Consequences of omnivory for trophic interactions on a salt marsh shrub

Although omnivory is common in nature, its impact on trophic interactions is variable. Predicting the food web consequences of omnivory is complicated because omnivores can simultaneously produce conflicting direct and indirect effects on the same species or trophic level. We conducted field and laboratory experiments testing the top-down impacts of an omnivorous salt marsh crab, Armases cinereum, on the shrub Iva frutescens and its herbivorous and predatory arthropod fauna. Armases is a "true omnivore," consuming both Iva and arthropods living on Iva. We hypothesized that Armases would benefit Iva through a top-down trophic cascade, and that this benefit would be stronger than the direct negative effect of Armases on Iva. A field experiment on Sapelo Island, Georgia (USA), supported this hypothesis. Although Armases suppressed predators (spiders), it also suppressed herbivores (aphids), and benefited Iva, increasing leaf number, and reducing the proportion of dead shoots. A one-month laboratory experiment, focusing on the most common species in the food web, also supported this hypothesis. Armases strongly suppressed aphids and consumed fewer Iva leaves if aphids were available as an alternate diet. Armases gained more body mass if they could feed on aphids as well as on Iva. Although Armases had a negative effect on Iva when aphids were not present, Armases benefited Iva if aphids were present, because Armases controlled aphid populations, releasing Iva from herbivory. Although Armases is an omnivore, it produced strong top-down forces and a trophic cascade because it fed preferentially on herbivores rather than plants when both were available. At the same time, the ability of Armases to subsist on a plant diet allows it to persist in the food web when animal food is not available. Because omnivores feed on multiple trophic levels, their effects on food webs may differ from those predicted by standard trophic models that assume that each species feeds only on a single trophic level. To better understand the complexity of real food webs, the variable feeding habits and feeding preferences of different omnivorous species must be taken into consideration.     

Grass shrimp (Palaemonetes spp.) play a pivotal trophic role in enhancing Ruppia maritima

Coupled trophic-engineer interactions are potentially important for maintaining habitat function and ecosystem services. As ephemeral submerged aquatic vegetation (SAV), Ruppia maritima has a short well-defined growth-senescence cycle and should benefit from any ecological interaction that enhances its physical condition and longevity. Grass shrimp (Palaemonetes spp.) are abundant facultative grazers of epiphytic algae and conveyors of nutrients in tidal marsh and SAV habitats. Grass shrimp addition consistently enhanced Ruppia biomass and shoot density in a series of three field experiments conducted in Grand Bay National Estuarine Research Reserve, Mississippi, USA. In two experiments, epiphyte grazing by grass shrimp enhanced Ruppia by inhibiting die-back during the mid- and latter stages of the Ruppia life cycle. Despite a nonsignificant epiphyte grazing effect, grass shrimp also enhanced Ruppia during its early growth stage in a third experiment. In that experiment, nutrient addition also significantly increased epiphyte biomass. Grass shrimp may have fostered the early growth of Ruppia through direct deposition of feces to the sediment in the third experiment. Grass shrimp play a pivotal trophic role in the maintenance of Ruppia through context-dependent interactions involving stage of the SAV life cycle, season, and nutrient limitation.     

Competition, predation and coexistence in a three trophic system

Simple ecological models that mostly operate with population densities using continuous variables, explain quite well the behavior of real populations. In this work we propose and discuss the continuous dynamics of a system of three species, which belongs to the well-known family of Lotka–Volterra models. In particular, the proposed model includes direct effects such as predation and competition among species, and indirect effects such as refuge. The model is proposed to explain recent studies about a group of crustacean (amphipods of genus Hyallela) found in all the plain streams and shallow lakes of the American continent. The studied system includes three compartments: algae, a strictly herbivore amphipod and an omnivore (herbivore and carnivore) one. The analysis of the model shows that there are stable extinction equilibria throughout all the parameters’ space. There are also equilibria with stable coexistence of the three species and two interesting binary equilibria: one with stable coexistence of algae and herbivore and other with coexistence between algae and omnivore amphipods. The presence of Allee effect in the algae growth and the existence of refuge for the herbivore amphipod (prey) determine a bottom-up control.     

