Influence of dietary carbohydrate on the metabolism of juvenile Litopenaeus stylirostris

Cockles Cerastoderma edule were fed two different concentrations ( approximately 0.8 and 2 mm(3) l(-1)) of two diets with different qualities ( approximately 10 and 60% of organic content) which were achieved by mixing different proportions of ashed silt particles with cells of the microalgae Tetraselmis suecica. Clearance, ingestion and absorption rates of organic matter and biochemical components were measured after 3 days (acute response) and 11 days (acclimated response) of exposure to the diets. With low quality diets cockles were found to reject part of the filtered matter ( approximately 25-35%) through pseudofaeces production both in the acute and acclimated responses. In the acute response, absorption rate of organic matter was positively dependent on food quality and quantity, but the physiological response to increasing food concentrations differed with food quality: with low qualities, increasing absorption rate resulted from the simultaneous increase of clearance ( approximately 2 times) and ingestion rate ( approximately 4 times) as well as absorption efficiency of organic matter ( approximately 22%). However, those fed high qualities, were found to compensate increasing food concentration by reducing ( approximately 50%) clearance rate. The resulting moderate increase of ingestion rate ( approximately 1.6 times) was accompanied with a reduction in absorption efficiency ( approximately 20%). Irrespective of food quality and quantity, protein and lipids were absorbed, respectively, with the highest (from 61.7 to 80.0%) and the lowest (from 42.6 to 66.8%) efficiency. Acclimated response was entirely affected by food quality: with low qualities, cockles greatly improved the energetic intake from available ration ( approximately 4 and 2 times, with low and high food concentrations, respectively). Both preingestive and digestive mechanisms were involved in this response: at the preingestive level, clearance rate and preingestive selection efficiency were significantly increased. At the digestive level, cockles were capable of maintaining absorption efficiency of organic matter with rising ingestion rate. On the contrary, acclimation to high quality diets brought about no significant increase in organic absorption rate: with low ration, clearance rate was kept constant, whereas with high ration the increase in clearance and ingestion rate ( approximately 2 times) promoted a compensatory reduction in absorption efficiency. However, the biochemical composition of the absorbed matter was found to be absolutely modified, both at low and high food rations, due to an strong reduction of lipid absorption efficiency. The observed modifications of absorption rate and/or the biochemical composition of the absorbed matter suggests the capability of cockles to adjust the digestive performance.     

Feeding behaviour of Cerastoderma edule in a turbid environment: physiological adaptations and derived benefit

Resuspension of bottom sediments by wind andtide-driven currents often occur in shallow waters,coastal embayments and estuaries. These processes maylead to dramatic variations in the concentration andorganic richness of suspended particle assemblages.Since resuspended matter is mainly inorganic,decreasing organic contents are usually associatedwith higher seston loads.Under this environmental context the feeding behaviourof bivalves shows a wide degree of plasticity, whichhas been interpreted as having high adaptive value. Inorder to evaluate benefits derived from thisbehaviour, we have used functional relationshipsobtained in previous studies, relating feedingparameters to characteristics of suspended food, topredict the effect that different feeding responseswould have exerted. In cockles, main processesdetermining energy acquisition are feeding rates andpreingestive food selection. Thus, the procedurefollowed in the present work consisted of simulatingrates of food absorption under alternative feedingbehaviours characterised by: (a) no preferentialingestion of filtered organic matter and (b) maintenance ofconstant clearance rates. In theabsence of selection of organic matter at thepreingestive level, ingestion rate of organics (OIR)would decline with increasing seston loads to 30% ofvalues predicted by functions fitted to experimentaldata; difference in absorption rate (AR) would be evengreater, falling to 10%, due to the strong effectthat the organic content of ingested matter exerts onabsorption efficiency. On the other hand, had theclearance rate (CR) kept constant despite theincreasing seston load, OIR and AR would have fallento values respectively 30% and 49% lower than actualvalues.From these results it is concluded that the ability ofsorting particles before ingestion and the capabilityof adjusting clearance rate are key elements in thefeeding behaviour that enable cockles to be welladapted to cope with changes in the water columncaused by resuspension events.     

