The effects of predation by juvenile fish on the meiobenthic community structure in a natural pond

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Top-down control of a meiobenthic community by two juvenile freshwater fish species

Top-down control of prey assemblages by fish predation has been clearly demonstrated for zooplankton and macroinvertebrates. However, in the benthic communities of freshwater ecosystems, the impact of fish predation on meiofaunal assemblages is nearly unknown. In this study, the predation effects of juvenile carp (Cyprinus carpio) and gudgeon (Gobio gobio) on meiofaunal abundance, biomass, community structure, and the diversity of nematodes were examined using microcosms that were sampled repeatedly over 64 days. Significant differences in abundance and biomass were found between the two fish treatments (carp and gudgeon) and their respective controls for nematodes, oligochaetes, and crustaceans (copepods, harpacticoids, ostracods, and cladocerans), but not for rotifers. These changes were consistent with top-down control of the freshwater meiofaunal assemblages in the microcosms over time. By contrast, small-bodied meiofauna was more abundant, suggesting indirect facilitation. Neither the species richness nor the diversity of the nematode community was affected by fish predation. The results indicate that predation by juvenile freshwater fish depresses the overall abundance and biomass of meiofaunal assemblages, except for rotifers, and alters the size structure of the meiofaunal community. Therefore, the meiofaunal assemblages of freshwater ecosystems may be influenced by bottom-feeding juvenile fish, e.g., carp and gudgeon, through top-down control of meiofaunal populations.     

An ecological and economic review of meiofauna as food for fish*

After briefly reviewing the evidence that meiofauna are an integral part of the complex food web of invertebrate infaunal and epifaunal organisms, it is shown that, at least in intertidal and shallow subtidal habitats, meiofauna are an important food for the small (30–60 mm) juveniles of some commercially important fishes (flatfish and salmonids) or for fish which are themselves food for commercial species. The evidence that fish feed preferentially on a few species of harpacticoid copepod rather than on the numerically dominant meiofaunal taxa is presented and discussed. It is argued that, in addition to benthic copepods being energetically efficient as food for fishes, it is differences in the activity and availability between major taxa of meiofauna as they relate to the feeding strategies of the fish which go a long way to explaining this apparent general preference of fish for harpacticoid copepods rather than nematodes. From literature estimates of consumption rates, it is concluded that the supply of meiofaunal food to juvenile fish is unlikely to be limiting although this may not be the case in some shallow subtidal muddy sand habitats. Whilst it is recognized that fish predation may influence the magnitude of the seasonal peaks in numbers, predation is not the principal cause of seasonal fluctuations in harpacticoid copepod species abundances. The effects of predation on other aspects of community structure is unknown. Finally, the possible role of meiofauna in mariculture systems is briefly discussed.     

上川岛潮间带不同生境底栖软体动物物种多样性初步研究

目的:研究上川岛潮间带不同生境底栖软体动物物种多样性。方法:对上川岛潮间带(沙滩、泥沙滩、红树林泥滩、岩石滩) 4种不同生境的7个样方的底栖软体动物进行了生物多样性的初步调查。结果:共鉴定底栖软体动物14科30种,上川岛潮间带底栖软体动物物种丰富度指数为岩石滩(平均为1.784 ) >泥沙滩(平均为1.5 2 1) >红树林泥滩(平均为1.2 35 ) >沙滩(平均为0 .6 92 ) ;多样性指数岩石滩(平均为1.5 5 1) >红树林泥滩(平均为1.30 4 ) >泥沙滩(平均为1.16 2 ) >沙滩(平均为0 .4 0 3)。结论:底栖软体动物的种类及数量分布受底质、浪击等因素的影响。     

