Food preference,food quality and diets of three herbivorous gastropods (Trochidae: Tegula) in a temperate kelp forest habitat

Summary The relationship between food preference and food quality (i.e. a food's contribution to growth and reproductive development) was examined in the laboratory for 3 species of herbivorous kelp forest gastropods (Tegula). The preference hierarchies of the 3 Tegula for 6 common algal species are the same: giant kelp (Macrocystis) is the most preferred food and brown algae are consumed at higher rates than red algae. Despite their strong preference for Macrocystis, the 3 species had significantly greater growth and/or reproductive development on a mixed-algae diet than on either brown or red algae alone. Laboratory preferences of the snails did not correspond closely with caloric content, estimated availability or quality of the algal species used in this study. However, the 3 Tegula are subject to strong benthic predation and Macrocystis provides an important spatial refuge in nature. The potential role of non-nutritional factors such as predator avoidance on the formation of food preferences is discussed.     

Effect of dietary history and algal traits on feeding rate and food preference in the green sea urchin Strongylocentrotus droebachiensis

Feeding behaviour is influenced by a variety of factors, including nutritional requirements, the quality of available foods, and environmental conditions. We examined the effect of two factors, food morphology and dietary history, on the feeding rate and preference of the sea urchin Strongylocentrotus droebachiensis. Standardizing food shape and structure did not alter urchins' expected preference for the native kelp Laminaria longicruris over the invasive alga Codium fragile ssp. tomentosoides. However, when foods containing L. longicuris were shaped to mimic the algae, the C. fragile mimic was consumed more rapidly than the kelp mimic. Dietary history had no effect on single diet feeding rate. Urchins feeding on C. fragile consistently consumed twice as much (by mass) as those fed kelp, regardless of their previous diet. Despite higher feeding rates on C. fragile, urchins feeding on this alga were unable to compensate for its low energetic content and ingested less energy. Dietary history had a short-term effect on food preference, with urchins tending to prefer less familiar foods. Our findings suggest that urchins feed on C. fragile at a high rate, due to ease of handling and/or compensatory feeding, and that they do not a have strict preference hierarchy. Rather, food choice appears to reflect active maintenance of a mixed diet.     

Food and habitat choice of the isopod Idotea baltica in the northeastern Baltic Sea

The isopod Idotea baltica is the most important benthic herbivore in the Baltic Sea. There exists a significant correlation between the distribution of the adult isopod and the belts of bladder wrack Fucus vesiculosus. However, following the eutrophication induced blooms of the filamentous macroalga Pilayella littoralis and the disappearance of F. vesiculosus a notable increase in idoteid abundances has been observed. The aim of this paper was (1) to evaluate experimentally whether F. vesiculosus provides either food, shelter or both to the isopods and (2) to estimate the role of associated filamentous algae in the habitat selection process. Amongst six abundant macroalgal species, about 50% of isopod population was attracted to F. vesiculosuscovered with the filamentous algae P. littoralis. The majority of the remaining part of the population was either swimming freely or attracted to non-epiphytic P. littoralis. When both live algae and artificial substrata were provided, P. littoralis growing on artificial substrata was clearly preferred by the isopods over epiphyte-free F. vesiculosus. In the grazing experiment where I. baltica was allowed to choose between F. vesiculosus and P. littoralis the latter contributed practically 100% of the diet of the isopod. The results indicate the importance of P. littoralis as a food item and F. vesiculosus as a shelter for I. baltica.     