Feeding and digestion by the mussel Mytilus edulis L. (Bivalvia: Mollusca) in mixtures of silt and algal cells at low concentrations

Mussels Mytilus edulis L. from two populations were exposed for 2 days or 2 wk to mixtures of silt and the diatom Phaeodactylum tricomutum Bohlin before measuring rates of feeding, the passage time for food in the gut, absorption efficiency and metabolic rate. Experimental diets were set up to span the range of organic content in seston from the natural habitats (% organic matter by weight: 7–55%), but were less than the total levels of natural seston. Absorption efficiencies were adequately modelled in an exponential relationship to food quality and to gut passage time, although at high proportional silt concentrations metabolic faecal losses led to negative net rates of absorption. Over short-term exposures the scope for growth was a simple function of food quality, and the nutritional quality of the diet was best expressed as organic content per unit volume of particles. Over a period of 2 wk physiological acclimation occurred, across all levels of experimental food quality (which were as low as 10% organic matter by weight), resulting in positive growth potential. Relevant mechanisms of compensation include increased rates of ingestion, increased absorption efficiency and an apparent increase in digestive capacity, estimated here as gut fullness. In experiments in which natural diets are simulated by adding silt to phytoplankton cells, the consequences for net rates of absorption depend on the balance between mean particle size and organic content per unit volume. Calculations show how, in some circumstances, growth may be enhanced by the addition of small silt particles to living phytoplankton cells.     

The functional role of burrowing bivalves in freshwater ecosystems

1. Freshwater systems are losing biodiversity at a rapid rate, yet we know little about the functional role of most of this biodiversity. The ecosystem roles of freshwater burrowing bivalves have been particularly understudied. Here we summarize what is known about the functional role of burrowing bivalves in the orders Unionoida and Veneroida in lakes and streams globally. 2. Bivalves filter phytoplankton, bacteria and particulate organic matter from the water column. Corbicula and sphaeriids also remove organic matter from the sediment by deposit feeding, as may some unionids. Filtration rate varies with bivalve species and size, temperature, particle size and concentration, and flow regime. 3. Bivalves affect nutrient dynamics in freshwater systems, through excretion as well as biodeposition of faeces and pseudofaeces. Excretion rates are both size and species dependent, are influenced by reproductive stage, and vary greatly with temperature and food availability. 4. Bioturbation of sediments through bivalve movements increases sediment water and oxygen content and releases nutrients from the sediment to the water column. The physical presence of bivalve shells creates habitat for epiphytic and epizoic organisms, and stabilizes sediment and provides refugia for benthic fauna. Biodeposition of faeces and pseudofaeces can alter the composition of benthic communities. 5. There is conflicting evidence concerning the role of resource limitation in structuring bivalve communities. Control by bivalves of primary production is most likely when their biomass is large relative to the water volume and where hydrologic residence time is long. Future studies should consider exactly what bivalves feed upon, whether feeding varies seasonally and with habitat, and whether significant overlap in diet occurs. In particular, we need a clearer picture of the importance of suspension versus deposit feeding and the potential advantages and tradeoffs between these two feeding modes. 6. In North America, native burrowing bivalves (Unionidae) are declining at a catastrophic rate. This significant loss of benthic biomass, coupled with the invasion of an exotic burrowing bivalve (Corbicula), may result in large alterations of ecosystem processes and functions.     

Dietary mixing in three generalist herbivores: nutrient complementation or toxin dilution?

We reared larvae of three generalist insect species on plants occurring in their habitats. Individuals of each species were kept either on mixed diets, or on each plant species separately. We measured food plant preference in the mixed-diet group and compared insect performance on single plants to the performance on the mixed diet. For all three insect species, food choice within the mixed-diet groups was non-random and delivered the best overall performance, thus fulfilling the criteria for self-selected diets. When a single diet was as good as the mixed diet for one particular aspect of performance (Adenostyles alliariae and Petasites albus for Miramella alpina; A. alliariae for Callimorpha dominula), it was never the most preferred food plant in the mixed-diet treatment. Whether the benefit achieved by mixing diets is due to nutrient complementation or toxin dilution, we argue that there is no easy way to distinguish between the two hypotheses on the basis of consumption and performance measurements, as has previously been proposed. From the interpretation of utilisation plots, the ANCOVA equivalent of nutritional indices, we were able to gain insight into where in the sequence from ingestion to growth (preingestive, predigestive or postdigestive) single diets caused differences from mixed diets. The elements of this control system which were influenced by single diets varied considerably, both within and between insect species. No food plant was toxic or deterrent to all experimental insect species; a food plant that caused consumption effects (preingestive) for one insect species could be dealt with metabolically (postdigestive) by another; different food plants could cause behavioural effects (preingestive), metabolic effects (postdigestive), or a combination of both effects, all within the same insect species. However, one generality did emerge: once a food was ingested, further growth-relevant effects occurred metabolically (postdigestive) rather than via differential egestion (digestibility). Received: 5 October 1998 / Accepted: 1 March 1999     

