Predicting and Correcting Electrocution of Birds in Mediterranean Areas

ABSTRACT Bird electrocution on power lines is an important conservation problem that affects many endangered species. We surveyed 3,869 pylons in the Barcelona Pre-littoral Mountains (Catalonia, NE Spain) and collected 141 carcasses of electrocuted birds, mainly raptors and corvids. Univariate analysis indicated that metal pylons with pin-type insulators or exposed jumpers, with connector wires, located on ridges, overhanging other landscape elements, and in open habitats with low vegetation cover were the most dangerous. A logistic regression model indicated that the probability of a pylon electrocuting a bird was mainly related to pylon conductivity, distribution of the conductive elements on the cross-arms, cross-arm configuration, habitat, topography, whether the pylon was overhanging other landscape elements, and presence of rabbits (Oryctolagus cuniculus). We validated the predictive power of this model by using a random sample of 20% of all pylons surveyed. We found that bird mortality was aggregated mainly on pylons assigned a high probability risk by the model. Pylons included in the very high electrocution risk category (9.2%) accounted for 53.2% of carcasses, whereas pylons classified in the low electrocution risk category (54.5%) only accounted for 3.5% of mortality. Power companies employed this classification to prioritize the correction of 222 pylons by installing alternate cross-arms and suspended jumpers and isolating wires and jumpers. We evaluated the effectiveness of this mitigation strategy. A significant fall in the mortality rate on corrected pylons combined with the lack of any reduction in the mortality rate in a sample of 350 noncorrected pylons indicated that the model selected adequately the most dangerous pylons and that the applied correction measures were effective. Consequently, our strategy may be a useful tool for optimizing efforts and resources invested in solving the problem of bird electrocution.     

Minimising mortality in endangered raptors due to power lines: the importance of spatial aggregation to optimize the application of mitigation measures

Electrocution by power lines is one of the main causes of non-natural mortality in birds of prey. In an area in central Spain, we surveyed 6304 pylons from 333 power lines to determine electrocution rates, environmental and design factors that may influence electrocution and the efficacy of mitigation measures used to minimise electrocution cases. A total of 952 electrocuted raptors, representing 14 different species, were observed. Electrocuted raptors were concentrated in certain areas and the environmental factors associated with increased electrocution events were: greater numbers of prey animals; greater vegetation cover; and shorter distance to roads. The structural elements associated with electrocutions were shorter strings of insulators, one or more phases over the crossarm, cross-shaped design and pylon function. Of the 952 carcasses found, 148 were eagles, including golden eagle (Aquila chrysaetos), Spanish imperial eagle (Aquila adalberti) and Bonelli's eagle (Aquila fasciata). Electrocuted eagles were clustered in smaller areas than other electrocuted raptors. The factors associated with increased eagle electrocution events were: pylons function, shorter strings of insulators, higher slopes surrounding the pylon, and more numerous potential prey animals. Pylons with increased string of insulators had lower raptor electrocution rates than unimproved pylons, although this technique was unsuccessful for eagles. Pylons with cable insulation showed higher electrocution rates than unimproved pylons, both for raptors and eagles, despite this is the most widely used and recommended mitigation measure in several countries. To optimize the application of mitigation measures, our results recommend the substitution of pin-type insulators to suspended ones and elongating the strings of insulators.     

A preliminary carbon budget for a part of the Ems estuary: The Dollard

During 1975, measurements were made to quantify all sources of input of organic matter in the Dollard. This made a comparison possible between in situ primary production, import from natural sources and organic waste discharges in terms of organic carbon. In order to make a carbon budget, mineralization and the amount of organic matter buried in the sediment was also measured. Input of organic carbon was mainly based on primary production on the tidal flats (measured in situ as O₂ production, 9.3×10⁶ kg C · year^–1), accumulation of suspended matter originating from the North Sea and the River Ems (maximal 37.1×10⁶ kg C · year^–1) and discharge of heavily polluted water (33.0×10⁶ kg C · year^–1). Input from primary production in the water phase was negligibly low (0.7×10⁶ kg C · year^–1). Loss of organic carbon was due to mineralization in the sediment (measured in situ as oxygen consumption, 18.2×10⁶ kg C · year^–1), mineralization in the water phase (using the BOD technique, 7.2×10⁶ kg C · year^–1) and burying of organic matter in the sediment (9.9·10⁶ kg C · year^–1). The loss of dissolved organic matter to the adjacent Waddensea was not measured but must be considerable. Allochthonous detritus was the main source of energy for the food-webs in the Dollard. The role of bacteria as an important source of food for higher organisms in the Dollard is discussed.     

