Detritus Food Chain Research Articles

Feeding interactions and foraging of juvenile fish and shrimp in the estuaries of Peter the Great Bay in the summer-fall period

The feeding interrelations and foraging of juvenile fish and shrimp were investigated in estuaries of Peter the Great Bay in summer and fall 2006 and 2007. In total 3483 specimens were analyzed. The trophic spectra of 34 fish species and 3 shrimp species were described for the first time. Three trophic groups were singled out depending on differences in diet compositions: (i) detritophages (3 species of bitterlings—Acanthorhodeus sp., A. chankaensis and Rhodeus sericeus, redlip mullet Liza haematocheila and two species of shrimp-Palaemon macrodactylus and Crangon cf. septemspinosa); (ii) predators (lookup Culter alburnus and Amur catfish Silurus asotus); (iii) euryphages (28 fish species and the shrimp Palaemon paucidens). The detritus food chain was found to be the primary one in estuaries of Peter the Great Bay. Most of the species had two peaks of feeding activity each day. Values of daily feeding rations were determined for 21 fish species; these varied from 1.6% to 8.0% of body weight. Similarity in diet composition was significant only for 10% of the species, which shows a low level of competition for food in the community.

Utilization of invasive tamarisk by salt marsh consumers

Plant invasions of coastal wetlands are rapidly changing the structure and function of these systems globally. Alteration of litter dynamics represents one of the fundamental impacts of an invasive plant on salt marsh ecosystems. Tamarisk species (Tamarix spp.), which extensively invade terrestrial and riparian habitats, have been demonstrated to enter food webs in these ecosystems. However, the trophic impacts of the relatively new invasion of tamarisk into marine ecosystem have not been assessed. We evaluated the trophic consequences of invasion by tamarisk for detrital food chains in the Tijuana River National Estuarine Research Reserve salt marsh using litter dynamics techniques and stable isotope enrichment experiments. The observations of a short residence time for tamarisk combined with relatively low C:N values indicate that tamarisk is a relatively available and labile food source. With an isotopic (15N) enrichment of tamarisk, we demonstrated that numerous macroinvertebrate taxonomic and trophic groups, both within and on the sediment, utilized 15N derived from labeled tamarisk detritus. Infaunal invertebrate species that took up no or limited 15N from labeled tamarisk (A. californica, enchytraeid oligochaetes, coleoptera larvae) occurred in lower abundance in the tamarisk-invaded environment. In contrast, species that utilized significant 15N from the labeled tamarisk, such as psychodid insects, an exotic amphipod, and an oniscid isopod, either did not change or occurred in higher abundance. Our research supports the hypothesis that invasive species can alter the trophic structure of an environment through addition of detritus and can also potentially impact higher trophic levels by shifting dominance within the invertebrate community to species not widely consumed.

The Coupling Between Grazing and Detritus Food Chains and the Strength of Trophic Cascades Across a Gradient of Nutrient Enrichment

A minimal food web model was constructed comprising one grazing and one detritus food chain coupled by nutrient cycling and generalist carnivores to investigate how prey preference by carnivores may affect the strength of trophic cascades across a gradient of nutrient enrichment. The equilibrium or mean abundance of each food web component and the magnitude of the carnivore effect on lower trophic levels were calculated for different values of the prey preference and nutrient input parameters. Our model predicts that nutrient enrichment increases the mean abundances of carnivores, autotrophs and detritus, but the magnitude of this effect is dependent on the prey preference term. On the other hand, herbivores and detritivores are relatively unaffected by enrichment but are strongly affected by carnivore preference. Carnivores have a negative effect on herbivores and a positive effect on autotrophs and detritus, whereas the effect on detritivores can be both positive and negative. At high preference for herbivores, carnivores have a positive effect on detritivores, because the positive effect of increased detritus availability due to reduced herbivore grazing outweighs the negative effect of predation. At high preference for detritivores, the balance is changed in the other direction. We argue that in systems where authochtonous primary production is the major source of detritus, herbivores can control the rates of detritus production and have indirect effects on detritivores, which may feed back into effects on herbivores through their shared enemies. This positive feedback is probably one mechanism affecting the resilience of alternative stable states in shallow lakes.

