Fluxes and transformations of nitrogen in a high-elevation catchment,Sierra Nevada

The fluxes and transformations of nitrogen (N) were investigated from 1985 through 1987 at the Emerald Lake watershed (ELW), a 120 ha high-elevation catchment located in the southern Sierra Nevada, California, USA. Up to 90% of annual wet deposition of N was stored in the seasonal snowpack; NO ₃ ^– and NH ₄ ⁺ were released from storage in the form of an ionic pulse, where the first fraction of meltwater draining from the snowpack had concentrations of NO ₃ ^– and NH ₄ ⁺ as high as 28 eq L^–1 compared to bulk concentrations of <5 eq L^–1 in the snowpack. The soil reservoir of organic N (81 keq ha^–1) was about ten times the N storage in litter and biomass (12 keq ha^–1). Assimilation of N by vegetation was balanced by the release of N from soil mineralization, nitrification, and litter decay. Mineralization and nitrification processes produced 1.1 keq ha^–1 yr^–1 of inorganic N, about 3 1/2 times the loading of N from wet and dry deposition. Less than 1% of the NH ₄ ⁺ in wet and dry deposition was exported from the basin as NH ₄ ⁺ . Biological assimilation was primarily responsible for retention of NH ₄ ⁺ in the basin, releasing one mode of H⁺ for every mole of NH ₄ ⁺ retained and neutralizing about 25% of the annual acid neutralizing capacity produced by mineral weathering in the basin. Nitrate concentrations in stream waters reached an annual peak during the first part of snowmelt runoff, with maximum concentrations in stream water of 20 eq L^–1, more than 4 times the volume-weighted mean annual concentrations of NO ₃ ^– in wet deposition. This annual peak in stream water NO ₃ ^– was consistent with the release of NO ₃ ^– from the snowpack in the form of an ionic pulse; however soil processes occurring underneath the winter snowpack were another potential source of this NO ₃ ^– . Concentrations of stream water NO ₃ ^– during the summer growing season were always near or below detection limits (0.5 eq L^–1).     

Influence of inorganic microelements on the production of camptothecin with suspension cultures of <Emphasis Type="Italic">Camptotheca acuminata</Emphasis>

The effects of eight microelements (I^–, BO₃ ^3–, MoO₄ ^2–, Co²⁺, Cu²⁺, Mn²⁺, Fe²⁺, Zn²⁺) on the biosynthesis of camptothecin and the growth of suspension cultures of Camptotheca acuminata were studied. The increase of I^– to 25 M l^–1, Cu²⁺ to 1 M l^–1, Co²⁺ to 2 M l^–1 and MoO₄ ^2– to 10 M l^–1 in Murashige and Skoog (MS) medium resulted in 1.66, 2.84, 2.53 and 2.04 times higher of camptothecin yield than that in standard MS medium respectively. Combined treatment of I^– (25 M l^–1), Cu²⁺ (1 M l^–1), Co²⁺ (2 M l^–1) and MoO₄ ^2– (10 M l^–1) lead to improve cell dry weight, camptothecin content, and camptothecin yield to 30.56 g l^–1, 0.0299%, and 9.15 mg l^–1, respectively, which were 20.2, 208.9 and 273.8% increment respectively when compared with those of control.     

Effect of whole catchment liming on the episodic acidification of two adirondack streams

