The impact of vertebrate and invertebrate predators on a stream benthic community

I assessed the impact of both vertebrate and invertebrate predators on a lotic benthic community in a 1-month-long experiment, using enclosures containing cobble/gravel bottoms, with large-mesh netting that allowed invertebrates to drift freely. Brown trout (Salmo trutta) and leeches (Erpobdella octoculata) were used as predators and four treatments were tested: a predator-free control, leeches only, trout only, and leeches and trout together. A density of 26.7 leeches/m² (20 leeches/enclosure) and 1.3 trout/m² (one trout per enclosure) was stocked into the enclosures. The total biomass of invertebrate prey was significantly lower in the trout and trout plus leech treatments than in the leech and control treatments, which were due to strong negative effects of trout on Gammarus. On the individual prey taxon level, both trout and leeches affected the abundance of Asellus , Baetis and Ephemerella, whereas the abundance of Gammarus was only affected by trout, and the abundance of Orthocladiinae and Limnephilidae was only affected by leeches. In the treatment with trout and leeches together, the abundance of Ephemerella and Baetis was higher than when trout or leeches were alone, which was probably due to predator interactions. Leeches and trout had no effects on prey immigration but did affect per capita emigration rates. Both trout and leeches indirectly increased periphyton biomass in enclosures, probably due to their strong effects on grazers. Both trout and leeches were size-selective predators, with trout selecting large prey, and leeches selecting small prey. Size-selective predation by trout and leeches affected the size structure of five commonly consumed prey taxa. Trout produced prey populations of small sizes owing to consumption of large prey as well as increased emigration out of enclosures by these large prey. Leech predation produced prey assemblages of larger size owing to consumption and increased emigration of small prey. These results suggest that in lotic habits, predatory invertebrates can be as strong interactors as vertebrate predators. Received: 23 June 1997 / Accepted: 4 May 1998     

The role of trout in stream food webs: integrating evidence from field surveys and experiments

1. We evaluated the effects of brown trout on boreal stream food webs using field surveys and enclosure/exclosure experiments. Experimental results were related to prey preference of uncaged trout in the same stream, as well as to a survey of macroinvertebrate densities in streams with vs. without trout. Finally, we assessed the generality of our findings by examining salmonid predation on three groups of macroinvertebrate prey (chironomid midges, epibenthic grazers, invertebrate predators) in a meta-analysis. 2. In a preliminary experiment, invertebrate predators showed a strong negative response to trout, whereas chironomids benefited from trout presence. In the main experiment, trout impact increased with prey size. Trout had the strongest effect on invertebrate predators and cased caddis larvae, whereas Baetis mayfly and chironomid larvae were unaffected. Trout impact on the largest prey seemed mainly consumptive, because prey emigration rates were low and independent of fish presence. Despite strong effects on macroinvertebrates, trout did not induce a trophic cascade on periphyton. Uncaged trout showed a strong preference for the largest prey items (predatory invertebrates and aerial prey), whereas Baetis mayflies and chironomids were avoided by trout. 3. Densities of invertebrate predators were significantly higher in troutless streams. Baetis mayflies also were less abundant in trout streams, whereas densities of chironomids were positively, although non-significantly, related to trout presence. Meta-analysis showed a strong negative impact of trout on invertebrate predators, a negative but variable impact on mobile grazers (mainly mayfly larvae) and a slightly positive impact on chironomid larvae. 4. Being size-selective predators, salmonid fishes have a strong impact on the largest prey types available, and this effect spans several domains of scale. Discrepancies between our experimental findings and those from the field survey and meta-analysis show, however, that for most lotic prey, small-scale experiments do not reflect fish impact reliably at stream-wide scales. 5. Our findings suggest that small-scale experiments will be useful only if the experimental results are evaluated carefully against natural history information about the experimental system and interacting species across a wide array of spatial scales.     