Seasonal variatins in seston and in the growth rate of the iceland scallop, Chlamys islandica (O.F. Müller) from balsfjord, 70°N

The absorption efficiency of Chlamys islandica (O.F. Müller) decreases with increasing “dilution” of particulate organic matter (POM) by particulate inorganic matter (PIM), indicating that the scallop's pallial organs do not select POM in preference to PIM. Since PIM constitutes a large but varyibg fraction of the seston, absorption efficiency would alter in the course of the year and contribute to a variation in growth rate. It is found that seasonal changes in the relative concentrations of POM and PIM in seston can explain the seasonal pattern of growth of C. islandica in the Balsfjord.     

Seston quality controls zebra mussel (Dreissena polymorpha ) energetics in turbid rivers

Feeding processes and energetic balance of zebra mussels were both related to the quantity and quality of natural seston. Filtration rate and pseudofeces production increased while clearance rate remained constant with increasing seston concentration. Ingestion rate, assimilation efficiency, and assimilation rate all increased with increasing food quality, measured as the ratio of organic to inorganic material in the seston. Respiration rate did not change with either food quantity or quality. As a result, scope for growth declined with decreasing food quality, and fell below 0 cal mg⁻¹ h⁻¹ at an organic:inorganic ratio of 0.5. The association between feeding processes and food quality appears related to a breakdown in the ability of zebra mussels to selectively ingest high-quality organic particles when the organic content of the seston is low. Ingestion, assimilation efficiency, assimilation rate and scope for growth were all higher when seston was amended with an addition of a natural assemblage of algae. Food quality may be a better indicator of environmental conditions suitable for growth than food quantity. These results suggest that the conditions of high suspended inorganic sediment concentrations in large turbid rivers represent a difficult growth environment for the zebra mussel. Received: 12 May 1997 / Accepted: 7 July 1998     

Variation of the physiological energetics of the bivalve Spisula subtruncata (da Costa, 1778) within an annual cycle

Spisula subtruncata is an infaunal filter-feeding bivalve, which lives in shallow sandy bottoms (2-20 m depth) from Norway to the Atlantic coasts of Morocco, including the Mediterranean Sea. Considering that fisheries of this species have become an important economic resource in some European countries (e.g. The Netherlands), it is of great interest to know the seasonal variation in its physiological energetics. For this purpose, individuals of S. subtruncata were collected and maintained under ambient temperature and seawater conditions of Dutch coastal waters. Physiological processes related to the acquisition and utilisation of energy (e.g. clearance rate [CR], absorption and oxygen uptake) were measured under ambient conditions of the period March 1999 to February 2000. Mean annual clearance and respiration rates (RR) were 0.99 l h⁻¹ and 0.23 ml O₂ h⁻¹ for a standard individual of 250 mg. Values for both clearance and respiration rate were high during spring and summer and low during autumn and winter. Stepwise multiple regression analyses indicated a significant relationship of the clearance rate with temperature and particulate organic matter (POM), whereas respiration rate was significantly related to temperature, absorption rate (AR) of the animals and their reproductive condition. Absorption efficiency (AE) of the food was significantly related to food quality. Scope for growth (SFG) of S. subtruncata, as well as flesh weight of the animals, was high in summer and low in winter.     