Role of meiofauna in estuarine soft-bottom habitats*

Meiofauna are ubiquitous in estuaries worldwide averaging 10⁶ m^?2. Abundance and species composition are controlled primarily by three physical factors: sediment particle size, temperature and salinity. While meiofauna are integral parts of estuarine food webs, the evidence that they are biologically controlled is equivocal. Top down (predation) control clearly does not regulate meiofaunal assemblages. Meiofauna reproduce so rapidly and are so abundant that predators cannot significantly reduce population size. Food quantity (bottom up control) also does not appear to limit meiofaunal abundance; there is little data on the effect of food quality. In estuarine sediments meiofauna: (i) facilitate biomineralization of organic matter and enhance nutrient regeneration; (ii) serve as food for a variety of higher trophic levels; and (iii) exhibit high sensitivity to anthropogenic inputs, making them excellent sentinels of estuarine pollution. Generally mineralization of organic matter is enhanced and bacterial production stimulated in the presence of meiofauna. Tannins from mangrove detritus in northern Queensland appear to inhibit meiofaunal abundance and therefore the role of meiofauna in breakdown of the leaves. Meiofauna, particularly copepods, are known foods for a variety of predators especially juvenile fish and the meiofaunal copepods are high in the essential fatty acids required by fish. The copepod’s fatty acid composition is like that of the microphytobenthos they eat; bacterial eaters (nematodes?) do not have the essential fatty acids necessary for fish. Most contaminants in estuaries reside in sediments, and meiofauna are intimately associated with sediments over their entire life-cycle, thus they are increasingly being used as pollution sentinels. Australian estuarine meiofauna research has been concentrated in Queensland, the Hunter River estuarine system in New South Wales, and Victoria’s coastal lagoons. Studies in northern Queensland have primarily concentrated on the role of nematodes in mineralization of organic matter, whereas those from southern Queensland have concentrated on the role of meiofauna as food for fish and as bacterial grazers. The New South Wales studies have concentrated on the Hunter River estuary and on the structure and function of marine nematode communities. In Victoria, several fish have been shown to eat meiofauna. The Australian world of meiofaunal research has hardly been touched; there are innumerable opportunities for meiofaunal studies. In contaminated estuarine sediments reduced trophic coupling between meiofauna and juvenile fish is a basic ecological question of habitat suitability, but also a question with relevance to management of estuarine resources. Because meiofauna have short lifecycles, the effects of a contaminant on the entire life-history can be assessed within a relatively short time. The use of modern molecular biology techniques to assess genetic diversity of meiofauna in contaminated vs uncontaminated sediments is a promising avenue for future research. Much of the important meiofaunal functions take place in very muddy substrata; thus, it is imperative to retain mudflats in estuaries.     

Putting the meio- into stream ecology: current findings and future directions for lotic meiofaunal research

• 1 There is a paucity of research on epigean freshwater lotic meiofauna. This may result from a previous emphasis on interstitial (groundwater and hyporheic) meiofauna and/or a reliance on sampling methodologies in lotic systems which are inappropriate for meiofauna.
• 2 Meiofauna contribute much to the diversity of lotic ecosystems. Species lists for seven streams reveal that meiofauna contribute 58–82% of total species numbers, with rotifers and chironomids dominating most systems. The absence of taxonomic keys for most meiofaunal taxa in large areas of the world precludes a wider analysis of their contribution to lotic diversity and an assessment of biogeographical patterns and processes.
• 3 The trophic and functional role of meiofauna in lotic ecosystems is unclear. There are few estimates of meiofaunal production in freshwaters and biomass spectra have produced conflicting results for lotic meiofauna. Present static estimates suggest that the contribution of meiofauna to lotic productivity and biomass is small to moderate, but further studies incorporating a temporal component may provide a more realistic picture of the total contribution of meiofauna to biomass size spectra.
• 4 Meiofauna differ from macroinvertebrates in several respects apart from size and conceptual models for lotic ecosystems should include all metazoans if they are to be truly representative.
• 5 Information on the basic ecology of certain lotic meiofauna (i.e. nematodes, tardigrades, microturbellarians) is urgently required. For those groups whose distributional patterns are better understood (e.g. microcrustaceans), the mechanisms underpinning these patterns should be explored. It is essential that the importance of meiofauna is recognised by lotic ecologists; the only realistic way forward is for greater collaboration among meiofaunal ecologists and taxonomists and other lotic scientists.

