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Mg C L (mg + c_l)
Selected AbstractsCalcium balance in Daphnia grown on diets differing in food quantity, phosphorus and calciumFRESHWATER BIOLOGY, Issue 11 2009XUE-JIA HE Summary 1.,The influences of dietary phosphorus (P) and food concentration on the calcium (Ca) balance in Daphnia magna were examined in this study at two different ambient Ca concentrations (0.5 and 10 mg Ca L,1). Daphnia were grown by feeding the young adults differentially under contrasting dietary P conditions [molar C : P ratio = c. 900 and c. 90 as low P (LP) and high P (HP), respectively], ambient Ca concentrations [0.5 mg and 10 mg Ca L,1 as low Ca (LCa) or high Ca (HCa), respectively] and food levels [0.15 or 1.5 mg C L,1 as low food (LF) or high food (HF), respectively] for 5 days. 2.,The specific Ca contents of daphniids (1.9,6.5% of dry weight,1) increased with increasing Ca concentration, food level and dietary P content, although the food level did not affect the Ca content in the HPHCa treatment. A radioactive tracer method showed that the food level did not affect the influx of Ca from the water under LP conditions, but the Ca influx under HP conditions doubled with a HF level. A LP condition also led to a decrease in Ca influx with a HF level. 3.,During the 3 days of efflux, generally only a small proportion of Ca (2.6,3.3%) was retained by the daphniids, but this retention increased (14,23%) under low ambient Ca concentrations and under P-limitation. Excretion was the most important pathway for Ca loss (accounting for 50,60% of body Ca), followed by moulting (20,47%), but the relative contribution of these two pathways (excretion and moulting) did not vary among all the different treatments. The absolute loss of Ca through excretion and moulting, on the contrary, differed with different ambient Ca concentrations and dietary P conditions. A HF level led to an increase in the loss rates in most cases. 4.,Our study strongly suggested that there is an interaction between an essential metal (Ca) and macronutrients (C and P) in freshwater crustaceans with HCa and P contents. The results imply that variation in environmental nutrient conditions may change the Ca budget in crustaceans and may affect the dynamics of Ca in the epilimnion of freshwaters. [source] Lability of organic carbon in lakes of different trophic statusFRESHWATER BIOLOGY, Issue 6 2009A. P. OSTAPENIA Summary 1. We used first-order kinetic parameters of biological oxygen demand (BOD), the constant of aerobic decomposition (k) and the asymptotic value of BOD (BODult), to characterise the lability of organic carbon pools in six lakes of different trophic state: L. Naroch, L. Miastro and L. Batorino (Belarus), L. Kinneret (Israel), L. Ladoga (Russia) and L. Mendota (U.S.A.). The relative contributions of labile and refractory organic carbon fractions to the pool of total organic carbon (TOC) in these lakes were quantified. We also determined the amounts of labile organic carbon within the dissolved and particulate TOC pools in the three Belarus lakes. 2. Mean annual chlorophyll concentrations (used as a proxy for lake trophic state) ranged from 2.3 to 50.6 ,g L,1, labile organic carbon (OCL = 0.3BODult) from 0.75 to 2.95 mg C L,1 and k from 0.044 to 0.14 day,1. 3. Our data showed that there were greater concentrations of OCL but lower k values in more productive lakes. 4. In all cases, the DOC fraction dominated the TOC pool. OCL was a minor component of the TOC pool averaging about 20%, irrespective of lake trophic state. 5. In all the lakes, most (c. 85%) of the DOC pool was refractory, corresponding with published data based on measurements of bacterial production and DOC depletion. In contrast, a larger fraction (27,55%) of the particulate organic carbon (POC) pool was labile. The relative amount of POC in the TOC pool tended to increase with increasing lake productivity. 6. Long-term BOD incubations can be valuable in quantifying the rates of breakdown of the combined particulate and dissolved organic carbon pools and in characterising the relative proportions of the labile and recalcitrant fractions of these pools. If verified from a larger number of lakes our results could have important general implications. [source] Periphyton as alternative food source for the filter-feeding cladoceran Daphnia magnaFRESHWATER BIOLOGY, Issue 1 2009SILVANA SIEHOFF Summary 1., Daphnia magna, a well-studied primary consumer, is mainly known as a filter feeder. In this study, we investigated the ability of D. magna to use periphyton as an alternative food source to phytoplankton. We examined the development of laboratory populations fed with different food sources (Desmodesmus subspicatus and/or periphyton or neither) over a period of 42 days, and observed the behaviour of the daphnids. 2.,The addition of periphyton to phytoplankton food led to an increase of daphnid population biomass. When fed with periphyton as the only food source, a small but stable D. magna population developed. 3.,The behaviour of daphnids fed with both food sources revealed a preference for feeding on D. subspicatus. Only below a concentration of D. subspicatus of approximately 0.05 mg C L,1 (0.4 × 107 cells L,1) did D. magna use periphyton as an alternative food source. 