中国西部电网电击猛禽致死事故调查

2004～2008年调查了西部电网电击猛禽情况.调查区域包括新疆、青海、四川、甘肃、西藏等省区.共统计10 kV和35 kV输电线路长度178 km、杆塔数1 800余根;收集线路附近鸟的尸体、羽毛、残骸等110件,用于鉴定鸟种.调查结果,被电死猛禽计79只(10 kv),平均每千米输电线路1.36只,11.31%的杆塔对鸟类构成威胁;35 kV输电线路上鸟巢数44个,平均每千米输电线路O.37个,至少6.06%杆塔被猛禽利用.已知24%～40%的跳闸停电事故是由于鸟类引发的.症结分析,西部电网结构设计不合理.线路裸露,缺乏防鸟措施.在西部约20%左右的猛禽(11种)受到电击的威胁,同时严重影响着输电线路安全运行.本文提出了合理化建议.     

Avian electrocution rates associated with density of active small mammal holes and power-pole mitigation: Implications for the conservation of Threatened raptors in Mongolia

Avian electrocution at power lines is a well-documented phenomenon, yet factors influencing the frequency of electrocution events and the efficacy of mitigation techniques remain relatively under-reported. During May-July, we surveyed a 56 km long 15 kV electricity distribution line running across open steppe in Mongolia recording electrocuted birds of prey under the power poles. We recorded high rates of electrocution of several Threatened raptor species, particularly the Endangered Saker Falcon Falco cherrug, which was killed at a monthly rate of 1.6 birds per 10 km during the period of our study. Electrocution frequency at line poles was associated with density of small mammal holes and the deployment of mitigation measures. It is likely that local prey abundance influences the frequency of birds of prey perching on power poles, which is consequently reflected in electrocution rate. We evaluated the efficacy of mitigation measures and found that the use of perch deflector spikes on the crossarms of line poles reduced electrocution rates when 3 or 4 spikes were deployed. Perch deflectors probably worked by reducing the opportunity for birds to perch adjacent to pin insulators rather than by reducing the frequency of birds perching on the crossarm per se. At anchor poles, reconfiguration of jump wires at two phases, so they passed under the crossarm rather than over, significantly reduced electrocution rates. These mitigation measures potentially represent a relatively inexpensive method to reduce the frequency of raptor electrocution events in regions where cost is a key factor for power line managers in determining whether or not any form of mitigation is used.     

Control of renal and extrarenal salt and water excretion by plasma angiotensin II in the kelp gull (Larus dominicanus)

Summary The osmoregulatory effects of intravenously (i.v.) administered angiotensin II (AII) at dose rates of 5, 15 and 45 ng · kg^–1 · min^–1 were examined in kelp gulls utilizing salt glands and/or kidneys as excretory organs.In birds given i.v. infusion of 1200 mOsmolal NaCl at 0.3 ml · min^–1 and utilizing only the salt glands to excrete the load, infusion of AII for 30 min consistently inhibited salt gland function in a dose-dependent manner.In birds given i.v. infusion of 500 mOsmolal NaCl at 0.72 ml · min^–1 and utilizing both salt glands and kidneys to excrete the load, each dose of AII given for 2 h inhibited salt gland function but stimulated the kidney, so that the overall outputs of salt and water were enhanced and showed significant (2P<0.01) positive correlations with plasma AII.In birds given i.v. infusion of 200 mOsmolal glucose at 0.5 ml · min^–1 and utilizing only the kidneys to excrete the load, low doses of AII (5 and 15 ng · kg^–1 · min^–1) caused renal salt and water retention, whereas a high dose (45 ng · kg^–1 · min^–1) stimulated salt and water output.The actions of plasma AII in kelp gulls support the concept that this hormone plays a vital role in avian osmoregulation, having effects on both salt gland and kidney function. Elevation of plasma AII consistently inhibits actively secreting salt glands, but its effects upon renal excretion depend primarily on the osmotic status as well as on the plasma AII concentration. In conditions of salt and volume loading doses of AII stimulate sodium and water excretion. With salt and volume depletion, the action of AII is bi-phasic with low doses promoting renal sodium and water retention but high circulating levels causing natriuresis and diuresis.     