Predation on mayfly nymph, Baetis rhodani, by native and introduced Gammarus: direct effects and the facilitation of predation by salmonids

1. In a series of laboratory experiments, we assessed the predatory nature of the native Irish amphipod, Gammarus duebeni celticus , and the introduced G. pulex , towards the mayfly nymph Baetis rhodani . We also investigated alterations in microhabitat use and drift behaviour of B . rhodani in the presence of Gammarus , and indirect predatory interactions with juvenile Atlantic salmon, Salmo salar .
2. In trials with single predators and prey, B. rhodani survival was significantly lower when Gammarus were free to interact with nymphs as than when Gammarus were isolated from them. The invader G. pulex reduced the survival of B. rhodani more rapidly than did the native G. d. celticus . Both Gammarus spp. were active predators.
3. In `patch' experiments, B. rhodani survival was significantly lower both when G. pulex and G. d. celticus were present, although the effect of the two Gammarus species did not differ. Again, active predation of nymphs by Gammarus was observed. Significantly more nymphs occurred on the top and sides of a tile, and per capita drifts were significantly higher, when Gammarus were present. Baetis rhodani per capita drift was also significantly higher in the presence of the introduced G. pulex than with the native G. d. celticus .
4.  Gammarus facilitated predation by salmon parr of B. rhodani by significantly increasing fish–nymph encounters on exposed gravel and in the drift. There were no differential effects of the two Gammarus spp. on fish – B . rhodani encounters or consumption.
5. We conclude that Gammarus as a predator can have lethal, nonlethal, direct and indirect effects in freshwaters. We stress the need for recognition of this predatory role when assigning Gammarus spp. to a `Functional Feeding Group'.     

Stonefly predation along a hydraulic gradient: a field test of the harsh—benign hypothesis

SUMMARY. 1. Microhabitat preferences of predatory stoneflies and four prey taxa were assessed by taking benthic samples along a hydraulic gradient in a Black Forest stream in West Germany. Densities of predator and prey species were estimated at twenty-one hydraulic regimes.
2. Enclosures containing the stonefly, Dinocras cephalotes , and control cages with no predators were placed in the substrate at hydraulic regimes favourable and unfavourable to predators. Cages received initial prey communities that were obtained from benthic samples taken at hydraulic regimes matching those intended for each cage.
3. Population densities of the two most numerically important prey taxa, the mayfly. Baetis rhodani , and the Chironomidae, were reduced in the presence of Dinocras , but only when enclosures were placed in the hydraulic regimes favourable to the predator. Thus, predation effects increased as the hydraulic regime became more benign to the predators.
4. Densities of two other prey species rare in the diets of Dinocras ( Hydropsyche instabilis and Gammarus fossarum ) were generally unaffected by predators regardless of the hydraulic regime.
5. These data provide support for the hypothesis that perception of the abiotic regime as harsh or benign to predators is a good predictor of predator impact on densities of preferred prey species. In harsher abiotic regimes, impact will be low, while impact will be high in benign abiotic regimes.     

Unidirectional prey-predator facilitation: apparent prey enhance predators' foraging success on cryptic prey

Food availability can strongly affect predator-prey dynamics. When change in habitat condition reduces the availability of one prey type, predators often search for other prey, perhaps in a different habitat. Interactions between behavioural and morphological traits of different prey may influence foraging success of visual predators through trait-mediated indirect interactions (TMIIs), such as prey activity and body coloration. We tested the hypothesis that foraging success of stream-dwelling cutthroat trout (Onchorhyncus clarki) on cryptically coloured, less-active benthic prey (larval mayfly; Paraleptophebia sp.) can be enhanced by the presence of distinctly coloured, active prey (larval stonefly shredder; Despaxia augusta). Cutthroat trout preyed on benthic insects when drifting invertebrates were unavailable. When stonefly larvae were present, the trout ate most of the stoneflies and also consumed a higher proportion of mayflies than under mayfly only treatment. The putative mechanism is that active stonefly larvae supplied visual cues to the predator that alerted trout to the mayfly larvae. Foraging success of visual predators on cryptic prey can be enhanced by distinctly coloured, active benthic taxa through unidirectional facilitation to the predators, which is a functional change of interspecific interaction caused by a third species. This study suggests that prey-predator facilitation through TMIIs can modify species interactions, affecting community dynamics.     

