Competition in Bembicium auratum (Gastropoda) and its effect on microalgal standing stock in mangrove muds

Summary Field experiments were undertaken on the intertidal gastropod Bembicium auratum, which is one of the dominant organisms in sheltered bays and mangroves in New South Wales. Animals were caged at various densities ranging from natural levels to about four times normal density. Increased density resulted in increased mortality (particularly of juveniles), reduced body weight, and (less conclusively) a decline in growth rate. Chlorophyll levels of the mangrove mud within the cages were monitored as an index of food availability, and increased rapidly in the control cages in the absence of Bembicium, remained steady at normal Bembicium densities, and declined at higher densities. Parallel experiments were conducted on the effects of substratum on Bembicium. One series of animals was allowed access to a hard substratum and the other only to mud. Body weights and survivorship were proportionally lower in animals deprived of a hard substratum. Juveniles were particularly susceptible to a shortage of food and more dependent on a hard substratum than adults. The population structure also suggests a high juvenile mortality but high adult survival and longevity. Bembicium is very abundant low on the shore but numbers decline and body weights increase higher up. Chlorophyll levels and amounts of hard substratum also decline up the shore. Possibly juveniles settle selectively (or survive better) low on the shore where oysters provide a dense hard substratum. Adults are less dependent on the hard substratum and may disperse up the shore, explaining the patterns of abundance and size.As Bembicium limits its own food supply and seemingly is limited by it, predation is unlikely to play an important role in restricting population density.     

Importance of fungi in the diet of Gammarus pulex and Asellus aquaticus

An important component of the interaction between macroinvertebrates and leaf litter in streams in the extent to which consumers can differentiate between undecomposed and decomposing leaves. The detritivores Gammarus pulex and Asellus aquaticus fed preferentially on conditioned rather on unconditioned leaf material. Growth in A. aquaticus was significantly reduced when unconditioned leaves were provided, but in G. pulex no significant effect of conditioning on growth was observed. The capacity of G. pulex to tolerate reductions in food quality seems to be a consequence of a compensatory system in which respiration rates change to compensate for reductions in food quality. In this way a constant growth rate is maintained. Increases in ingestion rates to compensate for low quality food were not observed.     

Importance of fungi in the diet of Gammarus pulex and Asellus aquaticus I: feeding strategies

The importance of fungi in the trophic biology of the freshwater detritivores Gammarus pulex and Asellus aquaticus was investigated. Inspection of leaves used in feeding trials indicated that whereas A. aquaticus scrapes at the leaf surface, G. pulex bites through the leaf material. Both species discriminated between fungal mycelia, fungally colonized and uncolonized leaf material but, although A. aquaticus selectively consumed fungal mycelia, G. pulex fed preferentially on leaf material. Fungi appear to be an important food source for A. aquaticus and selection of food material was positively correlated with fungal biomass. In contrast, for G. pulex, fungi appear to be more important as modifiers of leaf material. However, no significant correlations were found between food preference and any of the leaf modifications measured.     

Comparative ecology of Gammarus pulex (L.) and Asellus aquaticus (L.) I: population dynamics and microdistribution

Gammarus pulex and Asellus aquaticus generally occupy different zones in rivers; the former occurs in upper reaches but is replaced by the latter in lower reaches. Microdistribution and life-history patterns of G. pulex and A. aquaticus in sympatry and allopatry, were analyzed. Both species exhibited similar patterns of microhabitat selection, with larger individuals associated predominantly with large-sized substratum particles, and juveniles with weed. Coexisting populations of G. pulex and A. aquaticus had similar densities and population dynamics. Within each species, differences in population dynamics of allopatric and sympatric populations were observed. Although variation in population dynamics of G. pulex may be explained in terms of competition between the two species, the evidence is weak and equivocal. Differences in the dynamics of the two A. aquaticus populations were possibly a consequence of coal-mine and organic pollution, reducing the survival of offspring in the allopatric population.     

