10−6 m2 s−3, and the water color was 140 mg Pt L−1. The results demonstrated that in a system dominated by invertebrates, predation pressure on cladocerans increased considerably under intermediate turbulence. Under calm conditions, chaoborids caused only a minor reduction in the crustacean biomass. The effect of fish predation on cladocerans was slightly reduced by turbulence, while predation on cyclopoids was strongly enhanced. Surprisingly, under turbulent conditions fish reduced cyclopoid biomass, whereas in calm water it increased in the presence of fish. We thus concluded that turbulence affects fish selectivity. The results suggested that in dystrophic invertebrate-dominated lakes, turbulence may severely affect the abundance of cladocerans. In fish-dominated dystrophic lakes, on the other hand, turbulence-induced changes in planktivory may considerably affect copepods instead of cladocerans. In lakes inhabited by both invertebrates and fish, the response of top-down regulation to turbulence resembles that in fish-dominated systems, due to intraguild predation. The changes in planktivorous predation induced by abiotic factors may possibly cascade to primary producers.

Hyperbenthic and pelagic predators regulate alternate key planktonic copepods in shallow temperate estuaries

Although predation has been identified as an important community driver, the role of predator diversity in structuring estuarine zooplankton has not been assessed. As such, we investigated the effects of two different zooplanktivorous fish species on the estuarine zooplankton community during a 12-day mesocosm study. Three experimental treatments were established, whereby natural zooplankton communities were subject to either (1) no predatory pressure, (2) predation by a pelagic predator (Monodactylus falciformis) or (3) predation by a hyper-benthic predator (Glossogobius callidus). The pelagic feeding M. falciformis fed largely on the numerically dominant mid-water copepod species, Paracartia longipatella. In contrast, the hyper-benthic fish had a greater predatory impact on the less numerically dominant copepod, Pseudodiaptomus hessei, which demonstrates strong diel vertical migration. Variations in prey-population regulation are ascribed to the distinct behavioural differences of the predators, and mediated by the differences in behaviour of the copepod species.

Different tolerance of zooplankton communities to insecticide application depending on the species composition

Natural zooplankton communities are composed of many different species at different trophic levels in the aquatic food web. Several researchers have reported that in mesocosm/enclosure experiments, larger cladocerans tend to be more sensitive to carbamate insecticides than smaller ones (Daphnia > Moina, Diaphanosoma > Bosmina). In contrast, results from individual-level laboratory tests have suggested that large cladoceran species are more tolerant than small species. To clarify this inconsistency, we conducted a microcosm experiment using model zooplankton communities with different species compositions, where animals were exposed to lethal (near to the 24 h LC50, concentration estimated to kill 50% of individuals within 24-h for the small cladoceran Bosmina) and lower, sublethal concentrations of carbaryl. In the experiment, population densities of the small cladocerans (Bosmina and Bosminopsis) decreased subsequent to the applications of chemical, but no impacts were observed on the large cladoceran Daphnia. Our results supported the reports of previous individual level toxicity tests, and indicated that the sensitivity of zooplankton to the insecticide was unchanged by biological interactions but the response of population can be modified by compensation of population through hatching from resting eggs and/or the persistence of insecticide in the systems.

해양적조생물제어를 위한 살조물질 Thiazolidinedione 유도체(TD49) 평가

Worldwide development of harmful algal blooms causes serious problem for public health and fisheries industries. To evaluate the algicidal impact on the harmful algae bloom species in aquatic ecosystems of coast, a new algicide thiazolidinedione derivative (TD49) were tentatively examined in the growth stages (i.e., lag, logarithmic and stationary phase) of rapidophyceae , and sp..Three strains could easily destroy in the lag phase due to relatively weak cell walls than those of the logarithmic and stationary phase. It is thought that inoculation of TD49 substances into initial or developmental natural blooms with a threshold concentration () can maximize the algicidal activity. Also, bio-chemical assays revealed that the algicidal substances from all culture strains were likely to be extracellular substances because those cells have easily destroyed in cell walls. On the other hand, natural zooplankton communities were influenced within the exposure experiments of , which is showed the maximum algcidal activity of tested organisms. These results indicate that although the TD49 substance is potential agents for the control of , and sp. in the enclosed eutrophic bay and coastal water, more detailed research of acute toxicity effect on high trophic organism in marine ecosystems need to be conducted.

Consequences of anthropogenic impact on aquatic ecosystems in Khibiny Mountain Massif

Comprehensive physicochemical (Selector software package), paleoecological, and hydrobiological studies of two lakes-Pai-Kunjavr (Gol’tsovoe) and Bol’shoi Vudjavr (the Khibiny Mountain Massif) was carried out for the first time. Reconstruction of natural waters and wastes was performed, the present-day state of the ionic composition of natural waters, diatomic complexes, and zooplankton communities was assessed with the aim to determine the degree of transformation of Lake Bol’shoi Vudjavr under technogenic impact. The results are of practical and methodological significance for the assessment of long-term anthropogenic impact on subarctic aquatic ecosystems.