During the fall of 1989 7.7Mg/ha of calcium carbonate was applied on two tributary catchments (40 ha and 60 ha) to Woods Lake, a small (25 ha) acidic headwater lake in the western Adirondack region of New York. Stream-water chemistry in both catchment tributaries responded immediately. Acid-neutralizing capacity (ANC) increased by more than 200 eq/L in one of the streams and more than 1000 eq/L in the other, from pre-liming values which ranged from –25 to +40 eq/L. The increase in ANC was primarily due to increases in dissolved Ca²⁺ concentrations. Most of the initial response of the streams was due to the dissolution of calcite that fell directly into the stream channels and adjacent wetlands. A small beaver impoundment and associated wetlands were probably responsible for the greater response observed in one of the streams.After the liming of subcatchmentIV (60 ha), Ca²⁺ concentrations increased with increasing stream discharge in the stream during fall rain events, suggesting a contribution from calcite dissolved within the soil and transported to the stream by surface runoff or shallow interflow. Concentrations of other ions not associated with the calcite (e.g. Na⁺) decreased during fall rain events, presumably due to mixing of solute-rich base flow with more dilute shallow interflow. The strong relation between changes in Ca²⁺ and changes in NO ₃ ^– concentrations during spring snowmelt, (r² = 0.93, slope = 0.96, on an equivalent basis) suggests that both solutes had a common source in the organic horizon of the soil. Increases in NO ₃ ^– concentrations during snowmelt were balanced by increases in Ca²⁺ that was released either directly from the calcite or from exchange sites, mitigating episodic acidification of the stream. However, high ambient NO ₃ ^– concentrations and relatively low ambient Ca²⁺ concentrations in the stream during the spring caused the stream to become acidic despite the CaCO₃ treatment.In stream WO2 (40ha), Ca²⁺ concentrations were much higher than in stream WO4 because of the dissolution of calcite which fell directly into the upstream beaver pond and its associated wetlands. Calcium concentrations decreased as both NO ₃ ^– concentrations and stream discharge increased, due to the dilution of Ca-enriched beaver pond water by shallow interflow. Despite this dilution, Ca²⁺ concentrations were high enough to more than balance strong acid anion (SO ₄ ^– , NO ₃ ^– , Cl^–) concentrations, resulting in a positive ANC in this stream throughout the year. These data indicate that liming of wetlands and beaver ponds is more effective than whole catchment liming in neutralizing acidic surface waters.     

Diurnal variation in physical-chemical properties and primary production in the interconnected marine,mangrove and freshwater biotopes of Kakinada coast,Andhra Pradesh,India

Diurnal variation in hydrological variables and dissolved inorganic nutrients such as PO _inf4 ^sup3– -P, N O _inf2 ^sup– -N, NO _inf3 ^sup– -N and NH _inf4 ^sup+ -N were studied in three interconnected biotopes including freshwater, marine and mangrove brackish water of the Kakinada coastal zone, Andhra Pradesh. Samples were collected at intervals of 3 hours, for a period of 24 hours. In the marine environment salinity varied from 26 to 32 whereas in the mangrove waters it fluctuated from 12 to 20 and in both biotopes salinity showed bimodal type of oscillation. Dissolved oxygen content was high in the mangrove waters during day time but decreased rapidly during the night hours. In the marine environment PO_f4 ^p3–-P concentration varied from 0.345 to 1.195 g at l^–1, NO _inf3 ^sup– -N from 1.03 to 6.62 g at l^–1 and NO _inf2 ^sup– -N from 0.086 to 0.506 g at l^–1. The highest and the lowest concentrations of PO _inf4 ^sup3– -P, NO _inf3 ^sup– -N, NO _inf2 ^sup– -N recorded in the mangrove waters were 0.790 and 0.325 g at l^–1, 7.10 and 1.60 g at l^–1 and 0.278 and 0.060 g at l^–1, respectively. The concentration of PO _inf4 ^sup3– -P, NO _inf3 ^sup– -N and NO _inf2 ^sup– -N were high in the freshwater canal, the maximum and minimum values being 1.110 and 0.730 g at l^–1, 26.40 and 9.98 g at l^–1 and 0.520 and 0.252 g at l^–1 respectively. The concentration of ammonia was relatively high in the mangrove water. Gross and net primary production in the mangrove water was 4 times higher than in the marine biotope. There was no export of dissolved nutrients from the mangrove environment to the adjacent marine waters.     

Spatial and temporal variations in aluminum chemistry of a dilute,acidic lake

Elevated concentrations of Al have been observed in acidic surface waters. An assessment of the chemistry of aqueous Al is of interest because of its role as a toxicant to aquatic organisms, a pH buffer, and an adsorbent of orthophosphate and organic carbon. In this investigation we evaluated the spatial and temporal fluctuations of Al forms in an acidic drainage lake.High concentrations of NO ₃ ^– (51.0 ± 11 mol l^–1), H⁺ (14.9 ± 3.5 mol l^–1), and Al (19.6 ± 3.5 mol l^–1) were introduced to Dart's Lake through drainage water during the snowmelt period. During low flow periods microbially mediated depletions of nitrate served to neutralize H⁺ and aluminum base neutralizing capacity. Thus in Dart's Lake, NO ₃ ^– transformations were extremely important in regulating short-term changes in pH and subsequent changes in the inorganic forms of Al. During stratification periods Al appeared to be non-conservative within the lake system. Although we know very little about the character and transformations of alumino-organic solutes, these substances were correlated with dissolved organic carbon (DOC) concentrations. Alumino-organic substances appear to be introduced to the lake from both drainage water and sediments.     