     

Lotic meiofaunal community dynamics: colonisation, resilience and persistence in a spatially and temporally heterogeneous environment

1.  The spatial and temporal dynamics of lotic meiofaunal communities were examined with a focus on colonisation, maintenance of populations in lotic environments and persistence of meiofaunal communities.
2.  Lotic meiofaunal colonisation of new habitats may take place via a number of mechanisms and is rapid at both the patch scale (within hours–days) and the drainage basin scale (within 20 years). Successional patterns in lotic meiofaunal communities are evident although data are extremely limited.
3.  Lotic meiofaunal communities appear to be resilient to high flow disturbances. Resilience is moderated by the availability of in-stream refugia and habitat hydrology. Lotic meiofauna may also adopt a 'refuge as habitat' approach to such disturbances.
4.  Lotic cyclopoid copepods possess a common suite of life history characteristics that confers resilience to disturbances. Compared to pelagic planktonic species they have short generation times, many descendants per reproductive cycle and long lifespans. Females in 'source' populations are likely to survive disturbances and so could continuously reproduce over a long period of time producing large numbers of offspring which develop rapidly and recolonise 'sink' areas of the stream.
5.  Persistence of lotic meiofaunal communities is highly variable. Meiofaunal persistence does not increase with increasing proportions of in-stream flow refugia.     

Benthic diversity in the Rhine-Meuse estuary

Summary The benthic diversity of the Rhine-Meuse estuary has been investigated by means of three diversity indices: the Shannon-Weaver information function, an index obtained by SANDERS' rarefaction method and the number of the species per sample. Succession proved to be a very important factor in the realization of diversity. Also the median grain-size of the sediment proved to be correlated significantly with diversity, sandy sediments containing more diverse communities than muddy ones. When succession and median grain-size are included in the analysis, salinity, nevertheless, proves to be correlated very significantly with diversity. The low diversity of brackish ecosystems also appears from the relatively high number of introduced species in these habitats.Communication no. 117 of the Delta Institute for Hydrobiological Research.     

Nematode and macrofaunal diversity in central Arctic Ocean benthos

Deep-sea diversity studies have revealed intriguing patterns on both local and regional scales, but there is insufficient evidence with which to evaluate these trends in the Arctic Ocean basin. We collected data on the diversity of benthic macrofauna and meiofaunal nematodes along two transects from the shelf margin to the North Pole. Contrary to prevailing paradigms, there was no change in diversity with depth between 1000 and 4273 m. There was a trend, however, toward reduced taxonomic richness for both macrofauna and nematodes with increasing latitude. Regional (β-) diversity differences were not observed for nematodes, but significant contrasts in Bray-Curtis similarity-based community structure of macrofauna were seen between the Eurasian and Amerasian Basins, as well as between the Lomonosov and Mendeleev Ridges. Since fauna within the deep Arctic Ocean appear to represent a single species pool, we suggest that both local (α-) and β-diversity may be determined by ecological processes in the Arctic, and are not the consequence of historical or evolutionary processes. Furthermore, insights gained from studies of benthic-pelagic coupling, known to play a significant role in determining benthic community structure and function at high latitudes, may also be useful in investigations of Arctic biodiversity. This model may be valuable in designing future studies of biodiversity, and for predicting potential impacts of climate change on diversity patterns.     

The meiofauna of artificial water-filled tree holes: colonization and bottom-up effects