Trophic diversity in amphipods within a temperate eelgrass ecosystem as determined by gut contents and C and N isotope analysis

A conjoint analysis of gut contents and stable C and N isotopes was applied to determine the main food sources and feeding habits of dominant amphipods in an eelgrass bed (Zostera marina) in Gwangyang Bay, Korea. Gut content observations demonstrated that, while Gammaropsis japonicus and Jassa slatteryi are herbivorous, feeding on epiphytes and detritus, Pontogeneia rostrata and Monocorophium acherusicum are omnivorous, feeding on mesozooplankton fragments and detritus. Stable isotope data confirmed that epiphytes, detritus, and mesozooplankton fragments were major food sources for amphipods in the eelgrass bed. Isotopic mixing model calculations clearly showed an interspecific difference in diet composition. A high isotopic dissimilarity between amphipod taxa demonstrated interspecific trophic diversity, reflecting their herbivorous (G. japonicus and J. slatteryi) and omnivorous (P. rostrata and M. acherusicum) feeding habits and confirmed the detrivorous feeding habits of caprellids. Such trophic diversity at interspecific level of the amphipod species indicates that they use different food resources within their microhabitats and play species-specific functional roles as mediators in trophic pathways from producers to higher-level consumers of the eelgrass ecosystem. Finally, our findings suggest that information on the species-specific trophic ecology of amphipods is needed to better understand their potential role in the trophic dynamics and carbon flow of seagrass bed ecosystems.     

Relative and interactive effects of plant and grazer richness in a benthic marine community

The interactive effects of changing biodiversity of consumers and their prey are poorly understood but are likely to be important under realistic scenarios of biodiversity loss and gain. We performed two factorial manipulations of macroalgal group (greens, reds, and browns) and herbivore species (amphipods, sea urchin, and fish) composition and richness in outdoor mesocosms simulating a subtidal, hard-substratum estuarine community in North Carolina, U.S.A. In the experiment where grazer richness treatments were substitutive, there were no significant effects of algal or herbivore richness on final algal biomass. However, in the experiment in which grazer treatments were additive (i.e., species-specific densities were held constant across richness treatments), we found strong independent and interactive effects of algal and herbivore richness. Herbivore polycultures reduced algal biomass to a greater degree than the sum of the three herbivore monocultures, indicating that the measured grazer richness effects were not due solely to increased herbivore density in the polycultures. Taking grazer density into account also revealed that increasing algal richness dampened grazer richness effects. Additionally, the effect of algal richness on algal biomass accumulation was far stronger when herbivores were absent, suggesting that grazers can utilize the increased productivity and mask the positive effects of plant biodiversity on primary production. Our results highlight the complex independent and interactive effects of biodiversity between adjacent trophic levels and emphasize the importance of performing biodiversity-ecosystem functioning experiments in a realistic multi-trophic context.     

Cascading effects from predator removal depend on resource availability in a benthic food web

We tested joint effects of predator loss and increased resource availability on the grazers’ trophic level and the propagation of trophic interactions in a benthic food web by excluding larger predatory fish from cages and manipulating nutrients in the coastal zone of the Baltic Sea. The combination of nutrient enrichment and excluding larger predators induced an increase in medium-sized predatory fish (three-spined stickleback). The meso-predator fish in turn did not change the total abundance of the invertebrate herbivores, but did cause a substantial shift in their community composition towards the dominance of gastropods by reducing amphipods by 40–60%, while gastropods were left unchanged. The shift in grazer composition generated a 23 times higher producer biomass, but only under nutrient enrichment. Our results show that top-predator declines can substantially shift the species composition at the grazers’ level, but that cascading effects on producers by a trophic cascade strongly depend on resource availability.     

Predators have large effects on ecosystem properties by changing plant diversity, not plant biomass

Cascading effects of predators on total plant trophic-level biomass tend to be weaker in terrestrial than in aquatic systems. Accordingly, it is hypothesized that top predator effects on terrestrial plant diversity and on ecosystem function should likewise be weak or unimportant. This report presents a test of this hypothesis using data from a long-term field experiment. The five-year experiment manipulated the trophic structure of an old field ecosystem by excluding either predators or predators and herbivores relative to an unmanipulated, natural control. Long-term manipulations led to systematic treatment effects on community properties (plant trophic-level biomass, plant species biomass, plant species evenness) and on ecosystem properties (supply rate of solar radiation, N mineralization rate). The strengths of top predator effects on community properties were modest compared with nonterrestrial systems. But, predator-caused changes in plant community structure via alteration of plant dominance, and hence plant species evenness, strengthened effects on ecosystem properties. Counter to the hypothesis, weak trophic cascades do not necessarily lead to weak indirect effects of predators on ecosystem properties.     

