Short- and Long-Term Effects of the Pyrethroid Insecticide Fenvalerate on an Invertebrate Pond Community

ECOTOXICOLOGY AND ENVIRONMENTAL SAFETY

41, 137—156 (1998)

ENVIRONMENTAL RESEARCH, SECTION B ARTICLE NO.

ES981694

Short- and Long-Term Effects of the Pyrethroid Insecticide Fenvalerate on an Invertebrate Pond Community P. Woin Department of Ecology, Chemical Ecology and Ecotoxicology, University of Lund, S-223 62 Lund, Sweden Received April 9, 1996

Direct and secondary effects of fenvalerate on the structure of pond ecosystems were studied in six freshwater mesocosms simulating natural fish-free eutrophic ponds. Exposed mesocosms were compared with nonexposed ones and the effects of the added compound on the macroinvertebrate community were followed during three vegetation seasons (years) in two mesocosms. Exposure to fenvalerate at 1.3 and 0.54 lg liter21 resulted in structural changes in the macroinvertebrate community. The insecticide was directly lethal to insects and other arthropods, but indirect community changes were also observed. For example, after exposure there was a remarkable (> 10-fold) increase in oligochaetes (Stylaria lacustris), probably caused by reduced predation and interspecific competition for food. When predators (insects) recolonized the system, the oligochaetes decreased in abundance and were replaced by ostracods (Herpetocypris reptans), which use similar food resources but are less susceptible to predation. The marked increase in these two taxa is probably explained by the mass death of arthropods, resulting in increased food availability. More than 2 years after treatment, the most exposed system was still different compared with nonexposed ones, suggesting that nonpersistent pesticides may produce detrimental effects resulting in long-term changes at the ecosystem level of organization. ( 1998 Academic Press Key Words: insecticides; pyrethroids; fenvalerate; community; macroinvertebrates; indirect effects; secondary effects; mesocosms; freshwater; pond; ecosystem; similarity index.

INTRODUCTION

A fundamental goal of ecotoxicology and hazard assessment is to determine the ecological effects of toxic chemicals on natural communities and ecosystems. In recent years, it has become apparent that toxicity tests using single-species bioassays are not adequate for assessing the impact of pollutants on a long-term scale. Single-species toxicity tests do not include interactions within or between species or between species and their environment, and are limited in their ability to predict the extent or significance of toxic effects on ecosystems (Cairns, 1981, 1983; Kimball and Levin, 1985). Attention has therefore been directed toward

multispecies toxicity tests and the use of artificial ecosystems and enclosures for ecotoxicological studies (Cairns, 1983, 1988; Sheehan et al., 1984; Odum, 1984; Pratt, 1990; Cairns and Pratt, 1993). The applicability and reliability of these systems to facilitate hazard assessment have been discussed intensively during recent years by regulators, scientists, and industry (Cairns, 1988; Cairns and Pratt, 1993; DeNoyelles and Howick, 1993; AMEAC, 1993; Landis et al., 1993). Although no alternative procedures are currently available to detect adverse effects of chemicals on the structure and function of ecosystems resulting in long-term detrimental changes, some authors still question the necessity and usefulness of this ecosystem approach for regulation. However, as the development of cheaper systems and more effective methods continues, and as the knowledge of fundamental ecological principles and processes increases, the artificial ecosystem approach will probably survive as an instrument to detect potential long-term effects. Benthic organisms have been widely used as indicators of aquatic health and in toxicity tests and bioassays (Burton, 1991). The benthic invertebrate communities are ideal indicators of water/sediment quality because they comprise species ranging from pollution sensitive to pollution tolerant, and occupy multiple trophic levels and a myriad of niches in the ecosystem (Cummins, 1974). Measurement of community responses to stress, and of invertebrate responses in particular, however, becomes more difficult as system complexity increases. The use of self-sustaining systems with hundreds of interacting species in a complex environment is, of course, more realistic in an ecological sense but demands much greater effort to analyze in detail. Since the overall functional characteristics of ecosystems (e.g., nutrient turnover, pH, and P/B, P/R, and R/B ratios) do not always seem to fulfill their promises as sensitive ecosystem stress indicators, increased attention must be paid to community structure responses, despite the taxonomical, statistical, and economical problems attached to them (cf., Cairns, 1988; Pratt, 1990). Experimental studies of lentic freshwater macroinvertebrate community responses to pollutants are still in their infancy. However, some studies

137 0147-6513/98 $25.00 Copyright ( 1998 by Academic Press All rights of reproduction in any form reserved.

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have been published (e.g., Hurlbert et al., 1972; Crossland and Bennett, 1989; Van Wijngaarden and Leeuwangh, 1989; Lozano et al., 1992; Fairchild et al., 1992; Van Wijngaarden et al., 1995; Farmer et al., 1995; Wayland and Boag, 1995). A common feature of most published community and ecosystem level experiments is that the systems are followed for short periods ranging from a few weeks to several months. A characteristic of studies of this length is that conclusions about ecologically mediated long-term changes are not easy or even possible to make (Westman, 1978; Sheehan et al., 1984; Pusey et al., 1994). Certain studies on the effects of pyrethroids, however, have concluded that, in general, arthropods are the only invertebrates affected and that these effects in natural water bodies would be expected to be occasional, localized, minor, and transient (Hill, 1989; Hill et al., 1994). These conclusions were drawn because the concentrations seldom reach lethal levels in receiving systems, and partly because affected populations are suggested to recover quickly by recolonization and reproduction. These sweeping assumptions may be true for many natural water bodies but certainly not for all. The question of whether an effect is transient or not can hardly be answered only by such short-term studies alone, even if they are performed in complex mesocosms for several weeks. Furthermore, the perturbed ecosystem’s degree of isolation from other water bodies determines the rate and extent by which the population recovery occurs (MacArthur and Wilson, 1967; Sheehan, 1984). Some affected semivoltine species that have long life cycles (e.g., Anisoptera) and are important in the top-down regulation of other invertebrates (Benke, 1976) cannot recover within a year in an isolated system, and their postexposure succession may therefore be forced in an unexpected and unwanted direction. It is thus important to carry out studies that follow the development in perturbed ecosystems over longer periods. The intention of this study was to follow the direct and secondary effects of an insecticide on the structure of a eutrophic pond ecosystem for 3 years. The well-known insecticide fenvalerate was added to freshwater mesocosms and the investigation was

focused on structural long-term changes in the invertebrate community that may develop in natural ecosystems exposed to inseticidal pollutants. MATERIALS AND METHODS

The present study was part of a project aimed at investigating short- and long-term effects of modern nonpersistent pesticides on the structure and function of pond ecosystems. The work covered observations on water chemistry, macrophytes, invertebrates, phytoplankton, and zooplankton before, during, and after application of the pesticides fenvalerate and metazachlor.

The Mesocosms According to the SETAC classification (Hill et al., 1994), the systems should be termed microcosms because of their size. However, in this paper the term mesocosm is used according to the original definition by Odum (1984). Six identical plastic (Polyplan) swimming pools (Funny Pool, Sattler, Austria) were used as containers (ca. 7 m3, depth 0.8—0.9 m, diameter 3.5 m) and placed outdoors in the southern part of Sweden (Fig. 1). The pools were filled with ca. 1 m3 unpolluted eutrophic lake sediment, ca. 6 m3 of water from nearby eutrophic unpolluted ponds, 10 liters of freshly collected Elodea canadensis, and 12 adult plants of Stratiotes aloides. All the natural components were fresh and, thus, embodied a natural composition of organisms (algae, zooplankton, benthos, etc.). The plants were added to speed up the succession from r-strategic to K-strategic producer communities. Fish were excluded from the systems by electrofishing since the size of the mesocosms was not sufficient to keep fish in an ecologically sustainable manner (Parsons, 1982). The systems were static (no inflow or outflow) and never connected among themselves. They were open to air, precipitation, and sunlight. The water column depth varied

FIG. 1. Mesocosm setup and ‘‘traps’’ (artificial substrate) used for sampling of the invertebrate community.