The effect of substratum type on aspects of swimming performance and behaviour in shortnose sturgeon Acipenser brevirostrum

The swimming performance and associated swimming behaviour (i.e. substratum‐skimming, station‐holding and free swimming) were assessed in shortnose sturgeon Acipenser brevirostrum during critical swimming and endurance swimming tests over a rough and a smooth substratum. It was hypothesized that the addition of a rough substratum in the swimming flume may provide a surface for the A. brevirostrum to grip and offer an energetic advantage. Substratum type did not affect the critical swimming performance, but A. brevirostrum consistently performed more bottom behaviours (i.e. substratum‐skimming and station‐holding) while on a smooth substratum. Acipenser brevirostrum had little contact with the rough substratum until the velocity was >1 body length s^?1. Endurance swimming time was significantly lower for A. brevirostrum over the rough bottom at the highest velocity (30 cm s^?1) which may be attributed to the observed increase in free swimming and decrease in bottom behaviours. During endurance swimming, the rough substratum was mainly used at intermediate velocities, suggesting that there may be a stability cost associated with being in contact with the rough substratum at certain velocities.     

Studies on two species of Asellus in Rutland Water

The study comprised a comparison of the ecology of two species of Asellus (A. aquaticus and A. meridianus), and had two main aims. First the energy budget of the population of A. aquaticus, which is one of the most abundant species in the reservoir was assessed. Secondly, the two species appear to be ecologically very similar and it has been assumed by many workers that they are in competition. The study aimed to investigate how these two species might be able to coexist.The population dynamics of both species and the interaction between them is under study, as are their relations with the other benthic animals recorded in the reservoir (see Bullock et al. 1982). The sampling programme to cover variations in depth and substratum is also described by Bullock et al. 1982. A. aquaticus occurs in both arms while A. meridianus exists only in the south arm. A preliminary examination of the available data reveals that the density of A. aquaticus has increased in the south arm while the density of A. meridianus decreased drastically in February 1980. Since then low numbers of A. meridianus have been recorded every month even in the summer (breeding) season. It is noteworthy that simultaneously the number of predators, particularly Helobdella stagnalis, increased in comparison with previous years.It was therefore decided to study the predation on Asellus in the laboratory as well as in the field by using a serological technique. Antisera were produced against each species of Asellus by injections into rabbits of precipitated proteins (antigens). Thus a positive reaction obtained from a homogenised predator when tested with the specific antiserum indicates which species of Asellus had been consumed and the percentage of predators which had fed on Asellus in every sample could be calculated. The results obtained to date reveal that considerable numbers of Asellus spp. are removed from the environment by the predators (Helobdella stagnalis, Erpobdella octoculata, Polycelis tenuis. Dendrocoelum lacteum and Dugesia lugubris).Size class structures of the population of both species were constructed to study their life cycles and to estimate the population production. Comparitive respirometry of the two species was carried out at 4 °C, 10 °C and 16 °C, using a Gilson differential respirometer. This study aimed firstly to assess one of the essential parameters in the energy budget study of A. aquaticus and secondly, since A. aquaticus appears to be more active than A. meridianus, it would be expected that a comparison of their metabolic rate at different temperature would reveal marked differences between two species. It was found that there was no significant difference between the two species at 4 °C and 10 °C, but A. aquaticus had a significantly higher metabolic rate than A. meridianus at 16 °C.Consumption and assimilation of decaying oak leaves and of Cladophora glomerata (both of which are available in the environment) by two species of Asellus are being measured at present in the laboratory at three different temperatures (4 °C, 10 °C and 16 °C), and will contribute to the estimation of energy budgets.     

A Three Years' Study of Life Cycle,Population Dynamics and Production of Asellus aquaticus L. in a Macrophyte Rich Stream

Populations of A. aquaticus were sampled quantitatively in 1979–82 at two localities in the river Suså which has a slope of <1 m km⁻¹ and large variations in macrophyte biomass, discharge and stream velocity. The latter two differed significantly between years. A. aquaticus had (1)−2 life cycles per year and summer and winter cohort production intervals of 85 and 290 days. Populations of A. aquaticus varied between 0 and 28,000 ind. m⁻² with an exponential increase in the spring and an exponential decrease in the autumn-winter. The A. aquaticus rate of decrease varied between 0.62 and 5.61 % d⁻¹ and increased with increasing rate of elimination of the macrophyte biomass Differences between the two localities were due to differences in physical heterogeneity. Production varied between 2.8 and 9.1 g DW m⁻² yr⁻¹ and between 12.9 and 55 g DW m⁻² yr⁻¹ at the two localities and P/B ratios were 7.4–9.9 yr⁻¹. Physical limitations are thought to be most important for the populations of A. aquaticus in the Suså, and the macrophyte biomass played an important role in modifying the physical environment. Differences between streams and lake populations are discussed.     