Linyphiid spider populations in sustainable wheat‐clover bi‐cropping compared to conventional wheat‐growing practice

AbstractLinyphiid web densities in wheat‐clover bi‐crop systems where winter wheat was grown in an under‐storey of white clover were compared with web densities estimated in conventional wheat‐growing systems. The web densities in the wheat‐clover bi‐crop systems were on average between 200 and 250 webs per square‐metre when the densities peaked, while the estimated average web density peak levels under conventional growing practices were 100–150 webs per square‐metre. Repeated Measure anova tests show significant differences between the estimated mean web density levels of the bi‐crop systems compared to the mean density levels of the conventional growing systems in two consecutive growing seasons. Particularly, Bathyphantes gracilis and Tenuiphantes tenuis took advantage of the more heterogeneous conditions with high secondary vegetation layer density and high food animal supply in the bi‐crop plots compared to the more homogeneous conditions under conventional wheat‐growing practices with low food animal supply. Structural equation modelling with a data set pooled from four different wheat‐growing practices shows that the linyphiid juvenile production is influenced by the number of collembolans accessible as food animals for the adult females. The modelling demonstrates the importance of the detritus food chain, particularly for the juvenile recruitment.

Beneficial links for the control of aphids: the effects of compost applications on predators and prey

1. Polyphagous predators, such as spiders and beetles, perform a fundamental ecosystem service as regulators of agricultural pests, particularly aphids. They are most effective when they colonize the crop before the pest has reached its exponential growth phase. However, this is also when predators find themselves in a state of near-starvation. 2. Predator numbers can be enhanced by applications of different types of organic matter, but the mechanism is not clearly understood. One hypothesis is that compost applied to the field may introduce a new detrital food chain to maintain predators until the pest arrives, but this may also be detrimental to effective pest control, fostering a surplus of alternative prey and causing a switch away from the pest. To elucidate these possible outcomes, we report on the use of within-field compost applications on aphids and their predators, presenting 4 years of field-scale manipulations. 3. We found both direct and indirect links between compost, aphids and predators. In years when compost-treated plots had significantly higher numbers of predators, aphids were in significantly lower numbers than in plots without compost. Conversely, when there was a lack of response by predators, aphid numbers showed similar trends in all treatments. 4. In all years, alternative prey responded strongly to compost application and did not fluctuate at the level shown by predators, suggesting that these two prey groups were decoupled. Instead, the predicted positive feedback of compost on predators numbers was either weak or absent. 5. Synthesis and applications. The effect of compost on aphids clearly requires further practical refinement if it is to provide constant pest suppression, making it difficult to provide specific management recommendations at this stage. In the short term, compost application may not always confer immediate benefits in terms of pest control alone but this must be set against other better known benefits (moisture retention, nutrients). In the long term, experiments measuring the full trophic pathway are needed to unravel the effects of organic matter type, application time and the siting of compost relative to the crop in order to optimise pest suppression potential.

Detrital ontogenic model including decomposer diversity

Detritus influences the structure and dynamics of the living species. The quality of detritus tied by the degree of colonization such that it may be more useful in some cases to consider them as grouped entities as higher consumers in detrital branch may not distinguish between the consumption of detritus and micro-organisms that fed on it. Detritus and producer-based food models both suggest that energy supply limits the length of food chains. However, in detrital food chain the amount of dead organic matter is needed to support micro-detritivores (bacteria and fungi) is lower on average than would be required to support physiological and morphologically comparable herbivores in grazing food chain. Many questions concerning detritus and diversity are only recently beginning addressed. These include: (1) what are the factors that determine species richness in detrital communities? (2) How does diversity of detritivores affects key rates and fate of detritus processing and ultimately feed back to producer productivity and ecosystem diversity? (3) How does species diversity of organisms associated with detritus differ at different stages of its ontogeny? In this present work a dynamic model of two pools of detritus (recalcitrant and labile pool), and decomposers (fungus and bacteria) is considered. The behavior is explored after varying the consumption of labile material by fungi, the transfer rates from one pool of detritus to another and the amount of recalcitrant material entering the system. The model is also sensitive to changes in other parameters, for example changes in assimilation and production efficiencies of microbes to reflect changes in the quality of detritus. The model behavior shows the coexistence at equilibrium between fungal and bacterial population is possible which depicted as a function of the rate of transfer from recalcitrant pool to the labile. At low rates of fungi mediated transfer of detritus, fungi are favored over bacteria. Coexistence of bacteria and fungi also depends on the amount of recalcitrant material entering the system; if inputs are dominated by labile material (that is recalcitrant input is low), fungi cannot survive. However increasing input of recalcitrant material leads to coexistence and input labile material that are high relative to other inputs can generate bacteria dominated system.

