Aquatic mesocosm assessments of a neem (azadirachtin) insecticide at environmentally realistic concentrations--2: zooplankton community responses and recovery.

A neem-based insecticide, Neemix 4.5, was applied to forest pond enclosures at environmentally realistic concentrations (i.e., below the worst-case expected environmental concentration of 35 microg L(-1)). Crustacean zooplankton communities were examined by multivariate ordination (nonmetric multidimensional scaling) and time-course analyses of population trends among indicator taxa over two field seasons to determine application effects on community structure and recovery patterns. Significant effects on zooplankton community structure were detected at all main test concentrations (n = 5) of 28, 17, and 10 microg L(-1) azadirachtin. There was also evidence of adverse effects on zooplankton communities at an auxiliary test concentration (n = 2) of 5 microg L(-1) azadirachtin. Community-level effects resulted primarily from reductions in adult copepods with short-term, reciprocal increases in cladocerans. Copepod nauplii were not significantly affected. Response patterns suggested that the reductions in adult copepods resulted from growth-regulating effects of the active ingredient azadirachtin, or other neem compounds, and not from formulation ingredients. There was no evidence of recovery among adult copepods within the season of application. At the beginning of the second sampling season, there were apparent carryover effects similar to the community responses in the previous year. By the end of the second season, there was evidence indicating recovery of community structure at the two lower test concentrations of 10 and 17 microg L(-1), but not at 28 microg L(-1). The selective toxicity to adult copepods is problematic in that this group has a relatively long life cycle (1 year), contributes a major component of zooplankton biomass and respiration, and occupies critical functional guilds within zooplankton food web structures. Mitigation measures such as reductions in application rates and efforts to avoid deposition of sprayed materials on water bodies will be required to reduce the risk of harmful effects on zooplankton communities of forest ponds and other shallow, standing-water bodies.

Some ecological implications of a neem (azadirachtin) insecticide disturbance to zooplankton communities in forest pond enclosures.

A neem-based insecticide, Neemix 4.5, was applied to forest pond enclosures at concentrations of 10, 17, and 28 microg l(-1) azadirachtin (the active ingredient). At these test concentrations, significant, concentration-dependent reductions in numbers of adult copepods were observed, but immature copepod and cladoceran populations were unaffected. There was no evidence of recovery of adult copepods within the sampling season (May to October). The ecological significance of this disturbance to the zooplankton community was examined by determining biomass as a measure of food availability for higher predators, plankton community respiration, dissolved oxygen (DO) concentrations, and conductivity as functional indicators of ecosystem stress, and zooplankton food web stability as a measure of effects on trophic structure. The selective removal or reduction of adult copepods was sufficient to measurably reduce total zooplankton biomass for several weeks mid-season. During the period of maximal impact (about 4-9 weeks after the applications), total plankton community respiration was significantly reduced, and this appeared to contribute to significant, concentration-dependent increases in dissolved oxygen and decreases in conductivity among treated enclosures. The reductions in adult copepods resulted in negative effects on zooplankton food web stability through eliminations of a trophic link and reduced interactions and connectance. Comparing the results here to those from a previous study with tebufenozide, which was selectively toxic to cladocerans and had little effect on food web stability, indicates that differential sensitivity among taxa can influence the ecological significance of pesticide effects on zooplankton communities.

Mercury content and speciation in the plankton and benthos of Lake Superior.

As part of a study is to assess the importance of watersheds in controlling sources, transport, fate, and bioavailability of monomethyl mercury (MeHg) in Lake Superior, biotic samples were collected and analyzed to determine total mercury (HgT) and MeHg content, and to examine size, species, trophic and geographic trends. Plankton was collected in two ways: vertical tows of non-metallic, 153 microm mesh net (bulk zooplankton), and by passively filtering near-surface water through stacked Nitex sieves, generating size-fractionated seston (<35, 35-63, 63-112, and >112 microm). Benthos was sampled using a Ponar grab to collect sediment, and a non-metallic sieve to separate biota from substrate. Samples were processed to quantify dry weights, HgT and MeHg. Results for bulk zooplankton sampled offshore showed a range of approximately from 35 to 50 ng MeHg/gram dry weight (gdw) and from 80 to 130 ng HgT/gdw during April, and from 15 to 25 ng MeHg/gdw and from 20 to 70 ng HgT/gdw during August. Results from sieved, near-surface water from offshore sites in April showed a dominance by the <35 microm size fraction both in total mass and mass of MeHg compared to other size fractions. On a dry weight basis, however, we found little difference between the size fractions in April (MeHg ranges from 2 to 10 ng/gdw). During the summer cruise, we found similar concentrations in the <35 microm fraction, but higher in the 112-243 microm size fraction (MeHg 14-16 ng/gdw). The MeHg concentration in Mysis relicta ranged from 33 to 54 ng/gdw throughout the lake. Chironomid larvae were 8 ng MeHg/gdw and amphipods were 32 ng MeHg/gdw.

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.