Bioturbation as Driver of Zooplankton Recruitment,Biodiversity and Community Composition in Aquatic Ecosystems

In an experimental study we assessed if benthic bioturbating invertebrates affect the recruitment (hatching) of zooplankton from the sediment, and if this effect persists as differences in the zooplankton community in the water column, that is, if bioturbation quantitatively stimulates benthic–pelagic coupling. We investigated the effects of four different benthic invertebrates (Asellus aquaticus, Chironomus plumosus, Tubifex tubifex in the presence or absence of the predator Sialis lutaria). In total, 45 zooplankton taxa hatched from the sediment and the hatching success of some of these was dependent on the species identity of the bioturbating invertebrate. The predator Sialis reduced the abundance of all three invertebrate species, but tended to positively influence the zooplankton recruitment rates, possibly through increasing the activity of the bioturbating invertebrates. The most striking effect of bioturbation on the hatching and pelagic zooplankton community properties was that, on average, 11% more species hatched in the Asellus treatment than in any other treatment. This was also mirrored in the zooplankton water column community where, on average, 7% more species established a viable population in treatments with Asellus as bioturbator. In a complementary field survey, Asellus was more common in littoral than in profundal sediments. Because Asellus strongly affected recruitment of zooplankton in our experiment, we argue that bioturbation may partly explain why recruitment of resting stages of both phyto- and zooplankton is generally higher in littoral than in profundal areas.     

Relative influences of submersed macrophytes and bioturbating fauna on biogeochemical processes and microbial activities in freshwater sediments

1. Invertebrates and aquatic plants often play a key role in biogeochemical processes occurring at the water–sediment interface of aquatic ecosystems. However, few studies have investigated the respective influences of plants and bioturbating animals on ecological processes (nutrient fluxes, benthic oxygen uptake, microbial activities) occurring in freshwater sediments. 2. We developed a laboratory experiment in aquaria to quantify the effects of (i) one invertebrate acting as a bioturbator (Tubifex tubifex); (ii) one submersed plant with a high sediment‐oxygenating potential (Myriophyllum spicatum) and (iii) one submersed plant with a low sediment‐oxygenating potential (Elodea canadensis). 3. The tubificid worms significantly increased the fluxes of nitrogen at the water–sediment interface (influx of nitrate, efflux of ammonium), whereas the two plant species did not have significant influences on these nitrogen fluxes. The differences in nitrogen fluxes between tubificid worms and plants were probably due to the bioirrigation process caused by T. tubifex, which increased water exchanges at the water–sediment interface. Tubifex tubifex and M. spicatum produced comparable reductions of nutrient concentrations in pore water and comparable stimulations of benthic oxygen uptake and microbial communities (percentages of active eubacteria and hydrolytic activity) whereas E. canadensis had a very weak influence on these variables. These differences between the two plants were due to their contrasting abilities to increase oxygen in sediments by radial oxygen losses (release of oxygen from roots). 4. Our study suggests that the bioirrigation process and radial oxygen loss are major functional traits affecting biogeochemical functioning at the water–sediment interface of wetlands.     

A comparison of two tubificid oligochaete species as candidates for sublethal bioassay tests relevant to subtropical and tropical regions

Two tubificid oligochaetes, Branchiura sowerbyi and Tubifex tubifex, were evaluated as potential test organisms for sediment bioassays. We attempt to reproduce the sediment bioassay proposed by Reynoldson et al. (1991) using his strain of Tubifex tubifex and his reference sediment and to compare this technique using Branchiura sowerbyi. This species was chosen because it is more common and dominant in tropical and subtropical environments than Tubifex tubifex. Data on survival and reproduction were obtained for both species, and growth estimates were obtained for B. sowerbyi. The sublethal bioassay with T. tubifex confirmed earlier estimates of a test duration of 4 weeks at 22.5 °C. B. sowerbyi cultures can produce usable estimates in 21 days at 30 °C.     