EFFECTS OF FENVALERATE ON POND INVERTEBRATES

between summer (minimum 0.7 m) and winter (maximum 0.9 m) due to evaporation and precipitation. The water in each mesocosm was mixed by recirculation by pumps (RENA) that sucked water above the sediment surface and released it horizontally below the water surface. The water column was therefore presumed to be homogeneous with respect to water chemistry, temperature, and plankton distribution, allowing an adequate sampling strategy. To decrease night and day fluctuations in water temperature and to minimize the risk of freezing during the winter, the pools were sunk 0.4 m into the ground. Initial macrophyte structure and physicochemical characteristics. E. canadensis was the dominant macrophyte with a bottom cover of 30—50% in all mesocosms. S. aloides was also abundant, with 15—20 scattered plants in each pond. Other species with substantial cover or biomass were ¸emna trisulca and Cladophora sp., which formed joined patches covering 10—40% of the pond area; ¸emna minor and ¸emna polyrrhiza ((10% cover together); and Batrachium aquatile. Species with low abundance (a few plants) but found in all mesocosms were Ceratophyllum submersum, Myriophyllum sp., Potamogeton crispus, Potamogeton sp., Hydrocharis morsus-ranae, and Chara sp. Chlorophyll concentrations and selected physicochemical parameters in the six mesocosms during the week before treatment are presented in Table 1. Pesticide Application and Determination The mesocosms were allowed to stabilize for 3 months before adding the pesticide. The fenvalerate was added once in the later part of the growing season during the first year (August 1990). Fenvalerate is one of many similar photostable synthetic pyrethroid broad-spectrum insecticides used worldwide. Common features of these pesticides include low water solubility (40.020 mg liter~1), a high

TABLE 1 Selected Biological and Physicochemical Characteristics of the Mesocosms the Week before Fenvalerate Exposure Started (n 5 6) Characteristic

Mean (SD)

pH Conductivity [lS cm~1 (25°C)] Alkalinity (mmol HCO~ liter~1 3 Turbidity (NTU) Chlorophyll a (lg liter~1) Phosphate (lg liter~1) Total phosphorus (lg liter~1) Ammonia (lg liter~1) Nitrate (lg liter~1) Calcium (mg liter~1)

9.53 (0.18) 203.3 (11.7) 1.04 (0.19) 1.46 (0.62) 0.43 (0.16) 5.67 (3.20) 50.67 (14.11) 9.00 (1.41) 2.33 (0.82) 34.7 (5.7)

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octanol—water partition coefficient (log P 54), and high 08 acute toxicity to arthropods (0.005-5 lg liter~1) and fish (0.2—20 lg liter~1). A commercially available concentrated emulsifiable formulation of fenvalerate (Sumicidin 10FW, Dupont, 100 g liter~1) was used for making standard solutions with distilled water. These solutions were then slowly added with a multichannel peristaltic pump into the recirculating tubes of each mesocosm. This continuously simulated ‘‘leakage’’ of fenvalerate into the mesocosms lasted 14 days (13—27 August). The initial calculated concentrations after 2 weeks exposure were 0.02 (ultralow exposure), 0.2 (low exposure), 2 (medium exposure), and 20 (high exposure) micrograms of active substance per liter. These nominal concentrations assumed no partitioning, no degradation, as well as complete mixing of the substance over the water column. Two systems served as controls and received only distilled water. Unfiltered water was sampled at different intervals during and after the exposure period for determination of fenvalerate residue levels. The fenvalerate were extracted and determined according to the method of Woin (1994). Sampling of Macroinvertebrates Observation and sampling started in July and continued until November of the first year in six systems. One control mesocosm and the mesocosm exposed to 20 lg fenvalerate liter~1 were kept and followed for a further 2 years to allow possible long-term changes to be studied. The mesocosms were inspected visually for signs of acute effects during the exposure. Dead or dying animals and the behavior of the organisms were recorded. The invertebrate community was sampled at differing intervals during the study using specially designed artificial substrates (Fig. 1). In each mesocosm there were six of these ‘‘traps’’ made of a Plexiglas cylinder (diameter"200 mm, height"100 mm), with the bottom covered with a nylon net (1-mm mesh). The traps were hung in the water approximately 0.1 m above the sediment surface. Each trap was attached to a floating buoy and anchored in the sediment. The traps were randomly dispersed in the mesocosm. Each trap covered about 0.33% of the mesocosm area (9.5 m2), and when lifted from the system, approximately 0.29% of the water volume (ca. 7000 liters) was sampled. After a colonization period of at least 1 week the traps were rinsed and the organisms were collected and determined. Animals that were identified to species or genus level in the field (mainly larger invertebrates such as snails and predatory insects) were returned to the systems to minimize sampling interference on the community structure. Most of the organisms were identified to genus or species level. Certain abundant but difficult taxonomical groups (e.g., Diptera and eruciform Trichoptera) were superficially examined. In the mesocosms there were at least 10 different taxa of

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TABLE 2 Scheme for the Invertebrate Trap Sample Collections Year 1 (all mesocosms)

Period A

4]6 traps/ mesocosm

July—August 13—26 August Exposure Period B 3]6 traps/ mesocosm August—October

Year 2 (one control and high)

Year 3 (one control and high)

2]6 traps/ mesocosm

0

3]6 traps/ mesocosm

3]6 traps/ mesocosm

obvious (‘‘significant’’) than others. To compare changes in integrated community metrics, Shannon’s diversity index, S+rensen’s similarity index, and the Bray—Curtis similarity index (all described by Sheehan 1984), as well as simple species richness, were used. RESULTS

Fenvalerate Concentrations

Chironomidae but none was identified to species or genus. The results in terms of absolute abundance values of counted taxa in the traps (numbers per 0.186 m2 or 122 liters for each sampling occasion and mesocosm) are presented as means of specific periods separated by the date 13 August (the date exposure started in Year 1). To obtain a theoretical value of total numbers in the entire mesocosm at a given time, the reported values should therefore be multiplied by approximately 50. The periods are referred to as 1A and 1B (Year 1, before and after 13 August), 2A and 2B (Year 2, before and after 13 August), and 3B (Year 3, after 13 August) as described in the sampling scheme in Table 2. In August, 1 year after the fenvalerate exposure, all emerging dragonflies (Odonata: Aeshnidae and Libellulidae) were counted by picking all exuviae from the surface of the water and from plants in the control and the highexposure mesocosms. August was chosen since dragonflies of many different families emerge at this time. Data Interpretation: Considerations As no effects were seen and no fenvalerate was detected in the low- and ultralow-exposure treatments, these mesocosms were excluded from further analysis. A great effort was made to ensure that the mesocosms had ambient conditions (wind, precipitation, and sun exposure) as similar as possible and also to give them quantitatively and qualitatively similar startup components. The treatment mesocosms were selected randomly. Partly because of the design itself (no replication) and partly because of the lack of potentially dose-related effects in two low-exposure mesocosms, no inferential statistics, as recommended by Hurlbert (1984), have been applied to the data. Instead, common sense, biological and ecological knowledge and intuition, as well as comparisons with related investigations, have been applied in the interpretation and evaluation of raw data (viewed as plots of the response of the individual variables over time). Observed posttreatment changes are therefore assumed to be exposure related, with some changes more

The actual concentrations of fenvalerate in the water were lower than the nominal ones (Fig. 2). The highest concentration (1.3 lg liter~1) was found in the high-exposure mesocosm 1 day after the addition was completed. In the medium-exposure mesocosm, the highest concentration was found 2 days after the exposure started. In the low- and ultralow-exposure mesocosms, no fenvalerate was detected. The highest concentration detected relative to the nominal value was found in the early phase of exposure. Two days after the start of addition 25 and 20% of the applied amount were detected in the high- and medium-exposure mesocosms, respectively. Directly after the application (Day 15), 1—6% of the nominal concentrations was detected. The concentration of fenvalerate in the water column decreased continuously after exposure. Approximately 40 days after the exposure started the concentrations were below the detection-limit (+5 ng liter~1). Direct Effects on Invertebrates Two days after the exposure started, the fenvalerate concentration in the high-exposure mesocosm was lethal to many taxa, especially insects (Table 3). Large numbers of immobilized animals were floating on the water surface. The most frequently observed animals were Coleoptera, Hemiptera, and Ephemeroptera. Large numbers of living

FIG. 2. Levels of fenvalerate in the mesocosm water. The nominal concentrations after 14 days of continuous addition to the medium- and high-exposure mesocosms were 2 and 20 lg fenvalerate liter~1, respectively. No fenvalerate was detected in the low (0.2 lg liter~1)- and ultralow (0.02 lg liter~1)- exposure mesocosms.