A comparison of zooplankton densities and biomass in Lakes Peipsi and Võrtsjärv (Estonia): rotifers and crustaceans versus ciliates

The abundance and biomass of ciliates, rotifers, cladocerans and copepods were studied in Lake Peipsi and Lake Vortsjarv, both of which are shallow, turbid and large. Our hypothesis was that in a large shallow eutrophic lake, the ciliates could be the most important zooplankton group. The mean metazooplankton biomass was higher in Peipsi than in Vortsjarv (mean values and SD, 1.8 ± 0.7 and 1.3 ± 0.6 mg WM l−1). In Peipsi, the metazooplankton biomass was dominated by filtrators that feed on large-sized phytoplankton and are characteristic of oligo-mesotrophic waters. In Vortsjarv, the metazooplankton was dominated by species characteristic of eutrophic waters. The planktonic ciliates in both lakes were dominated by oligotrichs. The biomass of ciliates was much greater in Vortsjarv (mean 2.3 ± 1.4 mg WM l−1) than in Peipsi (0.1 ± 0.08 mg WM l−1). Ciliates formed about 60% of the total zooplankton biomass in Vortsjarv but only 6% in Peipsi. Thus, the food chains in the two lakes differ: a grazing food chain in Peipsi and a detrital food-chain in Vortsjarv. Consequently, top-down control of phytoplankton can be assumed to be much more important in Peipsi than in Vortsjarv. When the detrital food chain prevails, the planktonic ciliates become the most important zooplankton group in shallow, eutrophic and large lake. Neglecting protozooplankton can result in serious underestimates of total zooplankton biomass since two-thirds of the zooplankton biomass in Vortsjarv comprises ciliates.

The contribution of scarid herbivory to seagrass ecosystem dynamics in the Indo-Pacific

The functional importance of herbivory is well established within terrestrial ecosystems yet within marine ecosystems, specifically seagrass beds, consumption by herbivores has generally been considered to be of little importance due to its minor contribution to turnover of growth. The present paper challenges this assumption within the Indo-Pacific and shows that scarid (parrotfish) herbivory may have a functionally important role in the export of seagrass detrital material. Estimation of scarid herbivory on dominant Indo-Pacific seagrass species was conducted using a three pronged approach. We estimated: background levels of scarid seagrass bites; in-situ rates of grazing using tethering experiments; and seagrass grazing effect per individual scarid, which was scaled up to estimate the grazing effects of the local scarid population. All research was carried out within the Wakatobi Marine National Park, Indonesia. Research found scarid herbivory to be an important process within an Indo-Pacific seagrass bed causing the loss of up to ten times (average, four times) the daily seagrass growth. T. hemprichii was found to be lost at a rate of 0.28 ± 0.08 g DW day −1, while E. acoroides was lost at a rate of 2.85 ± 0.43 g DW day −1. We propose that such high rates of seagrass consumption are the combined effects of seasonally high grazing rates by resident scarids and short-term effects from shoals of juvenile reef scarids. Scarid grazing was indiscriminate between seagrass species and grazing was dependant on seagrass availability rather than nutritional quality. In conclusion, the present research finds scarids to be major herbivores within an Indo-Pacific seagrass bed; they not only take up carbon into the food chain through direct seagrass consumption, but also make an important indirect contribution to the detrital food chain through the export of discarded decaying seagrass material.

Utilization and flow of energy in tropical ecosystem: A test case for two Ox-bow lakes

To estimate the rate of organic production, two solar powered tropical natural aquatic systems (Oxbow lakes) have been studied for 24 months. The study of utilization of energy in those two tropical lakes has revealed that herbivores were more efficient in energy utilization than autotrophs. Silver carp and grass carp were found to be most efficient in conversion of energy in the grazing and detritus food chain of these lakes. The efficiency of the system to fish production was found to be 0.05 % of the total available solar radiation. Various forms of life are all accompanied by energy changes, even though no energy is created or destroyed in the studied system.