Functional responses associated with pallial organ variations in the Pacific oyster Crassostrea gigas (Thunberg, 1793)

The plasticity and function of the pallial organs were studied in the Pacific oyster Crassostrea gigas from three sites of Bourgneuf Bay (French Atlantic coast, 46-47°N, 1-2°W) characterized by different turbidity conditions. Labial palp area was closely and positively related to the turbidity gradient. No clear pattern was established between the gill area and the gradient of suspended particulate matter (SPM). The functional responses induced by these morphological variations were investigated in the laboratory by means of ecophysiological experiments and endoscopic observations. Oysters with different pallial organ areas were supplied with mixed suspensions of heat-killed Tetraselmis suecica and living Skeletonema costatum added to different concentrations of kaolinite to simulate low (SPM = 8.5 ± 0.4 mg l^− 1) and high (SPM = 48.3 ± 1.4 mg l^− 1) turbidity conditions. At each SPM concentration, heat-killed T. suecica were preferentially rejected in pseudofaeces compared to S. costatum, indicating a preingestive particle selection. At low seston load, clearance rate (CR) was closely and positively related to gill area and particle selection occurred only on the gills, between the ventral grooves and dorsal tracts. At higher seston load, palps exhibited a particle-sorting capacity dependent on gill area. Indeed, with small gills, an increase in selection efficiency (SE) and CR was positively related to palp area. On the other hand, large gills processed the particles without an effect of palps but with a decrease in CR. The functional responses associated with pallial organ variations clearly showed that the preingestive particle processing in oysters is an integrated mechanism dependant on the gill and labial palp areas.     

Feeding rhythms of the green-lipped mussel, Perna viridis (Linnaeus, 1758) (Bivalvia: Mytilidae) during spring and neap tidal cycles

Food availability and feeding responses of the green mussel Perna viridis were investigated for two complete tidal cycles during both spring and neap tides. Temporal changes in total particulate matter (TPM), particulate inorganic matter (PIM) and particulate organic matter (POM), were smaller during neap than spring tides. Seston characteristics at different times of a tidal cycle were compared for both spring and neap tides. Only during spring tides were TPM and PIM significantly higher at high tides while POM remained relatively constant (P>0.05). The clearance rate of the mussels underwent temporal variations with tides, and was a negative power function of TPM and a positive linear function of f (organic content), during both spring and neap tides. f was the key factor influencing filtration rate, organic ingestion rate, absorption rate and absorption efficiency. All feeding rates increased linearly with increases in organic content. Pseudofaeces were produced only during spring but not neap tides. Feeding rates and absorption efficiency were highest at low and lowest at high tides (P<0.01). There was no significant temporal change in the wet weight and protein content of the crystalline style with the tidal regime. For the digestive gland, alpha-amylase activity was higher at spring than at neap tides, and higher during high tides in a tidal cycle. The digestive gland cellulase activity did not change significantly with the tides. For the crystalline style, both the activity of cellulase and alpha-amylase were not significantly different (P>0.05) between spring and neap tides. Tidal rhythms in feeding and digestion in this species were likely controlled by temporal variations in food availability in the seawater. By adjusting feeding rates and enzymatic activities, absorption in Perna viridis remained constant, irrespective of the changes in food availability.     

Seston dynamics and bivalve feeding in the Bay of Marennes-Oléron (France)

Within the framework of an investigation into the carrying capacity of the Bay of Marennes-Oléron (France) for bivalve culture, thein situ uptake of suspended particulate material by oysters (Crassostrea gigas) and mussels (Mytilus edulis) was determined in experiments with benthic ecosystem tunnels. Very high fluctuations in seston quantity and quality were observed within and between tidal cycles. The percentage of organic carbon was inversely related to seston quantity at low concentrations. Organic carbon was diluted by resuspension of material rich in inorganic matter. At high seston concentrations a constant level of about 2% organic carbon was found. The C/N ratio was relatively constant throughout the seasons and fairly low (6.5 to 8.4). Owing to the presence of bivalves large fluxes of suspended particulate material were observed in thein situ measurements. Selective retention of organic carbon or nitrogen could not be demonstrated. Clearance rates based on chlorophyll uptake were within a normal range compared to other studies. A large contribution to the food of the bivalves seemed to be formed by resuspended microphytobenthos. Judged by the low C/N ratio, the food was of good quality. Although its quantity was variable by dilution, it may support largely the carrying capacity of the Bay of Marennes-Oléron for the cultivation of bivalves. Communication no. 687 of the Netherlands Institute of Ecology, Centre for Estuarine and Coastal Ecology, Yerseke, the Netherlands.     