Influence of acid rain and liming on fluxes of NO and NO2 from forest soil

Flux measurements of nitric oxide (NO) and nitrogen dioxide (NO₂) were performed in a coniferous forest (Höglwald) in southern Germany using a fully automated measuring system based on the dynamic chamber method. The forest soil was predominately a source of NO, but mean flux rates of NO ranged from –26.3 (deposition) to 55 g N m^-2 h_-1 (emission). NO₂ was deposited on the forest soil with mean flux rates ranging from –4 to –72 g N m^-2 h_-1 . Removal of forest floor vegetation did not influence NO or NO₂ fluxes. Apparently, forest floor vegetation was neither a source of NO nor a significant sink of NO₂. When the organic layer of the forest soil was removed, net NO flux changed from emission to deposition. Thus NO emitted to the atmosphere was produced almost exclusively in the organic layer of the forest soil. Liming caused a significant decrease in the rate of NO emission by 43 to 100%, whereas irrigation with simulated acid rain increased the emission of NO by a factor of 3.1. Irrigation with simulated normal rain decreased the emission of NO by 35 to 100%. No such effects could be detected for the deposition of NO₂.     

Ammonium and nitrate uptake in gap,generalist and understory species of the genus Piper

Summary We studied root net uptake of ammonium (NH ₄ ⁺ ) and nitrate (NO ₃ ^– ) in species of the genus Piper (Piperaceae) under high, intermediate and low photosynthetically active photon flux densities (PFD). Plants were grown hydroponically, and then transferred to temperature controlled (25° C) root cuvettes for nutrient uptake determinations. Uptake solutions provided NH ₄ ⁺ and NO ₃ ^– simultaneously (both) or separately (single). In the first experiment, seven species of Piper, from a broad range of rainforest light habitats ranging from gap to understory, were screened for mineral nitrogen preference (100 M NH ₄ ⁺ and/or 100 M NO ₃ ^– ) at intermediate PFD (100 mol m^–2 s^–1). Preference for NH ₄ ⁺ relative to NO ₃ ^– , defined as the ratio of NH ₄ ⁺ (both):NO ₃ ^– (both) net uptake, was higher in understory species than in gap species. Ammonium repression of NO ₃ ^– uptake, defined as the ratio of NO ₃ ^– (single): NO ₃ ^– (both) net uptake, was also higher in understory species as compared to gap species. In a second set of experiments, we examined the effect of nitrogen concentration (equimolar, 10 to 1000 M) on NH ₄ ⁺ preference and NH ₄ ⁺ repression of NO ₃ ^– net uptake at high (500 mol m^–2 s^–1) and low (50 mol m^–2 s^–1) PFD in a gap (P. auritum), generalist (P. hispidum) and understory species (P. aequale). All species exhibited negligible NH ₄ ⁺ repression of NO ₃ ^– net uptake at high PFD. At low PFD, NH ₄ ⁺ preference and repression of NO ₃ ^– net uptake occurred in all species (understory > generalist > gap), but only at intermediate nitrogen concentrations, i.e. between 10 and 200 M. Ammonium repression of net NO ₃ ^– uptake decreased or increased rapidly (in < 48 h) after transitions from low to high or from high to low PFD respectively. No significant diurnal patterns in NO ₃ ^– or NH ₄ ⁺ net uptake were observed.CIWDPB publication # 1130     

Growth inhibition of biomass adapted to the degradation of toluene and xylenes in mixture in a batch reactor with substrates supplied by pulses

A biomass adapted to degrade toluene and xylenes in mixture was grown in a batch reactor with substrates supplied by pulses. The inhibition of biomass growth in the course of substrate degradation was investigated. The maximal biomass concentration of 7 g l^–1 was obtained using 150 l of toluene and 15 l of a mixture of xylenes in one litre of liquid medium, and the maximal biomass productivity and yield were 53 mg l^–1 h^–1 and 0.32 g_DW g _s ^–1 , respectively. Higher quantities of substrate added by pulses, that is 200 l of toluene with 20 l of xylenes and 300 l of toluene with 30 l of xylenes, caused an accumulation of metabolites. These higher quantities of substrates caused inhibition of microbial growth. Among the metabolites produced, 4-methyl catechol was found in large quantities in the culture medium and in the cells.     