A Device to Study the Behavioral Responses of Zooplankton to Food Quality and Quantity

In order to explore the behavioral mechanisms underlying aggregation of foragers on local resource patches, it is necessary to manipulate the location, quality and quantity of food patches. This requires careful control over the conditions in the foraging arena, which may be a challenging task in the case of aquatic resource-consumer systems, like that of freshwater zooplankton feeding on suspended algal cells. We present an experimental tool designed to aid behavioral ecologists in exploring the consequences of resource characteristics for zooplankton aggregation behavior and movement decisions under conditions where the boundaries and characteristics (quantity and quality) of food patches can be standardized. The aggregation behavior of Daphnia magna and D. galeata x hyalina was tested in relation to i) the presence or absence of food or ii) food quality, where algae of high or low nutrient (phosphorus) content were offered in distinct patches. Individuals of both Daphnia species chose tubes containing food patches and D. galeata x hyalina also showed a preference towards food patches of high nutrient content. We discuss how the described equipment complements other behavioral approaches providing a useful tool to understand animal foraging decisions in environments with heterogeneous resource distributions.     

Eutrophication-induced changes in benthic algae affect the behaviour and fitness of the marine amphipod Gammarus locusta

This study, conducted in mesocosms, natural field sites, and in laboratory aquaria, showed that eutrophication altered the nutrient status and dominance patterns among marine macroalgae, which in turn, stimulated gammaridean density. Gammaridean abundance correlated positively with both nutrient addition and the amount of green algae (also stimulated by nutrient enrichment). Path analysis indicated that the direct effect of nutrients on gammaridean density was of less importance than the indirect effect through increased production of green algae. In cage colonisation experiments, either in the field or in a control mesocosm kept under ambient nutrient conditions, more gammarids colonised nutrient enriched algae (E-algae) than algae with ambient nutrient levels (A-algae). Gammarus locusta generally grew faster on nutrient enriched algal specimens and when reared on green rather than on brown algae (fucoids). The nutrient status of periphytic algae did not affect gammaridean growth significantly, but the number of egg-carrying females (and thus egg production) was significantly higher among gammarids reared on E-periphyton. The gammaridean habitat preference order (red > green > brown > periphyton) was almost the reverse of their growth rate in feeding assays (periphyton > green > brown). This implies that macroalgae may be more important as a habitat than as a food source for these animals, which then have to become mobile in search of optimal food items. In this process, algal nutrient content was important as the gammarids in our study actively chose high quality nutrient-rich food, which, in addition, increased their fitness. Stimulated growth rates and egg production may ultimately lead to population increase, which, combined with the preference for high nutrient food items may dampen the initial effect of nutrient enrichment (i.e. blooms of green macroalgae) in shallow coastal waters.     

Green urchin as a significant source of fecal particulate organic matter within nearshore benthic ecosystems

The role of green sea urchin Strongylocentrotus droebachiensis as a source of fecal particulate organic matter (POM) for the benthic nearshore ecosystems has been studied over a 3.5-month period. Three macroalgae were tested as food sources: Alaria esculenta, Laminaria longicruris and Ulvaria obscura. Urchins were fed ad libitum with either a single alga species or a mixture of all three algae. Consumption and defecation rates were determined as well as the feces/alga ratio in term of biomass and biochemical composition. Consumption rate increased exponentially with urchin size and also varied with alga species. In the single alga trial, consumption rate was higher for both brown algae (Laminaria and Alaria) compared to Ulvaria. Urchins feeding on the mixture of algae maintained their total ingestion rate (sum of the three algae) at the same level to those feeding on a single alga diet. The mixed algae trial showed that urchins clearly preferred Laminaria (72% of total ingestion) over Alaria (22%) and Ulvaria (6%). The defecation rate was tightly correlated with the food consumption rate and thus increased with urchin size. On average, 75% of the ingested algal biomass was released as fecal POM. The percentage of food defecated changed with alga species, with the highest value for Alaria (81%) and the lowest for Laminaria (67%). The percentage of food defecated by urchins feeding on the mixture of algae was generally comparable to those feeding on single alga diet. Biochemical composition (in soluble carbohydrates, proteins and lipids) of urchin fecal POM reflected that of the algae content. From 40% to 80% of macronutrients in algal food persisted in fecal matter. This proportion varied with the alga species and macronutrient considered. This study shows that the green sea urchin plays a significant role in the production of POM within nearshore benthic ecosystems, and it is a potentially nutritious food source for detritivores.     