Comparison of dietary mercury exposure in two sympatric top predator fishes, largemouth bass and northern pike: a bioenergetics modeling approach

Physical and ecologicalfactors, including lake temperature, fishphysiology, and diet, influence methylmercury(MeHg) exposure in fish. We employedbioenergetics modeling to compare dietary MeHgexposure in sympatric top predators, largemouthbass (Micropterus salmoides) and northernpike (Esox lucius). We comparedsimulations using field data to hypotheticalsimulations with (1) ± 25% change in meandaily lake temperature for juvenile and adultbass and pike; (2) ± 25% change inlong-term growth rate of pike; (3) adult bassdiet shift from generalist predator to strictpiscivore. Bass and pike MeHg exposures weresimilar in baseline simulations and reflectedpatterns in field tissue concentrations. Thisoccurred despite the fact that bass consumedhighly contaminated benthic invertebrates,while pike exclusively consumed lesscontaminated fish prey. Higher temperaturesincreased adult bass and pike MeHg exposures by35% and 27%, respectively. Shifting adultbass diets to 100% fish resulted in a 54%decrease in exposure, while increasing pikegrowth rates resulted in a 24% decrease. Bioenergetics modeling proved useful inunderstanding the influence of temperature,prey-base, and predator growth on differencesin Hg exposure across fish species.     

Habitat overlap between predatory benthic fish and their invertebrate prey in streams: the relative influence of spatial and temporal factors on predation risk

1. The spatial heterogeneity of ecosystems as well as temporal activity patterns of organisms can have far‐reaching effects on predator–prey relationships. We hypothesised that spatiotemporal constraints in mesohabitat use by benthic fish predators would reduce habitat overlap with benthic invertebrates and lead to mesohabitat‐specific predation risks. 2. We analysed the spatiotemporal activity patterns of two small‐bodied benthivorous fishes, gudgeon (Gobio gobio) and stone loach (Barbatula barbatula), and of benthic invertebrates in a small temperate stream during three 24‐h field experiments. By applying a novel method of field video observation, we monitored the spatiotemporal foraging behaviour of the fish in their natural environment. A parallel analysis of invertebrate mesohabitat use by means of small area Hess sampling allowed a direct estimation of habitat overlap at a pool–riffle scale. 3. Gudgeon showed a dominant spatial activity pattern preferring pools at all times of day, whereas stone loach used both mesohabitats but with a distinct temporal (nocturnal) activity pattern. The patterns of residence were not identical with those of active foraging. Invertebrate community composition differed significantly between mesohabitats but not between times of day. More than half of the total dissimilarity between pools and riffles was accounted for by six invertebrate taxa. Five of these were subject to higher fish predation in pools than in riffles. The total prey consumption of the two fish species together in pools was about three times as high as in riffles. Trophic niche breadth of stone loach and thus its predation range was broader than that of gudgeon. 4. These results indicate that the potential predation risk for stream invertebrates depends on the combination of spatial and temporal patterns of both predator and prey. Given the distinct differences in predation risk found between pools and riffles, we conclude that spatial heterogeneity at the mesohabitat scale can influence mechanisms and consequences of selective predation. We also suggest that the analysis of spatiotemporal predator–prey relationships should not be based on the premise that the main residence habitat and active foraging habitat of a predator are identical.     