Effects of <Emphasis Type="Italic">in vitro</Emphasis>rooting environments and irradiance on growth and photosynthesis of strawberry plantlets during acclimatization

Strawberry (Fragaria ananassaDuch. cv. Fengxiang) plantlets were cultured under two in vitroenvironments for rooting, and then acclimatized under two ex vitroirradiance conditions. At the end of rooting stage plant height, fresh weight and specific leaf area of T1-plants grown under high sucrose concentration (3 sucrose), low photosynthetic photon flux density (30 mol m^–2 s^–1) and normal CO₂ concentration (350–400 l l^–1) were significantly higher than those of T2-plantlets grown under low sucrose concentration (0.5), high photosynthetic photon flux density (90 mol m^–2 s^–1) and elevated CO₂ concentration (700–800 l l^–1). But T2-plantlets had higher net photosynthetic rate (Pn), effective photochemical quantum yield of PSII (_PSII), effective photosynthetic electron transport rate (ETR), photochemical quenching (q_P) and ratio of chlorophyll fluorescence yield decrease (Rfd). After transfer, higher irradiance obviously promoted the growth of plantlets and was beneficial for the development of photosynthetic functions during acclimatization. T2-plantlets had higher fresh weight, leaf area, _PSII and ETR under higher ex vitroirradiance condition.     

Tissue culture ofKarwinskia humboldtiana—a plant producing toxins with antitumoural effects

Isolated embryos ofKarwinskia humboldtiana were cultured in vitro. The growth of embryos and development to plantlets on woody plant medium supplemented with indole-3-acetic acid 6.10^-2 mol l^–1, gibberellic acid (GA₃) 3.10^-2 mol l^–1, and 6-benzylaminopurine (BA) 2 mol l^–1 was obtained. Multiplication of shoots and rooting of excised shoots has been achieved. Callus formation on modified Murashige-Skoog medium supplemented with 1-naphthaleneacetic acid 10 mol l^–1, GA₃ 14 mol l^–1, and kinetin 5 mol l^–1 on hypocotyls, or on root cultures on medium supplemented with 2.4-dichlorophenoxyacetic acid 10 mol l^–1 and BA 10 mol l^–1 was induced.Abbreviations BA 6-benzylaminopurine - 2,4-d 2,4-dichlorophenoxyacetic acid - GA₃ gibberellic acid - IAA indole-3-acetic acid - NAA 1-naphthaleneacetic acid - TEM transmission electron microscopy     

Ecotoxicity of lead to the zebra musselDreissena Polymorpha,Pallas

The effect of lead on the filtration rate of the zebra musselDreissena polymorpha was investigated, together with the accumulation of Pb in the soft tissues of the mussels. The NOEC-filtration was 116 g.l^–1 (0,56 mol.l^–1) and the EC₅₀-filtration was 370 g.l^–1 (1.79 mol.l^–1). The NOEC-accumulation was the concentration found in the control water (1.4g.l^–1). These experiments show that the EC₅₀-filtration for Pb is similar to that for Cd, higher than that for Cu and lower than that for Zn. The water quality criteria for lead allow 25 g Pb.l^–1 in surface water. This will not cause short-term effects. Long-term effects may, however, occur, since an accumulation of Pb as low as 16 g.l^–1 was recorded in this study.     

