Benthic nematodes in acidified lakes: case of a neglected grazer

Abundance and composition of nematode fauna were examined in the benthic microbial mats and upper sediment layer of the littoral of acidified (pH 4.6 to 5.7) lakes. Nematodes constituted from 58% to 90% of all the invertebrates present (excluding protozoans and rotifers). In the examined material, the majority of nematodes was represented by 3 taxa of which Ironus was found specifically associated with the mats. It was concluded that the persistence of benthic mats may be linked to the metabolic activity of the associated nematode fauna.     

Feeding of tropical freshwater fishes: seasonality in resource availability and resource use

Summary Food resources in the environment and in the diets of small fish inhabiting two water bodies in a tropical savanna were studied during both wet and dry seasons. During the wet season (high water, abundant food) most fish species in both habitats fed predominantly on vegetation-dwelling invertebrates. Most fish species switched to alternative foods (algae and detritus) following the drastic decline in invertebrate food available towards the end of the dry season. In one habitat, this change in diet was accompanied by an increase in the volume of food intake. In the second habitat, only two larger species foraged intensively, while smaller species showed low food intake or almost ceased feeding. These differences may be explained by the high risk of predation for small fish in the second habitat. Dietary overlaps among fish species were high at the end of the dry season and moderate in the wet season. However, critical analysis of such factors as food abundance, the size and number of shared prey, and diet breadth showed that all significant overlaps were ecologically unimportant i.e. there was only weak competition for food.     

Relationships between food and cellulase activity in freshwater fishes

The food and cellulase activity of five cyprinid and one salmonid species were examined. All Cyprinidae species showed cellulase activity. No activity was found in pelagic Coregonus albula (Salmonidae). Cellulase activity showed a positive correlation with the amount of highly processed plant detritus in the gut. No correlation was found between the activity level and the amount of fresh fragments of macrophytes and filamentous algae.     

Importance of predation in regulating density of meio- and macrofauna in seasonal tropical waters

Toward the end of the low water period in two seasonal tropical water bodies, we observed drastic declines in densities of dominant groups of macrofauna (Chironomidae, Oligochaeta and Ostracoda). This, however, was not accompanied by reductions in the densities of meiofauna (Nematoda, small (< 3 mm) Chironomidae and Oligochaeta). The decline of macrofauna was correlated with growing numbers of large, mobile, carnivorous invertebrates (Coleoptera, Heteroptera, Decapoda: Macrobrachium) and small, gape-limited fish, which emigrated from drying habitats and concentrated in places that predation pressure has been directed mainly towards macrofauna. Alternative hypotheses are discussed.     

Size-selective predation by roach (Rutilus rutilus) on zebra mussel (Dreissena polymorpha): field stuides

Summary Studies of predation by roach (Rutilus rutilus) on zebra mussel (Dreissena polymorpha) in a large, eutrophic lake showed that there was a clearly marked size threshold ( 160 mm SL) above which roach began to feed on mussels. Roach preying on various sizes of mussels selected them in proportions different from their abundance and accessibility in the habitat. The mean size of mussels ingested by roach of 220 mm and larger, which fed predominantly on Dreissena, closely followed the pattern expected for a constant ratio of mean prey size to mean predator mouth size = 0.59. To explain the size selection we applied an optimal foraging approach, based on the ability of different-sized fish to crush (cost) mussels of different sizes, and hence crushing resistance, and energy contents (benefit). We found that fish smaller than 160 mm, which showed no inclination to eat Dreissena, would only be able to take small mussels with a very high cost/benefit ratio. The real switch to Dreissena would be expected in fish of 230–240 mm that could take most of their prey from highly profitable, numerous, and easily accessible size classes while keeping the mean prey size at the optimal level relative to mean predator mouth size.     

Chemical Composition of Lipids from Peat-Forming Leafy Mosses of Eutrophic Swamps of Western Siberia and Altai

The molecular composition of lipids in three samples of leafy mosses (Aulacomnium palustre, Warnstorfia fluitans, and Calliergon giganteum) has been determined. The revealed acyclic compounds included normal and isoprenoid alkanes, isoprenoid alkenes, normal and isoprenoid ketones, carboxylic acids and their esters, alcohols, and aldehydes. Among cyclic compounds, bi-, tri- and tetracyclic polycycloaromatic hydrocarbons (PAHs), bicyclic and pentacyclic terpenoids, steroids and tocopherols have been observed. The identified organic compounds consisted mainly of carbocyclic acids and n-alkanes with the prevalence of C₂₇ homologues. A. palustre is characterized by a reduced content of isoprenoid compounds, alcohols, and ketones, while the content of unsaturated acids, pentacyclic terpenoids, and aldehydes is rather heightened. A. palustre differs from W. fluitans and C. giganteum in the steroid composition and contains eremophylene, a sesquiterpenoid, which is absent in the mosses of the family Amblystegiaceae. Compared to C. giganteum, W. fluitans has a higher content of lycopadiene, carboxylic acids, n-alkanes, phyt-2-ene, aldehydes, esters, squalene, diploptene, α-tocopherol, and triphenyl phosphates.     

Risk of predation and feeding rate in tropical freshwater fishes: field evidence

Summary In a previous study (A. Prejs and K. Prejs in press) we found that at the end of the dry season small fish species trapped together with piscivorous fish in small, shallow pool varied in their level of food intake. The two smallest of these species fed little, whereas larger species fed at high rate. By examining the species and size composition of the diet and gape limitations of predatory species, I found that the decreased feeding rate of small fish was in response to a high risk of predation by gape-limited predators. The reduction of feeding rate was very substantial when compared to that of the same species in predator free area.     

Nematodes as a possible cause of rhizome damage in three species of Potamogeton

Tissue damage, seen as differently coloured necrotic patches on rhizomes of three Potamogeton species, was found to result from nematode feeding.A high density of plant-feeding Hirschmanniella gracilis was found in the underground parts of Potamogeton pectinatus.The abundance and composition of the nematode aggregations seem to be related to the morphology of Potamogeton rhizomes.     

Hypolimnetic anoxia hampers top–down food-web manipulation in a eutrophic lake

SUMMARY 1. A biomanipulation experiment was carried out in a small (10 ha), but relatively deep (17 m) and highly eutrophic lake in northern Poland. The lake had been stocked in 1996, 1997 and 1998 with a variety of piscivorous fish (pike, catfish, trout and pikeperch), in order to reduce numbers of cyprinid planktivores.
2. Piscivore stocking was associated with a threefold decrease in the offshore fish density (night echosounding). Despite this reduction, the large planktonic cladoceran, Daphnia hyalina , remained scarce, whereas the density of small-sized zooplankton increased greatly.
3. The lack of demographic response in D. hyalina was probably due to the anoxia in the hypolimnetic refuge of this vertically migrating species. The anoxic hypolimnion, below 3–4 m depth, was inhabited day and night by numerous Chaoborus flavicans larvae.
4. Changes in zooplankton were associated with shifts in the taxonomic composition (from single-cell green algae to filamentous cyanobacteria), size structure (from nano- to net phytoplankton) and seasonal dynamics of phytoplankton, but not in the average biomass of planktonic algae. A clear-water phase, which was absent in the prestocking years, developed in spring, with Secchi depth reaching 2.5 m, a value which had never been recorded in the 20 years preceding the biomanipulation. In general, the lake's status was switched from hypertrophic to eutrophic.
5. Deteriorating food conditions, resulting from qualitative changes in the phytoplankton community, combined with predation pressure by the remaining fish and Chaoborus larvae were associated with the ultimate elimination of D. hyalina from the lake.