Top-down and bottom-up impacts of juvenile fish in a littoral reed stand

1. Research has often focused on pelagic food chains and processes of lakes; less is known about the contribution of benthic energy flows to whole‐lake ecosystem energetics. This stems from the fact that the shoreline and littoral habitats, which provide a key linkage between sediment and water column, have only recently become a significant focus for study. 2. This study aimed to quantify the feeding and phosphorus allocation of a juvenile fish community in a littoral zone of a shallow lake in response to the biomass succession of the invertebrate prey community. Habitats comprising reed and adjacent open water were sampled over two consecutive years during day and night. 3. Although there were substantial year‐to‐year differences in the biomass of invertebrates, the fish community composition, diet consumption rates and phosphorus allocations were very similar in both study years. Biomasses and predation impacts by juvenile fish on prey groups were substantially higher within the reeds than in the adjacent open water habitat. This may be explained by the refuge‐seeking behaviour of the fish. 4. In general, invertebrates were negligibly influenced by fish feeding, with the exception for a strong top‐down control of large cladocerans. In response to the resulting low Daphnia biomass, fish were forced to switch to a higher degree of benthivory. Consequently, juvenile fish in littoral reed stands may shift benthic‐derived energy and phosphorus via the excretion of soluble reactive phosphorus into the open water.     

Feeding ecology of a vertebrate assemblage inhabiting a stream of NW Spain (Riobo; Ulla basin)

The food resource use of a stream in NW Spain by fish (Salmo trutta L. and Anguilla anguilla L.), birds (Cinclus cinclus L. and Motacilla cinerea L.) and mammals (Galemys pyrenaicus G. and Neomys anomalus C.) was studied. Data on seasonal diets and stream benthos prey were used to determine prey selection patterns.Caddisfly larvae are the main resource for Cinclus and Galemys, but these predators also consumed other benthic prey. Salmo fed on a wide range of benthic invertebrates, emergent pupae and terrestrial prey, whereas Anguilla consumed primarily benthic invertebrates, especially Lumbricids. Neomys fed mainly on terrestrial prey (Gasteropods and Lumbricids), but also consumed aquatic prey. Motacilla captured aquatic insects both in larval and aerial stages, as well as terrestrial prey.Both prey availability and selection led to seasonal differences in the use of food resources. All species showed a marked prey selection of aquatic taxa. Prey size plays an important role in this selection, most species consuming the largest of available prey sizes. In spite of the fact that all species feed upon freshwater invertebrates, substantial resource partitioning was observed in all seasons. This partitioning may be attributable to morpholological and physiological differences. Nevertheless, Anguilla and Galemys, two quite different animals, did feed on the same prey much of the time.     

Seasonal Trophic Niche Shift and Cascading Effect of a Generalist Predator Fish

Few studies have examined how foraging niche shift of a predator over time cascade down to local prey communities. Here we examine patterns of temporal foraging niche shifts of a generalist predator (yellow catfish, Pelteobagrus fulvidraco) and the abundance of prey communities in a subtropical lake. We predicted that the nature of these interactions would have implications for patterns in diet shifts and growth of the predator. Our results show significant decreases in planktivory and benthivory from late spring to summer and autumn, whereas piscivory increased significantly from mid-summer until late autumn and also increased steadily with predator body length. The temporal dynamics in predator/prey ratios indicate that the predation pressure on zooplankton and zoobenthos decreased when the predation pressure on the prey fish and shrimps was high. Yellow catfish adjusted their foraging strategies to temporal changes in food availability, which is in agreement with optimal foraging theory. Meanwhile the decrease in planktivory and benthivory of yellow catfish enabled primary consumers, such as zooplankton and benthic invertebrates, to develop under low grazing pressure via trophic cascading effects in the local food web. Thus, yellow catfish shifts its foraging niche to intermediate consumers in the food web to benefit the energetic demand on growth and reproduction during summer, which in turn indirectly facilitate the primary consumers. In complex food webs, trophic interactions are usually expected to reduce the strength and penetrance of trophic cascades. However, our study demonstrates strong associations between foraging niche of piscivorous fish and abundance of prey. This relationship appeared to be an important factor in producing top-down effects on both benthic and planktonic food webs.     

