Feeding rates of amphipods in boreal lakes: is there a seasonal shift independent of temperature and photoperiod?

In boreal lakes, temperature and photoperiod are important environmental cues affecting reproduction, growth and feeding rates of benthic invertebrates; however, how invertebrates cope with seasonal changes in the availability of their food sources has rarely been addressed. In this study, where temperature and light conditions were controlled, we aimed at assessing whether Hyalella azteca amphipods from the littoral zone of boreal lakes adjust their feeding rates in response to the dynamics of their food sources and their population structure. Using a gravimetric approach, we compared the ingestion and absorption rates of amphipods from a natural population collected periodically during the ice-free season. Amphipods were fed in the laboratory with conditioned leaf detritus. Feeding rates increased regardless of seasonal variations of temperature and photoperiod in the lake. Field data suggest this increase could be linked to seasonal changes in the population structure or in the diet of amphipods. This suggests that H. azteca amphipods adjusted their feeding rates with their energy and nutrient requirements and/or developed a compensatory feeding trade-off to cope with the dynamics of their different food sources in lakes.     

Morphological and trophic specialization in a subterranean amphipod assemblage

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Trophic relationships between macroinvertebrates and fish in St. Petersburg methane seep community in abyssal zone of Lake Baikal

Trophic relationships were examined for macroinvertebrates and fish inhabiting the St. Petersburg methane seep (central part of Lake Baikal, ~1400 m depth). The analyses of the values of carbon and nitrogen stable isotopes showed that all animals associated with the seep were heterotrophs with different feeding strategies; symbiotrophes were absent. Seep animals consumed combined food with different portion of methanederived carbon ranged from 2.7 to 89%. The average δ¹³С values varied in the range of–26.2‰ (in benthopelagic amphipods) to–64.5‰ (in gastropods). The trophic food web in the methane seep consists of filter feeders (pelagic amphipods, trophic position (TP) is 1.9), detritophages (gastropods, TP of 2.2, and burrowing amphipods, TP of 2.1), polyphages and necrophages (nectobenthic and benthopelagic amphipods of TP 2.8–3.2), and predators (planarians of TP 3.6–4.2 and cottoid fish of TP 3.0–3.8). Animals occupying similar trophic positions significantly differed in the δ¹³С values and have partially overlapping components of food.     

Reproductive strategies and energetic adaptations of polar zooplankton

Summary

Key factors governing polar ocean ecosystems are low temperatures and a pronounced seasonal variability of ice cover, light regime and primary production. Depending on their ecological niche and trophic position, zooplankton species at high latitudes have developed a variety of reproductive strategies and energetic adaptations to cope with these extreme environmental conditions. Life-cycle strategies of the herbivorous copepods and euphausiids, which make up the major portion of polar zooplankton biomass, include seasonal vertical migration, dormancy (diapause, quiescence) and the accumulation of energy reserves. These lipid stores help to buffer the pulsed seasonal food supply, and they play an important role in fueling reproduction independent of phytoplankton. Only a smaller fraction of the lipid reserves accumulated during spring and summer are usually catabolized for metabolic maintenance during the food-limited dark season. These deposits are retained until the end of winter and allow early egg production and spawning prior to—or coinciding with—the onset of vernal primary production. It enables the new generation to make full use of the short productive season for growth and development to reach viable overwintering stages. The Antarctic krill Euphausia superba is an exception since it uses its depot lipids for metabolic maintenance during the dark season. It therefore relies on external resources (Primary production) for reproductive processes, resulting in a later spawning period as compared to the other euphausiids. Another important component of the herbivorous Antarctic zooplankton, the salps, have developed a very different reproductive strategy. They are able to switch from sexual reproduction to asexual budding (metagenesis), which allows extreme multiplication rates under favourable feeding conditions. Due to these successful adaptations, herbivores are able to build up huge stocks, in spite of the short productive period. Omnivorous and carnivorous zooplankton species, e.g., amphipods or chaetognaths, are not much constrained by the seasonality problem, but experience a more constant food supply. They show a tendency towards K strategies with a prolonged reproductive period, reduced egg numbers and increasing parental care. However, they do not exhibit such typical “polar adaptations” as developed by the herbivorous species.     