EFFECTS OF FENVALERATE ON POND INVERTEBRATES

TABLE 3 Dead Animals Collected on the Water Surface of the HighExposure Mesocosm at Different Times Relative to the Start of Exposure (13 August, Year 1)a Dead animals collected Colymbetes Corixa Dytiscus Agabus Acilius Aeshna Trichoptera Notonecta Sum

Day 3 Day 5 Day 8 Day 9 Day 15 (16 August) (18 August) (21 August) (22 August) (28 August) 28 6 1 1

36

16 3 2 1 4

26

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fly larvae (Ephemeroptera) and caddis larvae (Trichoptera), the abundances of which were markedly reduced after exposure (Fig. 5). The dipteran Chaoborus crystallinus was also reduced in number while the chironomids were unaffected. In nonarthropod taxa, no direct negative effects were recorded in either the high-exposure or the medium-exposure mesocosm.

2

Changes in Community Metrics 2 7 1 4

2 1

1

3

10

16

6

11

a The nominal fenvalerate concentration in water at Day 3 (August 16) was 2—3 lg liter~1.

animals (including Copepoda and Cladocera) were concentrated just below the surface of the water, and many animals, with heavily disturbed locomotory behavior, were observed. However, dead or disturbed individuals of the class Malacostraca (e.g., Asellus) were not observed. Exposure to fenvalerate at nominal concentrations of 20 and 2 lg liter~1 resulted in direct structural changes of the macroinvertebrate community. The arthropods, especially the insects (Fig. 3), were those directly affected. Insects such as Cloeon dipterum (Ephemeroptera, Figs. 4—6) and Chironomidae (Diptera, Figs. 4—6) were almost eliminated in the high-exposure mesocosm. In the high-exposure mesocosm, the first-year (1B) four zygopterans, four chironomids, and two individuals of ¹ipula sp. were found after exposure. No Asellus aquaticus were recorded after exposure, in contrast to the 10 individuals that were caught before exposure (Fig. 7, high). In the medium-exposure mesocosm the clearest direct effects were observed on may-

About 70 taxa were recognized in the mesocosms (Appendix). The mean ‘‘species’’ richness (defined as the mean number of taxa caught per sampling and mesocosm) normally varied, between 11 and 18 (Fig. 8). Directly after the fenvalerate exposure in the first year (1B), the mean number of taxa caught in the high-exposure mesocosm dropped to below 7 (+50% of normal) (Fig. 8, high). After this initial response, the number of taxa increased to values comparable to the control values. In the medium-exposure mesocosm, the effects of fenvalerate were not reflected in the ‘‘species’’ richness. The value of Shannon’s diversity index decreased in both the high- and medium-exposure mesocosms directly after exposure (Fig. 8). In the high-exposure mesocosm during the year following exposure, the diversity index indicated a recovery, approaching the control value, but the value was lower than the control value throughout the study. The estimates of mesocosm community similarity revealed high similarity between the systems before exposure followed by decreased similarity after exposure, at both treatment levels (Fig. 9). The Bray—Curtis index (qualitative and quantitative estimate) indicated, in contrast to S+rensen’s index (qualitative estimate), that the posttreatment change exceeded the system’s (high exposure) threshold of self-repair, resulting in divergence from the control community throughout the study.

FIG. 3. Mean abundance of Insecta in the mesocosms. Each bar represents the mean numbers of individuals per sample, with two to four samples per time period. In Year 1, the values are based on the mean of two control mesocosms; otherwise, one mesocosm. The light-gray bars (Medium) represent the 2 lg fenvalerate liter~1 nominal exposure (one mesocosm, Year 1) and the dark-gray bars (High) represent the 20 lg fenvalerate liter~1 nominal exposure. The arrow separates pre- and postexposure.

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FIG. 4. Mean abundance of insect orders in the control mesocosms. In Year 1, the values are based on the mean of two control mesocosms; otherwise, one mesocosm. Each bar represents the mean numbers of individuals per sample, with two to four samples per time period. The arrow separates pre- and postexposure.

Indirect Effects and Long-Term Scenario Oligochaeta. Oligochaetes were represented in the mesocosms by four taxa, of which Stylaria lacustris was the dominant taxon during the whole study. One taxon, Naididae, was found only in the control mesocosm in the third year. After medium and high fenvalerate exposure there were 8.5- and 6.2-fold increases, respectively, in oligochaetes relative to preexposure values (Fig. 10, 1B). The corresponding increase in the control was 1.5-fold. The increase was more than 11-fold in the high-exposure mesocosm during the second year compared with both preexposure and control values. The increase was accounted for mostly by S. lacustris but Chaetogaster sp. also contributed, especially in the medium-exposure mesocosm, where 30%

of the increase was due to this taxon. In Year 3, the abundance of oligochaetes was reduced and comparable to that of the control. Insecta. In the season following exposure (Fig. 3, 2A and 2B), the total insect abundance increased but was still less than half of the abundance in the control mesocosm. The increase was partly a recolonization by flying adult insects (Hemiptera, Coleoptera) and by larvae of species with short generation cycles (uni- and multivoltine species). The latter were dominated by ceratopogonids and Chaoborus crystallinus and, to some extent, also by chironomids and the mayfly Cloeon dipterum, which principally appeared in period 2B (Fig. 6). The Hemiptera recolonization consisted of an assemblage of locally occurring species, both carnivorous

FIG. 5. Mean abundance of insect orders in the medium-exposure mesocosm during Year 1. Each bar represents the mean numbers of individuals per sample, with two to four samples per time period. The arrow separates pre- and postexposure.

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FIG. 6. Mean abundance of insect orders in the high-exposure mesocosm. Each bar represents the mean numbers of individuals per sample, with two to four samples per time period. The arrow separates pre- and postexposure.

(Notonecta sp.) and herbivorous/omnivorous species (Corixa punctata, Hesperocorixa sahlbergi, Sigara falleni). The coleopteran colonizers were dominated by carnivorous species (Colymbetes fuscus, Acilius sulcata, Hydroporus sp.) but some algal feeders were also present (Haliplus sp.). In the following year (Year 3), the insect fauna recovered quantitatively and was denser than in the control mesocosm (Fig. 3). However, the insect species composition was not similar to that of the control mesocosm. The main difference between the control and high-exposure mesocosms 2 years after exposure was in the abundance of Ephemeroptera and Diptera (Figs. 4 and 6). In the control mesocosm, Ephemeroptera (C. dipterum) were dominant, while the opposite was valid for the exposed mesocosm, where the domination of Diptera consisted mainly of C. crystallinus (Figs. 4 and 6). The other dipterans, the chironomids (considered as one taxon), recovered 1 year after the exposure, when their abundance became comparable to that of the control. The trichopterans in the community consisted mainly of Polycentropodidae, Holocentropus dubius, and ¸eptocerus sp. before exposure and in the control during the whole study period, but Limnephilidae and Phryganeidae were

FIG. 7. Mean abundance of Malacostraca in the mesocosms. For details see Fig. 3.

also present. The trichopterans disappeared completely in the high-exposure mesocosm and did not reappear until Year 3, when their abundance was 41% of that in the control (Fig. 6). The reappearing Trichoptera were dominated by eruciform (case-building) species. The campodeiform (caseless) types of Trichoptera that were abundant in the control during the whole period and in the high-exposure mesocosm before fenvalerate exposure did not reappear (except one individual of H. dubius found in period 2B). Only 6 of 28 individual colonizers were of previously determined taxa. The rest were of a different case-building species not observed earlier in the mesocosms. The number of dragonflies (order Odonata, families Aeshnidae and Libellulidae) caught in all mesocosms was low during the whole study. ‘‘Direct’’ effects on dragonflies were discovered 1 year after exposure in the high-exposure mesocosm. At that time, two individual emerging dragonflies (Libellulidae: Sympetrum sp.) were observed in the highexposure mesocosm compared with 27 individuals (mainly Aeshnidae) in the control mesocosm. All odonates caught in the invertebrate traps in the high-exposure mesocosm after exposure were of the damselfly group (Zygoptera). After exposure (period 1B), 4 damselflies were recorded in the high-exposure mesocosm compared with 12 in the control. During the year after exposure no zygopterids were found in the high-exposure mesocosm. However, there were only a few individuals of the group in the control as well that year. Two years after exposure, 40 individual zygopterans were found in the exposed mesocosm compared with 58 in the control. Ostracoda. The ostracods in this study were represented only by Cypris (Herpetocypris) reptans. Before exposure, ostracods were occasionally found in all of the mesocosms. In the second and third years there was a marked increase in