The production of 2,3-butanediol by fermentation of high test molasses

Summary Klebsiella oxytoca fermented 199 g·l^–1 high test or invert molasses using batch fermentation with substrate shift to produce 95.2–98.6 g 2,3-butanediol·l^–1 and 2,4–4.3 g acetoin·l^–1 with a diol yield of 96–100% of the theoretical value and a diol productivity of 1.0–1.1 g·l^–1·h^–1. Fermentation was performed numerous times with molasses in repeated batch culture with cell recovery. Such repeated batch fermentation, in addition to a high product yield, also showed a very high product concentration. For example, 118 g 2,3-butanediol·l^–1 and 2.3 g acetoin·l^–1 were produced from 280 g·l^–1 of high test molasses. The diol productivity in this fermentation amounted to 2.4 g·l^–1·h^–1 and can undoubtedly be further increased by increasing the cell concentration. Because the Klebsiella cultures ferment 2,3-butanediol at an extremely high rate once the sugar has been consumed, the culture was inhibited completely by the addition of 15 g ethanol·l^–1 and switching off aeration. Offprint requests to: A. S. Afschar     

Water chemistry and nutrient budgets in an undisturbed evergreen rainforest of southern Chile

In a pristine evergreen rainforest of Nothofagus betuloides, located at the Cordillera de los Andes in southern Chile (41 °S), concentrations and fluxes of nutrients in bulk precipitation, cloud water, throughfall water, stemflow water, soil infiltration and percolation water and runoff water were measured. The main objectives of this study were to investigate canopy-soil-atmosphere interactions and to calculate input-output budgets. From May 1999 till April 2000, the experimental watershed received 8121 mm water (86% incident precipitation, 14% cloud water), of which the canopy intercepted 16%. Runoff water volume amounted 9527 mm. Bulk deposition of inorganic (DIN) and organic (DON) nitrogen amounted 3.6 kg ha^–1 year^–1 and 8.2 kg ha^–1 year^–1 respectively. Occult deposition (clouds + fog) contributes for 40% to the atmospheric nitrogen input (bulk + occult deposition) of the forest. An important part of the atmospheric ammonium deposition is retained within the canopy or converted to nitrate or organic nitrogen by epiphytic bacteria or lichens. Also the export of inorganic (0.9 kg ha^–1 year^–1) and organic (5.2 kg ha^–1 year^–1) nitrogen via runoff is lower than the input to the forest floor via throughfall and stemflow water (3.2 kg DIN ha–1 year^–1 and 5.6 kg DON ha^–1 year^–1). The low concentrations of NO ₃ ^– and NH ₄ ⁺ under the rooting depth suggest an effective biological immobilization by vegetation and soil microflora. Dry deposition and foliar leaching of base cations (K⁺, Ca²⁺, Mg²⁺) was estimated using a canopy budget model. Bulk deposition accounted for about 50% of the total atmospheric input. Calculated dry and occult deposition are both of equal value (about 25%). Foliar leaching of K⁺, Ca²⁺, and Mg²⁺ accounted for 45%, 38% and 6% of throughfall deposition respectively. On an annual basis, the experimental watershed was a net source for Na⁺, Ca²⁺ and Mg²⁺.     

Recruitment,mortality and growth of mangrove (<Emphasis Type="Italic">Rhizophora</Emphasis> sp.) seedlings in Ulugan Bay,Palawan, Philippines

Propagule dispersal, establishment and recruitment to the sapling stage are critical steps in the life cycle of mangroves. Specific (i.e., per capita) rates of recruitment and mortality, and the growth rates of Rhizophora seedlings in three mangrove stands in Ulugan Bay (Palawan, Philippines) were estimated between March 1999 and February 2001. Recruitment and mortality were variable in space and time, with mortality exceeding rates of recruitment at all sites. The specific rates of seedling recruitment and mortality were higher in Buenavista (0.66 year^–1 and –1.67 year^–1) than in Umalagan (0.05 year^–1 and –0.33 year^–1) and Oyster Bay (0.13 year^–1 and –0.24 year^–1). The annual rate of production of internodes by the main stem was similar at the three sites (5.4–5.5 internodes year^–1), but the annual rate of elongation of the main stem was higher in Buenavista (10.6 cm year^–1) than in Oyster Bay (7.6 cm year^–1) and Umalagan (5.6 cm year^–1).     

Continuous underwater light measurement near Helgoland (North Sea) and its significance for characteristic light limits in the sublittoral region

Underwater irradiance was measured at intervals of 20 min for one year at 2 water depths (2.5 and 3.5 m below M.L.W.S.) and in 3 spectral regions in the sublittoral region of the rocky island of Helgoland. Data are presented for spectral and total irradiance at water depths ranging from 2 to 15 m (below M.L.W.S.). 90% of the total annual light reaching sublittoral habitats is received during the period from April to September, when Jerlov water type 7 (occasionally water type 5) dominates. During the other half of the year, the water is very turbid, and transparency is so low that long dark periods occur even at moderate water depths. The total annual light received at the lower kelp limit (Laminaria hyperborea), at 8 m water depth, is 15 MJ m^–2 year^–1 or 70 E m^–2 year^–1, which corresponds to 0.7% of surface irradiance (visible). At the lower algal limit (15 m water depth) these values are 1 MJ m^–2 year^–1 or 6 E m^–2 year^–1, corresponding to 0.05% of surface irradiance. These data are similar to measurements at the same limits in several different geographical areas, and may determine the depth at which these limits occur.     