The meiofaunal community of artificial water-filled tree holes was determined, and the bottom-up effects of different amounts of leaf litter on abundance and diversity were estimated. We assume a positive impact of leaf litter on meiofaunal abundances, species diversity, and trophic links. Plastic cups with different amounts of leaf litter were placed in a beech forest (Teutoburg Forest, Bielefeld, Germany) for 24 weeks. As early as 1 week later, the artificial tree holes were colonized by bdelloid rotifers, tardigrades, and nematodes. Rotifers were dominant throughout the experiment, followed by nematodes and tardigrades. The 29 nematode species that were identified included bacterial and hyphal feeders, with common species such as Plectus cirratus/accuminatus and Aphelenchoides parietinus predominating. Impacts of water volume (up to complete desiccation), pH, and O₂ on the meiofaunal community were not detected, whereas the addition of leaf litter resulted in bottom-up effects. Nematode abundance, especially that of bacterial feeders, and species number increased with increasing leaf input. The predatory nematode Prionchulus muscorum was found only in treatments containing high leaf content. Rotifer abundances were partly negatively affected by the amount of added leaves and, like tardigrades, showed a reversal in their correlation at higher leaf inputs. Our study revealed the fast colonization of small water bodies by meiofaunal organisms and the importance of passively distribution. Furthermore, the results provide a comparison with the meiofaunal community in lakes and soil.     

Recent trends in Lake Ladoga ice cover

This contribution reviews diversity of turbellarian species by biogeographical regions, with comments on species biology. The review draws on the database available at http://www.devbio.umesci.maine.edu/styler/turbellaria. Comparisons between regions suggest that species richness may be at least one order of magnitude higher than the currently reported number of species. In the context of the recent reconstructions of phylogeny of Platyhelminthes based on molecular data, the paper allows inferences as to the history of colonization of freshwaters by turbellarians. Specifically, four, or perhaps six, major invasions of freshwater habitats may have occurred in the Pangean period, each of which gave rise to a monophyletic freshwater taxon. In addition, several occasional invasions by representatives of marine taxa must have taken place.     

Is there competition among ciliates and nematodes?

1. Biotic interaction between the ciliate Cyclidium glaucoma and the nematode Caenorhabditis elegans was investigated by manipulating the densities of the organisms in microcosms with and without sediment. 2. After 11 days the abundance of ciliates, nematodes and bacteria as well as extracellular enzyme activity were determined. Ciliates had a negative effect on nematode abundance in microcosms without sediment and in microcosms with sandy sediment, whereas in muddy sediment the effect was less distinctive. An effect of nematodes on ciliates was not observed. 3. The common resource bacteria were not affected negatively by the activity of the grazers. Overall grazer biomass increased with the addition of sediment to the microcosms, suggesting a rise of the carrying capacity in the experimental system. Especially in muddy sediment the abundance of bacteria and extracellular enzyme activity was higher compared to the microcosms without sediment. 4. The results of the experiment suggest a strong interspecific competition between nematodes and ciliates, where nematodes are, at least temporary, strongly affected.     

Plant nematode communities

Most plant-parasitic nematodes live in polyspecific communities. They are abundant in most habitats. Their numbers and diversities are influenced greatly by the host and diversity of the habitats. Although populations are affected by interactions with other organisms working through the host plant, it is doubtful that there is much direct competition among plant-parasitic nematodes. Except were damage is severe as a result of parasitism, there usually is an abundance for food. Damage to plants may be additive or partly so by two or more species of nematodes.     

Effects of chronic bottom trawling disturbance on benthic biomass, production and size spectra in different habitats

Bottom trawling has widespread impacts on benthic communities and habitats. It is argued that the impact of chronic bottom trawling on benthic infauna depends on the natural disturbance levels to which benthic communities are adapted. We analysed biomass, production and size structure of two communities from a muddy and a sandy habitat, in relation to quantified gradients of trawling disturbance on real fishing grounds. We used an allometric relationship between body mass and individual production to biomass ratio to estimate community production. Chronic trawling had a negative impact on the biomass and production of benthic communities in the muddy habitat, while no impact was identified on benthic communities from the sandy habitat. These differences are the result of differences in size structure within the two communities that occur in response to increasing trawling disturbance.     