Balancing food availability and hydrodynamic constraint: phenotypic plasticity and growth in Simulium noelleri blackfly larvae

Organisms through phenotypic plasticity can cope with multiple changed environmental conditions. Theory predicts that animals in streams and rivers should be able to balance demands of the needs to obtain food efficiently and to adjust response to hydrodynamic variability. This study examined effects of variations in food availability and current velocity on the feeding structure and growth rate of Simulium noelleri blackfly larvae. The larvae developed larger labral fans and more rays under slow current and low food regimes than in fast current and high food conditions. In both fast and slow current regimes, growth rates were higher and development periods to the final-instar stage were shorter in high food treatments. The estimated flux rates of food particles through labral fans under high food treatments for both fast and slow current regimes were higher than those under low food treatments. Although both food and current velocity appeared to have selected for flexibility of feeding structure and growth rate, food availability was a more important factor for phenotypic and developmental plasticity than current velocity. The results indicate a strong link between environmental changes in food availability and current velocity, phenotypic plasticity, and growth rate of S. noelleri. This study suggests that plasticity of ecomorphs with macroevolutionary significance may play a role in the early evolutionary stages of blackfly larvae.     

Acute Toxicity for some Evaporating Aromatic Hydrocarbons for Freshwater Snails and Crustaceans

Some typical and more frequent freshwater invertebrates of running waters were studied to examine the influence of styrene, xylene and benzene on their mortality. Snails Amphimelania holandri FÉR. and Lymnaea stagnalis L. and crustacenas Asellus aquaticus L. and Gammarus fossarum KOCH. , were used in the semi-static test. Compounds were added in volume concentrations of 0.005 to 0.4% v/v. For all concentrations LC₅₀ was calculated by probit method, which demonstrated that mortality depends much more on increased concentrations (depending on the initial concentration) than on the length of exposure. Styrene was the most toxic, followed by xylene and then benzene. The species G. fossarum showed marked sensitivity, followed by A. aquaticus, and the species A. holandri and L. stagnalis showed less sensitivity.     

Feeding and growth of Asellus aquaticus (Isopoda) on food items from the littoral of Windermere, including green leaves of Elodea canadensis

SUMMARY. In the laboratory, Asellus aquaticus devoured intact green leaves from growing shoots of the aquatic macrophyte Elodea canadensis. In four collections of A. aquaticus on Elodea in a lake (Windermere), c. 20% of the specimens contained in their guts fragments of green Elodea leaves; this material and pieces of oak (Quercus) were identified from characteristic leaf hairs. Some specimens had also eaten the filamentous alga Oedogonium. Fluorescence microscopy is a useful aid for screening invertebrates that may have eaten living plant tissues. Immature A. aquaticus, with an initial mean body length of c. 3 mm, wet weight c. 1 mg, were grown through sexual maturity over a 49-day period at 15°C in a series of twenty-two experiments (six to twelve isolated specimens in each experiment) comparing growth rates on different foods, including instances where no food was given. Animals were fed on a variety of items collected from the littoral of Windermere, plus some laboratory cultures of algae and bacteria. The highest mean specific growth rate (5.8% day^?1) was obtained on young Elodea leaves mechanically shaken to remove epiphytes. Other diets yielding fast growth rates (3.7–5.3% day^?1) were young growing leaves of Elodea with few epiphytes and older green and brown living leaves covered with a thick growth of epiphytic algae, epiphytic algae removed from Elodea, plastic imitation Elodea immersed in the lake until covered with attached algae, epilithic algae on stones, Oedogonium, and decaying oak leaves. Slower growth (1.3–2.2% day^?1) and poorer survival was obtained on the following: a pure culture of the bacterium Sphaerotilus natans; cultured bacteria from lakewater; the filamentous algae Cladophora and Stigeoclonium both with and without epiphytes; faecal matter from Asellus; freshly killed Asellus; lake sediment. Some growth (mean = 0.7% day^?1) and 50% survival for 21 days occurred in ‘starved’ animals kept in filtered, sterilized lakewater. Better survival and slightly faster growth (1.0–1.5% day^?1) occurred in ‘starved’ animals kept in filtered and unfiltered lakewater. Growth of A. aquaticus was also experimentally determined from birth in animals fed on young green Elodea leaves and on decaying oak leaves. On both diets, growth was curvilinear and approximately exponential from birth to sexual maturity reached at c. 2mg wet weight in 46–60 days at 15°C. In older specimens the relative growth rate gradually fell over a period of 50 days, representing a more linear phase of growth during sexual maturity.     