Accumulation of metals by the seagrass, Z. Capricorni in Lake Illawarra

The concentrations of arsenic, cadmium, copper, lead, manganese, nickel and zinc in Zostera capricorni leaves and rhizomes were analyzed in order to document the accumulation of metals in Lake Illawarra seagrass tissues and to evaluate the potential role of Z. capricorni in the cycling of metals within the lake. Metal concentrations in seagrass tissues were compared with porewater metal concentrations and previously documented sediment concentrations to determine if the seagrass tissues reflect ambient metal concentrations. Zostera leaves and/or rhizomes reflected ambient concentrations of arsenic, cadmium, copper, manganese and zinc. Lead and nickel were not accumulated to high concentrations in most tissue samples from Lake Illawarra, although differences in ambient lead concentrations were reflected in some groups of leaf samples. Metal concentrations in seagrass tissues collected near to the Port Kembla industrial complex, and adjacent to the former Tallawarra power station, were significantly higher than concentrations in seagrass samples collected from the lake entrance. The majority of metals was accumulated to higher concentrations in leaves than in rhizomes; however, arsenic concentrations were highest in rhizomes. Z. capricorni in Lake Illawarra is likely to play a significant role in the cycling of copper, manganese, and possibly lead, via uptake from sediments, acropetal transferrance to leaves, and the release of metals from leaves into the water column or into the detrital food chain via leaf litter.

Fishes and salinities in the St Lucia estuarine system—a review

The recorded salinity ranges of freshwater, estuarine and marine fish species in Lake St Lucia, a Ramsar and World Heritage Site, are documented. The freshwater group is most diverse and abundant under oligohaline conditions, although the Mozambique tilapia (Oreochromis mossambicus) was common un- der all salinity regimes. Estuary resident species also favoured oligohaline conditions but, in contrast to the freshwater taxa, were well represented in salinities up to 40 &. The marine group was most diverse and abundant within the salinity range 10-40 &, but a large number of species could also be found in salin- ities up to 70 &. Very few fish species were able to tolerate salinities between 70 & and 110 &, with only O. mossambicus surviving for extended periods in salinities above 110 &. All the aquatic macrophytes and most of the zoobenthos within the lake appear to die out within the salinity range of 50-60 &, thus creating additional stress to those fish present under such conditions. The food resources least affected by extreme hypersalinity are the microphytobenthos and detritus food chains, with detritivorous fishes being dominant when the lake is in this state. Mass mortal- ities of fishes in Lake St Lucia have been recorded under both low ( 70 &) conditions. The fish kills are often triggered by exceptionally low or high water temperatures which affect the osmoregulatory abilities of these species. Hypersaline conditions and fish mortalities under the most recent closed estuary mouth conditions (2002- 2005) are reviewed. If the surface area of St Lucia (35,000 ha) is compared to the total surface area of all South African estuaries (approximately 70,000 ha), then the possibility exists that the loss of the Lake St Lucia nursery area for estuary-associated marine fish species over the past few years may cause significant short-term declines in the future abundance of these taxa on both a local and regional scale.

INFLUENCE OF STREAMSIDE SURFACE AREA ON AQUATIC BIOTA AND BIOFILM ACTIVITY

In this study, fillers (gravel and brick) of different surface areas were installed on both banks of two test channels so as to have the same capacity, and effects of differences in surface area and porosity of the streamside on aquatic biota and activity of the attached biofilm were investigated at downstream distance. The results showed that an increase in surface area and porosity causes change in the composition of microbial community, increasing the number of total bacteria and nitrifying bacteria and improving oxygen uptake activity. Although there was no evident difference found in the biofilm microfauna between the two test channels, a result was obtained that oxygen uptake rate by protozoa and metazoa evaluated by laboratory batch experiments quadruples farther downstream in the gravel-filled channel. Furthermore, it was clear, from the result that there observed many Physidae and Ephemeroptera that prey on deposits such as detritus precipitated in the interstices, that the amount and diversity of macrobenthos increased in the gravel-filled channel. These results suggest that surface area and porosity are extremely important factors in the detritus food chain constructed by settling and accumulation of particulate organic matter, and it is considered that controlling them can create a diverse aquatic biota.