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FIG. 8. Species richness (bars) and Shannon’s diversity index (lines) analysis of data from the mesocosms. In Year 1, the control (Y, X) consisted of two mesocosms, whereas one mesocosm was used as control during Years 2 and 3. The medium-exposure mesocosm (Z, h ) represents the 2 lg fenvalerate liter~1 nominal exposure (one mesocosm, Year 1) and the high-exposure mesocosm (Z, r) represents the 20 lg fenvalerate liter~1 exposure. The arrow separates pre- and postexposure.

ostracods in the high-exposure mesocosm compared with pre-exposure and the control. The mean number of trapped specimens in the high-exposure mesocosm increased from zero in the preexposure period to 148 and 700 in Years 2 and 3, respectively (Fig. 11). The increase in abundance had already started in the high-exposure mesocosm in the first postexposure period (1B), with a mean of 13 trapped animals per sampling. The largest number of captured ostracods in the control was 11 specimens per sampling (during year 2). Mollusca (Gastropoda and Bivalvia). No negative effect of fenvalerate on mollusks was recorded in the mesocosms.

On the contrary, there was a clear long-term tendency to increased abundance, illustrated by period 2B (Fig. 12). The gastropod community in unexposed mesocosms was dominated by ¸ymnaea stagnalis, ¸. peregra, Bithynia sp., and two different species of Planorbis. The increased abundance in the high-exposure mesocosm in period 2B was caused mainly by ¸ymnaea species. ¸. stagnalis dominated during both Years 2 and 3. Mussels were not recorded in any of the mesocosms during the first year. In the high-exposure mesocosm, 3 and 27 individuals per sampling, belonging to at least three different species of the Sphaeriidae family, were found in Years 2 and 3, respectively. Only one animal was recorded in the control during the same period.

FIG. 9. Changes in two community comparison indexes during the study. Comparison of control mesocosm versus high-exposure mesocosm (1!Bray—Curtis index, j ; S+rensen’s index, h) and medium-exposure mesocosm (1-Bray—Curtis index, r; S+rensen’s index, e) by indexes calculated using the original data. Similarity between two communities is indicated with values approaching 1 (1"identical).

EFFECTS OF FENVALERATE ON POND INVERTEBRATES

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FIG. 10. Mean abundance of Oligochaeta in the mesocosms. For details see Fig. 3.

Malacostraca. The higher crustaceans, class Malacostraca, represented in the mesocosms only by the isopod A. aquaticus, which became ‘‘extinct’’ in the high-exposure mesocosm directly after the exposure, did not recover during the following 2 years (Fig. 7). Hirudinea. Two species of leeches (Erpobdella octoculata and Glossiphonia complanata) were found in the mesocosms. In common with the taxonomically closely related Oligochaeta their postexposure abundance compared with the control was higher (Fig. 13). E. octoculata was the dominant species during Years 1 and 2. The population density of G. complanata increased in the high-exposure mesocosm in Year 2 and it was the dominant species in Year 3. No free-living animals were caught in the control mesocosm during Years 2 and 3. However, the presence of E. octoculata was documented by the presence of cocoons on the pool walls. Arachnida. The abundance dynamics of water mites (Arachnida: Hydrachnellae), clumped as one taxon in this study, are noteworthy. Their population was quite stable in the control during the whole study, with 1 to 5 trapped

individuals each period. In the high-exposure mesocosm, the population increased temporarily from 4 trapped individuals preexposure to 12 in period 1B and to '100 in period 2B. In the third year, the population was back to normal (3 individuals caught). DISCUSSION

Application and Fate of Fenvalerate The large difference between nominal (e.g., 20 lg liter~1) and detected concentrations (in this case 1.3 lg liter~1) of fenvalerate is explained mainly by the rapid adsorption to particulate organic matter (POM) such as plankton (Muir et al., 1985; Hinckley and Bidleman, 1989), to surface films (Crossland, 1982), to macrophytes and animals (Muir et al., 1985; Caquet et al., 1992), and to system walls (Sharom and Solomon, 1981). Photodegradation, chemical hydrolysis, and metabolism in organisms may also contribute to the dissipation of fenvalerate, to a lesser extent (Muir et al., 1985; Caquet et al., 1992). The photodegradation of fenvalerate in water can be rather rapid; the half-life is 4—15 days (WHO, 1990). However, substantial decomposition of

FIG. 11. Mean abundance of Ostracoda in the mesocosms. For details see Fig. 3.

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FIG. 12. Mean abundance of Mollusca in the mesocosms. For details see Fig. 3.

the substance by UV light should occur mainly in the upper layer of the water body since the UV rays are absorbed quickly in natural waters. Chemical hydrolysis in pure water is relatively slow (half-lives of 65—67 days at pH 9) and may therefore, like photodegradation, be of minor importance in explaining the detected concentrations. Bioaccumulation of fenvalerate in aquatic organisms is known to be of great significance (BCF 120-4700, WHO, 1990). However, rapid metabolism is known to occur mainly in vertebrates like mammals. Adsorption onto and uptake into sensitive living organisms, such as crustacean plankton, will accelerate the dissipation of fenvalerate from the water column. For example, when the substance affects cladocerans and copepods, which consume POM, they die (unpublished data) and sink to the sediment. This adsorption—bioaccumulation—mortality—sedimentation process leads to decreased concentrations of fenvalerate in the water. Consequently, the low concentrations found in the water were obviously a result of a combination of different factors.

However, adsorption to POM and surfaces, followed by trophic transfer and sedimentation, is probably of major importance. A rather unexpected condition was that the highest concentrations, relative to the added amounts, were found in the beginning of the exposure period. The reason for this could be that the disappearance of fenvalerate, as the pyrethroid deltamethrin (Caquet et al., 1992), is biphasic, with a rapid initial decrease phase (minutes to hours) due to adsorption, followed by a slower (days to weeks) degradation-generated decrease. This phenomenon is normally observed in ‘‘single application’’ designs with substances that have low water solubility and high affinity to surfaces and particles. However, in this study the application of fenvalerate was continuous over 14 days, and consequently, the initial adsorption process occurred during this period parallel to the slower second degradation process, thereby explaining the relatively low concentrations recovered at Day 15.

FIG. 13. Mean abundance of Hirudinea in the mesocosms. For details see Fig. 3.