Experimental study of 110mAg transfer from sediment to biota in a simplified freshwater ecosystem

Radioecological field studies show that most of the ^110m in contaminated freshwater ecosystems occurs in the sediment, which therefore represents a potential source of radioactive pollution. Laboratory experiments were carried out to quantify ^110mAg uptake and retention by three species of bottom-dwelling organisms, a limicolous midge larva (Chironomus luridus), a benthic crustacea (Gammarus pulex) and an omnivorous fish (Cyprinus carpio), and to assess the potential incorporation of this radionuclide in freshwater food webs. The rate of contamination of each organism was described using a two-compartment model. Concentration factors (^110mAg in organism: ^110mAg in water) reached a maximum (1100 ww) in 30 and 60 d respectively for gammarid and midge larva. The concentration factor for Cyprinus carpio increased as a function of time to reach a maximum value of 106 after 180 d. ^110mAg uptake from the sediment suggests a real possibility of direct transfer from this compartment to the three organisms. The maximum value of the transfer factor (^110mAg in organism: ¹¹⁰Ag in sediment) was about 1.9, 0.17 and 0.022, for gammarid, midge larva and carp, respectively. The ingestion of contaminated gammarids and midge larvae by carp results in a ^110mAg accumulation corresponding to a trophic transfer factor (^110mAg in predator: ^110mAg in prey) of 0.023 and 0.135, respectively. Comparison of the different factors that influence the ^110mAg transfer suggests that water is the most important contamination vector. However, taking into account their ^110mAg concentration, prey and sediment can lead to a greater level of contamination in consumer organisms than uptake from water.     

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.     

Turbidity amplifies the non‐lethal effects of predation and affects the foraging success of characid fish shoals

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Prey location and prey choice by the freshwater leech Erpobdella octoculata using foraging kairomones

1. The freshwater leech, Erpobdella octoculata , is a generalist predator feeding on prey organisms such as Tubifex spp., Chironomus spp. and Asellus aquaticus. Using different experimental designs, we studied the use of foraging kairomones by the leeches for prey location.
2. Leeches were attracted to living as well as to freshly killed larvae of Chironomus sp., to Tubifex sp., and to A. aquaticus offered in Petri dishes covered with gauze. The leeches also reacted to an extract of macerated Tubifex sp. presented in agar.
3. Using ion exchange chromatography, the presence of histidine and glutamic acid was demonstrated in water contaminated by living larvae of Chironomus sp. and Tubifex sp. Agar blocks containing a synthetic mixture of these compounds at concentrations above 5 mg mL⁻¹ were attractive to the leeches.
4. Choice tests showed that leeches preferred chemical signals from Tubifex sp. over larvae of Chironomus sp. No difference was found between chemicals from Tubifex sp. and A. aquaticus , and A. aquaticus and larvae of Chironomus sp.
5. The results demonstrate that E. octoculata uses specific foraging kairomones in searching for prey and indicate that amino acids serve as foraging kairomones.     

THE TROPHIC ROLE OF MARINE MAMMALS IN THE NORTHERN GULF OF ST. LAWRENCE

The trophic role of apex predators was evaluated in the northern Gulf of St. Lawrence ecosystem. An Ecopath model was developed for the period 1985–1987 prior to the collapse of commercially exploited demersal fish stocks in this area. Marine mammal trophic levels were estimated by the model at 4.1 for cetaceans, 4.4 for harp seals (Pagophilus groenlandicus), 4.7 for hooded seals (Cystophora cristata), 4.5 for gray seals (Halichoerus grypus), and 4.3 for harbor seals (Phoca vitulina). Harp seals were the third most important predator on vertebrate prey following large Atlantic cod (Gadus morhua) and redfish (Sebastes spp.). Different seal species preyed on different levels of the food chain. Harp seals preyed on most trophic groups, whereas larger seals, such as gray seals and hooded seals, mainly consumed higher trophic levels. The model suggested that apex predators had a negative effect on their dominant prey, the higher trophic level fish, but an indirect positive feedback on the prey of their preferred prey, mainly American plaice (Hippoglossoides platessoides), flounders, skates, and benthic invertebrates. Our results suggest that both marine mammals and fisheries had an impact on the trophic structure.     