Food acquisition responses of the suspension-feeding bivalve Placopecten magellanicus to the flocculation and settlement of a phytoplankton bloom

In situ technologies were employed to monitor suspended particle flocculation and floc settlement and utilization by a cohort of sea scallops (Placopecten magellanicus) during the 2000 spring phytoplankton bloom in Bedford Basin, Nova Scotia, Canada. The objectives were to determine the effect of bloom flocculation and settling on food acquisition and utilization by scallops, and to assess the potential role of flocculation in enhancing the bioavailability of trophic resources and particle-reactive contaminants to bivalve filter feeders. The development and flocculation of the phytoplankton bloom were monitored within the surface layer (10 m depth) by in vivo chlorophyll fluorescence and silhouette camera observations. Sedimentation rate, seston abundance and composition, and sea scallop functional responses were monitored at 20 m depth (below the bloom) to provide insight into the potential forcing of feeding and digestion processes by changes in the abundance, composition and properties of the ambient food supply. The bloom began in mid-March and median floc diameter at 10 m depth increased rapidly from 200 μm to greater than 400 μm between 21 and 28 March. Flocs were observed to be abundant in the surface layer up to 4 April. Daily vertical particle flux was high during the last week of March and declined to near zero by 1 April. Clearance rates of scallops held at 20 m depth were relatively high (average ± S.D.; 11.7 ± 4.0 L h^− 1) during the period of bloom settlement and declined rapidly to low levels (0.4 ± 0.9 L h^− 1) after 31 March. Average absorption efficiency also declined (0.88 ± 0.01 to 0.78 ± 0.05) after bloom settlement. Daily biodeposition rates by scallops were poorly correlated with temporal variations in the quantity (total particulate matter and chlorophyll a concentration) or quality (organic content) of seston available to the scallops, but were significantly correlated with sedimentation rate. Comparison of disaggregated inorganic particle size distributions for suspended particulate matter, settled particles, and scallop feces indicated that fine-grained particles (1 to 4 μm) were effectively ingested by sea scallops—an indication of whole floc ingestion. The settlement of flocs produced during the spring bloom appears to be important in regulating this species physiological energetics and for enhancing the bioavailablility of fine particles (including picoplankton) and particle-reactive contaminants.     

Reach-scale geomorphology affects organic matter and consumer δ13C in a forested Piedmont stream

1. We investigated the spatial (longitudinal position and reach geomorphology) and seasonal (spring and autumn) influences on the variation of δ¹³C among organic matter sources and consumers in a forested Piedmont river, South Carolina, U.S.A. 2. Six sites were sampled along a continuum and varied in basin area from approximately 30 to 300 km². Sites fell into two geomorphic categories (i) high‐gradient, rock bed (‘rock’) or (ii) low‐gradient, sand bed (‘sand’) sites. 3. Variation in δ¹³C was more strongly related to reach geomorphology than longitudinal position. δ¹³C of biofilm and consumers was consistently enriched at rock sites. Leaf litter (i.e. coarse particulate organic matter, CPOM) δ¹³C did not vary with bed type. There was significant δ¹³C enrichment at rock sites for biofilm, seston, fine benthic organic matter (FBOM), and eight of nine consumer trophic guilds (e.g. grazing invertebrates, insectivorous fishes). δ¹³C of biofilm and four trophic guilds was also positively correlated with drainage area, but the magnitude of enrichment was less than between bed types. 4. δ¹³C was generally enriched in spring, but this varied among organic matter types, consumers, and by bed type. CPOM and seston were enriched in spring, FBOM was enriched in autumn, and biofilm showed no trend. Five consumer guilds were enriched in spring, and only one fish guild, generalised carnivores, showed enrichment of muscle tissue in autumn. 5. Consumer δ¹³C enrichment at rock sites suggests greater reliance on algal carbon than for consumers at sand sites, but we also found δ¹³C enrichment of biofilm at rock sites. Thus, differences in consumer δ¹³C between bed types could be related to (i) increased consumption of biofilm at rock compared with sand sites, or (ii) consumption of biofilm at rock sites that is enriched relative to biofilm at sand sites or (iii) both mechanisms. 6. δ¹³C signatures in local food webs appear to respond to processes operating at multiple spatial scales. Overall downstream enrichment of biofilm and consumers was disrupted by strong local effects related to bed morphology. These results suggest that human alteration of channel habitat will have corresponding effects on stream food webs, as assessed by changes in δ¹³C.     