Review of the potential use of brown algal ecotoxicological assays in monitoring effluent discharge and pollution in southern Australia

The temperate near-shore reefs of southern Australasia possess diverse assemblages of brown algal macrophytes which underpin major grazing and detrital food chains in medium to high energy coastal habitats. These `kelp' communities are at risk from effluent discharge with documented decline of macrophyte stands in areas subject to outfall. Recent studies utilising reproductive endpoints of several Australian species in ecotoxicological assays have highlighted their sensitivity to pollution. Results indicate an acceptable degree of reproducibility for assays with variation in response between taxa presumably related to differing habitat requirements and potential for stress resulting from toxicant/pollution exposure. Species studied are drawn from the orders Laminariales and Fucales, with the latter in particular offering substantial scope for further investigation. The greatest diversity of fucoid algae is found in south-eastern Australia and closely related taxa occur in similar latitudes in both the southern and northern hemispheres. The broad distribution of taxa from both taxonomic groups enables ready comparison of assays from disparate locations and offers the potential for simple, routine toxicological testing and habitat monitoring across broad geographic ranges.

Enhancement of Planktonic Rotifers by Microcystis aeruginosa Blooms: An Enclosure Experiment in a Shallow Eutrophic Lake

ABSTRACT Enclosure experiments with three treatments (sediment addition, sediment + nitrogen enrichment, sediment + phosphorus enrichment) and unfertilized controls were performed in shallow hypereutrophic Lake Donghu during the summer of 2000. Dense Microcystis aeruginosa blooms occurred in all the enclosures during the experimental period but not in the surrounding lake water. Generally, the dominant rotifers were Polyarthra vulgalis, Filinia longiseta, Proales sp. and Asplanchna sp. at the beginning of the experiment, followed by a shift to Brachionus calyciflorus, Trichocerca similis, Cephalodella catellina and Anuraeopsis fissa, and finally to F. longiseta, Proales sp. and Keratella cochleris. M. aeruginosa blooms strongly suppressed the larger Diaphanosoma brachyurum but enhanced the development of the smaller cladocerans and rotifers that probably efficiently utilized organic matter from M. aeruginosa through the detritus food chain. The smaller cladoceran and rotifers coexisted successfully throughout the experimental period.

MTI sees first reversal in steep upward trend in profits over past 10 years

Tonle Sap Great Lake (TSL) is the largest freshwater lake ecosystem in Southeast Asia and receptacle of impressive biodiversity. However, there is surprisingly little knowledge of its ecosystem structure and functioning. The main objective of the current work was to quantify the food web structure and assess the ecosystem health status of the TSL system by constructing the first holistic food web model using Ecopath with Ecosim (EwE). The results indicate that the ecotrophic efficiency (EE) values were very high for most of functional groups (EE > 0.5) except molluscs (0.146) and macrophytes (0.102). The high EE values together with the MTI (Mixed Trophic Impact) analysis indicated the overexploitation and degradation of fishery resources in the TSL system. The discrete trophic levels varied from 1 (phytoplankton, macrophytes and detritus) to 3.17 (snakehead). The energy transfer in the TSL food web was based mostly on the detrital food chain (77.9%) rather than the grazing food chain (22.1%), with an average transfer efficiency of 8.27%. The ratios of total primary production to respiration (TPP/TR) and to biomass (TPP/TB) were 1.23 and 2.04, respectively, while the Ascendency and Finn cycling index (FCI) of the system were estimated at 27.4% and 23.62%. Nevertheless, the connectance index (CI: 0.253) and system omnivory index (SOI: 0.075) were in-between compared to other lake ecosystems, which indicated that the food web structure was characterized by linear, rather than web-like features. Systematic analysis and indicators suggested that the ecosystem was a relatively healthy ecosystem achieving a certain stage of maturity, albeit with a vulnerable food web structure. Accordingly, some ecosystem-based strategies are presented for the improvement of fishery management and ecosystem conservation in TSL.