Seasonal adaptations and the role of lipids in oceanic zooplankton

Oceanic zooplankton species exhibit quite diverse life history traits. A major driving force determining their life strategies is the seasonal variability in food supply, which is most pronounced in polar oceans where fluctuations in primary production are extreme. Seasonal adaptations are closely related to the trophic level of zooplankters, with strongest pressures occurring on herbivorous organisms. The dominant grazers, calanoid copepods and krill (Euphausiacea), have developed fascinating solutions for successful overwintering at higher latitudes. They usually exhibit a very efficient storage and utilization of energy reserves to reduce the effect of a highly seasonal primary production. The predominant larger Calanus species from the Arctic and Calanoides acutus from the Antarctic biosynthesize large amounts of high-energy wax esters with long-chain monounsaturated fatty acids and alcohols (20:1 and 22:1 isomers) as major components. They survive the dark season at depth in a stage of dormancy called diapause. In contrast, the Antarctic Calanus propinquus, a winter-active species, synthesizes primarily triacylglycerols, which are dominated by long-chain monounsaturated fatty acids with 22 carbon atoms (2 isomers) and yield even higher calorific contents. The omnivorous and carnivorous species, which are less subjected to seasonal food shortage, usually do not exhibit such an elaborate lipid biosynthesis. Herbivores usually do not utilize much of their enormous lipid reserves for overwintering, but channel this energy towards reproductive processes in late winter/early spring. Timing of reproduction is critical especially at high latitudes due to the short production period, and lipid reserves ensure early spawning independent of external resources. These energetic adaptations (dormancy, lipid storage) are supplemented by other life strategies such as extensive vertical migrations, change in the mode of life, and trophic flexibility.     

δ13C and δ15N indicators of fish and shrimp community diet and trophic structure in a mangrove ecosystem in Thailand

Stable carbon and nitrogen isotope ratios were used to elucidate primary carbon sources and trophic relationships of the fish and shrimp community in the Klong Ngao mangrove ecosystem, southern Thailand. There were no significant differences in isotopic compositions of biota between mangrove and offshore sites (Welch–Aspin test). The δ¹⁵N values of eight fish species and two shrimp species at both sites were also not significantly different by the test, meaning that at both sites they feed on the same diets due to the discharge of large quantities of mangrove sediments. The δ¹⁵N isotopic enrichment of consumers suggested that there are four trophic levels in the Klong Ngao food web, with at least two fish species capable of switching feeding strategies and thus altering their apparent trophic positions. Phytoplankton culture experiments indicated that mangrove-derived sediments could play an important role in stimulating phytoplankton growth for low turbidity offshore areas, thus providing an alternate food source. The isotopic associations among sources and consumers indicated that mangroves were the major carbon source supporting aquatic food webs in the Klong Ngao ecosystem.     

Nutrient Dynamics of Estuarine Invertebrates Are Shaped by Feeding Guild Rather than Seasonal River Flow