The predation on Asellus aquaticus (L.) by perch,Perca fluviatilis (L.), in a small forest lake

Asellus aquaticus (L.) was the most important benthic food item for perch, Perca fluviatilis (L.), in a small, extremely humic forest lake in southern Finland. The proportion of A. aquaticus in the diet of perch varied according to the former's availability, which, in turn, depended on its life cycle. Perch 11.0–12.9 cm (total length) most frequently fed on A. aquaticus; smaller and larger perch fed more frequently on crustacean zooplankton and aquatic insect larvae, respectively. The high density of the perch population, the importance of A. aquaticus in the benthos of the study lake and the general high activity level of the prey resulted in a high predation (0.1–1.8% per day). The population of A. aquaticus was also limited by the scarcity of macrophytic vegetation and the small area of oxygenated littoral in the lake.     

Respiration of Senecio Shoots: Inhibition during Photosynthesis, Resistance to Cyanide and Relation to Growth and Maintenance

Respiration and dry matter producation were measured in shoots of senecia aquaticus Hill, which is flood tolerant and in shoots of S. jacobaea L., which is flood- sensitive. Both species were grown in culture solutions of high and of low oxygen concentration Growth of food of S. jacobaca was unaffected by a low oxygen supply bur growth of S. jacobaca was severly hampered by a low oxygen concentration in the root medium. Kinetic data about the rate of apparent photosynthesis at low oxygen conetration and different carbon dioxide concentrations indicated that at light saturation respiration was strongly repressed during photosynthesis. Shoot growth respiration, i.e. the amount of carbon dioxide produced for synthesis of shoot dry, matter appeared to be absent on S. jacobaea and to be very low (13.mg CO₂/g dry shoots) in S. aquaticus. In comparison with values prepiration rate was 2.8. 2.0. 1.5 and 1.3mg CO₂/h.g dry shoots in aerobically and anaerobically growth S. jacobaea and in aerobically and anaerobically growth S. aquabaea respectively. These values were also low in comprision with values previously found for roots of the same species. Shoot dark respiration on S. aquaticus was inbihitedd by a com bination on CN and salicylhydroxamic acid (SHAM), but not by application on one of these inhibitors alone. It was therefore concluded that an alternative oxidative pathway was present but not active in shoots of S. aquaticus. In the absence of inhibited of the cylochorome pathway. The low value of growth respiration and maintenance respiration rate in the shoots as compared with those in the roots of the investigated Sencio species are discussed in relation to the activity of the alternative oxidative pathway and to the possibilbity of a direct supply of ATP by photosynthesis intead of respiratory meta bolism.     

Disturbance history influences the distribution of stream invertebrates by altering microhabitat parameters: a field experiment

1. We investigated the effects of local disturbance history and several biotic and abiotic habitat parameters on the microdistribution of benthic invertebrates after an experimental disturbance in a flood‐prone German stream. 2. Bed movement patterns during a moderate flood were simulated by scouring and filling stream bed patches (area 0.49 m²) to a depth of 15–20 cm. Invertebrates were investigated using ceramic tiles as standardized substrata. After 1, 8, 22, 29, 36 and 50 days, we sampled one tile from each of 16 replicates of three bed stability treatments (scour, fill and stable controls). For each tile, we also determined water depth, near‐bed current velocity, the grain size of the substratum beneath the tile, epilithic algal biomass and standing stock of particulate organic matter (POM). 3. Shortly after disturbance, total invertebrate density, taxon richness and density of the common taxa Baetis spp. and Chironomidae were highest in stable patches. Several weeks after disturbance, by contrast, Baetis spp. and Hydropsychidae were most common in fill and Leuctra spp. in scour patches. The black fly Simulium spp. was most abundant in fill patches from the first day onwards. Community evenness was highest in scour patches during the entire study. 4. Local disturbance history also influenced algal biomass and POM standing stock at the beginning of the experiment, and water depth, current velocity and substratum grain size throughout the experiment. Scouring mainly exposed finer substrata and caused local depressions in the stream bed characterized by slower near‐bed current velocity. Algal biomass was higher in stable and scour patches and POM was highest in scour patches. In turn, all five common invertebrate taxa were frequently correlated with one or two of these habitat parameters. 5. Our results suggest that several ‘direct’ initial effects of local disturbance history on the invertebrates were subsequently replaced by ‘indirect’ effects of disturbance history (via disturbance‐induced changes in habitat parameters such as current velocity or food).     