Regional analysis of inorganic nitrogen yield and retention in high-elevation ecosystems of the Sierra Nevada and Rocky Mountains

Yields and retention of dissolved inorganic nitrogen (DIN: NO₃ ^– + NH₄ ⁺) and nitrate concentrations in surface runoff are summarized for 28 high elevation watersheds in the Sierra Nevada of California and Rocky Mountains of Wyoming and Colorado. Catchments ranged in elevation from 2475 to 3603 m and from 15 to 1908 ha in area. Soil cover varied from 5% to nearly 97% of total catchment area. Runoff from these snow-dominated catchments ranged from 315 to 1265 mm per year. In the Sierra Nevada, annual volume-weighted mean (AVWM) nitrate concentrations ranged from 0.5 to 13 M (overall average 5.4 M), and peak concentrations measured during snowmelt ranged from 1.0 to 38 M. Nitrate levels in the Rocky Mountain watersheds were about twice those in the Sierra Nevada; average AVWM NO₃ ^– was 9.4 M and snowmelt peaks ranged from 15 to 50 M. Mean DIN loading to Rocky Mountain watersheds, 3.6 kg ha^–1 yr^–1, was double the average measured for Sierra Nevada watersheds, 1.8 kg ha^–1 yr^–1. DIN yield in the Sierra Nevada, 0.69 kg ha^–1 yr^–1, was about 60% that measured in the Rocky Mountains, 1.1 kg ha^–1 yr^–1. Net inorganic N retention in Sierra Nevada catchments was 1.2 kg ha^–1 yr^–1 and represented about 55% of annual DIN loading. DIN retention in the Rocky Mountain catchments was greater in absolute terms, 2.5 kg ha^–1 yr^–1, and as a percentage of DIN loading, 72%.A correlation analysis using DIN yield, DIN retention and surface water nitrate concentrations as dependent variables and eight environmental features (catchment elevation, slope, aspect, roughness, area, runoff, soil cover and DIN loading) as independent variables was conducted. For the Sierra Nevada, elevation and soil cover had significant (p > 0.1) Pearson product moment correlations with catchment DIN yield, AVWM and peak snowmelt nitrate concentrations and DIN retention rates. Log-linear regression models using soil cover as the independent variable explained 82% of the variation in catchment DIN retention, 92% of the variability in AVWM nitrate and 85% of snowmelt peak NO₃ ^–. In the Rocky Mountains, soil cover was significantly (p < 0.05) correlated with DIN yield, AVWM NO₃ ^– and DIN retention expressed as a percentage of DIN loading (%DIN retention). Catchment mean slope and terrain roughness were positively correlated with steam nitrate concentrations and negatively related to %DIN retention. About 91% of the variation in DIN yield and 79% of the variability in AVWM NO₃ ^– were explained by log-linear models based on soil cover. A log-linear regression based on soil cover explained 90% of the variation of %DIN retention in the Rocky Mountains.     

Pumping behaviour and filtration rate of the freshwater mussel Potomida littoralis as a tool for rapid detection of water contamination

In order to evaluate the ability of the freshwater bivalve Potomida littoralis for use as a biodetector of aquatic pollution, the short-term effects of some pollutants (metals, antifouling product and effluents) on pumping activity of this mussel were studied qualitatively (pumping behaviour) and quantitatively (filtration rate). The method of the flow sensing at the exhalent siphon showed that the normal activity of P. littoralis was characterized by a continuous pattern of pumping with occasional and brief interruptions. When exposed to toxic stress, the exhalent flow immediately shifted to an intermittent pattern with periods of pumping alternating with periods of pause that could last several minutes. The threshold concentrations of heavy metals/antifouling agent inducing this type of activity within 2 h were: 25 g l^–1 for copper, 90 g l^–1 for cadmium, 5 g l^–1 for mercury, 3000 g l^–1 for zinc and 700 g l^–1 for the antifouling agent Mexel. Both the effluent samples of the urban wastewater of Marrakech city tested were found to be toxic.The filtration rate of control animals ranged from 0.91 to 1.72 l h^–1 g^–1 animal, while after 3 h exposure to Cu, Cd, Hg and the effluent E1 and E2, filtration rates were decreased by concentrations above the threshold levels set as toxic for the pumping activity.The sensitivity of P. littoralis to pollution and its usefulness as biodetector for rapid toxicity screening is discussed and a procedure for the detection of pollution in Moroccan freshwater is proposed.     