Consumers affect prey biomass and diversity through resource partitioning

Consumer presence and nutrient availability can have contrasting and interactive effects on plant diversity. In a factorial experiment, we manipulated two levels of nutrient supply and the presence of two moderately specialized grazers in different combinations (no grazers, two species in monoculture, and both in combination). We tested how nutrients and grazers regulated the biomass of marine coastal epiphytes and the diversity of algal assemblages, based on the prediction that the effect of consumers on prey diversity depends on productivity and consumer specialization. Nutrient enrichment increased the epiphytic load, while monocultures of single grazer species partly prevented epiphyte growth. However, only the presence of two species with complementary feeding preferences effectively prevented epiphyte overgrowth. The epiphytes comprised micro- and macroalgal species, and the diversity of these algal assemblages differed, depending on grazer identity. For the microalgae, diversity was reduced by nutrient addition when grazer control was inefficient, but not when specialist microalgal grazers were present. Macroalgal diversity was reduced in ambient water with specialist macroalgal grazers compared to the treatment with inefficient ones. These results indicate that grazer composition and productivity are crucial in determining whether consumer pressure will have a positive or negative effect on algal diversity.     

Direct and indirect effects of pesticides on a benthic grazer during its life cycle

Background

Macroinvertebrates in aquatic ecosystems are repeatedly exposed to pesticides during their life cycle. Effects of consecutive exposure during different life stages and possible synergistic effects are not addressed in the standardized hazard assessment. The present study investigated two environmentally relevant exposure scenarios in batch (microcosm) and artificial indoor stream (mesocosm) experiments using the larvae of the mayfly Rhithrogena semicolorata (grazer) and natural aufwuchs. Grazers were analysed regarding growth, physiological condition, and drift behaviour, while the aufwuchs was analysed in terms of biomass using the particulate organic carbon as well as the chlorophyll a content. The aim was to reveal direct and indirect effects of an herbicide exposure during autumn on juvenile grazers and an insecticide exposure during spring on semi-juvenile grazers.

Results

Direct and indirect effects were found in both exposure scenarios at environmentally relevant concentrations. In the herbicide exposure scenario with terbutryn, clear direct effects on the aufwuchs community with a LOEC of 0.38 µg L^?1 were found. Effect levels of grazers due to indirect effects were equal, with the overnight drift being the most sensitive grazer endpoint. In the insecticide exposure scenario, clear lethal and sub lethal effects of lambda-cyhalothrin were evident. Derived LC₅₀ values for the artificial indoor stream and batch experiment were 2.42 µg g^?1 OC (69 days) and 1.2 µg g^?1 OC (28 days), respectively. Sub lethal effects in terms of increased drift as well-reduced growth and triglyceride levels were found at concentrations of 1.4 and 0.09 µg g^?1 OC (LOECs). These results were confirmed by the batch experiment, which revealed effect values in the similar range. Finally, a clear indirect effect of the insecticide on the aufwuchs was evident in the batch experiment with an LOEC at 0.9 µg g^?1 OC.

Conclusion

Toxicity Exposure Ratios calculated with the derived effect values indicate a risk for the investigated grazer by both pesticides. Moreover, observed indirect effects during the herbicide exposure seem to be able to affect the grazers during a second exposure with an insecticide, due to reduced physiological conditions. We suggest further research with time-shifted exposure scenarios to gain a better understanding of the complex interactions of pesticides with the life cycle and the food webs of macroinvertebrates.