Effects of the deposit-feeding benthic bivalve Macoma balthica on meiobenthos

To evaluate the effects of a deposit-feeding bivalve on meiobenthic assemblage structure in muddy habitats, a laboratory experiment was performed at the Askö Laboratory in the northwestern Baltic proper. Microcosms, surface area 104 cm², containing a c. 7-cm thick layer of sieved (0.5 mm) sublittoral mud were established in June 1990. Two months later the tellinid bivalve Macoma balthica was added in quantities varying from 0 to 40 individuals per microcosm. After 5 months the effects of the bivalves on the meiofauna were surprisingly small. The density of harpacticoid copepods was lowest (P <0.05) in microcosms containing a high density of M. balthica. It is suggested that competition for food resources was responsible for this pattern. For all other meiofaunal groups, including nematodes which were the most abundant taxon (99%), no significant differences (P>0.05) were observed among treatments. The assemblage structure of the nematodes was similar between treatments. The vertical distribution of both major taxa and nematode species appeared to be unaffected by the presence of the bivalve.     

The effects of nutrient enrichment on a freshwater meiofaunal assemblage

1. The effects of eutrophication on phytoplankton, zooplankton and fish in lakes are well known. By contrast, little is known about the response of the zoobenthos to nutrient enrichment, while smaller organisms, such as the meiofauna, have for the most part been neglected. 2. In a long‐term (16 months) microcosm experiment, we assessed the effects of five levels of nutrients [total phosphorus (TP), 7–250 μg L^?1; nitrate, 2–8 mg L^?1] on a freshwater meiofaunal assemblage and on nematode diversity in particular. 3. Within the first 8 months, meiofaunal succession was only weakly affected, whereas, during the last 4 months, nutrient addition influenced most of the main taxa, with a concomitant change in the assemblage structure. 4. The density of the numerically dominant nematodes decreased upon nutrient enrichment, whereas ostracods became more numerous. Other taxa, including copepods, reached a maximum at intermediate nutrient levels or, in case of oligochaetes, were almost unaffected by nutrient enrichment. However, the changes in the density of the main taxa were usually insufficient to alter their biomass. Consequently, meiofaunal biomass was remarkably unresponsive to nutrient addition, while meiofaunal density displayed a unimodal relationship, with a peak at a TP concentration of 30 μg L^?1. In addition, nematode species richness decreased significantly with increasing nutrient concentrations. 5. We hypothesise that the response of meiofaunal taxa to nutrients is attributable to the development of primary producers, which shifted with enrichment from low densities of edible diatoms and unicellular green algae to large standing stocks of inedible forms, such as Lemna minor and Cladophora spp.     

What are the effects of substrate and grass removal on recruitment of <Emphasis Type="Italic">Acacia mellifera</Emphasis> seedlings in a semi-arid environment?

Acacia mellifera is one of the most important encroaching woody plants in southern African savannas. Previous studies found that this species encroaches far more readily on rocky areas than on sandy substrates, although it grows larger on sandy substrates. Rocky substrates are known to retain more water than sandy substrates, which may be of vital importance during recruitment in semi-arid and arid environments. A number of studies have also indicated that competition with grasses may reduce the recruitment and biomass of tree seedlings. We created an experiment in a semi-arid environment (mean annual rainfall = 388 mm) that tested for the effects of rockiness on A. mellifera recruitment. We also tested the hypothesis that grasses effectively compete with A. mellifera in this environment by simulating the effect of grazing by clipping grasses from half the plots in both the rocky and sandy treatments. Significantly more A. mellifera seedlings established in plots where grasses were clipped than in control plots. A. mellifera seedlings had greater biomass on sandy substrates than on rocky substrates. No significant interaction effects were found between substrate and grass clipping treatment for either seedling number or biomass. We conclude that A. mellifera seedlings are more likely to encroach in habitats with low grass density, although they may achieve greater biomass on sandy soils. Thus, it may be the lower grass density rather than rockiness, which increases the encroachment observed in naturally rocky habitats. These results are also consistent with our observations that adult A. mellifera trees are larger on sandy soils than on rocky soils.     