Application of N-15 dilution for simultaneous estimation of nitrification and nitrate reduction in soil-water columns

Nitrification and denitrification rates were estimated simultaneously in soil-floodwater columns of a Crowley silt loam (Typic Albaqualfs) rice soil by an¹⁵N isotopic dilution technique. Labeled NO ₃ ^– was added to the floodwater of soil-water columns, half were treated with urea fertilizer. The (NO ₃ ^– +NO ₂ ^– )–N and (NO ₃ ^– +NO ₂ ^– )–N concentrations in the floodwater were measured over time and production and reduction rates for NO ₃ ^– calculated. Nitrate reduction in the urea amended columns averaged 515 mol N m^–2h^–1 and nitrification averaged 395 mol N m^–2h^–1 over the 35–153 d incubation. The nitrification rate for 4–19 d sampling period (1,560 mol N m^–2h^–1) in the urea amended columns was almost 9 times greater than the reduction rate (175 mol N m^–2h^–1) over the same period. Without the addition of urea the NO ₃ ^– production rate averaged 32 mol N m^–2h^–1 and reduction 101 mol N m^–2h^–1.     

Sisal plant regeneration via organogenesis

Callus was initiated from in vitro grown immature leaf and ex vitro grown mature leaf and rhizome explants of Agave sisalana Perr. ex. Engelm, on MS medium containing 2,4-D (9.05 M) and kinetin (4.6 M) or 2,4-D (9.05 M), kinetin (4.6 M) and CH (1000 mg l^–1) or mod. MS (NH₄NO₃, 1500 mg l^–1) containing 2,4-D (9.05 M) and kinetin (4.6 M). Light was essential for callus formation which, however, was different in three types of explants on three different media compositions. Increasing NH₄ ⁺had a negative impact while addition of CH had a positive impact on callus formation. Shoot regeneration from callus from CH-supplemented medium only was achieved for rhizome and immature leaf tissues. The highest rate of regeneration was obtained with BA (26.6 M) as the sole hormone. Shoot buds g^–1 callus varied according to BA concentrations. Shoot proliferation rate increased on half-strength MS medium containing BA (8.9 M). Microshoots developed on MS medium containing BA (2.22 M) and GA₃ (1.44 M) and finally rooted on MS medium containing IAA (11.42 M). Acclimatized rooted plantlets are growing satisfactorily in ex vitro. This is the first report on plant regeneration via organogenesis of A. sisalana.     

Denitrification in a South Louisiana wetland forest receiving treated sewage effluent

Although denitrification has the potential to reduce nitrate (NO ₃ ^– ) pollution of surface waters, the quantification of denitrification rates is complex because it requires differentiation from other mechanisms and is highly variable in both space and time. This study first measured potential denitrification rates at a wetland forest site in south Louisiana before receipt of secondary wastewater effluent, and then, following 30 months of effluent application, landscape gradients of dissolved nitrate (NO ₃ ^– ) and nitrous oxide (N₂O) were measured. A computer model was developed to quantify N transformations. Floodwater NO ₃ ^– and N₂O concentrations were higher in the forest receiving effluent than in the adjacent control forest. Denitrification rates of NO ₃ ^– -amended soil cores ranged from 0.03–0.45 g N m^–2 d^–1 with an overall mean of 0.10 g N m^–2 d^–1. Effluent N is being applied at a rate of approximately 0.034 g N m^–2 d^–1, with approximately 95% disappearing along a 1 km transect. In the treatment forest, floodwater NO ₃ ^– concentrations decreased from 1000 M at the inflow point to 50 M along the 1 km transect. Nitrous oxide concentrations increased from 0.25 M to 1.2 M within the first 100 m, but decreased to 0.1 M over the next 900 m. The initial increase in N₂O was presumably a result ofin situ denitrification. Model analyses indicated that denitrification was directly associated with nitrification and was limited by the availability of NO ₃ ^– produced by nitrification. Due to different redox potential optima, coupling of nitrification and denitrification was a function of a balance of environmental conditions that was moderately favorable to both processes. N removal efficiency was largely dependent on the proportion of effluent NH ₄ ⁺ to NO ₃ ^– . When NH ₄ ⁺ /NO ₃ ^– was 1, average N removal efficiency ranged from 95–100%, but ratios that were >1 reduced average efficiencies to as low as 57%. Actual effluent NH ₄ ⁺ /NO ₃ ^– loading ratios at this site are approximately 0.2 and are consistently <1.     