Predator impacts on stream benthic prey

The impact that predators have on benthic, macroinvertebrate prey density in streams is unclear. While some studies show a strong effect of predators on prey density, others show little or no effect. Two factors appear to influence the detection of predator impact on prey density in streams. First, many field studies have small sample sizes and thus might be unable to detect treatment effects. Second, streams contain two broad classes of predators, invertebrates and vertebrates, which might have different impacts on prey density for a variety of reasons, including availability of refuge for prey and prey emigration responses to the two types of predators. In addition, predatory vertebrates have more complex prey communities than predatory invertebrates; this complexity might reduce the impact that predatory vertebrates have on prey because of indirect effects. I conducted a meta-analysis on the results of field studies that manipulate predator density in enclosures to determine (1) if predators have a significant impact on benthic prey density in streams, (2) if the impacts that predatory invertebrates and vertebrates have differ, and (3) if predatory vertebrates have different impacts on predatory prey versus herbivorous prey. The results of the meta-analysis suggest that on average predators have a significant negative effect on prey density, predatory invertebrates have a significantly stronger impact than predatory vertebrates, and predatory vertebrates do not differ in their impact on predatory versus herbivorous invertebrate prey. Three methodological variables (mesh size of enclosures, size of enclosures, and experimental duration) were examined to determine if cross correlations exist that may explain the differences in impact between predatory invertebrates and vertebrates. No correlation exists between mesh size and predator impact. Over all predators, no correlation exists between experimental duration and predator impact; however, within predatory invertebrates a correlation does exist between these variables. Also, a correlation was found between enclosure size and predator impact. This correlation potentially explains the difference in impact between predatory invertebrates and predatory vertebrates. Results of the meta-analysis suggest two important areas for future research: (1) manipulate both types of predators within the same system, and (2) examine their impacts on the same spatial scale.     

Food web complexity affects stoichiometric and trophic interactions

The stoichiometry of trophic interactions has mainly been studied in simple consumer–prey systems, whereas natural systems often harbour complex food webs with abundant indirect effects. We manipulated the complexity of trophic interactions by using simple laboratory food webs and complex field food webs in enclosures in Lake Erken. In the simple food web, one producer assemblage (periphyton) and its consumers (benthic snails) were amended by perch, which was externally fed by fish food. In the complex food web, two producer assemblages (periphyton and phytoplankton), their consumers (benthic invertebrates and zooplankton) and perch feeding on zooplankton were included. In the simple food web perch affected the stoichiometry of periphyton and increased periphyton biomass and the concentration of dissolved inorganic nitrogen. Grazers reduced periphyton biomass but increased its nutrient content. In the complex food web, in contrast to the simple food web, perch affected periphyton biomass negatively but increased phytoplankton abundance. Perch had no influence on benthic invertebrate density, zooplankton biomass or periphyton stoichiometry. Benthic grazers reduced periphyton biomass and nutrient content. The difference between the simple and the complex food web was presumably due to the increase of pelagic cyanobacteria ( Gloeotrichia sp.) with fish presence in the complex food web, thus fish had indirect negative effects on periphyton biomass through nutrient competition and shading by cyanobacteria. We conclude that the higher food web complexity through the presence of pelagic primary producers (in this case Gloeotrichia sp.) influences the direction and strength of trophic and stoichiometric interactions.     

Nocturnal drift of mayfly nymphs as a post-contact antipredator mechanism

1. The predominantly nocturnal constrained drift of stream invertebrates is commonly regarded as a behaviour that avoids encounters with visually foraging fish in the water column. The alternative explanation, that drift peaks are caused by bottom-feeding, nocturnal predators, has rarely been tested.
2. We examined these hypotheses by collecting invertebrate drift in five streams in northern Finland: one with brown trout ( Salmo trutta , a drift-feeding fish), one with alpine bullhead ( Cottus poecilopus , a benthic fish), one with both species, and two fishless streams.
3. Drift by Baetis mayflies was aperiodic or slightly diurnal in both fishless streams on all sampling occasions. In contrast, drift was nocturnal in streams with trout and, to a lesser extent, in the stream with bullhead. Non-dipteran prey drifted mainly nocturnally in all streams with fish, whereas Diptera larvae were less responsive to the presence of fish.
4. In laboratory experiments, bullheads were night-active, causing a much higher frequency of drift by touching Baetis at night than during the day. Thus, increased nocturnal drift may serve to avoid both visual predators (a pre-contact response) and benthic fish (a post-contact response). In streams with bottom-feeding fish, nocturnal drift should be caused by increased drift by night rather than by reduced drift by day.     