     

Feeding biology of <Emphasis Type="Italic">Ophiura sarsii</Emphasis> Lütken, 1855 on Banquereau bank and the effects of fishing

Ophiura sarsii was collected in May/June from Banquereau Bank off Nova Scotia, Canada as part of the benthic fauna analyzed in a 2 year clam dredging-impact experiment. In total, 529 O. sarsii specimens were analyzed; 174 (33%) had stomach contents while the rest were empty. Ophiura sarsii is a trophic generalist feeding on at least 31 taxa (52% identified to species) drawn from four phyla: Arthropoda (81.8%), Annelida (13.2%), Mollusca (3%), and Cnidaria (1.5%). However, the diet was largely restricted to two amphipod families (Ampeliscidae and Lysianassidae), accounting for 54–70% of stomach contents at any one sampling period. This diet provides indirect evidence for carnivory. Limited spatial and temporal heterogeneity in the proportion of animals feeding showed no consistent pattern. Where diet composition differed (ANOSIM) it was attributed to the proportion of lysianassid and ampeliscid amphipods consumed. Dredging introduced new dietary items, although diet composition was not significantly altered.     

Feeding ecology of the grooved tiger shrimp <Emphasis Type="Italic">Penaeus semisulcatus</Emphasis> De Haan (Decapoda: Penaeidae) in inshore waters of Qatar,Arabian Gulf

The feeding ecology of the green tiger shrimp Penaeus semisulcatus was studied in inshore fishing grounds off Doha, Qatar, using a combination of stable isotope (δ¹³C and δ¹⁵N) analysis and gut contents examination. Samples of post-larvae, juvenile and adult shrimp and other organisms were collected from intertidal and subtidal zones during the spawning season (January–June). Shrimp collected from shallow water seagrass beds were mostly post-larvae and juveniles and were significantly smaller than the older juveniles and adults caught in deeper macroalgal beds. Gut content examination indicated that post-larvae and juvenile shrimp in seagrass beds fed mainly on benthos such as Foraminifera, polychaetes, benthic diatoms and small benthic crustaceans (amphipods, isopods and ostracoda), whereas larger shrimp in the macroalgal beds fed mainly on bivalve molluscs and to a lesser extent polychaetes. In shrimp from both seagrass and algal beds, unidentifiable detritus was also present in the gut (18, 32%). δ¹³C values for shrimp muscle tissue ranged from −9.5 ± 0.26 to −12.7 ± 0.05‰, and δ¹⁵N values increased with increasing shrimp size, ranging from 4.1 ± 0.03 to 7.7 ± 0.11‰. Both δ¹⁵N values and δ¹³C values for shrimp tissue were consistent with the dietary sources indicated by gut contents and the δ¹³C and δ¹⁵N values for primary producers and prey species. The combination of gut content and stable isotope data demonstrates that seagrass beds are important habitats for post-larvae and juvenile P. semisulcatus, while the transition to deeper water habitats in older shrimp involves a change in diet and source of carbon and nitrogen that is reflected in shrimp tissue stable isotope ratios. The results of the study confirm the linkage between sensitive shallow water habitats and the key life stages of an important commercially-exploited species and indicate the need for suitable assessment of the potential indirect impacts of coastal developments involving dredging and land reclamation.     

Conflict of interest between a nematode and a trematode in an amphipod host: test of the "sabotage" hypothesis

Microphallus papillorobustus is a manipulative trematode that induces strong behavioural alterations in the gamaridean amphipod Gammarus insensibilis, making the amphipod more vulnerable to predation by aquatic birds (definitive hosts). Conversely, the sympatric nematode Gammarinema gammari uses Gammarus insensibilis as a habitat and a source of nutrition. We investigated the conflict of interest between these two parasite species by studying the consequences of mixed infection on amphipod behaviour associated with the trematode. In the field, some amphipods infected by the trematode did not display the altered behaviour. These normal amphipods also had more nematodes, suggesting that the nematode overpowered the manipulation of the trematode, a strategy that would prolong the nematode's life. We hypothesize that sabotage of the trematode by the nematode would be an adaptive strategy for the nematode consistent with recent speculation about co-operation and conflict in manipulative parasites. A behavioural test conducted in the laboratory from naturally infected amphipods yielded the same result. However, exposing amphipods to nematodes did not negate or decrease the manipulation exerted by the trematode. Similarly, experimental elimination of nematodes from amphipods did not permit trematodes to manipulate behaviour. These experimental data do not support the hypothesis that the negative association between nematodes and manipulation by the trematode is a result of the "sabotage" hypothesis.     