Visual and olfactory preference of Harmonia axyridis (Coleoptera: Coccinellidae) adults to various companion plants

Natural enemies can be attracted to companion plants that can provide them with suitable habitat including food and shelter. This study was conducted to determine if the multicolored Asian lady beetle, Harmonia axyridis Pallas (Coleoptera: Coccinellidae), could be attracted to olfactory and visual cues of various companion plant species. Ten potential plants that can attract and provide H. axyridis with alternate food sources (i.e. nectar and pollen) were selected for olfactory preference tests. These plants included dill (Anethum graveolens), yarrow (Achillea spp.), butterfly weed (Asclepias tuberosa), dandelion (Taraxacum officinale), bugleweed (Ajuga reptans), marigold (Tagetes tenuifolia), tansy (Tanacetum vulgare), goldenrod (Solidago spp.), morning glory (Ipomoea spp.), and sunflower (Helianthus annuus). Visual preference tests were conducted with six different colors: white, yellow, blue, red, green, and orange. The result of the olfactory preference test showed that H. axyridis adults significantly preferred sunflower and dill among the ten potential companion plants tested in this study. In a visual preference test, H. axyridis significantly preferred yellow to any other colors. The result of this study suggested that odor and color of some companion plants (olfactory and visual cues) could assist in the use of biological control by attracting H. axyridis adults to the cropping area.     

Maternal and neonate diatom diets impair development and sex differentiation in the copepod Temora stylifera

Development and sex differentiation in the copepod Temora stylifera was studied in the presence of maternal and larval diets of the diatoms Thalassiosira rotula and Skeletonema marinoi, either provided alone or supplemented with the control dinoflagellate Prorocentrum minimum. Both diatoms had deleterious effects on growth compared to the control when used as pure diets, inducing very low or even zero survival from hatching to adulthood. This effect was deleted when the diet was supplemented with a good food (P. minimum) only in the case of T. rotula. By contrast, with a maternal or larval diet of S. marinoi, nauplii did not pass metamorphosis even when this alga was mixed with P. minimum. Arrested development was not due to lack of feeding since early and late nauplii (NII and NV) ingested all three algae at similar rates when used as single diets, and did not show any preference when the algae were offered as mixtures. Since mortality rates with a mixed diet of S. marinoi + P. minimum were similar or even higher than those obtained with a single diet of S. marinoi, we suggest that this diatom is more toxic than nutritionally deficient for T. stlyfera development. No males were produced in cohorts reared on pure diatom diets or with a mixture of S. marinoi + P. minimum, and intermediate male:female sex ratios were obtained with the mixed T. rotula + P. minimum diet. Possibly some diatoms produce compounds such as oxylipins or new molecules that alter sex differentiation in T. stylifera.     

Grass,mosses, algae,or leaves? Food preference among shredders from open-canopy streams

Shredder feeding is a vital process in making decomposition products available to biota in streams. To investigate which food sources shredders in open-canopy streams exploit, we conducted a feeding preference experiment with the invertebrate detritivores Limnephilus bipunctatus and Nemoura sp., which are commonly found in open-canopy streams on the Swedish island of Öland in the southern Baltic Sea. Leaves of birch, Swedish whitebeam, and shrubby cinquefoil; dead and fresh grass; water moss; and algae were offered to the shredders in multi- and single treatments. We hypothesized that food with high nutritional value would be preferred. Both taxa preferred leaves of shrubby cinquefoil, a bush common in the riparian zone of Öland streams; additionally Nemoura sp. also chose algae. Dead grass, the most abundant food source in the streams during the whole year, was the least consumed food type. The fresh food types had highest nutritional value, measured as carbon to nitrogen content. Therefore, food quality could not alone explain the preference of shrubby cinquefoil. However, among the detritus type offered, shrubby cinquefoil had the highest nutritional value. Shrubby cinquefoil may constitute one important energy source to these open-canopy stream ecosystems and may be essential in maintaining an abundant shredder community in these streams. Thus, the results of this study indicate that detrital resources are indeed important in open-canopy stream systems.     