Hydraulic influences on periphyton and benthic macroinvertebrates: simulating the effects of upstream bed roughness

1. Hydraulic conditions, periphyton biomass and invertebrate communities were compared on artificial substrates exposed to a range of upstream roughness conditions across an area of uniform current velocity and depth in a gravel-bedded river. The effect of river bed roughness was simulated by installing roughness elements upstream of artificial substrates.
2. Increasing upstream roughness reduced the average near-bed velocity above the substrates and increased short-term variability in velocity (i.e. turbulence).
3. Periphyton chlorophyll a density showed a general decline with near-bed velocity and was significantly lower on the substrates exposed to the river bed reference and 0 mm roughness treatments than the 110 mm roughness elements. Chlorophyll a was also negatively correlated with the abundance of larger collector-browsing invertebrates. This indicates that effects of the changes in hydraulic conditions on invertebrates may have contributed to the observed treatment effects on periphyton.
4. Invertebrate abundance and diversity declined with increasing upstream roughness. Filter-feeders, collector-browsers and predatory invertebrates all declined in abundance with increasing upstream roughness, but the effect was strongest for filter-feeders. Eight of the nine most common taxa showed significant treatment effects. The orthoclad chironomid, Eukiefferiella sp., was not influenced strongly by upstream roughness, but its abundance was correlated significantly with periphyton biomass.     

Hydraulic influences on periphyton and benthic macroinvertebrates: simulating the effects of upstream bed roughness

1. Hydraulic conditions, periphyton biomass and invertebrate communities were compared on artificial substrates exposed to a range of upstream roughness conditions across an area of uniform current velocity and depth in a gravel-bedded river. The effect of river bed roughness was simulated by installing roughness elements upstream of artificial substrates.
2. Increasing upstream roughness reduced the average near-bed velocity above the substrates and increased short-term variability in velocity (i.e. turbulence).
3. Periphyton chlorophyll a density showed a general decline with near-bed velocity and was significantly lower on the substrates exposed to the river bed reference and 0 mm roughness treatments than the 110 mm roughness elements. Chlorophyll a was also negatively correlated with the abundance of larger collector-browsing invertebrates. This indicates that effects of the changes in hydraulic conditions on invertebrates may have contributed to the observed treatment effects on periphyton.
4. Invertebrate abundance and diversity declined with increasing upstream roughness. Filter-feeders, collector-browsers and predatory invertebrates all declined in abundance with increasing upstream roughness, but the effect was strongest for filter-feeders. Eight of the nine most common taxa showed significant treatment effects. The orthoclad chironomid, Eukiefferiella sp., was not influenced strongly by upstream roughness, but its abundance was correlated significantly with periphyton biomass.     

Impacts of introduced brown and rainbow trout on benthic invertebrate communities in shallow New Zealand lakes

1. Brown and rainbow trout have been introduced to many inland waters in New Zealand, but research on the impacts on native communities has focused mainly on streams. The purpose of this study was to compare the benthic communities of trout and troutless lakes. Based on previous studies in North America and Europe, we predicted that the benthic biomass, and especially the abundance of large invertebrates, would be lower in lakes with trout as compared to those without. We surveyed the invertebrate fauna of 43 shallow, high‐elevation lakes (26 with and 17 without trout) in four geographic clusters on the central South Island and then conducted a detailed quantitative study of invertebrate biomass and community structure in 12 of these lakes. 2. Benthic community composition and diversity of lakes with and without trout were nearly identical and biomass was as high or higher in the lakes with as without trout. There was no evidence that trout have caused local extinctions of benthic invertebrates. Although the proportional abundance of large‐bodied aquatic was slightly lower in lakes with than without trout, the abundance of several groups of large‐bodied benthic taxa (dragonflies, caddisflies and water bugs) did not differ. 3. Our findings are in contrast to those in North American and Europe where trout introductions into previously troutless lakes have led to declines in the abundance of benthic invertebrates, especially large‐bodied taxa. We propose that the modest effects of trout in New Zealand could be explained by (i) the high areal extent of submergent vegetation that acts as a benthic refuge, (ii) low intensity of trout predation on benthic communities and/or (iii) characteristics of the benthic invertebrates that make them relatively invulnerable to fish predation. 4. Regardless of the relative importance of these hypotheses, our results emphasise that the same invertebrates occurred in all of the lakes, regardless of size, elevation and presence of trout, suggesting habitat generalists dominate the benthic fauna in shallow New Zealand lakes.     