Nitrogen fixation in a large shallow lake: rates and initiation conditions

The fixation of molecular nitrogen (N₂fix) by cyanobacteria in situ and in PO₄-P enrichment experiments was investigated in large shallow Lake Võrtsjärv in 1998–2000. In this lake, N₂fix started when TN/TP mass ratio was about 20, which is much higher than Redfield mass ratio 7. The rate of N₂fix varied between 0.81 and 2.61 gN l^–1 d^–1 and maximum rate (2.61 gN l^–1 d^–1) was measured in 15.08.2000. In L. Võrtsjärv a lag period of a couple of weaks occurred between the set-up of favourable conditions for N₂fix as the appearance of N₂-fixing species and depletion of mineral nitrogen, and the real N₂fix itself. However, if the favorable conditions for N₂fix occurred in the lake, N₂fix started after enrichment with PO₄-P in mesocosms even then when no N₂fix was detected in the lake. N₂fix in mesocosms was also more intensive than in lake water. In our experiments PO₄-P concentrations higher than 100 gP l^–1started to inhibit N₂fix.     

Effects of cadmium on foraging behaviour of lake charr, Salvelinus namaycush

Synopsis Foraging behaviour of lake charr, Salvelinus namaycush, exposed to 0.5 and 5.0 g l^–1 cadmium concentrations for 106–112 d, was examined in a flow-through stream channel using fathead minnows, Pimephales promelas, as prey. Five modes of foraging behaviour were observed and quantified (ending at fixate-orient; ending at approach and/or chase; ending at failed attack; ending at capture and loss; successful consumption). Each mode represented an increasingly progressive end-point from initial sighting of the prey to final consumption. Compared to controls., 0.5 g l^–1 Cd-exposed predators captured significantly fewer prey, while 5 g l^–1 Cd-exposed predators made fewer attacks. Both Cd-exposed groups consumed fewer prey than controls.     

Arsenic concentrations in water and fish from Chautauqua Lake,New York

Synopsis Arsenic persists in Chautauqua Lake, New York waters 13 years after cessation of herbicide (sodium arsenite) application and continues to cycle within the lake. Arsenic concentrations in lake water ranged from 22.4–114.81 g l^–1, = 49.0 ag l^–1. Well water samples generally contained less than 10 g l^–1 arsenic. Arsenic concentrations in lake water exceeded U.S. Public Health Service recommended maximum concentrations (10 g l^–1) and many samples exceeded the maximum permissible limit (50 g l^–1). Fish accumulated arsenic from water but did not magnify it. Fish to water arsenic ratios ranged from 0.4–41.6. Black crappie (Pomoxis nigromaculatus) contained the highest arsenic concentrations (0.14–2.04 g g^–1 ), X = 0.7 g g^–1) while perch (Perca flavescens), muskellunge (Esox masquinongy) and largemouth bass (Micropterus salmoides) contained the lowest concentrations (0.02–0.13 g g^–1). Arsenic concentrations in fish do not appear to pose a health hazard for human consumers.     

Effect of Naloxone on the Activity of Cl–-Activated Mg2+-ATPase from Plasma Membrane Fraction of Bream Brain (Abramis brama L.) in the Presence of GABAa-ergic Substances

We studied the effect of naloxone—an antagonist of the opioid receptors—on sensitivity of Cl^–-activated Mg²⁺-ATPase from the plasma membrane fraction of bream brain (Abramis brama L.) to GABA_a-ergic substances. Preincubation of the plasma membranes with 1–100 M naloxone increased the basal Mg²⁺-ATPase activity and suppressed its activation by chloride ions. The same effects were observed in the presence of the agonists of GABA_a/benzodiazepine receptors: 0.1–100 M GABA, 1–500 M pentobarbital, and 0.1–100 M phenazepam. Naloxone (10 M) inhibited activation of the basal Mg²⁺-ATPase by the studied ligands and restored the enzyme sensitivity to Cl^–. However, the effect of naloxone was not observed in the presence of high concentrations of pentobarbital (500 M) and phenazepam (100 M). The obtained data show that naloxone modulates the activity of Cl^–-activated Mg²⁺-ATPase from the plasma membranes of bream brain and antagonizes the GABA_a receptor ligands.