C. Davids

Chronic toxicity of cadmium to Chironomus riparius (Diptera: Chironomidae) at different food levels.

The interacting effects of cadmium toxicity and food limitation on the midge, Chironomus riparius, were studied during chronic exposure in laboratory experiments. If the food was supplied ad libitum, both larval developmental time and mortality of the larvae were negatively affected by cadmium concentrations of 2.0–16.2 μg/L. The number of eggs deposited per female and the mean life span of the imagines were not affected by cadmium. Integration of these separate effects into a population growth rate showed a clear reduction with increasing cadmium concentrations. Food limitation of unexposed larvae at high population density reduced fitness, judged on all parameters studied and consequently reduced the population growth rate (up to 85%).The effects on larvae, which were exposed to both cadmium and food limitation, differed considerably from the response to the individual stress factors. Exposure to cadmium increased mortality among food-limited first and second instar larvae. Consequently, the amount of food available for each surviving larva increased. At the two lowest concentrations studied (2.0 and 5.6 μg Cd/L), these indirect positive effects of cadmium overruled the direct negative effects and caused an increase of the fitness of the food-limited exposed larvae compared to the food-limited, unexposed controls. At a concentration of 16.2 μg Cd/L, the negative effects of cadmium on food-limited midges balanced the positive effects of reduced food limitation. At this concentration, the population growth rate did not differ significantly from the food-limited control any more. It is concluded that the indirect positive effects of cadmium on food limitation could eliminate negative, direct effects of low cadmium concentrations on food-limited chironomid populations.

Short-term effects of metals on the filtration rate of the zebra mussel Dreissena polymorpha.

In order to study the short-term ecotoxicity of metals to the freshwater mussel Dreissena polymorpha, the effects of Cu, Zn and Cd on the filtration rate of this mussel were determined in laboratory experiments. Filtration rate was chosen as the endpoint, because it is a sensitive sublethal parameter compared to mortality and it is an important parameter given the ecological role D. polymorpha fulfills. The filtration rate was calculated from the decrease in algal concentration, fed to mussels in aquaria, containing different metal concentrations. The EC50 for Cu (41 microg litre(-1)) was lower than for Cd (388 microg litre(-1)) and Zn (1350 microg litre(-1)). The NOEC(accumulation) for the essential metal Zn was higher than for the essential metal Cu. Cadmium, a non-essential metal, was accumulated at all elevated water concentrations, so the NOEC(accumulation) was the concentration in the control water (<0.2 microg litre(-1)). All (no) effect concentrations found in this study were above the quality criteria set for metal concentrations in Dutch surface water, suggesting that the zebra mussel is sufficiently protected by these quality criteria.

Chronic ecotoxicity of copper and cadmium to the zebra mussel Dreissena polymorpha

In order to evaluate ecological consequences of the long-term presence of metals in aquatic ecosystems, we investigated the filtration rate and survival of zebra mussels (Dreissena polymorpha) during chronic exposure to Cu and Cd. The filtration rate was measured once a week in laboratory experiments lasting 9–11 weeks. The lowest Cu concentration tested (13 μg/L) did not affect the filtration rate and survival of D. polymorpha, but the lowest Cd concentration (9 μg/L) did affect the filtration rate, but had no effect on survival. The EC50 for Cd decreased markedly from 388 μg/L to 27 μg/L when the exposure time was lengthened from 48 hours to 10 weeks. The largest decrease in EC50 for Cd was observed during the first week of exposure. In contrast, the EC50 for Cu did not decrease with increasing exposure time (chronic EC50: 43 μg/L). Since the chronic LC50 for Cd was 130 μg/L, the filtration rate appeared to be a far more sensitive endpoint for ecotoxicological laboratory experiments than mortality. D. polymorpha was capable of regulating the body concentration of the essential metal Cu at low concentrations in the water (13 μg/L). Cd was accumulated at every Cd concentration in the water, suggesting that Cd could not be regulated by D. polymorpha. It is concluded that the relation between short-term and long-term ecotoxicity was different for each metal and could not be predicted from the results of the short-term experiments.

The relations between mites of the genus Unionicola and the mussels Anodonta and Unio

The locations in mussels of four species of watermites, Unionicola aculeata, U. bonzi, U. intermedia and U. ypsilophora and their degrees of parasitism have been compared. Unionicola aculeata and U. bonzi do not live parasitic and need Anodonta anatina and Unio pictorum respectively as a shelter for their transformation stages. Unionicola intermedia and U. ypsilophora live parasitic on the gills of Anodonta anatina and A. cygnea respectively. Every female of Unionicola ypsilophora has a territory in the mussel, as contrasted with U. intermedia, many females of which are found together. The consequences for the population density are discussed.

The effect of the parasite Phyllodistomum macrocotyle (Trematoda) on heavy metal concentrations in the freshwater mussel Dreissena polymorpha

The freshwater mussel Dreissena polymorpha can be parasitized by the digenetic trematode Phyllodistomum macrocotyle, which forms sporocysts in the gill tissue. The gill tissue of the mussel is seriously damaged by the parasite and infested animals loose about one-third of their total dry weight. Infested and non-infested mussels were analysed for the heavy metals Cu, Zn, Cd and Pb. In a number of infested and non-infested animals the gills were separated from the rest of the soft tissues. Concentrations of Cu, Zn, Cd and Pb in total soft tissues and in the gills were higher in parasitized than in non-parasitized animals. In soft tissues without gills only Cd and Pb concentrations were higher in infested than in non-infested animals.