Refilling, ageing and water quality management of Brucher Reservoir

AbstractRefilling of the formerly oligo‐mesotrophic, softwater Brucher Reservoir commenced in April 1993 and took 11 months to completely fill. A severe ‘trophic upsurge’ in the sense of nutrient enrichment (phosphorus, dissolved organic material) as a result of the decomposition of the inundated vegetation occurred. However, algal crop and phosphorus utilization efficiency, expressed as chlorophyll concentrations per unit of total phosphorus, remained very low. In the absence of any fish stock, a single species, Daphnia galeata, monopolized the resources. Sustained by the detritus food chain, daphnids exerted a severe‚‘top‐down’ control upon phytoplankton, thereby preventing any net algal growth. In 1994, artificial mixing prevented the occurrence of anoxic water conditions and internal nutrient enrichment of the lake. Although the decay of the inundated vegetation was still of importance, phosphorus concentrations in the water column approached equilibrium with the external input while dissolved organic material concentrations clearly declined. That year, the reservoir became stocked with minnows, sun bleak (Leucaspius delineatus) and trout. As food limitation, as a result of reduced heterotrophic production, became more severe in the face of an increasing predation pressure, the daphnid population density declined, resulting in a decreasing but still adequate community filtering rate providing pronounced ‘clear‐water phases’ of up to 10 m that were features of the period 1995–1997. Although D. galeata defended its key position in the food web, its life‐history traits (e.g. body size) changed. Submerged macrophytes, which since 1995 gradually colonized suitable areas of the reservoir, provided a favourable refuge for minnows from trout predation that resulted in reduced predation pressure upon pelagic daphnids. However, in 1998, ungrazeable algae became prominent, adversely affecting transparency. That year, the significant inverse relationship between chlorophyll : total phosphorus ratios and daphnids became uncoupled during the summer (July–August) by indigestible chlorococcalean algae. Nevertheless, the fishery management that was implemented was successful in sustaining not only the lowest yield of algae at the given nutrient concentration but also the most favourable species composition with respect to water quality.

Potential dietary effects on the fatty acid composition of the common jellyfish Aurelia aurita

Fatty acid composition of the natural and aquarium-reared common jellyfish Aurelia aurita was investigated. Fatty acid composition of the aquarium-reared A. aurita clearly reflected that of the diet, brine shrimp (Artemia). In the same way, fatty acid composition of the natural A. aurita was assumed to reflect those of natural diets. Samples of natural A. aurita were collected from April 1995 to September 1995 in the Seto Inland Sea, Japan, and their fatty acids were analyzed by gas chromatography and mass spectrometry. Variation of fatty acid compositions was seasonal rather than dependent on body size. Two major seasonal groups were divided by the cluster analysis of the A. aurita fatty acid composition: the April–June and the August–September clusters. The April–June cluster was characterized by high contents of the (n − 3)-fatty acids of diatom origin, accumulated via the grazing food chain. By contrast, the August–September cluster was characterized by an increase in (n − 6)-fatty acids of macroalgal origin, probably transferred via the detritus food chain. These results suggest that the diet of natural A. aurita may shift between the diatom-based food chain and the detritus-based food chain.

Relationship between morphology and diets of six neotropical loricariids

Most loricariids are algivorous and detritivorous and play an important role in both the grazer and detritus food chains of neotropical waters. Relationships between morphological variation and diet were analysed in six syntopic species (Rhinelepis aspera, Hypostomus regani, H. ternetzi, H. margaritifer, H. microstomus and Megalancistrus aculeatus) fished commercially in a 10 km2 area of the Upper Paraná River upstream from the Itaipu Reservoir. Species feeding on fine grained detritus use suction to obtain food, and possess a well‐developed respiratory membrane, long gill rakers, rudimentary labial and pharyngeal teeth, a thin stomach wall, and a long intestine. Species feeding on coarser material (e.g. periphyton), ingest food by scraping the substratum. Their morphological characteristics are large, strong, spatulate teeth, short gill rakers, a well‐developed stomach, and a shorter intestine. The first species group is composed by R. aspera while the latter by M. aculeatus and H. microstomus. The others species had an intermediary position.