Cyst formation of Tubifex tubifex (Müller) — an adaptation to survive food deficiency and drought

Adult individuals of Tubifex tubifex (Müller) (Oligochaeta, Tubificidae) could be induced to form cysts in the laboratory. Hiding in cysts, Tubifex tubifex survived a five-month drought period in the field. Encysted Tubifex tubifex had a lower super-cooling-point than mobile individuals. Food shortage is believed to be the main factor in cyst formation.     

Sediment toxicity in lakes along the river Kolbäcksån,central Sweden

The River Kolbäcksån system is located in a historical mining and steel works district of central Sweden. Ten years ago, intensive limnological studies indicated that the sediments in many of the lakes of this system were contaminated with metals and oil (grease). More recently a very high toxicity was found in the sediments from some of these lakes in tests with Daphnia magna and Tubifex tubifex. The objective of this study was to determine the toxicity to Daphnia magna of surficial sediments from representative locations (N = 39) within this system of lakes and to look for possible correlations with concentrations of metals (Cd, Cr, Cu, Hg, Ni, Pb, Zn) and oil (total) analysed in parallel samples. These results were also compared with spiked sediment toxicity data generated for these metals and with the bottom fauna at 8 of the 39 locations. Among the metals analyzed, only Cd, Pb and Zn were correlated with whole sediment toxicity. Only for Zn and Cr did the maximum concentrations approach those that were toxic in the spiked sediments. Correlations using single and combined metal contamination and toxicity (additive models) explained 16% of the sediment toxicity (r ² = 0.16; N = 39). Significant correlations with the bottom fauna in situ (gross abundance, biomass and species richness) were found for whole sediment toxicity, but not for metal contamination (N = 8). The use of the sediment quality triad approach for assessment of sediment quality is exemplified with a numerical normalization procedure for the latter 8 sites.     

Predation and drift of lotic macroinvertebrates during colonization

Summary A field experiment was carried out to determine the effect of an invertebrate predator on the colonization and drift of benthic macroinvertebrates in experimental stream channels. Lotic invertebrates colonized four replicate channels: two controls with no predators, and two channels with low densities (2.8 m^–2) of predatory stonefly nymphs, Doroneuria baumanni (Perlidae). Immigration rates were measured at the inflow of two other channels. Drift rates of invertebrates immigrating to and emigrating from channels were measured daily, and benthic samples were collected every five days. Over a 25-day colonization period, benthic densities of Baetis nymphs and larval Chironomidae were reduced by D. baumanni. Colonization curves were fit with a power function and significantly different colonization rates were indicated for both Baetis and chironomids in predation and control channels. A predator-induced drift response was exhibited by Baetis only and this response was size-dependent. In the presence of D. baumanni, large Baetis drifted more frequently than small nymphs and, correspondingly, small nymphs were more frequent in the benthos. Net predator impacts on invertebrate densities in channel substrates were partitioned into predator-induced drift and prey consumption. These estimates suggest that predator avoidance by Baetis is a prominent mechanism causing density reductions in the presence of predators. Reductions in the density of Chironomidae, however, were attributed to prey consumption only. A rainstorm during the experiment demonstrated that stream flow disruptions can override the influence of predators on benthic invertebrates, at least temporarily, and re-set benthic densities.     

Inducible defences are a stabilizing factor for predator and prey populations: a field experiment

1. Based on mathematical models, antipredator defence mechanisms are commonly believed to have stabilizing effects on communities. However, empirical data are still lacking. 2. We tested stabilizing effects of an inducible vertical migration defence in two Daphnia pulex clones in a 5‐week field enclosure experiment. A defended (migrated down into darker water layers in the presence of fish chemicals in both laboratory and field experiments) and non‐defended (no ability to react to fish chemicals) clone were directly exposed to fish predators and compared to control enclosures (no fish). 3. In the absence of planktivorous fish, both defended and non‐defended clones exhibited boom‐and‐bust dynamics, probably owing to over‐exploitation of the food source. Predation almost led to extinction of the non‐defended Daphnia clone during the experiment and the fish, deprived of food, lost weight. However, the population density of the defended clone was stable and it did not over‐exploit the algal food source, while there was a continuous supply of food to the fish, which consequently gained weight. 4. We conclude that both consumptive and non‐consumptive (also called non‐lethal or trait‐mediated) predator effects, coupled with prey defences, are key contributors to prey stability. This has a positive effect on both the predator and the food organism of the prey.     