Experimental identification and in silico prediction of bacterivory in green algae

While algal phago-mixotrophs play a major role in aquatic microbial food webs, their diversity remains poorly understood. Recent studies have indicated several species of prasinophytes, early diverging green algae, to be able to consume bacteria for nutrition. To further explore the occurrence of phago-mixotrophy in green algae, we conducted feeding experiments with live fluorescently labeled bacteria stained with CellTracker Green CMFDA, heat-killed bacteria stained with 5-(4,6-dichlorotriazin-2-yl) aminofluorescein (DTAF), and magnetic beads. Feeding was detected via microscopy and/or flow cytometry in five strains of prasinophytes when provided with live bacteria: Pterosperma cristatum NIES626, Pyramimonas parkeae CCMP726, Pyramimonas parkeae NIES254, Nephroselmis pyriformis RCC618, and Dolichomastix tenuilepis CCMP3274. No feeding was detected when heat-killed bacteria or magnetic beads were provided, suggesting a strong preference for live prey in the strains tested. In parallel to experimental assays, green algal bacterivory was investigated using a gene-based prediction model. The predictions agreed with the experimental results and suggested bacterivory potential in additional green algae. Our observations underline the likelihood of widespread occurrence of phago-mixotrophy among green algae, while additionally highlighting potential biases introduced when using prey proxy to evaluate bacterial ingestion by algal cells.Subject terms: Microbial ecology, Cellular microbiology     

A choice chamber experiment on the selection of algae as food and substrata by Nais elinguis (Oligochaeta: Naididae)

SUMMARY.

• 1 A choice chamber experiment was performed in the laboratory to quantify the selection of benthic algae as food and substrata by Nais elinguis.
• 2 Most worms actively chose filamentous algae and/or diatoms as substrata whereas unicellular or colonial chiorophycean algae were discriminated against. Diatoms were selected in preference to latex beads but the worms could not discriminate between living filaments and nylon monofilaments. A thigmotactic response to filaments and a chemotactic response to diatoms may be implicated.
• 3 The worms ingested unicellular algae at a greater rate than colonial and filamentous algae. The rates at which the algae were ingested were negatively correlated with the lengths of the algae. A filament 200 μm long was the largest item ingested, representing about 63% of the length of the pharynx of the worm.
• 4 Ivlev's electlvity indices were positive when the rates at which the algae were ingested exceeded about 20 individuals h^-1. The index was negatively correlated with the lengths of the algae, but the time periods spent by the worms in association with the algae were unrelated to the electivity index.
• 5 The data confirmed previous observations on the selection of algae as food and substrata by Nais elinguis in a natural habitat.

     

Field evaluation of three-component solid food-based dispenser versus torula yeast for capturing Mediterranean and oriental fruit flies (Diptera: Tephritidae)

Certain species of true fruit flies (Diptera: Tephritidae) are worldwide pests of fruits and vegetables, and many countries operate trapping programs to detect and monitor invasions. Torula yeast-borax (TYB) solution is a widely used food bait, although its effective field longevity is relatively short (1–2 weeks). This led to the development of a synthetic cone-shaped food dispenser that contains ammonium acetate, putrescine, and trimethylamine. The food cone has received limited testing, and the objective of this study was to compare captures of the Mediterranean fruit fly (Ceratitis capitata (Wiedemann)) and the oriental fruit fly (Bactrocera dorsalis (Hendel)) in traps baited with the fresh TYB solution versus traps baited with fresh or weathered food cones. Captures showed a female bias for both baits and both species. For C. capitata, fresh food cones generally attracted more individuals than fresh TYB. Weathered food cones attracted similar or greater numbers of medflies than TYB for the initial two weeks, but food cones weathered for 4 or 6 weeks attracted fewer flies than TYB. For B. dorsalis, TYB-baited traps captured more females than fresh or weathered food cone-baited traps in each of six weekly sampling periods, and the differences were statistically significant in weeks 1–4. In a release-recapture experiment, both C. capitata and B. dorsalis were captured in higher numbers in traps baited with fresh TYB than those baited with food cones weathered for 0 (fresh) to 6 weeks. The implications of these results for fruit fly surveillance are discussed.     