Effects on benthic scavengers of discards and damaged benthos produced by the beam-trawl fishery in the southern North Sea

The beam-trawl fishery for flatfish produces large amounts of dying discards as well as damaged and disturbed benthos. The importance of these food sources to scavenging benthic species was investigated. To identify epibenthic species showing scavenging behaviour and to detect their food preferences, traps (370 overall) baited with different kinds of carrion (fish, crustaceans, molluscs, echinoderms, polychaetes) were deployed at 14 locations in the southern North Sea. Liocarcinus holsatus, Pagurus bernhardus, Asterias rubens, ophiurids, and small gadoids were the main active scavengers feeding on different kinds of food, while lyssianid amphipods (Orchomene nanus, Scopelocheirus hopei) fed mainly on crustacean carrion. Estimated attraction areas were largest for gadoids, hermit crabs, and swimming crabs. Immigration of fish and swimming crabs into experimentally trawled areas was observed by sampling repeatedly in a trawled strip. Differences in the response to trawling (feeding activity, food niche breadth, and resource partitioning) between selected fish species are described. Limanda limanda, Pleuronectes platessa, Merlangius merlangus, Callionymus lyra, and Eutrigla gurnardus all showed strong responses, using different feeding strategies. In situ clearance rates of dead discarded fish exposed on the sea floor were estimated. Discards as well as moribund benthos were consumed within a few days. The balance between the amount of food materials produced by beam-trawl fishery and the amount of food consumed by potential scavengers was translated in a model of the food flows as mobilized by beam trawling. In general, the direct importance of the additional food resource for populations of scavengers is considered to be relatively small. However, the importance may be relatively larger for scavenging fish than for invertebrates. It is estimated that after a single beam trawling about 1.27 g m 2 ash-free dry biomass, or 6% to 13% of the annual secondary production of macrobenthos per unit area, would suddenly become available to scavengers and to the detritus food chain. It is suggested that beam trawling leads to shortcuts in trophic relationships and therefore may enhance secondary production. 2000 International Council for the Exploration of the Sea

Produção e consumo de carbono orgânico na comunidade planctônica da represa da Pampulha, Minas Gerais, Brasil

O presente estudo tem por objetivo investigar a importância relativa do fitoplâncton como recurso alimentar para o zooplâncton em um reservatório hipereutrófico tropical. Para atingir esta meta, dois enfoques foram adotados. O primeiro deles consistiu num monitoramento regular (mensal) dos teores de biomassa do zooplâncton e das suas fontes de alimento expressas em termos de biomassa de carbono de partículas sestônicas. O segundo enfoque consistiu numa abordagem experimental com o objetivo de determinar o balanço entre a produção primária e o consumo de carbono orgânico na comunidade planctônica do reservatório. Tal avaliação deu-se através da mensuração simultânea das taxas de produção primária líquida potencial do fitoplâncton e das taxas de assimilação da comunidade mesozooplanctônica, derivadas a partir das taxas de respiração. A produção primária foi estimada em duas frações distintas: a) 50-160 µm e b) < 50 µm. Tanto a produção primária quanto a respiração do zooplâncton foram estimadas simultaneamente no laboratório sob condições controladas de iluminação e temperatura. O programa de monitoramento indicou que, excetuando breves períodos nos quais ocorreu o florescimento de Cyanobacteria coloniais (Microcystis spp.), a biomassa da comunidade fitoplanctônica foi quase sempre reduzida na zona limnética do reservatório (< 30 µg.L-1). A faixa de oscilação da clorofila-a restringiu-se a 0-126 µg.L-1. Adicionalmente, a maior parte do carbono particulado foi de origem não pigmentada. Os estudos experimentais indicaram que a demanda energética do zooplâncton, em termos de carbono, não pode ser suprida pela produção devida ao fitoplâncton no Reservatório da Pampulha. A produção primária potencial oscilou entre 2,0 e 37,3 mgC.m-3.h-1, valores estes que ficaram muito abaixo das taxas de assimilação devidas ao zooplâncton em diferentes épocas do ano. Considerando as evidências obtidas no campo e aquelas provenientes dos estudos experimentais, o presente estudo conclui que a demanda de carbono do mesozooplâncton durante o período de estudos foi em grande parte coberta através da cadeia de detritos.