Efficiency of Root Respiration of a Flood-Tolerant and a Flood-Intolerant Senecio Species as Affected by Low Oxygen Tension

A method was described to determine root growth respiration and root maintenance respiration rate of plants, grown in culture solutions of high and low oxygen concentrations, during linear growth. Root growth respiration of aerobically grown plants was three to four times lower in the flood-intolerant Senecio jacobaea L. than in the flood-tolerant species. Senecio aquaticus Hill. Root growth respiration of Senecio aquaticus Hill decreased by a factor two to three upon transplantation to a culture solution of low oxygen tension. The difference in root growth respiration between aerobically and anaerobically grown Senecio aquaticus Hill was ascribed to the presence of a highly active non-phosphorylating oxidase.     

Crayfish in lakes and streams: individual and population responses to predation, productivity and substratum availability

1. In a correlative study, we investigated the relative importance of fish predation, refuge availability and resource supply in determining the abundance and size distributions of the introduced and omnivorous signal crayfish (Pacifastacus leniusculus) in lakes and streams. Moreover, the biomass and food selection of predatory fish was estimated in each habitat type and stable isotopes of carbon and nitrogen were measured in perch (Perca fluviatilis), the dominant predator in the lakes, and in its potential food sources (crayfish, juvenile roach and isopods). 2. In lakes, crayfish were the most frequent prey in large perch (46%), followed by other macroinvertebrates (26%, including the isopod Asellus aquaticus) and small fish (25%). Crayfish and fish dominated the gut contents of large perch with respect to biomass. Nitrogen signatures showed that perch were one trophic level above crayfish (approx. 3.4‰) and a two‐source mixing model using nitrogen isotope values indicated that crayfish (81%) contributed significantly more to perch isotope values than did juvenile roach (19%). A positive correlation was found between the abundance of crayfish and the biomass of large perch. Crayfish abundance in lakes was also positively correlated with the proportion of cobbles in the littoral zone. Lake productivity (chlorophyll a) was positively correlated with crayfish size, but not with crayfish abundance. 3. In streams, brown trout (Salmo trutta) were the most abundant predatory fish. Gut contents of large trout in a forested stream showed that terrestrial insects were the most frequently found prey (60%), followed by small crayfish (27%) and isopods (27%). In contrast to lakes, the relative abundance of crayfish was negatively correlated with the total biomass of predatory fish and with total biomass of trout. However, abundance of crayfish at sites with a low biomass of predatory fish varied considerably and was related to substratum grain size, with fewer crayfish being caught when the substratum was sandy or dominated by large boulders. The mean size of crayfish was greater at stream sites with a high standing stock of periphyton, but neither predator biomass nor substratum grain size was correlated with crayfish size. 4. Our results suggest that bottom‐up processes influence crayfish size in lakes and streams independent of predator biomass and substratum availability. However, bottom‐up processes do not influence crayfish abundance. Instead, substratum availability (lakes) and interactions between predation and substratum grain size (streams) need to be considered in order to predict crayfish abundance.     

Successful methods for transplanting fragments of Acropora formosa and Acropora hyacinthus

In order to establish a successful method for the transplantation of branching and tabular coral fragments, we tested the effects of orientations of attachment, seasons of transplantation, and size of fragments on survival, growth, and spawning using Acropora formosa and A. hyacinthus. Vertically attached, large-sized fragments of A. formosa showed 98–100% survival rate after 18 months. The fragments transplanted in August exhibited better survival than those transplanted in November. The larger fragments had the higher percentage of spawning. The fragments that spawned had lower growth rate, while those resorbed the oocytes carried at the time of transplantation showed higher growth rate, suggesting the trade-off between growth and reproduction. Half of the fragments spawned 1 month earlier than the donor colonies. Only the vertically attached fragments of A. hyacinthus fused to the substratum, and those transplanted in February showed 100% survival rate after 14 months, indicating that this species is well suited for transplantation.     