Population dynamic aspects of chironomid larvae of the littoral zone of Lake Maarsseveen I

Population dynamics of chironomid larvae in the littoral zone of Lake Maarsseveen I were investigated to a depth of 6 m. No essential differences in the composition and density between various sampling stations in the shallow sand flats were found. Two species,Cladotanytarsus mancus andStictochironomus sticticus, were found to be the main components of the chironomid population to a depth of 4 m. The density ofCladotanytarsus, which occurred predominantly to a depth of 1 m, increased from June to a maximum in December. In May and June, the entireCladotanytarsus population emerged.Stictochironomus had its peak of abundance in June. From this cohort, a part, differing in size for each of the various years, emerged in August. The remainder of this population, together with the offspring of the August adults, emerged in May. The size of the August midges, and the number of its eggs was smaller than that of the May midges. At depths greater than 4 m,Tanytarsus bathophillus was found to be the dominant chironomid.In summer the abundance of the small (second and third instars)Stictochironomus was primarily affected by water mite predation. In autumn and winter,Cladotanytarsus was the main food source for water mites. In autumn, whenStictochironomus larvae have molted into the fourth instar, predation by bream decreased the density of these larvae. In winter, the impact of this fish is negligible.

Food and Feeding Strategies of Water Mites of the Genus Hygrobates and the Impact of Their Predation On the Larval Population of the Chironomid Cladotanytarsus Mancus (Walker) in Lake Maarsseveen

The role of several aquatic animals, as food for the water mites Hygrobates nigromaculatus and H. trigonicus, was examined. Both water mite species are able to enter the sediments and prey on different larval stages of chironomids. H. nigromaculatus fed on both cladocerans and chironomids, while H. trigonicus preyed only upon chironomid larvae. H. trigonicus burrowed in the sediment independent of the presence of prey. In H. nigromaculatus on the contrary, the burrowing activity entirely disappeared in vials with a high number of Daphnia. Prey vulnerability is significantly increased in experiments without sediment. In a temperature range from 5 to 20°C, prey consumption rates increased about five times. The density of the chironomid Cladotanytarsus mancus declined by about 50% (27,000 to 13,000 per m 2 ) in winter. Hygrobates (about 1000 per m 2 ) was found to be the most important predator, despite the fact that Cladotanytarsus larvae inhabit tubes constructed of sand grains, which inhibits predation. Due to their small size, benthivorous fish will not feed on these larvae. Virtually, no other predators of these chironomids were present. The numerical decline of the chironomid closely matched the population food requirements of the water mite Hygrobates between November and May. It is concluded that the mite is almost entirely responsible for this decline. Cladotanytarsus is an indispensable food source for the mite during winter and spring, and is a source of energy when the eggs of the new generation are formed.

Habitat differentiation and feeding strategies in water mites in Lake Maarsseveen I

SummaryThe habitat preference as regards depth and vegetation areas is discussed for a number ofPiona species.P. conglobata mainly walks over the water plants and undertakes swimming excursions of short duration from there.Arrenurus crassicaudatus is a bottom dweller.This behaviour is related to the searching activities for preys. The main food ofA. crassicaudatus consists of ostracods.P. conglobata prefersDaphnia aboveCyclops, while forUnionicola crassipes the reverse holds true.P. conglobata andU. crassipes do not recognize ostracods as possible preys. The relative prey size greatly influences the rate of catching success.The feeding strategies (catching behaviour, prey preferences and handling time) of these three species are discussed. Differences in these strategies are also considered in view of species diversity.

TEMPORAL AND SPATIAL PATTERNS OF WATER MITES IN LAKE MAARSSEVEEN I

The paper analyses the distribution and seasonal changes in numbers of the water mite fauna of Lake Maarsseveen I. Water mites were collected from 1977 until 1986 by dipnet, underwater light trap, hydraulic lift sampler and by frame net. Fifty-two species belonging to the Hydrachnellae and one to the Halacaridae were sampled. The faunal picture obtained is characteristic of mesotrophic to eutrophic lakes.Arrenurus nobilis. Piona paucipora andP. imminuta can be characterised as lake species. The dominant species in the littoral sand flat areHygrobates nigromaculatus andH. trigonicus, at a depth of 0.5–1 m, andMideopsis orbicularis at depths of 2–7 m.Limnesia maculata is also very common on depths between 2 and 7 m. In early summer, however, they prefer shallow water up to 1 m depth.Some species likeHygrobates nigromaculatus andH. trigonicus were not captured by the underwater light trap. The hydraulic lift sampler gave underestimations of the densisty of certain speciese.g. Hygrobates spp. andLimnesia maculata. Unionicola spp. even can avoid this sampler. Sampling by frame net gave the most realistic data.The highest number of water mites was observed in the littoral sand flat, generally up to 1000 ind. m−2. The genusHygrobates was represented with an average of 800 adult ind. m−2 throughout the year, and appeared to be the main invertebrate predator in this habitat.

Ecotoxicity of lead to the zebra musselDreissena Polymorpha, Pallas

The effect of lead on the filtration rate of the zebra musselDreissena polymorpha was investigated, together with the accumulation of Pb in the soft tissues of the mussels. The NOEC-filtration was 116 μg.l−1 (0,56 μmol.l−1) and the EC50-filtration was 370 μg.l−1 (1.79 μmol.l−1). The NOEC-accumulation was the concentration found in the control water (1.4μg.l−1). These experiments show that the EC50-filtration for Pb is similar to that for Cd, higher than that for Cu and lower than that for Zn. The water quality criteria for lead allow 25 μg Pb.l−1 in surface water. This will not cause short-term effects. Long-term effects may, however, occur, since an accumulation of Pb as low as 16 μg.l−1 was recorded in this study.