Preliminary Evaluation of the Use of Phosphogypsum for Reef Substrate. I. A Laboratory Study of Bioaccumulation of Radium and Six Heavy Metals in an Aquatic Food Chain

Phosphogypsum (PG), a solid by-product of phosphoric acid production, contains radionuclides and trace metals in concentrations which may pose a potential hazard to human health and the environment. to investigate the possibility of bioaccumulation of radium and six heavy metals over time when aquatic organisms experience both trophic and environmental exposure to PG, we designed a laboratory experiment representing three levels of an aquatic food chain. During the 135 day experiment, a meiobenthic copepod species (Amphiascoides atopus) was cultured in the presence of PG. the copepods were subsequently fed to grass shrimp (Palaemonetes vulgaris and P. pugio) which were in turn fed to gulf killifish (Fundulus grandis); both the grass shrimp and the killifish also experienced an environmental PG exposure. Other than elevated radium levels in the experimental grass shrimp, the experiment demonstrated little effect of environmental or trophic exposure to PG on microinvertebrates, macroinvertebrates, or fishes that could be attributed to PG. in all cases where increased concentrations were indicated within the experimental group, roughly equivalent increases in metal concentrations also occurred in the control group.     

Preliminary Evaluation of the Use of Phosphogypsum for Reef Substrate. I. A Laboratory Study of Bioaccumulation of Radium and Six Heavy Metals in an Aquatic Food Chain

Phosphogypsum (PG), a solid by-product of phosphoric acid production, contains radionuclides and trace metals in concentrations which may pose a potential hazard to human health and the environment. to investigate the possibility of bioaccumulation of radium and six heavy metals over time when aquatic organisms experience both trophic and environmental exposure to PG, we designed a laboratory experiment representing three levels of an aquatic food chain. During the 135 day experiment, a meiobenthic copepod species (Amphiascoides atopus) was cultured in the presence of PG. the copepods were subsequently fed to grass shrimp (Palaemonetes vulgaris and P. pugio) which were in turn fed to gulf killifish (Fundulus grandis); both the grass shrimp and the killifish also experienced an environmental PG exposure. Other than elevated radium levels in the experimental grass shrimp, the experiment demonstrated little effect of environmental or trophic exposure to PG on microinvertebrates, macroinvertebrates, or fishes that could be attributed to PG. in all cases where increased concentrations were indicated within the experimental group, roughly equivalent increases in metal concentrations also occurred in the control group.     

The effect of micrograzers on algal community structure in a coral reef microcosm

The effect of amphipod grazing on algal community structure was studied within a 75 l refuge tank connected to a 6500 l closed-system, coral reef microcosm. When amphipods (Ampithoe ramondi) were absent or present in low numbers, a high biomass of mostly filamentous algal species resulted, including Bryopsis hypnoides, Centroceras clavulatum, Ceramium flaccidum, Derbesia vaucheriaeformis, Enteromorpha prolifera, Giffordia rallsiae, and Polysiphonia havanensis. These microalgae disappeared when amphipod density increase beyond approximately 1 individual cm^-2 of tank surface. The macroalga Hypnea spinella germinated in the system in association with amphipod tube sites. H. spinella plants remained rare until filamentous species were eliminated by amphipod grazing. Feeding trials confirmed that H. spinella was protected from grazing by its size rather than a chemical defense strategy. The H. spinella community we observed is similar to the flora described on algal ridges where physical conditions exclude fish grazing. We suggest that amphipods and similar micrograzers are responsible for the algal community structure of these ridges. Caging experiments may be subject to similar effects from increased amphipod grazing on the algae. Introduction of fish that are amphipod predators into the refuge tank caused an increase in algal species diversity but total H. spinella growth rates fell from 25 g dry wt month^-1 to less than 8 g dry wt month^-1. We describe amphipod behavior in relation to changes in population density and food supply, and we stress the potential for increasing the productivity of commercial seaweeds through maintenance of appropriate amphipod species in mariculture facilities.     