Biofilm development on leaf and wood surfaces in a boreal river

SUMMARY 1. Biofilms are organic layers that develop on submerged surfaces. They are composed of micro-organisms, exoenzymes, and detritus particles enclosed within a gelatinous matrix. While much is known about mineral surface biofilms, those developing on organic surfaces have not been extensively studied. We examined the influences of current velocity and substratum composition on biofilm development in a fourth-order North American boreal river. 2. Arrays of white birch ice-cream sticks and sugar maple leaves were placed at fast and slow current sites. Samples were collected periodically, analysed for mass loss, and assayed for microbial biomass (ATP, ergosterol, chlorophyll a) and exoenzyme activity associated with lignocellulose degradation (exo- and endocellulase, β-glucosidase, phenol oxidase, peroxidase). 3. Biofilms developed rapidly on both surfaces. On leaves, biomass peaked within 30 days of exposure. On wood, ATP and chlorophyll a concentrations peaked within 30–70 days, whereas ergosterol increased throughout the study (161 days). On leaves, current velocity had little influence on biofilm development, although breakdown rates were greater at the fast flow site. On wood, ATP and chlorophyll a concentrations were greater at the fast flow site, whereas ergosterol concentrations and breakdown rates were similar at both sites. Microbial biomass was consistently greater on wood than leaves, Exoenzyme activity developed rapidly on both surfaces; current velocity had little influence on activity. Except for β-glucosidase, activities were greater on wood than leaves. 4. Our results suggest that fungi are an important structuring element of organic surface biofilms and the physical stability of the substratum strongly influences biofilm development. Leaf surfaces are susceptible to softening and fragmentation, truncating biofilm development. In contrast, abrasion of wood surfaces removes senescent material exposing fresh substratum for colonization. Thus, wood surfaces with their greater physical stability, permit the development of more extensive biofilms. Wood surfaces may represent an overlooked but important site of metabolic activity in streams.     

Is foraging innovation lost following colonization of a less variable environment? A case study in surface‐ vs. cave‐dwelling Asellus aquaticus

Behavioral innovation is a key process for successful colonization of new habitat types. However, it is costly due to the necessary cognitive and neural demands and typically connected to ecological generalism. Therefore, loss of behavioral innovativeness is predicted following colonization of new, simple, and invariable environments. We tested this prediction by studying foraging innovativeness in the freshwater isopod Asellus aquaticus. We sampled its populations along the route of colonizing a thermokarstic water‐filled cave (simple, stable habitat with only bacterial mats as food) from surface habitats (variable environment, wide variety of food). The studied cave population separated from the surface populations at least 60,000 years ago. Animals were tested both with familiar and novel food types (cave food: bacterial mats; surface food: decaying leaves). Irrespective of food type, cave individuals were more likely to feed than surface individuals. Further, animals from all populations fed longer on leaves than on bacteria, even though leaves were novel for the cave animals. Our results support that cave A. aquaticus did not lose the ability to use the ancestral (surface) food type after adapting to a simple, stable, and highly specialized habitat.     

Effects of territorial damselfish on an algal-dominated coastal coral reef

Territorial damselfish are important herbivores on coral reefs because they can occupy a large proportion of the substratum and modify the benthic community to promote the cover of food algae. However, on coastal coral reefs damselfish occupy habitats that are often dominated by unpalatable macroalgae. The aim of this study was to examine whether damselfish can maintain distinctive algal assemblages on a coastal reef that is seasonally dominated by Sargassum (Magnetic Island, Great Barrier Reef). Here, three abundant species (Pomacentrus tripunctatus, P. wardi and Stegastes apicalis) occupied up to 60% of the reef substrata. All three species promoted the abundance of food algae in their territories. The magnitudes of the effects varied among reef zones, but patterns were relatively stable over time. Damselfish appear to readily co-exist with large unpalatable macroalgae as they can use it as a substratum for promoting the growth of palatable epiphytes. Damselfish territories represent patches of increased epiphyte load on macroalgae, decreased sediment cover, and enhanced cover of palatable algal turf.     

Factors affecting the occurrence of Dendrocoelum lacteum (Turbellaria) in Swedish lakes

By discriminant analysis on data from 115 lakes in South Sweden it was shown that the distribution and abundance of the triclad Dendrocoelum lacteum was predominantly governed by the density of the isopod Asellus aquaticus, its main food. The occurrence of the latter was mainly correlated with the nutrient richness of the lakes, most probably via presence of appropriate detritus, chiefly allochtonous material. As opposed to British oligotrophic lakes, most Swedish nutrient-poor lakes are surrounded by deciduous vegetation, which indirectly promotes the existence of A. aquaticus and consequently D. lacteum. Presence of mat-forming plants generally seemed to depress triclad density, whereas smaller stones promoted their occurrence, at least in nutrient-rich lakes.