Cyanobacterial chemical warfare affects zooplankton community composition

Summary1. Toxic algal blooms widely affect our use of water resources both with respect to drinking water and recreation. However, it is not only humans, but also organisms living in freshwater and marine ecosystems that may be affected by algal toxins.2. In order to assess if cyanobacterial toxins affect the composition of natural zooplankton communities, we quantified the temporal fluctuations in microcystin concentration and zooplankton community composition in six lakes.3. Microcystin concentrations generally showed a bimodal pattern with peaks in early summer and in autumn, and total zooplankton biomass was negatively correlated with microcystin concentrations. Separating the zooplankton assemblages into finer taxonomic groups revealed that high microcystin concentrations were negatively correlated with Daphnia and calanoid copepods, but positively correlated with small, relatively inefficient phytoplankton feeders, such as cyclopoid copepods, Bosmina and rotifers.4. In a complementary, mechanistic laboratory experiment using the natural phytoplankton communities from the six lakes, we showed that changes in in situ levels of microcystin were coupled with reduced adult size and diminished juvenile biomass in Daphnia.5. We argue that in eutrophic lakes, large unselective herbivores, such as Daphnia, are ‘sandwiched’ between high fish predation and toxic food (cyanobacteria). In combination, these two mechanisms may explain why the zooplankton community in eutrophic lakes generally comprise small forms (e.g. rotifers and Bosmina) and selective raptorial feeders, such as cyclopoid copepods, whereas large, unselective herbivores, such as Daphnia, are rare. Hence, this cyanobacterial chemical warfare against herbivores may add to our knowledge on population and community dynamics among zooplankton in eutrophic systems.

The use of 14C tracer technique to assess the functional response of zooplankton community grazing to toxic impact

The use of functional endpoints in risk assessment of pollutants for marine pelagic communities is scarce, especially for the function of zooplankton communities. This work presents a rapid, inexpensive and ecologically relevant technique to assess the effect of toxic compounds on the grazing rates of zooplankton communities. The combination of a (14)C-tracer labelling technique with a fast (< 3 h) and representative sub-sampling and handling makes it possible to measure changes in food uptake of freshly collected natural zooplankton communities in a short-term assay. The methodology is described and its validity is determined by a full account of the fate of isotope tracers (up to 15% taken up by zooplankton). Egg production of Acartia tonsa cultures exposed to DCOI yielded EC(50) values of 118+/-33 nM DCOI after three days, where EC(50) values from grazing of natural zooplankton communities was 136+/-29 nM DCOI, indicating that grazing was the more sensitive variable. Finally the method was tested in a mesocosm experiment designed to evaluate the toxicity of the antifouling compound zinc pyrithion (ZPT) with an EC(50) value of 17 nM ZPT (SD=3.3 nM, n=3). The method was found to be reproducible both at the community (CV=8-39%) and individual level (CV=23-53%), and could detect small changes in response to a toxicant in a natural pelagic system. The use of natural communities in combination with the fast and cost-effective procedure makes this technique a powerful tool in risk assessment, where lack of ecological relevance or reproducible results often results in unwanted high uncertainties.

Community-level disruptions among zooplankton of pond mesocosms treated with a neem (azadirachtin) insecticide

A natural, plant-derived insecticide, neem, is being evaluated as an alternative insect pest control product for forestry in Canada. As part of the process to investigate the environmental safety of neem-based insecticides, a mesocosm experiment was conducted to assess the effects of neem on natural zooplankton communities. Replicate ( n=5), shallow (<1 m) forest pond enclosures were treated with Neemix® 4.5, at concentrations of 0.035 (the expected environmental concentration), 0.18, 0.70, and 1.75 mg/l active ingredient, azadirachtin. Zooplankton communities were quantitatively sampled over a 4-month experimental period in treated and control enclosures, and water samples were collected to track azadirachtin concentrations. Concentrations in water declined linearly with estimated DT 50 values of 25–29 days. Trends in abundance over time among populations of cladocerans, copepods, and rotifers were found to differ significantly among treatments. At the two highest test concentrations, adverse effects were obvious with significant reductions in several cladoceran species, and near elimination of the three major copepod species present. More subtle effects at the two lowest test concentrations were determined by comparing the community structure of enclosures across treatment levels and over time through an analytical process based on the multivariate statistical software, PRIMER. Significant effects on community structure were detected at both of these lower concentrations, including the expected environmental concentration of 0.035 mg/l azadirachtin. Differential responses among species (some increases, some decreases) caused detectable disruptions in community structure among zooplankton of treated enclosures. Perturbations to zooplankton communities were sufficient to cause measurable differences in system-level metabolism (midday dissolved oxygen concentrations) at all but the lowest test concentration.

Top-down control of natural phyto- and bacterioplankton prey communities by Daphnia magna and by the natural zooplankton community of the hypertrophic Lake Blankaart

Biomanipulation, through the reduction of fish abundance resulting in an increase of large filter feeders and a stronger top-down control on algae, is commonly used as a lake restoration tool in eutrophic lakes. However, cyanobacteria, often found in eutrophic ponds, can influence the grazing capacity of filter feeding zooplankton. We performed grazing experiments in hypertrophic Lake Blankaart during two consecutive summers (1998, with and 1999, without cyanobacteria) to elucidate the influence of cyanobacteria on the grazing pressure of zooplankton communities. We compared the grazing pressure of the natural macrozooplankton community (mainly small to medium-sized cladocerans and copepods) with that of large Daphnia magna on the natural bacterioplankton and phytoplankton prey communities. Our results showed that in the absence of cyanobacteria, Daphnia magna grazing pressure on bacteria was higher compared to the grazing pressure of the natural zooplankton community. However, Daphnia grazing rates on phytoplankton were not significantly different compared to the grazing rates of the natural zooplankton community. When cyanobacteria were abundant, grazing pressure of Daphnia magnaseemed to be inhibited, and the grazing pressure on bacteria and phytoplankton was similar to that of the natural macrozooplankton community. Our results suggest that biomanipulation may not always result in a more effective top-down control of the algal biomass.