The effects of foods consumed after adult eclosion on the mate-searching behavior and feeding preferences of the white-spotted longicorn beetle Anoplophora malasiaca (Coleoptera: Cerambycidae)

The effects of changes in host plants on the mate-searching behavior and feeding preferences of the white-spotted longicorn beetle Anoplophora malasiaca (Thomson) (Coleoptera: Cerambycidae) were examined. All individuals were raised on the same artificial diet until they became pupae. Analysis of the mate-searching behavior of the males showed that many more newly emerged males were attracted to the odor of the artificial diet than to an unbaited control. We prepared three groups of beetles and fed each group on different host plants for one week. The host plants used included the following: an artificial diet (containing Morus alba Linné), Citrus unshiu Marc. branches, and Vaccinium spp. branches. The mate-searching behavior of the males changed in relation to the plant supplied for feeding. Simultaneously, the preference among the three host plants was tested. The newly emerged males preferred the artificial diet. After a week of feeding on one of the three plants, however, the adult males selected and consumed significantly more of the plant that they had just experienced than the other plants. These results suggest that the male mate-location cue can be acquired after adult eclosion. In addition, the male beetles are capable of changing their host-plant preference. If they do so, they use different odor cues for mate location. Newly emerged A. malasiaca females showed no preference for their first choice of food among the three host plants presented, whereas the consumption was significantly larger on C. unshiu branches. After one week of feeding on different host plants, females chose their host plant after the adult stage as well as C. unshiu, but consumed mostly C. unshiu. These results suggest that the food preferences of females are different from those of males. The behavior of females may not be affected by chemical signals from their original host-plant species (as pupae) or from the host-plant species acquired when they emerge as adults.     

Comparative feeding ecology of two herbivorous damselfishes (Pomacentridae: Teleostei) from the Gulf of California,Mexico

Two species of benthic damselfishes from the Gulf of California, Mexico, use contrasting behaviors when feeding on benthic algal communities. The small (±70 g) Cortez damselfish, Eupomacentrus rectifraenum (Gill, 1862), feeds selectively from a multi-species algal mat, eats fleshy red and green algae and ignores brown and calcareous algae. The giant blue damselfish, Microspathodon dorsalis (Gill, 1862), is a large (±450 g), lethargic, nonselective feeder which grazes on a near monoculture of a fleshy red alga, Polysiphonia sp. Feeding activity for both species is low in the morning peaks during late afternoon, and drops sharply as night approaches. Based on feeding rates, gut-filling times, and weights of gut contents, Cortez damselfish process six to eight full guts of food and giant blue damselfish three full guts of food per day. The algal mat exhibits high standing crops (291–618 g dry wt · m^?2) and low productivity, but the preferred food of the Cortez damselfish (Ulva) appears to colonize the mat frequently and grow rapidly. The Polysiphonia dominated community on giant blue damselfish territories exhibits low standing crops (23 g · m^?2) and high productivity (34–47 times that of the mat per gram algae). Even though the feeding behaviors and resources used by the two damselfishes differ, both species eat similar food (delicate red and green fleshy algae, and depend on rapid colonization and/or high productivity to maintain their primary foods in the grazed algal community.     