枸杞岛潮下带沙地生境鱼类群落结构和季节变化

为了解岛礁水域沙地生境的鱼类群落结构特征,评估该生境对鱼类资源养护的潜在作用,于2009年采用多网目组合刺网对枸杞岛潮下带沙质区域进行了逐月采样,同步设置岩礁为对照生境.应用α和β多样性指数结合相对重要性指数、相对渔获率、ABC曲线和聚类排序方法对两种生境中的鱼类组成、多样性变化和群落格局与变化进行了全面探讨.全年在沙地和岩礁生境共采集鱼类63种,隶属11目38科56属,2种生境各自出现的鱼类皆为46种.受暖水种频繁出现在沙地生境的影响,潮下带沙质区域鱼类区系比岩礁生境略显丰富,且春夏季的渔获量普遍高于岩礁生境;由于种类组成均匀度较低,沙地生境各季节的α多样性普遍较低,夏季显著低于岩礁生境.日本须鳎是沙地生境的指示种,为早春、夏末和秋冬季沙地底层优势鱼种.5-7月鳀、多数月份鲻和10月份鳗鲇等种类对沙地生境的阶段性利用,使其形成了区别于以趋礁性鱼类为优势类群的岩礁生境的群落格局和季节动态.沙地是多种鱼类幼鱼阶段的庇护所和饵料场,是鲆鲽类的良好栖息地.沙地生境在维持鱼类区系和养护鱼类资源方面具有重要作用.     

Above- and belowground insect herbivores differentially affect soil nematode communities in species-rich plant communities

Interactions between above‐ and belowground invertebrate herbivores alter plant diversity, however, little is known on how these effects may influence higher trophic level organisms belowground. Here we explore whether above‐ and belowground invertebrate herbivores which alter plant community diversity and biomass, in turn affect soil nematode communities. We test the hypotheses that insect herbivores 1) alter soil nematode diversity, 2) stimulate bacterial‐feeding and 3) reduce plant‐feeding nematode abundances. In a full factorial outdoor mesocosm experiment we introduced grasshoppers (aboveground herbivores), wireworms (belowground herbivores) and a diverse soil nematode community to species‐rich model plant communities. After two years, insect herbivore effects on nematode diversity and on abundance of herbivorous, bacterivorous, fungivorous and omni‐carnivorous nematodes were evaluated in relation to plant community composition. Wireworms did not affect nematode diversity despite enhanced plant diversity, while grasshoppers, which did not affect plant diversity, reduced nematode diversity. Although grasshoppers and wireworms caused contrasting shifts in plant species dominance, they did not affect abundances of decomposer nematodes at any trophic level. Primary consumer nematodes were, however, strongly promoted by wireworms, while community root biomass was not altered by the insect herbivores. Overall, interaction effects of wireworms and grasshoppers on the soil nematodes were not observed, and we found no support for bottom‐up control of the nematodes. However, our results show that above‐ and belowground insect herbivores may facilitate root‐feeding rather than decomposer nematodes and that this facilitation appears to be driven by shifts in plant species composition. Moreover, the addition of nematodes strongly suppressed shoot biomass of several forb species and reduced grasshopper abundance. Thus, our results suggest that nematode feedback effects on plant community composition, due to plant and herbivore parasitism, may strongly depend on the presence of insect herbivores.     

Nearshore fish habitat utilization and species associations in Lake Opinicon (Ontario,Canada)

Synopsis The bulk of Lake Opinicon fish biomass is concentrated in the physically diverse inshore areas. Quantitative analysis of the community compositions of the various inshore habitat types (weedy inlets, sandy shallows, rock shelf, gravel, etc.) showed that each supported a characteristic assemblage of fishes with a relatively constant species and year class composition. This was maintained throughout the season despite a drop in fish biomass in late summer when there was a progressive movement into the offshore waters.Weedbed areas supported the highest biomass, and greatest species and year class diversity. Thereafter, in declining order of richness, were rocky and sandy areas. A few fish species were restricted to single habitat types, e.g.Notropis heterodon to inshore weedbeds.Lepomis macrochirus, the commonest species in the lake was, by contrast, versatile and occurred in all inshore habitats. Commonly the numbers of a species in a habitat differed between day and night. Significant diel movements between habitats characterized two nocturnal feedersPomoxis nigromaculatus andIctalurus nebulosus.