EFFECTS OF FENVALERATE ON POND INVERTEBRATES

The fenvalerate concentrations in the mesocosms were similar to those estimates of ‘‘expected environmental concentrations’’ (potential pond concentrations +0.175—5.25 lg fenvalerate liter~1) determined by Hill (1989). Thus, results obtained in this study may be used to predict effects in natural ponds and small waters receiving pesticides after runoff or spray drift from surrounding agricultural areas. Direct Effects on Invertebrates Direct toxicity effects were observed particularly on insects in the high-exposure mesocosm 2 days after the exposure started. At a concentration of approximately 0.6 lg fenvalerate liter~1 (measured), most of the arthropods were affected to such an extent that they died (Table 2). However, it was only possible to record the death of large and hardbodied animals. Smaller and soft-bodied animals were either too degraded or too small to allow a reliable quantification. Nevertheless, heavily disturbed locomotion behavior was taken as an indication of subsequent mortality. This behavior malfunction was preceded by a movement toward the water surface, as a substantial number of living arthropods gathered just below the water surface. This active movement toward the surface is suggested to indicate an escape behavior comparable to catastrophic drift in lotic ecosystems (Thure´n and Woin, 1991). Similar direct effects were not observed in Asellus, probably due to their habitat selection (dwelling on the sediment surface) and their rudimentary swimming capabilities. These factors make visual observations and quantification difficult. However, the numbers of A. aquaticus (Fig. 7) collected on the traps in the high-exposure mesocosm (no posttreatment occurrences) indicate a high sensitivity of Asellus to fenvalerate. On the other hand, in the medium-exposure mesocosm some insects (Fig. 5; Ephemeroptera, Trichoptera, Diptera) seemed to be severely affected, while no acute negative effect was recorded for A. aquaticus (Fig. 7). This circumstance indicates a higher sensitivity in insects than crustaceans. In general, fenvalerate has been suggested to be most toxic to arthropods and the acute toxicity of fenvalerate to nontarget freshwater organisms is reflected in the high mortality in insects and crustaceans (WHO, 1990; Mian and Mulla, 1992). Taxonomically related amphipods (e.g., Gammarus) have been suggested to be more sensitive to fenvalerate than Ephemeroptera (Ephemerella sp.), Plecoptera (Pteronarcys dorsata), and mollusks (Heliosoma trivolvis) (Anderson, 1982). Most studies so far suggest that mayfly nymphs, Gammarus, and cladocerans are among the most sensitive among aquatic organisms (WHO, 1990). Perhaps the observed difference in sensitivity between Asellus and insects in this study is a consequence of the test conditions (i.e., all ecosystem compartments involved), thus leading to unequal exposure conditions for different species. The isopods that live mainly in the sediment are therefore exposed to less

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bioavailable fenvalerate (because of binding to particles) than insects that live mainly in the water column. This reasoning may also be valid for the chironomids, which likewise were unaffected in the medium-exposure mesocosm. However, several species of this group do not live in sediment and, therefore, as the animals were collected in the water column, a more likely explanation is that the chironomids comprised species that were less sensitive to fenvalerate than other arthropods in the mesocosms. The dead animals collected in the high-exposure mesocosm were dominated by large predatory insects such as coleopterans, Aeshna and Notonecta (Table 3). These relatively large animals with good locomotory capability are able to climb up on floating macrophytes to escape. They are easier to observe than small ephemeropterans and trichopterans that remain in the water. Consequently, the relative numbers of dead individuals recorded after exposure reflected not a pattern of sensitivity but also differences in mobility, resulting in sampling skewness for certain invertebrate groups. Changes in Community Metrics The ‘‘species’’ richness displayed an increasing trend during Years 1 and 2, followed by a smaller decrease in Year 3 (Fig. 8). This is a pattern expected in a new habitat, where a succession from an r-selected environment (founder controlled) to a more complex K-selected community (nicheand dominance-controlled) results in increased species richness followed by stabilization at a level below the peak (Yodzis, 1986). The fenvalerate exposure affected the ‘‘species’’ richness only in the high-exposure mesocosm, and the species richness recovered to its initial value by the season after exposure (less than 1 year). This rapid ‘‘recovery’’ and the fact that no changes in species richness occurred in the medium-exposure mesocosm indicate that this variable, compared with others in this study, is not reliable and sensitive enough when toxic stress on aquatic macroinvertebrate communities is to be estimated. However, Pratt (1990) suggested, in contrast to others (Pusey et al., 1994), that species richness is a sensitive and generally useful indicator of toxic stress. His conclusions and recommendations were based mainly on laboratory microcosm studies, and the response variables that species richness were compared with included changes in enzyme activity, macronutrient dynamics, productivity, and other functional responses. His study cannot therefore be directly compared with those performed in the field and including cascading trophic interactions. However, it is important to note that species richness, from a general point of view, is not a useful indicator of toxic stress in ecosystems, on either a short-term or a long-term basis. Species richness is a rough measure that gives no information about the composition of the community, which is clearly seen in the present study. Further, in natural

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communities time acts against the acutely depressed species richness through processes such as immigration (shorter time scales) and adaptation (longer time scales), resulting in increased species richness but not necessarily with the same composition as before the perturbation. Caution must therefore be taken when using species richness change as an endpoint for determining ecotoxicity. The diversity index seemed somewhat more sensitive as a stress indicator than species richness, since it clearly responded negatively in the two most exposed mesocosms, and since a lower value relative to the control in the highexposure mesocosm persisted throughout the study. The index, which includes information about the partitioning of individuals among species (evenness) indicates, in accordance with the other data presented, that the community in the high-exposure mesocosm recovers slowly after the perturbation. This may also indicate that the diversity is a more sensitive estimator of community change. However, the critical arguments about species richness, stressed above and by others (Pusey et al., 1994), may also apply to diversity indices. Consequently, the diversity index values reported here should be regarded only as additional evidence of fenvalerate-induced change in the macroinvertebrate community. The Bray—Curtis index, displayed in Fig. 9, indicated a strong similarity between the mesocosm communities before fenvalerate exposure. This index has been regarded as a relatively sensitive similarity index because it is sensitive to dominant taxa (Pontasch et al., 1989). The value of the index is based on both qualitative and quantitative comparisons between selected systems, in contrast to the S+rensen index, which only compares the species composition between the systems (cf. diversity index with species richness). The Bray—Curtis index value and also the weaker S+rensen index value strongly indicate long-lasting fenvalerate-induced community changes. The Bray—Curtis index value also indicates that the change in the high-exposure mesocosm was substantial and too pronounced to permit a recovery toward the original community composition. This strongly supports the idea that the perturbation, through cascading disturbance of ecologically important self-organizing mechanisms, resulted in a ‘‘permanently’’ changed ecosystem, in the infancy of a newly started secondary succession. Indirect Effects and the Long-Term Scenario This section discusses changes in community structure that were not supposed to be created by direct fenvalerate toxicity. These indirect structural changes are suggested to be caused by trophic interactions and changes in abiotic factors. The community composition at a given point is therefore also dependent on, and related to, several different taxon-specific factors (tolerance, trophic status, life history strategies, mobility, competition, predation, habitat selec-

tion). However, the complexity of the mechanisms behind the responses and the limited knowledge of aquatic community ecology would necessarily leave many questions unanswered. Oligochaeta. After fenvalerate exposure there was a remarkable increase in oligochaetes dominated by S. lacustris (Fig. 10). This species has selected habitats among water plants and mud and is a collector and deposit feeder. Aquatic oligochaetes, in general, seem to have low sensitivity to pollutants and are often used as indicators (¹ubifex tubifex) of organic pollution (Burton, 1991). The increase in oligochaetes was probably due to their insensitivity to fenvalerate in combination with reduced predation and lower competition for food. The reduced predation was a consequence of the direct effects on the predatory insects. In the high-exposure mesocosm, where the insects were more or less completely eliminated, it was obvious that the predation pressure on the oligochaetes was reduced and that this probably favored the population increase. However, in the medium-exposure mesocosm where the increase in Oligochaeta was slightly higher than in the high-exposure mesocosm, there was not such a clear connection with a decrease in the numbers of predators. Therefore, the main reason for the marked increase in Oligochaeta after fenvalerate exposure was probably an effect of reduced competition for food. Arthropod competitors were reduced due to direct effects of the fenvalerate. Abundant competitors such as Cloeon dipterum (Ephemeroptera) and Chironomidae (Diptera) died and were thereby converted to potential food. However, reduced competition does not completely explain the oligochaete increase in the mediumexposure mesocosm, as some competitors were present in that system after exposure. Consequently, the most probable single factor responsible for the increase must have been a sudden increase in food resources that the opportunistic oligochaetes were able to exploit. The food of the dominant species (S. lacustris) is probably dominated by dead organic matter, bacteria, and algae. Since no increase in phytoplankton was observed after exposure during the first year, a probable food resource was the increased amounts of dead and decaying arthropods (insects, Cladocera, Copepoda). Insecta. A recovery of the entomofauna occurred in the season following exposure (2A). In Year 3 the insect fauna recovered quantitatively and became comparable to the control mesocosm. However, the recovery rates were different for different taxa, resulting in an unbalanced succession of the community composition compared with the unperturbed mesocosm. The first colonizers (recorded during the second year), apart from possible late autumn introduction of eggs, were mainly larger predatory aquatic insects such as water bugs (Hemiptera: Notonecta sp. and certain corixids) and diving beetles (Coleoptera: C. fuscus, A. sulcata, and