Countering morphological ambiguities: development of a PCR assay to assist the identification of <Emphasis Type="Italic">Tubifex tubifex</Emphasis> oligochaetes

The freshwater oligochaete, Tubifex tubifex, is a common resident of organic-rich sediments worldwide. Although it is a familiar species to fish enthusiasts, toxicologists and parasitologists, T. tubifex often confounds definitive identification due to the similarity that immature specimens bear to several other common oligochaetes, and given the degree of plasticity of key morphological characters due to environmental conditions and/or age of specimen. To solve this identity crisis, we used a polymerase chain reaction based molecular approach and developed a T. tubifex specific assay with primers that amplify a 192 bp fragment of the internal transcribed spacer region 1 ribosomal DNA. We tested these primers on four T. tubifex mitochondrial genotypes, and on other oligochaete species from nine genera. The primers amplified all specimens identified morphologically asT. tubifex. They did not amplify any other species, including morphologically similar worms possessing hair chaetae (Dero digitata, Ilyodrilus templetoni, Tubifex ignotus or Rhyacodrilus spp.) or other oligochaetes often found with T. tubifex (Lumbriculus variegates, Limnodrilus hoffmeisteri, Stylodrilus heringianus or Trichodrilus sp.). This technique should remove the uncertainties all too often associated with identification of T. tubifex.     

Fish predation regimes modify benthic diatom community structures: Experimental evidence from an in situ mesocosm study

Diatoms are important primary producers in shallow water environments. Few studies have assessed the importance of biological interactions in structuring these communities. In the present study, benthic diatom community structure in relation to manipulated food webs was assessed using in situ mesocosms, whereby predator‐free environments and environments comprising two different fish species were assessed. Zooplankton abundance, settled algal biomass and the diatom community were monitored over a 12‐day period across each of the three trophic scenarios. Differences among treatments over time were observed in zooplankton abundances, particularly copepods. Similarly, the benthic diatom community structure changed significantly over time across the three trophic treatments. However, no differences in total algal biomass were found among treatments. This was likely the result of non‐diatom phytoplankton contributions. We propose that the benthic diatom community structure within the mesocosms was influenced by trophic cascades and potentially through direct consumption by the fish. The study highlights that not only are organisms at the base of the food web affected by predators at the top of the food web, but that predator identity is potentially an important consideration for predator–prey interaction outcomes with consequences for multiple trophic levels.     

Predator avoidance costs and habituation to fish chemicals by a stream isopod

1. We examined whether long-term exposure to chemical cues of predatory longear sunfish (Lepomis megalottis) affected growth and rates of leaf processing by the isopod, Lirceus fontinalis, an important facultative shredder in low-order streams in the eastern United States, and whether isopods habituated to sunfish chemicals. 2. Long-term (24 days) exposure to fish chemicals did not affect growth or the extent of leaf processing by isopods in the laboratory. Thus, chemical cues alone may not be important in triggering long-term predator avoidance behaviours that could potentially reduce fitness or affect community processes like Litter breakdown. 3. Isopods exposed to fish chemicals for 3, 9 and 15 days were significantly more active than individuals unexposed to fish chemicals, when transferred to another environment with fish chemicals. This result, coupled with results of the growth experiment, suggests that isopods habituate to fish chemical stimuli and that time to habituation is c. 3 days or less. Because fish chemicals can mislead prey about predator presence, and hence be a non-threatening stimulus, they are probably important in eliciting only short-term antipredatory behaviours by isopods.