The Effects of Food Concentration and Quality on the Feeding Rates of Three Size Classes of the Greenshell<Superscript>TM</Superscript> Mussel, <Emphasis Type="Italic">Perna canaliculus</Emphasis>

In order to calibrate carrying capacity models, investigations were conducted into the effects of food concentration and food quality on the feeding rates of small (25–50 mm), medium (60–85 mm) and large (90–115 mm) Greenshell mussels (Perna canaliculus). Experimental diets varying from 3.3 to 6.0 μg l⁻¹ chlorophyll a concentration and 12–25% organic content were fed to mussels housed in individual flow through chambers. Not surprisingly, this study found that the main factor affecting feeding rates is mussel size. Small mussels were observed to maintain a constant filtration rate of approximately 20 mg h⁻¹ irrespective of food concentration or quality, whereas mussels of greater than 60 mm length had more variable filtration rates between 30 and 80 mg h⁻¹. The filtration rates of these large mussels were also observed to increase positively with organic content, and showed no sign of levelling out, even at the highest organic content tested (25%). Highest rejection rates (50–70 mg h⁻¹) were observed when the organic content of the available seston was low, suggesting that P. canaliculus are able to selectively reject organic material, thereby organically enriching their diet. It appears that the organic content of the seston is the primary determinant of the net efficiency with which food is selected from the available seston by the mussel. The present study shows that P. canaliculus of all sizes are capable of adapting their feeding behaviour to compensate for changes in the food supply, which may occur over relatively short time periods, in the culture environment.     

Post-bloom feeding of Calanus finmarchicus copepodites: Selection for autotrophic versus heterotrophic prey

Feeding and faecal pellet production of late copepodite stages of Calanus finmarchicus were measured in mixtures of cultured autotrophic and heterotrophic food, as well as in a natural post-bloom plankton assemblage, in order to evaluate food selection and its potential effect on sedimentation of organic matter. Calanus finmarchicus consistently selected for diatoms, both in mixtures with the heterotrophic dinoflagellate Oxyrrhis marina, and in natural seston containing dinoflagellates, ciliates and flagellates. Similarly, the filtration, ingestion and faecal pellet production rates were significantly higher feeding on diatoms than when feeding on other food species. Calanus finmarchicus selection appeared relatively inflexible, so that changes in seston composition induced large changes in diet quantity and composition. Our results support the traditional view of C. finmarchicus as a major grazer of diatoms, and suggest potentially high post-bloom faecal pellet production rates.     

The functional role of native freshwater mussels in the fluvial benthic environment

1. Freshwater mussels are the dominant consumer biomass in many fluvial systems. As filter feeding grazers, mussels can remove large amounts of particulate matter from the water column and transfer these resources to the substrate as biodeposits (agglutinated mussel faeces and pseudofaeces). Mussel biodeposits are a nutrient rich and easily assimilated food source and therefore may have significant relevance to benthic community structure. This study examines the functional role of Margaritifera falcata in the South Fork Eel River, California. 2. We addressed two main questions: (i) Do mussels increase benthic resources in this system? (ii) If so, does this alter macroinvertebrate community structure? 3. Measurements and enclosure experiments in the South Fork Eel River show that mussels can play a significant role in local food webs by increasing available fine particulate matter (both organic and inorganic) on the substrate. We document increased benthic macroinvertebrate biomass for predators and collectors (Leptophlebidae) in the presence of mussels, but only in late summer.     