Brown trout and food web interactions in a Minnesota stream

1. We examined indirect, community‐level interactions in a stream that contained non‐native brown trout (Salmo trutta Linnaeus), native brook trout (Salvelinus fontinalis Mitchill) and native slimy sculpin (Cottus cognatus Richardson). Our objectives were to examine benthic invertebrate composition and prey selection of fishes (measured by total invertebrate dry mass, dry mass of individual invertebrate taxa and relative proportion of invertebrate taxa in the benthos and diet) among treatments (no fish, juvenile brook trout alone, juvenile brown trout alone, sculpin with brook trout and sculpin with brown trout). 2. We assigned treatments to 1 m² enclosures/exclosures placed in riffles in Valley Creek, Minnesota, and conducted six experimental trials. We used three designs of fish densities (addition of trout to a constant number of sculpin with unequal numbers of trout and sculpin; addition of trout to a constant number of sculpin with equal numbers of trout and sculpin; and replacement of half the sculpin with an equal number of trout) to investigate the relative strength of interspecific versus intraspecific interactions. 3. Presence of fish (all three species, alone or in combined‐species treatments) was not associated with changes in total dry mass of benthic invertebrates or shifts in relative abundance of benthic invertebrate taxa, regardless of fish density design. 4. Brook trout and sculpin diets did not change when each species was alone compared with treatments of both species together. Likewise, we did not find evidence for shifts in brown trout or sculpin diets when each species was alone or together. 5. We suggest that native brook trout and non‐native brown trout fill similar niches in Valley Creek. We did not find evidence that either species had an effect on stream communities, potentially due to high invertebrate productivity in Valley Creek.     

Dispersal in drift-prone macroinvertebrates: a case for density-independence

1. Studies of dispersal of macroinvertebrates in streams and rivers tend to be focused on drift, whilst benthic movements are usually considered to be less important.
2. Field-enclosure experiments with the mayfly Baetis rhodani indicate that net dispersal in this species is simply a proportional loss of individuals from the benthos.
3. Neither net upstream or downstream movements exhibited evidence of density-dependence in the form of curvilinear relationships between initial and final densities.
4. The net number of animals moving upstream did not differ significantly from the net number moving downstream.
5. The probable mechanisms behind density-independent dispersal are discussed, as are the implications for our understanding of population dynamics in relation to invertebrate drift.     

Fish predation affects the structure of a benthic community

1. We conducted an experimental study of predation by benthivorous fish on a natural community of stream invertebrates using a reach‐scale approach. Over a 2‐year period (experimental phase), the benthic invertebrate community of a stretch containing two species of benthivorous fish was compared with a fishless stretch. Thereafter, all fish were removed and benthic community structure was analysed again to account for natural differences between the two stretches (reference phase). 2. Benthivorous fish at the moderate densities investigated did not affect total benthic biomass or density, but did alter species composition. In addition, the fish effect differed between pool and riffle habitats, with larger effects in the pools indicating a habitat‐specific predation effect. In the reference phase, when all fish were removed from the stream, the difference between the two stretches was reduced. 3. The benthivorous fish reduced the densities of four taxa (Pisidium sp., Dugesia gonocephala, Gammarus pulex, Limoniidae), representing 29% of total biomass. It is possible that density reductions of other species were masked by prey migration despite the relatively large spatial scale. Indeed, higher drift activity in the upstream fishless stretch could have increased the density of Baetis rhodani in the fish stretch, as indicated by the results of a drift model. 4. Our results provide insights into stream food web ecology because fish predation showed effects even in a natural system where habitat complexity was high, environmental factors were highly variable and many predator and prey species interacted and because benthivorous fish were the focus, whereas the majority of previous predation experiments in streams have used drift‐feeding trout.     