Grazing rates of organic matter and living fungal biomass of decaying Spartina alterniflora by three species of salt-marsh invertebrates

 The pathway for the flow of salt-marsh grass production into marsh food-webs is still not well defined. We compared the abilities of three marsh macroinvertebrates [salt marsh periwinkles, Littoraria irrorata (Say) (=Littorina irrorata), salt-marsh coffee-bean snails, Melampus bidentatus (Say); and a talitrid amphipod, Uhlorchestia spartinophila Bounsfield and Heard] to access standing-dead leaves of smooth cordgrass (Spartina alterniflora Loisel). The invertebrates were incubated with naturally-decaying leaves, and the rates of removal of organic matter and living fungal biomass (ergosterol) were measured. The impact of invertebrate activity upon fungal growth rates was measured as rates of fungal-membrane synthesis (incorporation of radioacetate into ergosterol). The removal rates of organic leaf biomass per mg individual biomass were highest for amphipods (700 μg mg⁻¹ d⁻¹) and lowest for periwinkles (90 μg mg⁻¹ d⁻¹), but the relatively large biomass of the snails made their removal rates per individual greater than those of amphipods. Net removal of ergosterol by all three invertebrates was >50% for yellow-brown (early-decay) leaf blades. For fully-brown (advanced-decay) blades, >50% removal of ergosterol was found only for periwinkles; exposure to coffee-bean snails and amphipods resulted in a net ergosterol reduction of ≤20%. The lower net reduction of living fungal biomass by coffee-bean snails and amphipods may have been due to fungal-growth stimulation (2.3-fold stimulation in coffee-bean snails and 1.5-fold stimulation in amphipods). Grazing by periwinkles did not stimulate fungal growth, possibly because of its high intensity. Grazing by these three salt-marsh shredders may affect marsh-grass shoot-decay in different ways. Periwinkles may abbreviate the period of fungal production, and incorporate the decaying material relatively quickly into snail biomass and fecal-pellet rain to the sediments. Coffee-bean snails and amphipods may enhance and prolong fungal production, along with the formation of fecal-pellet rain. All three invertebrates fed preferentially on leaf blades rather than leaf sheaths, and feeding rates of gastropods were higher during the night than during the day. Received: 25 November 1998 / Accepted: 4 November 1999     

Trophic relationships among invertebrates at the Kairei hydrothermal vent field (Central Indian Ridge)

Exploration of hydrothermal vent systems in locations remote from well-studied sites allows ecologists to determine the degree of site-specific variation in trophic relationships among communities. A preliminary outline of the trophic structure of the Kairei hydrothermal vent community on the Central Indian Ridge (25°19.23′S; 70°02.42′E) is provided here, based on analysis of collections from an April 2001 expedition. Invertebrate biomass at Kairei is dominated by organic carbon with a δ¹³C isotopic value of about –13‰, due to the abundance of primary consumers (shrimp: Rimicaris aff. exoculata) and secondary consumers (anemones: Marianactis n. sp.) with this isotopic composition. Filamentous thiotrophic episymbionts on shrimp have been interpreted to be the major diet items of the shrimp and hence are the dominant primary producers within the community. Free-living autotrophic microorganisms are implicated as the dietary base for other invertebrate species. Four trophic groups are identified within the Kairei invertebrates based on carbon- and nitrogen-isotope ratios, but these groups do not always define discrete trophic levels. Ontogenetic shifts in diet are documented for R. aff. exoculata and brachyuran crabs (Austinograea n. sp.). Diets of symbiont-bearing mussels (Bathymodiolus aff. brevior) and two species of gastropods are isotopically constant throughout the range of sizes analyzed. There is a consistent but unexplained pattern of increasing nitrogen isotopic composition with increasing carbon isotopic composition in vent communities from geographically disjunct oceanic regions. Given the assumptions associated with interpretations of isotopic data, there remains a missing pool of carbon (presumably unsampled bacterial biomass) that contributes to the maintenance of the ¹³C- and ¹⁵N-enriched primary consumers in these ecosystems.     