This study aimed to determine the variability of carbon and nitrogen elemental content, stoichiometry and diet proportions of invertebrates in two sub-tropical estuaries in South Africa experiencing seasonal changes in rainfall and river inflow. The elemental ratios and stable isotopes of abiotic sources, zooplankton and macrozoobenthos taxa were analyzed over a dry/wet seasonal cycle. Nutrient content (C, N) and stoichiometry of suspended particulate matter exhibited significant spatio-temporal variations in both estuaries, which were explained by the variability in river inflow. Sediment particulate matter (%C, %N and C:N) was also influenced by the variability in river flow but to a lesser extent. The nutrient content and ratios of the analyzed invertebrates did not significantly vary among seasons with the exception of the copepod Pseudodiaptomus spp. (C:N) and the tanaid Apseudes digitalis (%N, C:N). These changes did not track the seasonal variations of the suspended or sediment particulate matter. Our results suggest that invertebrates managed to maintain their stoichiometry independent of the seasonality in river flow. A significant variability in nitrogen content among estuarine invertebrates was recorded, with highest % N recorded from predators and lowest %N from detritivores. Due to the otherwise general lack of seasonal differences in elemental content and stoichiometry, feeding guild was a major factor shaping the nutrient dynamics of the estuarine invertebrates. The nutrient richer suspended particulate matter was the preferred food source over sediment particulate matter for most invertebrate consumers in many, but not all seasons. The most distinct preference for suspended POM as a food source was apparent from the temporarily open/closed system after the estuary had breached, highlighting the importance of river flow as a driver of invertebrate nutrient dynamics under extreme events conditions. Moreover, our data showed that estuarine invertebrates concentrated C and N between 10–100 fold from trophic level I (POM) to trophic level II (detritivores/deposit feeders) and thus highlighted their importance not only as links to higher trophic level organisms in the food web, but also as providers of a stoichiometrically homeostatic food source for such consumers. As climate change scenarios for the east coast of South Africa predict increased rainfall as a higher number of rainy days and days with higher rainfall, our results suggest that future changes in rainfall and river inflow will have measurable effects on the nutrient content and stoichiometry of food sources and possibly also in estuarine consumers.     

Trophic structure of the macrobenthic community of Hornsund,Spitsbergen, based on the determination of stable carbon and nitrogen isotopic signatures

Hornsund is a cold-water fjord in southwestern Spitsbergen, Svalbard Archipelago, with a resident biota that exhibit typical low-temperature Arctic features. Carbon (δ¹³C) and nitrogen (δ¹⁵N) isotopic signatures of macrobenthic fauna and its potential food sources were measured in summer 2008 to delineate the trophic structure of the bottom community and to identify its principal carbon sources. The soft-bottom fauna at a water depth of 100 m was found to rely primarily on detritus, which is supplied by sedimentation of suspended organic matter from the water column and horizontal transport of refractory macroalgae from euphotic coastal habitats. Through resuspension by bottom currents, deposited particles also contributed to the diet of benthic filter-feeders. Since benthic organisms were significantly enriched in ¹³C compared to epibenthic zooplankton (mainly amphipods and decapods), the stable carbon signature provides a tool to differentiate benthic and pelagic feeding habits. The benthic food web was characterized by a conventional trophic structure with decreasing species numbers in increasing trophic levels. Primary consumers feeding on a mixture of plant matter, fecal pellets, decaying animal tissue, bacteria, and protists accounted for the greatest biomass share (62 % of the total macrobenthic biomass), followed by secondary consumers (38 %). Based on δ¹⁵N signatures, three trophic levels were detected, corresponding to the following feeding guilds: filter-feeders and feeding generalists (mainly bivalves, crustaceans, polychaetes, and some fish), mixed detritivore–carnivores (polychaetes, priapulids, crustaceans, and ophiuroids) and obligate carnivores (ascidians). The average food chain length (4.5 trophic levels) suggests that high-quality food is readily available in this Arctic fjord ecosystem.     

A stable isotope approach to the eastern Weddell Sea trophic web: focus on benthic amphipods

Stable isotope (¹³C/¹²C and ¹⁵N/¹⁴N) analyses were performed on 90 species belonging to different benthic communities sampled in the eastern Weddell Sea. The study focused on eight amphipod species whose isotopic composition was compared to their previously described respective gut contents. Amphipod stable isotope ratios correspond fairly accurately to the trophic classification based on gut contents and attest to their wide spectrum of feeding types. Since the fundamental difference between the isotope and the gut content approaches to diet studies is the time scale each method addresses, this coincidence indicates that there would be no significant changes in feeding strategies over time. Three levels of the food web are covered by the eight species and, instead of belonging strictly to one trophic category, amphipods display a continuum of values from the suspension-feeder to scavengers.     