Induction of defenses and within-alga variation of palatability in two brown algae from the northern-central coast of Chile: Effects of mesograzers and UV radiation

Macroalgae possess different defense mechanisms in response to herbivory. Some species produce anti-herbivore secondary metabolites, but production of these substances can be costly. Therefore, algae may produce defensive metabolites only in response to herbivory (inducible defense) or defend particular parts of the alga differentially (within-alga variation). In the present study, we examined whether two species of brown algae from the SE-Pacific show evidence of inducible chemical defense (non-polar compounds) or within-alga variation of defense, which we estimated in form of palatability of differently treated algae to amphipod grazers (with live algae and agar-based food containing non-polar algal extracts). In Glossophora kunthii (C. Agardh) J. Agardh, we observed an increase in palatability after algae were acclimated for 12 days without grazers. Subsequent addition of grazers for 12 days then resulted in a reduction of palatability indicating the existence of inducible defense. After removal of grazers for 12 days, these induced effects again disappeared. The reaction of G. kunthii was triggered even by the mere presence of grazers, which suggests that this alga can respond to waterborne cues by reducing palatability. Effects were only found for agar-based food containing non-polar extracts, but not for live algae, suggesting that some parts of the algae are undefended. Our second experiment on within-alga variation confirmed that only apical (growth region) and basal parts (near the holdfast region) of G. kunthii are defended against herbivores. For the second species, Macrocystis integrifolia Bory, the first experiment revealed no induction of defense, while the second experiment on within-alga variation showed that amphipods avoided basal parts and in particular stipes of M. integrifolia but only in live algae. Although both studied algal species differed substantially in their defensive strategies, their reaction was independent of the presence or absence of UV radiation. Thus, it appears that UV effects play only a minor role in anti-herbivore defense, which is in accordance with most previous studies.     

Substratum selection and egg-capsule deposition in Nassarius reticulatus (L.)

When offered a range of 14 pairs of different substrata consisting of five brown algae, five red algae, three bivalve shells, and sandstone pebbles, spawning Nassarius reticulatus (L.) selected only four species of red algae on which to deposit egg capsules. There was some evidence of a greater preference for the red alga Gigartina stellata (Stackhouse). When offered a choice between only Gigartina and another red weed, Polyides rotundus (Hudson), there was a highly significant preference for the former. Some reasons for the selectivity are suggested and discussed.     

Food choice,feeding rates,and the turnover of macrophyte biomass by a surf-zone inhabiting amphipod

The amphipod Allorchestes compressa Dana inhabits large accumulations of detached macrophytes in the surf-zone of sandy beaches in southern Western Australia. A. compressa is most abundant on branching red algae and least abundant on intact thalli of the kelp Ecklonia radiata (Turn.) J. Agardh., yet the major component of the gut contents is brown algae (probably E. radiata) and decomposing E. radiata ranked first in laboratory food preference experiments. Observations on the feeding behaviour of Allorchestescompressa indicated that the amphipods obtain their food by feeding on small pieces (< 3 cm) of macrophyte tissue trapped within the highly branched algae, or amphipods may move with and feed on the plant particles as they are swept around in the surf. In a particle selection experiment, using plant particles 1–3 mm sieved from the surf, A. compressa selected particles of Ecklonia radiata, leached Ulva sp., Sargassum spp., and seagrass leaves but avoided branching red algae. The influence of potential foods on the darwinian fitness of Allorchestes compressa was assessed on the basis of adult survival, the percentage of females which carried eggs, growth rates, and time to maturity measured in laboratory rearing experiments. Fitness increased in the order red algae → intact seagrass leaves → mixed particles (1–3 mm) sieved from the surf → Eckloniaradiata tissue. Given the constraints of fish predation and the fluctuating supply of E. radiata, amphipods in the surf consume close to their theoretically optimum diet by feeding mainly on E. radiata from amongst the available particles of different macrophytes. Estimates of the significance of the Allorchestes compressa population in the turnover of Ecklonia radiata biomass in the surf-zone (estimated as g Ecklonia consumed per g Ecklonia per day) showed that amphipods could turnover E. radiata biomass twice per month in summer and once every 1 to 2 months during spring and autumn. These rates are comparable with those measured for the physical breakdown and microbial decomposition of E. radiata and, except during winter, grazing by Allorchestes compressa must, therefore, be considered an important process during the remineralization of nutrients tied up in kelp biomass in the surf-zone.     