Food web structure of a restored macroalgal bed in the eastern Korean peninsula determined by C and N stable isotope analyses

Loss of macroalgae habitats has been widespread on rocky marine coastlines of the eastern Korean peninsula, and efforts for restoration and creation of macroalgal beds have increasingly been made to mitigate these habitat losses. Deploying artificial reefs of concrete pyramids with kelps attached has been commonly used and applied in this study. As a part of an effort to evaluate structural and functional recovery of created and restored habitat, the macroalgal community and food web structure were studied about a year after the establishment of the artificial macroalgal bed, making comparisons with nearby natural counterparts and barren ground communities. Dominant species, total abundance, and community structure of macroalgal assemblage at the restored macroalgal bed recovered to the neighboring natural bed levels during the study period. The main primary producers (phytoplankton and macroalgae) were isotopically well separated. δ¹³C and δ¹⁵N values of consumers were very similar between restored and natural beds but varied greatly among functional feeding groups. The range of consumer δ¹³C was as wide as that of primary producers, indicating the trophic importance of both producers. There was a stepwise trophic enrichment in δ¹⁵N with increasing trophic level. A comparison of isotope signatures between primary producers and consumers showed that, while suspension feeders are highly dependent on pelagic sources, invertebrates of other feeding guilds and fishes mainly use macroalgae-derived organic matter as their ultimate nutritional sources in both macroalgal beds, emphasizing the high equivalency of trophic structure between both beds. Isotopic signatures of a few mollusks and sea urchins showed that they use different dietary items in macroalgal-barren grounds compared with macroalgal beds, probably reflecting their feeding plasticity according to the low macroalgal biomass. However, isotopic signatures of most of the consumers at the barren ground were consistent with those at the macroalgal beds, supporting the important trophic role of drifting algae. Our results revealed the recoveries of the macroalgal community and trophic structure at the restored habitat. Further studies on colonization of early settlers and the following succession progress are needed to better understand the process and recovery rate in the developing benthic community.     

Grazing regulates the spatial variability of periphyton biomass

The presence of consumers not only alters the mean biomass of the prey assemblage, but also affects the spatial heterogeneity of biomass distribution. Whereas the mean prey biomass is generally reduced by consumer presence, the effect on spatial heterogeneity is less clear-cut. A meta-analysis of almost 600 field experiments manipulating the presence of benthic invertebrate or vertebrate grazers was conducted to analyze the effect of grazers on both the absolute spatial variability of periphyton biomass and the relative variability, which was standardized to the mean. Effects on absolute variability were measured as the log response ratio of the standard deviation of biomass (LR-SD), whereas effects on relative variability were measured as the log response ratio of the coefficient of variation of biomass (LR-CV). The overall magnitude and range of LR-SD and LR-CV indicated that grazers not only reduced periphyton biomass, but also substantially altered their spatial distribution. However, grazer effects differed strongly for absolute and relative variability. On average, grazers reduced the absolute spatial variability in prey biomass by 50% (average LR-SD = -0.68) but increased the relative variability by 24% (average LR-CV = 0.22). The magnitude of LR-SD strongly depended on the efficiency of grazing, with strong biomass removal leading to strong homogenization. Moreover, LR-CV and LR-SD were significantly affected by habitat type (freshwater vs. coastal) and substrata. Given the importance of spatial heterogeneity for resource uptake, competition and the maintenance of diversity, grazer presence has potentially strong indirect effects on the interactions within prey assemblages.     

Midwater food web in McMurdo Sound,Ross Sea,Antarctica

The trophic structure of the midwater ecosystem of McMurdo Sound, Ross Sea, Antarctica in February, 1983 was examined through diet analysis of 35 species of zooplankton and micronekton. Ten feeding groups were suggested through cluster analysis. The two largest clusters consisted of small-particle grazers and omnivore generalists; the eight remaining clusters were of carnivores specializing on one or several types of metazoan prey. Diet composition often shifted with ontogeny and though exceptions occurred, diet diversity usually increased either during early growth or throughout development. Comparison with a krill-dominated area along the Antarctic Penisula (Croker Passage) indicated that species common to the two areas occupied approximately the same trophic position. Biomass in McMurdo Sound was much lower than in Croker Passage and large-sized particle grazers such as krill and salps were trophically less dominant in McMurdo Sound. Krill in Croker Passage in the fall entered the midwater food web primarily as detritus (molts); in McMurdo Sound it was mostly as live furcilia. Sampling in McMurdo Sound was during a plankton bloom and calculations of grazing rates suggest that much of the primary production settled through the water column uneaten.