Comparative Feeding Ecology of Bull Sharks (Carcharhinus leucas) in the Coastal Waters of the Southwest Indian Ocean Inferred from Stable Isotope Analysis

As apex predators, sharks play an important role shaping their respective marine communities through predation and associated risk effects. Understanding the predatory dynamics of sharks within communities is, therefore, necessary to establish effective ecologically based conservation strategies. We employed non-lethal sampling methods to investigate the feeding ecology of bull sharks (Carcharhinus leucas) using stable isotope analysis within a subtropical marine community in the southwest Indian Ocean. The main objectives of this study were to investigate and compare the predatory role that sub-adult and adult bull sharks play within a top predatory teleost fish community. Bull sharks had significantly broader niche widths compared to top predatory teleost assemblages with a wide and relatively enriched range of δ¹³C values relative to the local marine community. This suggests that bull sharks forage from a more diverse range of δ¹³C sources over a wider geographical range than the predatory teleost community. Adult bull sharks appeared to exhibit a shift towards consistently higher trophic level prey from an expanded foraging range compared to sub-adults, possibly due to increased mobility linked with size. Although predatory teleost fish are also capable of substantial migrations, bull sharks may have the ability to exploit a more diverse range of habitats and appeared to prey on a wider diversity of larger prey. This suggests that bull sharks play an important predatory role within their respective marine communities and adult sharks in particular may shape and link ecological processes of a variety of marine communities over a broad range.     

Arthropod food webs become increasingly lipid‐limited at higher trophic levels

Understanding why food chains are relatively short in length has been an area of research and debate for decades. We tested if progressive changes in the nutritional content of arthropods with trophic position limit the availability of a key nutrient, lipid, for carnivores. Arthropods at higher trophic levels had progressively less lipid and more protein in their bodies compared with arthropods at lower trophic levels. The nutrients present in arthropod bodies were directly related to the nutrients that predators extracted when feeding on those arthropods. As a consequence, nutrient assimilation shifted from lipid‐biased to protein‐biased as arthropods ascended trophic levels from herbivores to secondary carnivores. Successive changes in the nutritional consequences of predation may ultimately set an upper limit on the number of trophic levels in arthropod communities. Further work is needed to examine the influence of lipid and other nutrients on food web traits in a range of ecosystems.     

Fish determine macroinvertebrate food webs and assemblage structure in Greenland subarctic streams

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Relative contribution of lipid sources to eggs of lesser scaup

Studies of how birds mobilize nutrients to eggs have traditionally considered a continuum of possible allocation strategies ranging from income breeding (rely on food sources found on the breeding grounds) to capital breeding (rely on body reserves stored prior to the breeding season). For capital breeding, stored body reserves can be acquired either on or away from the breeding grounds, but it has been difficult to quantify the relative contribution of each, precluding identification of key habitats for acquiring nutrients for clutch formation. During 2006–2009, we explored the importance of spring‐staging habitats versus breeding‐area habitats for egg‐lipid formation in female lesser scaup Aythya affinis using stable carbon (δ¹³C) isotope analyses. Although δ¹³C values for abdominal lipid reserves brought to the breeding grounds overlapped those of local amphipods, we were able to quantify the importance of local plant carbohydrates (seeds of emergent wetland plants) to the production of eggs. We compared the importance of local wetland seeds (overall δ¹³C: ?29.1 ± 0.9‰ SD) to combined lipid stores and lipids from local amphipods (overall δ¹³C: ?23.8 ± 2.2‰). Local seeds and stored body lipids contributed equally to egg lipid formation across years but we found evidence of annual variation in their relative importance. Wetland seeds contributed 39% (SE = 10%) to egg lipid production, and the importance of this source varied by year (90% CI = 47–75% in 2006, 13–42% in 2007, 29–65% in 2008, and 7–30% in 2009). In contrast to earlier studies that suggest lesser scaup predominantly employ a capital breeding strategy, our results suggest that in some years females may attain half of their energy for clutch formation from foods on the breeding grounds.     