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Hydroporus sp.). These semiaquatic, air-dispersed insects may not colonize new established waters at an early stage of the successional development, because of lack of their preferred food and attractive spatial structure. However, the selectively perturbed mesocosm was rich in potential prey (e.g., oligochaetes) and the spatial structure (macrophytes) was well developed and similar to unexposed systems. The well-established macrophyte community and the pool walls with a presumably high content of periphytic algae may have attracted new algal feeders such as the beetle Haliplus sp. and certain herbivorous hemipterans. Due to the large amount of dead and decaying organisms after exposure, detrivorous and omnivorous (e.g. corixids) species may also have been favored. Therefore, the high-exposure mesocosm may have been attractive for exploitation by these semiaquatic insects as soon as the fenvalerate concentration reached a nontoxic level (approximately 1—2 months after the initial application). The other significant source of early colonization was probably the natural deposition of eggs by uni- and multivoltine species during the autumn of the first year and the following spring. Insects responsible for this kind of colonization are predominantly mayflies (C. dipterum) and different dipterans (mainly ceratopogonids and C. crystallinus). C. dipterum is known to have a multivoltine life cycle, with one slow-growing winter generation followed by one or more, rapidly growing summer generations (Elliott et al., 1988). Eggs are normally laid by adults of the winter generation in spring (May—June). These eggs had probably not hatched in July—August or had hatched into larvae too small to be caught in the traps. The winter generation was probably extinct after the fenvalerate exposure during the first year since no larvae were found in period 2A. The larvae detected in the following period (2B) originated from eggs laid in the spring. Factors affecting subsequent favorable population growth of these animals were probably the extended niche space (competitive advantage), an initial decrease in predation, and an increase in accessible food after the large-scale death of arthropods. In the third year there was a difference between the control and high-exposure mesocosms in the abundance of Ephemeroptera and Diptera. The normal development in a nonexposed mesocosm seemed to be toward an Ephemeroptera-dominated community, while the insect community in the exposed mesocosm developed toward Diptera domination. The dominating Diptera was Chaoborus crystallinus, which is a predator on small crustaceans and certain small benthic animals (Wetzel, 1975; Engelhardt, 1976). It has been demonstrated that Chaoborus can be conspicuously selective of the food it consumes, preferring copepods and oligochaetes over ostracods and Daphnia (Wetzel, 1975). Their contribution to the regulation of macrozooplankton has also been found to be significant, particularly in fish-free water bodies (Wetzel, 1975). The large abundance of C.

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crystallinus cannot be explained by reduced predation pressure because predators were present in Year 3. It is therefore likely that the exposure mesocosm had evolved toward a more zooplankton-rich system that favored C. crystallinus. It may also be possible that the previously highly abundant S. lacustris contributed as a food source. The origin of the change is, however, probably found in the recirculation of nutrients from decayed organisms that died after the fenvalerate exposure. The chironomids seemed to be among the least sensitive insects in the study since, as a taxon, they survived the exposure in the medium-exposure mesocosm without decreasing in abundance. There were also a few records of chironomids in the high-exposure mesocosm directly after exposure that confirm this conclusion. Chironomids are known to reside in relatively polluted areas and, as would be expected, are often more resistant to toxicants compared with other insects (Cairns et al., 1984). However, the Chironomidae consist of many species with different demands on abiotic and biotic factors and probably also with different sensitivities to toxicants. Some recent investigations have confirmed that many chironomids are particularly sensitive to certain pesticides (cf., Ward et al., 1995). The fast recovery, on a long-term scale, in the high-exposure mesocosm may be explained by the continuous input of eggs from adult animals in the terrestrial surroundings. Because of the possibility of may species depositing eggs at different times of the year and the relatively fast egg and larval development in the majority of chironomids, it might be suggested that the postexposure species composition was different, as found by others (Ward et al., 1995). However, because the species composition was not analyzed, this question needs to be investigated further despite the problem of taxonomy and the poor general knowledge about the ecology of the group Chironomidae. Trichoptera seemed to be among the most sensitive orders on a long-term basis. Recovery of this group was slow and the species composition changed. It is therefore reasonable to assume that certain caddis flies are extremely sensitive to fenvalerate, but the changes may also be derived from ecological processes. The shift from one dominant species (Polycentropodidae) to another, the eruciform type, completely new for the ‘‘ecosystem,’’ indicates a genuine change toward a new species composition. The fact that the former frequent species (Polycentropodidae) did not reappear during the remaining study also supports this scenario of real community change. The mechanisms behind this fundamental change of the trichopteran community are probably explained by basic and persistent secondary changes of the ecosystem that created new biotic and abiotic conditions, unsuitable for the former species. For example, a change in the macrophyte structure, which occurred in the high-exposure mesocosm (unpublished data), may account for the absence of some phytophilous species as well as the

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introduction of others. Studies have suggested that insects living in submerged vegetation can exhibit specificity for particular plant species, and that morphological form and chemical composition of the plants are the major factors that influence the specificity (Ward, 1992; Hutchinson, 1993). Furthermore, different types of substrate not only harbor different assemblages of species, but the density and biomass of the fauna may vary over several orders of magnitude. Former studies (see Ward, 1992) have found that the relative standing crops of entomofauna on Elodea and Ranunculus (Batrachium) were 13 and 5.5 times higher, respectively, than on Chara, and all these macrophytes are relevant to the present study. E. canadensis dominated the species-rich macrophyte community during the first year. It was then covered and replaced by Cladophora during the second year, and, after a rapid retrogression of Cladophora, a community based on Potamogeton crispus and Chara was established during the third year. This change in vegetation may contribute to significant changes in feeding and substrate conditions for the trichopterans, resulting in a functional discrepancy (herbivory/carnivory, eruciform/ campodeiform). Chara, for example, is known to be avoided by sessile microzoobenthos such as rotifers and bryozoans (Hutchinson, 1993), animals that may act as food for Polycentropodidae and as food competitors to the eruciform species. The new colonizers that completely dominated the caddis fly fauna 2 years after exposure belonged to the species-rich family of Limnephilidae, all of which are eruciform. In contrast to the majority of the preexposure fauna (Polycentropodidae, H. dubius, Phryganeidae), mainly known to be carnivorous and omnivorous (Edington and Hildrew, 1981), these limnephilid species are in general herbivorous and detrivorous (Hickin, 1968; Clegg, 1974). Many of them feed on diatoms and other epiphytic algae, but even Cladophora may act as a major food source (Lepneva, 1970). The amount of Cladophora increased during the second year after exposure, probably partly due to release of nutrients from decaying arthropods. The pronounced Cladophora growth during the spring and summer of the second year may have produced an attractive habitat for dispersing adult limnephilids to settle their eggs in, resulting in captured late-instar larvae the following season. However, it is hard to believe that habitat selection by adult caddis flies was the only reason for finding mainly limnephilids in the exposed mesocosm during the third year, and it does not explain why the former carnivorous and omnivorous species did not return. Neither do the life history cycles explain the 2-year delay between acute fenvalerate effects and reappearance of new caddis flies as a 1-year life cycle is typical of the majority of species. Records of univoltine larvae, originating from eggs laid in the preceding autumn or spring, should therefore have been made in the summer of the year after exposure, but that was not done. The fact that the macrophyte structure changed after expo-