Leaf breakdown and associated macroinvertebrates in alpine glacial streams

1. The relationship between macroinvertebrate assemblages and the breakdown of alder [Alnus viridis (Chaix), Dc.] leaves was examined by exposing leaf packs in four streams in an alpine glacial floodplain over 8 months. Although glacially fed, the four sites (pro-glacial, glacial lake outlet, main channel, and a side-channel with a mix of water sources) differed physically and contained different benthic communities.
2. Leaf breakdown and associated fungal properties differed widely among sites. Leaf decay rate varied by an order of magnitude ( k ranged from 0.0029 to 0.0305 day^–1), and was fastest at the lake outlet (< 20% leaf mass remaining by day 45) and slowest at the pro-glacial site (> 75% remaining on day 45). Rapid processing at the lake outlet was because of the presence of Acrophylax zerberus Brauer, a shredding caddisfly.
3. There were few macroinvertebrate taxa at the pro-glacial site (two to four taxa present in packs) and leaf breakdown was attributed primarily to micro-organisms. Leuctra abundance in leaf packs was strongly correlated with fungal biomass but not with the sporulation activity of any specific aquatic hyphomycete. Other taxa, such as Baetis and chironomids, showed no relationship with any leaf characteristic, suggesting that leaf packs were used mainly as a habitat and not as a food resource.
4. The predatory stonefly Isoperla was significantly associated with the abundance of macroinvertebrate prey ( Baetis , Chironomidae and Leuctra ) in leaf packs at the main and side-channel sites. The results indicate that leaf breakdown can vary widely in alpine lotic environments, reflecting site-specific differences in habitat characteristics, and in macroinvertebrate and fungal composition.     

The effects of hydrological disturbance on the densities of macroinvertebrate predators and their prey in a coastal stream

1. Environmental Stress Models (ESMs) predict that abiotic disturbance or harshness will differentially affect predators and prey. Consumer Stress Models (CSMs) predict that consumers will be relatively more inhibited by disturbance than prey, and therefore predator impacts will be reduced. Conversely, Prey Stress Models (PSMs) predict that prey will be more adversely affected and consequently predator impacts will increase in disturbed habitats. This study compared the relative tolerances of lotic invertebrate predators and their prey to hydrological disturbance in an Australian coastal stream to test the initial predictions of ESMs.
2. Macroinvertebrates were sampled with a suction sampler at monthly intervals and immediately following four high flow events at five sites on the Cumberland River, in south-west Victoria, Australia. Various statistical procedures were used to compare the relative resistance and resilience of predatory and prey taxa to each high flow event.
3. The relative resistances of seven predator and nine prey taxa to four floods over a 12-month period were highly variable between floods and between runs within the same flood. Prey taxa appeared to be more resilient than predators to the largest flood event, but there were no differences in the resilience of predators and prey following smaller floods. If disturbance tolerance is determined by resistance and resilience, then there was no consistent pattern of differential tolerance to floods among invertebrate predators and prey in this system.
4. The variability in the relative tolerances of taxa to different disturbance events makes general predictions about the effects of disturbance on the community-wide impact of predation extremely difficult.     

Spatial variation in abundance of prey and diet of trumpeter (Pelates sexlineatus: Teraponidae) associated with Zostera capricorni seagrass meadows

Abstract The diet of juveniles of Pelates sexlineatus was examined at six estuaries, separated by tens to hundreds of kilometres, and two sites within each estuary, separated by hundreds of metres to kilometres. Fish were collected in Zostera capricorni seagrass meadows along the coast of New South Wales (Australia). Spatial variability in diet was compared with the abundance of prey. Pelates sexlineatus had a broad diet (27 different prey) but generally preferred crustaceans (harpacticoid copepods, gammarid amphipods, ostracods and tanaids), although at some sites other prey items were important (e.g. polychaetes, nematodes and foraminiferans). Composition of the diet varied among estuaries and between sites. Proportional representation of the different size of prey eaten by the three sizes of juveniles (40‐ 54 , 55–74 and 75–94 mm standard length) was similar. Fish generally preferred prey smaller than 1 mm. Abundance of prey also varied at both spatial scales. At five of the 12 sites, there was a significant correlation between abundance of prey in the seagrass meadows and abundance of prey in the diet. Variation in the composition of the diet was partly explained by spatial variation in abundance of prey. When crustaceans were not abundant in the seagrass, P. sexlineatus had a broad diet, taking both benthic and planktonic prey items. It is concluded therefore that trophic linkages between P. sexlineatus and benthic invertebrates may vary greatly with spatial scales from hundreds of metres to hundreds of kilometres, and are strongly related to availability of prey in seagrass habitats.     