Lipid‐rich zooplankton subsidise the winter diet of benthivorous Arctic charr (Salvelinus alpinus) in a subarctic lake

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Food web structure of a shallow eutrophic lake (Lake Taihu,China) assessed by stable isotope analysis

Lake Taihu is a large, shallow, and eutrophic lake in China. It has provided local communities with valuable fisheries for centuries, but little is known of the trophodynamics, or of its faunal communities. Carbon and nitrogen isotopic composition was used to assess its trophic pathways and the food web structure [food sources and trophic levels (TL)]. Basal food sources were distinguishable based on their δ¹³C values, ranging from −27.2 to −15.2‰. Consumers were also well separated in δ¹³C (−26.9 to −17.9‰ for invertebrates and −25.7 to −18.1‰ for fishes), which allowed for an effective discrimination of carbon sources between these fauna. An average trophic enrichment factor of 3.4‰ was used to calculate the TLs based on δ¹⁵N of zooplankton, with results indicating a food web having four TLs. Although δ¹⁵N values overlap and cover a large range within trophic compartments, the isotopic signatures of the species assessed revealed a general trend of ¹⁵N enrichment with increasing TL. Stable isotope signatures were also used to establish a general food web scheme in which five main trophic pathways were analyzed.     

Trophic structure of shallow-water benthic communities in the sub-Antarctic Strait of Magellan

Trophic structure is among the most fundamental characteristics of an ecosystem since it is a useful way to determine the main energy flow at the ecosystem level. In the Magellan Strait, the local spatial heterogeneity at the shallow-waters ecosystems may have a great variety of potential food sources; however, knowledge about their biological communities and their structure is still unclear. We examined the trophic structure of shallow-water-mixed bottom communities at two sites in the sub-Antarctic Magellan Strait based on carbon (δ ¹³C) and nitrogen (δ ¹⁵N) stable isotope ratios. The benthic communities were composed of 46 species from 20 major taxa at Bahía Laredo (BL) and 55 species from 18 major taxa at Punta Santa Ana (PSA). Benthic macroalgae and organic matter associated with sediment are the major primary food sources at both sites. Although both sites are quite similar in their food sources and in their vertical trophic structure (≥three trophic levels), the food web structure varied distinctly. Functionally, predators and grazers dominated both communities, but top predators were shorebirds, carnivore anemones and predatory nemerteans at BL, and sea stars, shorebirds, crabs and fishes at PSA. The distinct differences in the trophic structure at BL and PSA highlight the important variability of δ ¹⁵N at the base of the benthic food web, the role of local environmental conditions and community dynamics in structuring shallow-water communities.

     

Decreased trophic position as a function of increasing body size of a benthic omnivorous fish from the largest freshwater lake in China

Potential body size-trophic position relationships of the Darkbarbel catfish Pelteobagrus vachelli (Richardson 1846) were examined using stable isotope analysis. Pelteobagrus vachelli is a benthic feeding fish from Lake Poyang, the largest freshwater lake in China. Two-source mixing model with mussel (Corbicula fluminea) and snail (Bellamya aeruginosa) as baseline primary consumers of planktonic and benthic food webs, respectively, was used to estimate contribution of carbon derived from planktonic vs. benthic food web. Results showed that as an indicator of trophic position, δ¹⁵N was negatively correlated with the body length and weight of the fish; on the other hand, as an indicator of the end-member food sources, δ¹³C was not correlated with fish size. The mixing model results showed that the averaged trophic position of our sampled 3.3–12.7 cm Pelteobagrus vachelli was 3.1 ± 0.2 and derived 68 ± 27% of their food from the benthic food web, confirming that the feeding behavior of the catfish favors benthic food sources.     