4.,Periphyton showed distinct reactions to grazing by D. magna. The thickness of the periphyton layer was reduced from about 4 to 1 mm and we observed a change in species composition due to grazing. 5.,The ability of D. magna to graze on periphyton could serve to stabilize its population density and reinforce its competitive advantage over other cladocerans. By switching between food sources, D. magna can act as a coupler between pelagic and benthic habitats and food webs. [source] Body size and food thresholds for zero growth in Dreissena polymorpha: a mechanism underlying intraspecific competitionFRESHWATER BIOLOGY, Issue 12 2008ALEXANDER WACKER Summary 1. Dreissena polymorpha is an extraordinarily successful invasive species that shows high recruitment of small juvenile mussels on established mussel banks. Such juvenile settlement on, and overgrowth of, large adult mussels; however, leads to competition with adults, and often at high densities and low-food concentrations. 2. The concept of food thresholds for zero growth has been a powerful approach to explaining size-related exploitative competition in different zooplankton species. We applied it to investigate whether food threshold concentrations for zero growth (C0) differ between juvenile and adult zebra mussels. 3. By determining body mass growth at various concentrations of a diet mixture (Nannochloropsis limnetica and Isochrysis aff. galbana) we demonstrate that the threshold food concentration for growth of juvenile mussels (C0 = 0.08 mg C L,1) is substantially lower than that for adults (C0 = 0.36 mg C L,1). 4. This indicates that, at low food availability, juvenile zebra mussels are competitively superior to their larger conspecifics. Within zebra mussel banks plankton food is substantially depleted and so the observed mechanism might ensure juvenile success and therefore the regeneration of mussel banks in nature. [source] Survival and development of five species of cyclopoid copepods in relation to food supply: experiments with algal food in a flow-through systemFRESHWATER BIOLOGY, Issue 9 2005ULRICH HOPP Summary 1. Cyclops spp. generally develop and grow during favourable food conditions in spring and undergo a diapause in summer, while Acanthocyclops robustus, Mesocyclops leuckarti and Thermocyclops crassus develop and grow in summer when they face poorer food conditions and more competition from Cladocera. Since nauplii are the bottleneck in copepod development, we tested the hypothesis that Cyclops abyssorum and C. vicinus nauplii have higher food requirements for survival and development than the nauplii of A. robustus, M. leuckarti and T. crassus. We also tested survivorship and development from hatching to adulthood. 2. Survivorship and development of the copepods was studied in a flow-through system using five concentrations of the phytoflagellate Chlamydomonas reinhardtii in the range from 1 × 104 to 4.5 × 105 cells mL,1 (approximately 0.5,22.5 mg C L,1). 3. Nauplii of both species of Cyclops died at intermediate to low (C. abyssorum) and low (C. vicinus) food concentrations, while nauplii of A. robustus, M. leuckarti and T. crassus survived at all concentrations. 4. The negative effects of low food concentration were also reflected in development. In C. abyssorum and C. vicinus, development duration increased at low food concentration while development was much less affected in A. robustus and T. crassus. Mesocyclops leuckarti was intermediate between Cyclops spp. and A. robustus/T. crassus, with an increase in development duration at the lowest food concentration. 5. Our results support the hypothesis that summer diapause in Cyclops spp. has developed as a strategy to avoid a food bottleneck for nauplii. [source] Humic acids crossinteractions with root and organic acidsANNALS OF APPLIED BIOLOGY, Issue 2 2008L.P. Canellas Abstract The apparent high molecular mass of humic acids (HAs) hardly seems compatible with their direct effects in plant physiology. However, previous evidence has indicated that HAs are non-covalent associations of relatively small molecules, which can be broken down by the action of organic acids. The aim of this work was to evaluate the effects of organic acids on the structure of HAs by spectroscopy and on their bioactivity by following the responses of maize root growth. Changes in the exudation of organic acids from maize seedlings treated with HAs at 50 mg C L,1 were evaluated by high-performance liquid chromatography. The results are in agreement with the concept that HAs are chemical aggregates that acquire characteristics typical of low-molecular-mass humic substances when exposed to organic acids exuded by the roots. Maize seedlings grown in solutions supplemented with HAs plus citric acid at 0.0005, 0.005 and 0.05 mM exhibited significant changes in their root area, primary root length, number of lateral roots and lateral root density and increases in plasma membrane H+ -ATPase activity. Furthermore, the root exudation profile of plants treated with HAs exhibited an increase in the efflux of oxalic and citric acids, with a concurrent decrease in malic and succinic acids. These data reveal a crosstalk between HAs and plants where the exudation of organic acids from the roots influences and is influenced by bioactive molecules released from HAs during root development. [source] | ||||||||||