The effect of bladderwort (Utricularia) predation on microcrustacean prey

SUMMARY 1. The effects of the carnivorous plant Utricularia ( bladderwort) on its microcrustacean and macroinvertebrate prey were studied under seminatural and natural conditions. The results suggest that Utricularia is a strong interactor in littoral communities that influences its prey populations by direct predation and indirect facilitation.
2. In an 8-week enclosure experiment, effects on prey density were compared in three treatments with (1) U. vulgaris with intact trapbladders, (2) U. vulgaris without bladders and (3) no Utricularia present.
3. Utricularia predation caused a decrease in prey density over time, whereas presence of Utricularia without bladders increased prey density. In the controls without Utricularia , prey density was relatively constant over time.
4. Field samples were collected to quantify predation rates of three Utricularia species on two natural prey populations. Daily consumption rates on prey peaked from mid-July to mid-August for all Utricularia species, but were low in June and September. This pattern was explained mainly by a high number of trapbladders at this time, but also by a slight increase in the number of prey caught per bladder. Per capita prey mortality rates caused by Utricularia were substantial and ranged between 0.14 and 0.43 day⁻¹ for copepods, 0.1–0.27 day⁻¹ for ostracods and 0.04–0.2 day⁻¹ for chydorid cladocerans.
5. Predation and facilitation effects were observed for total prey and separately for epiphytic and benthic prey. Planktonic microcrustaceans showed no response to Utricularia presence.     

Effects of nutrients and grazers on periphyton phosphorus in lake enclosures

SUMMARY. 1. Periphyton. measured as particulate phosphorus (PP) and expressed as periphyton PP, growing on vertically oriented substrata (polyvinyl impregnated nylon) under different nutrient loadings, light intensities (exposures), and grazer communities was examined in eight large enclosures (750 m3) where nutrients (N and P) and planktivorous fish (1+yellow perch) were added in a 2x2 factorial design.
2. During the first 3 weeks of the experiment (25 June to 15 July), there was a significantly higher accumulation of phosphorus into periphyton (periphyton PP) with fertilization, but fish addition had no effect. During the fourth to seventh weeks (16 July to 12 August), addition of fish was associated with lower abundance of amphipods and chironomids and higher concentration of periphyton PP. In the enclosures without fish, these invertebrates were over 25 times more abundant, and periphyton PP decreased substantially compared to the June-July period. Fertilization increased periphyton PP only at high exposures in the enclosures with fish.
3. Exposure had a significant effect on periphyton PP. In the enclosures with fish, high abundance of nanoplankton reduced water transparency, and periphyton PP was lower in the deeper waters which may have been due to limitation by low light. Lower periphyton PP was also observed at the surface on sunny sides of enclosures without fish, and therefore with high water transparency. This pattern may have been due to inhibitory effects of high light intensity.
4. Periphyton communities in the enclosures with fish had higher uptake rates for planktonic phosphorus, and lower rates of phosphorus release, suggesting that periphyton with high phosphorus demand may have high internal cycling of assimilated phosphorus.     

Effects of fish predation and habitat type on stream benthic communities

The influence of habitat on interactions between a fish predator (brown trout Salmo trutta) and a benthic invertebrate community was studied in nine field enclosures (8 ×3 m) in a creek in southern Sweden. Three habitat treatments were tested, a shallow sandy habitat, a deep habitat containing a mixture of large and small cobbles and a moderately deep habitat with large cobbles. The one month-long experiment showed that there were no major differences in the abundance and biomass of the benthic macroinvertebrate fauna among these habitats as no functional groups of invertebrates and only a few taxa differed between treatments. Invertebrate drift rates decreased over time, which was probably related to seasonal changes in invertebrate life cycles or to effects of predation independent of habitat type, as there was no difference between treatments.