Physiological energetics of the zebra musselDreissena polymorpha in lakes II. Food uptake and gross growth efficiency

Food uptake ofDreissena polymorpha from two shallow water sites and one site at a 9 m depth in lakes was monitored over the course of 1 1/2 years. Feeding was characterized by the filtration capacity and ingestion capacity (i.e. the unrestricted maximum rates) at ambient temperature. The available food was estimated as seston dry weight, as seston ash free dry weight and as seston volume (sum of the volumes of the particles in the water column). The filtration capacity and ingestion capacity varied strongly during the course of the year by more than 1:10, with maximum values during late spring and early summer. The filtration capacity was correlated with water temperature, the ingestion capacity with temperature and seston concentration. However, at identical water temperatures both capacities were higher in spring than in autumn. At the 9 m site, filtration and ingestion capacity was also significantly influenced by lack of oxygen caused by stratification during summer. In the allometric equations by which both capacities (C) were related to body weightW (C=a W ^b ), an exponent b of 0.89±0.14 (mean±95% confidence interval) was calculated for the filtration capacity and 1.00±0.18 for the ingestion capacity. Both exponents did not vary significantly during the course of the year. The seasonal trend of the incipient limiting concentration (i.e. the seston concentration at which the intestine is filled to its capacity when the animal filters at maximum rate) was less pronounced. It showed a significantly positive correlation with seston concentration and a negative correlation with temperature. Except during the spring bloom, the incipient limiting concentration was in the range of the seston volume registered at the sites. Gross growth efficiencies were highly variable. An annual average between 5 and 20% was calculated. Growth efficiencies were lower at the shallow water sites with the highest seston concentrations.     

Influence of food size and food quantity on the feeding of the mussel Dreissena polymorpha

Summary In common with many other suspension feeders, the freshwater mussel Dreissena polymorpha has a maximum filtration rate at low food concentrations and a maximum ingestion rate at high food concentrations. These high rates, which reflect the potential maximum food uptake of the animal, are called the filtration capacity and the ingestion capacity respectively. The ingestion capacity was attained without forming pseudofaeces with Chlamydomonas reinhardii as food. The incipient limiting level could be calculated as the quotient of these two values. A decrease of the filtration rate at high food concentrations was correlated with changes in pumping activity, which showed more frequent interruptions, or a lower level of water transport. Dreissena can filter out particles of diameter greater than 0.7 m from the water. Retention reaches a plateau at about 5 m particle diameter. Scanning electron micrographs of the arrangement of the cilia on the gill filaments are given.     

Transport of seston in the karstic hydrosystem of the Plitvice Lakes (Croatia)

Research into spatial and temporal variation in seston transport was carried out during the year 2000 on three reaches on the longitudinal profile of the karstic cascading system of the Plitvice Lakes in Croatia. The three investigated reaches were (i) a channel over a barrier with low gradient; (ii) flow through a deep lake; and (iii) a channel with cascades and a steep gradient. The aim of the study was to establish the influence of physiographical and hydrological differences of these reaches on the quality and quantity of seston transport and food resources in the seston. To calculate the seston transport, we measured: total suspended matter (TSM), particulate inorganic matter (PIM), particulate organic matter (POM), chlorophyll-a (chl-a), heterotrophic bacteria and discharge. The PIM contribution from TSM ranged between 60 and 90%, while the percentage of POM in TSM was the highest in summer and ranged from 33 to 46%. POM and discharge were significantly negatively correlated (r = −0.43, P < 0.05). For the transport of TSM, PIM, POM and chlorophyll-a statistically significant differences between the three reaches were established. In a principal component analysis, 86% of the variance was explained by the first two factors. The first factor corresponded well with net transport of TSM, PIM, POM and chl-a and distinguished investigated reaches in two groups: the 1^st group with increasing (reaches with low gradient and with high gradient), and the 2nd group with decreasing net seston transport (reach with flow through a deep lake). The second factor corresponded strongly with discharge and distinguished investigated reaches according to their temporal variability. Electronic Supplementary Material Supplementary material is available in the online version of this article at and is accessible for authorized users Handling editor: J. Padisak     

Ingestion rate of the burrowing mayfly Hexagenia limbata as determined with14C

The effects of nymph size and temperature on the ingestion rate ofHexagenia limbata were investigated using¹⁴C labelled algae mixed into sediment. Ingestion rate increased with nymph length and temperature. Daily ingestion rates of large 19 mm nymphs burrowing in silt averaged 58 and 192% of the dry body weight at 16° and 21 °C, respectively. Ingestion of seston due to filter feeding in artificial burrows was insignificant compared to that ingested by burrowing nymphs.