sure and that the original vegetation structure did not return during the whole study period may be the most ecologically significant explanation for the change in the caddis fly species composition. The apparently small numbers of dragonflies (suborder Anisoptera) caught in the mesocosms were probably dependent on the trap construction, which not was optimal for Anisoptera collection. The members of the family Libellulidae are sluggish, live mainly on the sediment surface, and do not naturally find their way to the traps hanging above the sediment. The family Aeshnidae, with well-developed eyes and the ability to quickly escape dangerous situations, probably escaped from the traps at sampling. Besides the dead individuals observed directly after exposure, ‘‘direct’’ effects on dragonflies were not discovered until emergence a year after exposure. The small number of emerging dragonflies in the exposed mesocosm compared with the control indicates a direct toxic effect that resulted in acute mortality, or, if the concentration was sublethal, the effect should be regarded as chronic and, for example, caused by reduced predation efficiency (Woin and Larsson, 1987), which could result in delayed or impaired emergence. Considering the large number of dead aeshnids found directly after the exposure and their low abundance during the whole study in the high-exposure mesocosm, the most plausible explanation for the reduction in emerging odonates, however, was a direct lethal action of fenvalerate that reduced or eliminated the population. The individuals of the genus Sympetrum that emerged in the high-exposure mesocosm were probably not subjected to any fenvalerate toxicity. Members of this genus have a 1-year generation cycle, and they usually reproduce in August—September and under warm conditions, even in October (Corbet, 1962). This means that the emerging individuals originated from eggs laid at least some weeks after the fenvalerate exposure started, when the concentration was low or not detectable. Thus, these larvae were subjected to low or insignificant exposure. The members of the family Aeshnidae, on the other hand, have a 2- to 3-year generation period (Corbet, 1962) and therefore no aeshnid could emerge 1 year after the exposure since all late-instar larvae were probably extinct. The life history cycles of different organisms are therefore of great importance when impact of environmental pollutants is to be interpreted. The damsefly (suborder Zygoptera) abundance dynamics were similar in the exposed mesocosm and in the control. The lower abundance, compared with the control, found in the first year after the exposure may indicate a real population decrease due to fenvalerate toxicity. However, the individuals recorded in the postexposure period (IB) must have survived the highest exposure levels during their larval development. The damselflies, predators on small invertebrates, normally have a 1-year generation cycle in southern Sweden. The adults fly from May to the first half of September and eggs laid during this period hatch after 4 to

EFFECTS OF FENVALERATE ON POND INVERTEBRATES

6 weeks (Corbet, 1962). Consequently, the individuals collected about 4 weeks after exposure cannot have been laid as eggs after exposure, but more probably, due to their size, were laid before exposure in the first part of the summer. The results thus indicate that larval damselflies are less sensitive to fenvalerate than other insects. The small abundances in both the control and the exposed mesocosms during the second year and the conflicting large abundances during the third year in both systems may indicate that external factors (equal in all mesocosms) such as climate played a major role in the population growth regulation of the Zygoptera. Ostracoda. The marked population increase in ostracods that occurred in the exposed mesocosm could have been caused by at least three factors: a modest sensitivity to fenvalerate, an increased amount of food, and a changed competition situation in Year 3. The suggestion of low sensitivity is based on the fact that ostracods, as other arthropods in general, should be sensitive to fenvalerate and therefore should not have demonstrated population growth as soon after exposure as they did. Nevertheless, having a low sensitivity as juveniles and adults, they were not only able to survive the exposure but also able to quickly exploit the new resources in the perturbed and changed ecosystem. An alternative possibility is that the population survived the exposure only as resting eggs (Havel and Talbott, 1995) that hatched when the fenvalerate concentration declined. The increased amount of food, as discussed in the section on Oligochaeta, is suggested to be the main reason for population growth, as ostracods are omnivores feeding mainly on decaying organic matter (Clegg, 1974; Wetzel, 1975; Engelhardt, 1976). However, their population growth was lower in period 1B and Year 2 compared with the extensive increase in Year 3. This depression is probably due to competition from the Oligochaeta that were highly abundant during this period (1B and 2), but it could also be due to sublethal fenvalerate effects acting on reproduction, for example. However, the rapid decrease in fenvalerate concentration does not support this latter explanation. The decline of the oligochaete population during the third year probably caused a changed competition situation favorable to the ostracods. When predators recolonized the system (mainly during the later part of Year 2 and subsequently) the oligochaetes decreased, allowing the ostracods to dominate. They use similar food resources but are less susceptible to predation due to their bivalve shell that covers the body (Clegg, 1974). Mollusca (Gastropoda and Bivalvia). As expected, the mollusks seemed not to be adversely affected by fenvalerate exposure (cf. Tagatz, 1986). The long-term increase in numbers of mollusks in the high-exposure mesocosm was distinct in contrast to the control. The two dominant gastropod species (¸. stagnalis and ¸. peregra) probably

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took advantage of the increasing amount of dead, decaying organisms and used them as food. Most gastropods are herbivorous, whereas ¸ymnaea are omnivorous (Burton, 1991). The increase could also be explained by reduced food competition after the reduction of the Ephemeroptera that feed on epiphytic algae. Epiphytic algae are regarded as an important food source for ¸ymnaea (Bro¨nmark, 1989). Another factor, probably also relevant to the increase in numbers, was the change in the plant species composition that occurred slowly during the second year. The dominant macrophyte species that was established (P. crispus) is suggested to be more favorable and optimal as a food source for lymnaeids than E. canadensis, which dominated in the control mesocosm and, in fresh form, is generally rejected by gastropods (Hutchinson, 1993). Consequently, the increase in gastropods was probably a secondary effect mediated by a combination of increased quality (changed vegetation) and quantity (decaying arthropods, Elodea, and Cladophora, and decreased competition) of accessible food. The increase in Bivalvia, which was most pronounced during the third year in the exposed mesocosm, was probably caused by an improved food supply. Mussels are restricted to feeding on particles in the water and it is therefore likely that they used the suspended detritus and microbial decomposers that increased due to the mass death of arthropods. A temporary increase in phytoplankton during the second year, due to a conceivable release of nutrients from the decaying plant and animal material, may also add to this effect. An increase in turbidity in the exposed mesocosm during the second year (unpublished data) supports the assumption of increased amounts of suspended food particles. Malacostraca. The fenvalerate exposure probably harmed the Asellus population so much that no reproductive recovery was possible during the following 2 years. Fenvalerate has been suggested to be very toxic to crustaceans (Tagatz, 1986) and other synthetic pyrethroids have been found to be highly toxic to crustaceans such as Gammarus (Muirhead-Thomson, 1978) and crayfish (Jolly et al., 1978). As these animals disperse by water, the isolated mesocosms prevented immigration of new animals. Isolated freshwater ecosystems like ponds and pools in agricultural areas are therefore supposed to be more sensitive to pesticide pollution on a long-term basis than aquatic habitats connected to each other through streams and canals. Hirudinea. The sampling technique, which offered efficient swimmers a good opportunity to escape, was not ideal for catching E. octoculata. The abundance of larger (mature) E. octoculata was therefore probably underestimated in relation to immature ones, which constituted the majority in the samples. This could also explain the lack of animals caught in the control during Years 1 and 2, assuming that the level of available food in this mesocosm was too low to

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support a growing population, thus resulting in low survival rates for the immature E. octoculata. The leeches seemed to be insensitive to fenvalerate exposure and tended to increase in abundance in the high-exposure mesocosm during Years 1 and 2. However, the low overall registered population densities (resulting in increasing variance) and the absence of captured animals in the control mesocosm during Years 2 and 3 make reliable interpretation of the observed pattern difficult. Nevertheless, the coherence in the observed dynamics in the highexposure mesocosm seemed, in an ecological sense, logical. Leeches, in general, are known to tolerate a wide range of physical and chemical factors and the most important factors, affecting their distribution and abundance, are the availability and abundance of food (Elliott and Mann, 1979). E. octoculata reproduces annually during the warm season (May to October). This opportunistic reproduction strategy made it possible for the mesocosm population to grow when the conditions became more favorable. E. octoculata also often attains high population densities in organically enriched waters because their prey (oligochaetes and chironomids) are often highly abundant in such environments (Elliott and Mann, 1979). After the fenvalerate exposure, the mesocosm was organically enriched by dead and decaying arthropods and the oligochaete population began to grow. The increase in numbers of E. octoculata during the first year was therefore not a direct response to the organic enrichment, but was most probably an indirect effect of an increase in food organisms, especially oligochaetes. During the following two seasons the abundance of E. octoculata closely followed that of the Oligochaeta, indicating a probable predator—prey relationship between the two species. The appearance of G. complanata in the high-exposure mesocosm during the second year was assumed to be a result of the increase in gastropod abundance. G. complanata is a sanguivorous parasite mainly on mollusks, but also oligochaetes and larger insects (Elliott and Mann, 1979). The population density of G. complanata increased slightly during the third year probably because the gastropods were still abundant and the mesocosm became inhabited by large insects which they were also able to parasitize. However, the assumed causality between the species and gastropod abundance should be regarded with caution because of the small numbers of G. complanata caught and also because the species was not found in the control mesocosm that was inhabited by a robust gastropod community. Arachnida. The increase in water mites (Arachnida: Hydrachenellae) in the exposure mesocosm was probably a secondary effect induced by the fenvalerate exposure. The pronounced increase in numbers in period 2B could be comparable to the increase in oligochaetes and ostracods—a quite massive increase followed by a ‘‘crash.’’ Knowledge of the taxonomy and ecology of water mites is