The influence of fish predation on the abundance cycles of an algal turf invertebrate fauna

Summary A fauna of small invertebrates in a perennial algal turf habitat showed marked seasonal fluctuations in abundance. These were characterised by a summer (December) peak in the abundance of gammarid amphipods and an autumn (April) peak in polychaete densities. Invertebrate abundance was lowest during the period February–March which coincided with an influx of very high densities of juveniles (0⁺ year class) of the sparid fish Chrysophrys auratus. Both juvenile C. auratus and also the mullid Upeneichthys porosus achieved their highest densities over coralline turf areas and fed on the associated invertebrates. Gammarid amphipods constituted the main food items of each species. All these observations suggested a key role of fish predators in determining the observed seasonal patterns of invertebrate abundance.This hypothesis was investigated by the use of replicated fish exclusion shields and cages which excluded both species from the algal turf and also permitted an assessment of Upeneichthys porosus feeding in the absence of Chrysophrys auratus. The experiment ran from November until June and covered the period of invertebrate and fish abundance changes. Little evidence of a fish predation effect on either the densities of or the timing of abundance peaks of the invertebrate fauna was detected. Sediment analysis of the cage and control experimental sites revealed no evidence of a cage effect on the microhabitat. It is suggested that seasonal changes in the abundance of invertebrates occurs independently of the high densities of predatory fish recorded in this habitat.     

Effects of a top invertebrate predator (<Emphasis Type="Italic">Dytiscus alaskanus</Emphasis>; Coleoptera: Dytiscidae) on fishless pond ecosystems

We investigated the predatory effects of Dytiscus alaskanus, a large predaceous diving beetle, on the biomass, species composition and diversity of fishless pond communities. The effects were tested using presence and absence treatments of D. alaskanus in 24 mesocosms distributed among six ponds. We sampled phytoplankton, zooplankton and macroinvertebrates every two weeks for a six week period. Periphyton was sampled from the mesocosm walls on the final day. Total macroinvertebrate biomass decreased in the presence of dytiscids while species richness was not affected. Macroinvertebrate predators, snails and Gammarus lacustris decreased in the dytiscid treatments. Laboratory feeding experiments confirmed feeding preferences consistent with the mesocosm results. Periphyton biomass was six times greater in the dytiscid enclosures, concomitant with the decreased grazing by gastropods and other invertebrate primary consumers indicating a benthic trophic cascade. Top–down effects of dytiscids on other predatory invertebrates led to increased total zooplankton biomass, largely due to increased abundances of large and small cladocerans. Zooplankton species richness increased in the dytiscid enclosures. Inconsistent with trophic cascade theory, phytoplankton did not respond to top–down effects of D. alaskanus within the study period. Overall, the results show D. alaskanus predation caused trophic effects via two distinct food chains, a dytiscid–snail–periphyton trophic cascade, and a dytiscid–predatory macroinvertebrates–zooplankton partial trophic cascade.     

Stable isotopes and trophic positions of littoral fishes from a Mediterranean marine protected area

Stable isotope analyses were employed to explore feeding and foraging habitats and trophic levels of littoral fishes in a western Mediterranean Marine Protected Area (Egadi Islands, Sicily, Italy). Carbon and nitrogen stable isotope ratios were measured in primary producers, invertebrates and fishes collected in December 2001 and January 2002. Fishes of the littoral region of the Egadi Islands had isotopic signatures that fell into a wider range for δ ¹³C (about 6‰) than for δ ¹⁵N (about 3‰). Carbon isotope ratios were consistent with a food web based on mixed sources and two trophic pathways leading to different fish species. Differences in the isotopic composition between islands were higher for benthivorous than for planktivorous fishes. The overall picture gained from this study is of a isotopic distinction between planktivorous and benthivorous fishes, resource partitioning facilitating the coexistence of similar species within the same ecosystem, and spatial variability in the isotopic signatures and trophic level of fishes. Asymmetrical analysis of variance showed that estimated trophic levels were lower in the area with the highest level of protection (Zone A) for only two out of the nine fishes analysed. As a consequence, overall spatial differences do not seem to be a consequence of protection, since in most cases trophic levels did not change significantly between zone A and zones C where professional fishing (trawling apart) is permitted, but of natural sources of variation (e.g. variability in food availability and site-specific food preferences of fishes). However, the results of this study suggest a different response at the species compared to the community level.