The Symbiosis between Lophelia pertusa and Eunice norvegica Stimulates Coral Calcification and Worm Assimilation

We investigated the interactions between the cold-water coral Lophelia pertusa and its associated polychaete Eunice norvegica by quantifying carbon (C) and nitrogen (N) budgets of tissue assimilation, food partitioning, calcification and respiration using ¹³C and ¹⁵N enriched algae and zooplankton as food sources. During incubations both species were kept either together or in separate chambers to study the net outcome of their interaction on the above mentioned processes. The stable isotope approach also allowed us to follow metabolically derived tracer C further into the coral skeleton and therefore estimate the effect of the interaction on coral calcification. Results showed that food assimilation by the coral was not significantly elevated in presence of E. norvegica but food assimilation by the polychaete was up to 2 to 4 times higher in the presence of the coral. The corals kept assimilation constant by increasing the consumption of smaller algae particles less favored by the polychaete while the assimilation of Artemia was unaffected by the interaction. Total respiration of tracer C did not differ among incubations, although E. norvegica enhanced coral calcification up to 4 times. These results together with the reported high abundance of E. norvegica in cold-water coral reefs, indicate that the interactions between L. pertusa and E. norvegica can be of high importance for ecosystem functioning.     

Population Growth of the Cladoceran,Daphnia magna: A Quantitative Analysis of the Effects of Different Algal Food

In this study, we examined the effects of two phytoplankton species, Chlorella vulgaris and Stephanodiscus hantzschii, on growth of the zooplankton Daphnia magna. Our experimental approach utilized stable isotopes to determine the contribution of food algae to offspring characteristics and to the size of adult D. magna individuals. When equal amounts of food algae were provided (in terms of carbon content), the size of individuals, adult zooplankton, and their offspring increased significantly following the provision of S. hantzschii, but not after the provision of C. vulgaris or of a combination of the two species. Offspring size was unaffected when C. vulgaris or a mixture of the two algal species was delivered, whereas providing only S. hantzschii increased the production of larger-sized offspring. Stable isotope analysis revealed significant assimilation of diatom-derived materials that was important for the growth of D. magna populations. Our results confirm the applicability of stable isotope approaches for clarifying the contribution of different food algae and elucidate the importance of food quality for growth of D. magna individuals and populations. Furthermore, we expect that stable isotope analysis will help to further precisely examine the contribution of prey to predators or grazers in controlled experiments.     

Littoral-pelagial interchange and the decomposition of dissolved organic matter in a polyhumic lake

The small, polyhumic lake, Mekkojärvi (southern Finland), is bordered by a moss vegetation zone (Warnstorfia and Sphagnum species) which provides a habitat-rich and productive environment for many planktonic and periphytic animals. Impacts of moss on the metabolism of bacterioplankton, phytoplankton and zooplankton in polyhumic water were investigated in laboratory throughflow systems. Growing Warnstorfia (together with epiphytic algae and bacteria) suppressed the production of planktonic algae but had no clear effect on leucine uptake, and hence bacterial production, or on the decomposition of humic substances. Phenol uptake and mineralization rates, however, were lower in the littoral water than in the pelagial water. Excretion of organic carbon by Warnstorfia algae or Daphnia longispina (the predominant crustacean in the pelagial water) provided only a minor contribution to bacterial production; therefore, a major contribution had to be from humic substances. A bacterial production efficiency of 31–38% could account for the microbial respiration in the water. The results indicated that bacterial, or detrital matter (originating largely from the littoral zone), could not obviate the need for algal food, and that a great deal of particulate matter in the water was poor or useless food for Daphnia. In all, the bulk of dissolved organic matter in Lake Mekkojärvi was biochemically highly recalcitrant. Our results indicate that humic substances (from watershed or littoral area) which, through bacterial degradation, enter the planktonic food web of the lake are mainly lost through respiration by microorganisms.