limited; therefore, it is difficult to draw any reliable conclusions from the observations. However, most adult members of the group are known to be active predators, attacking cladocerans, chironomid larvae, and small oligochaetes (Clegg, 1974). Many of the free-living adults also prefer habitats dominated by filamentous algae such as Cladophora (Hutchinson, 1993), which were abundant in the exposed mesocosm during the second year. The larval stage of most mites is a parasite on other aquatic invertebrates, usually insects (Clegg, 1974). It thus seems difficult to explain the increase in terms of abundant hosts because of the preceding small numbers of insects. It seems more probable that the mites found in the mesocosms parasitized mainly nonarthropod hosts, such as the more abundant gastropods, leeches, and oligochaetes. Integration and Consequences Effects of insecticides on nontarget organisms in different freshwater systems have been widely investigated and reported during the last decade. In general it is accepted that the insecticides used today can temporarily reach acute toxic concentrations in natural water bodies following spray drift and surface runoff from agricultural areas, and that the major effects that could occur in such waters are reductions in the arthropod fauna. This investigation has confirmed, on a short-term scale, that the arthropod fauna is severely affected at concentrations that are possible to encounter in natural freshwater environments. However, investigations within this field have rarely concerned long-term effects: the influence of insecticides on population dynamics resulting in secondary effects and cascading trophic interactions in the exposed communities. This study has added some indications of such insecticide-induced long-term changes that should be regarded as ecotoxicologically important. Some authors (Hill, 1989; Farmer et al., 1995) have drawn conclusions about the potential hazard of pyrethroids in aquatic environments, suggesting that they ‘‘will not constitute a hazard to aquatic ecosystems’’ and the possible effects will only be localized, minor and transient.’’ The present study, however, highlights evidence and indications of pronounced, long-term changes in aquatic ecosystems caused by the pyrethroid fenvalerate, one of today’s widely used ‘‘nonpersistent’’ pesticides, that are of great importance to consider and further investigate in risk and hazard assessment. The results indicate, for instance, that isolated freshwater ecosystems in areas treated with pesticides may be ‘‘permanently’’ changed, taken into account that the fight against the pests usually is repeated annually. Small water bodies and their immediate surroundings are, in certain urban regions, very important refuges for wildlife and other organisms, enabling the maintenance of species and genetic diversity that would otherwise be lost. The probable impoverishment of the community structure in such waters due

EFFECTS OF FENVALERATE ON POND INVERTEBRATES

to repeated inputs of pesticides, followed by secondary changes, may therefore be a threat to the successful maintenance of a species-rich and diverse environment. To improve the hazard and risk assessment processes and to decrease the risk of losing species it is important to increase, improve, and prolong this rare type of long-term study. Interest in effects of nonpersistent pesticides has so far predominantly been focused on direct toxicity to organisms that are directly related to human health and/or activity. The ultimate goal of ecotoxicology, producing information fundamental to protecting species and ecosystems in a sustainable manner, has suffered because of the high costs of performing the necessary objective research (i.e., long-term artificial ecosystem studies). The low cost of this mesocosm study (1—2 man-years of effort for six mesocosms) suggests that the mesocosm approach does not necessarily have to be expensive. Not only the pesticide industry itself, but even academic and government authorities should be able to afford such important studies. Besides the regulatory need to develop reliable test systems for prediction of environmental damage and suitability for environmental protection (requested by the general public, regulatory agencies, and decision makers), the mesocosm approach, used for basic research in ecology, promises to increase understanding of how natural ecosystems function. Without this basic ecological knowledge, humankind will have great problems to restore or protect ecosystems in a sustainable manner.

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CONCLUSIONS

This investigation proposes, with the support of many independent indications, that a relatively short-lived pesticide (fenvalerate), in concentrations that are possible to find in natural farmland ponds, generates effects that result in long-term changes at the ecosystem level. These long-term changes result from direct toxic effects on sensitive animals (arthropods) followed by secondary effects in the community structure due to altered interspecies relationships and differences in life history cycles. The weakness of this study is that the results were based mainly on observations in two separate systems. However, as many of the observed changes confirmed changes observed in other, comparable investigations, the reliability of the results is considered high. The major and most obvious results, apart from the direct lethal effects on arthropods, are summarized in a conceptual way in Fig. 14. Non-arthropod taxa (Oligochaeta, Hirudinea, Mollusca) increased temporarily due to reduced predation, increased food resources and released niche-space. Decomposers, shredders and filter feeders were favoured and dominated the community after exposure. Impoverishment of the community composition lasted for at least 2 years with a slow community and ecosystem recovery. The observed pattern of change is probably not unique for fenvalerate-exposed systems, but may apply in general to most insecticides with low persistence and similar toxicities in comparable ecosystems. The described ecological phenomena and their role in ecosystem sustainability, sensitivity, and reconstruction are

FIG. 14. Conceptualized summary of effects and invertebrate community development in an isolated eutrophic pond after lethal insecticide exposure, based on findings in the present study.

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by far the most important to understand if the goal is to prevent species extermination and ecosystem breakdown. It is therefore important to focus further on modern shortlived (‘‘harmless’’) pesticides and their role in the change and degradation of ecosystems. Order

APPENDIX Order Invertebrate taxa collected from the experimental ponds from May to November during 3 years. Class

Class

Class

Class

Class

Class Class Order

Order

Order

Order

Order

TURBELLARIA Dendrocoelum lacteum Planaria sp. OLIGOCHAETA ¹ubificidae spp. Nais sp. Stylaria lacustris Chaetogaster sp. HIRUDINEA Erpobdella octuculata Glossiphonia complanata ARACHNIDA (Chelicerata) Hydrachnellae spp. Arrenurus sp. MALACOSTRACA (Crustacea) Asellus aquaticus Gammarus sp. OSTRACODA Cypris (Herpetocypris) reptans INSECTA Ephemeroptera Cloeon dipterum Caenis horaria Odonata Zygoptera spp. Aeshnidae spp. Libellulidae spp. Hemiptera Notonecta sp. Corixidae spp. Corixa punctata Cymatia coleoptrata Callicorixa praeusta Hesperocorixa sahlbergi Hesperocorixa linnaei Sigara sp. Sigara nigrolineata Sigara fossarum Sigara limitata Sigara lateralis Sigara dorsalis Sigara falleni Coleoptera Coleoptera spp. Colymbetes ( fuscus) sp. Acilius sulcata Agabus bipustulatus Hydroporus sp. ¸accephilus sp. Haliplus sp. Trichoptera Trichoptera (campodeiform) spp.

Class

Class

Holocentropus sp. Holocentropus dubius ¹richoptera (eruciform) spp. ¸eptocerus sp. Limnephilidae spp. Phryganeidae spp. Lepidoptera Lepidoptera spp. Paraponyx stratiotata Diptera Chironomidae (52,5 mm) Ceratopogonidae spp. Culicidae spp. Anopheles sp. Chaoborus crystallinus ¹ipula sp. Diptera (pupae) spp. GASTROPODA ¸ymnaea stagnalis (53 mm) ¸ymnaea pereger Planorbis corneus Planorbis sp. Bithynia sp. Physa fontinalis Acroloxus lacustris BIVALVIA Sphaeriidae spp. Sphaericum lacustre Pisidium sp.

ACKNOWLEDGMENTS The author thanks the following people for their practical help in the mesocosm construction and sample collection: P. Larsson, G. Ewald, O. Regnell, L. Okla, C. Agrell, A. Go¨ransson, S. Douwes. The author also thanks Professor A. So¨dergren, Dr C. Bro¨nmark, Dr. P. H. Enckell, and Dr. P. Larsson for valuable comments on the manuscript. The work was sponsored by the Swedish Environmental Protection Agency, Crafoordska Stiftelsen, and Gyllenstiernska Krapperupsstiftelsen.

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