Activities of benthic nitrifiers in streams and their role in oxygen consumption

The in situ rates of oxygen consumption by benthic nitrifiers were estimated at 11 study sites in 4 streams. Two methods were used: an in situ respiration chamber method and a method involving conversion of nitrifying potential measurements to in situ rates. Estimates of benthic nitrogenous oxygen consumption (BNOC) rate ranged from 0–380 mmol of O₂ m^–2·day^–1, and BNOC contributed between 0–85% of the total benthic oxygen consumption rate. The activity of nitrifiers residing in the sediments was influenced by O₂ availability, temperature, pH, and substrate. Depending upon site, nitrification could approximate either first-order or zero-order kinetics with respect to ammonium concentration. The source of ammonium for benthic nitrifiers could be either totally from within the sediment or totally from the overlying water. Nitrate produced in the sediments could flux to the water above or be lost within the sediment. The sediments could act as a source (positive flux) or sink (negative flux) for both ammonium (–185 mmol·m^–2·day^–1 to +195 mmol·m^–2·day^–1) and nitrate (–135 mmol·m^–2·day^–1 to +185 mmol·m^–2·day^–1).This study provides evidence to suggest that measurements of down-stream mass flow changes in inorganic nitrogen forms may give poor estimates of in situ rates of nitrification in flowing waters.     

Lateral hydraulic conductivity of early metaxylem vessels in Zea mays L. roots

The hydraulic conductivity of the lateral walls of early metaxylem vessels (Lp_x in m · s^–1 · MPa^–1) was measured in young, excised roots of maize using a root pressure probe. Values for this parameter were determined by comparing the root hydraulic conductivities before and after steam-ringing a short zone on each root. Killing of living tissue virtually canceled its hydraulic resistance. There were no suberin lamellae present in the endodermis of the roots used. The value of Lp_x ranged between 3 · 10^–7 and 35 · 10^–7 m · s^–1 · MPa^–1 and was larger than the hydraulic conductivity of the untreated root (Lp_r = 0.7 · 10^–7 to 4.0 · 10^–7 m · s^–1 · MPa^–1) by factor of 3 to 13. Assuming that all flow through the vessel walls was through the pit membranes, which occupied 14% of the total wall area, an upper limit of the hydraulic conductivity of this structure could be given(Lp_pm=21 · 10^–7 to 250 · 10^–7 m · s^–1 · MPa^–1). The specific hydraulic conductivity (Lp_cw) of the wall material of the pit membranes (again an upper limit) ranged from 0.3 · 10^–12 to 3.8 · 10^–12 m² · s^–1 · MPa^–1 and was lower than estimates given in the literature for plant cell walls. From the data, we conclude that the majority of the radial resistance to water movement in the root is contributed by living tissue. However, although the lateral walls of the vessels do not limit the rate of water flow in the intact system, they constitute 8–31% of the total resistance, a value which should not be ignored in a detailed analysis of water flow through roots.Abbreviatations and Symbols k_wr (T ₁ ^2/W ) rate constant (half-time) of water exchange across root (s^–1 or s, respectively) - Lp_cw specific hydraulic conductivity of wall material (m² · s^–1 · MPa^–1) - Lp_pm hydraulic conductivity of pit membranes (m · s ^–1 · MPa^–1) - Lp_r hydraulic conductivity of root (m · s^–1 · MPa^–1) - Lp_x lateralhydraulic conductivity of walls of root xylem (m · s ^–1 · MPa^–1) This research was supported by a grant from the Bilateral Exchange Program funded jointly by the Natural Sciences and Engineering Research Council of Canada and the Deutsche Forschungsgemeinschaft to C.A.P., and by a grant from the Deutsche Forschungsgemeinschaft, Sonderforschungsbereich 137, to E.S. The expert technical help of Mr. Burkhard Stumpf and the work of Ms. Martina Murrmann and Ms. Hilde Zimmermann in digitizing chart-recorder strips is gratefully acknowledged.     

Effects of specific versus cross-training on running performance

The cross-training (XT) hypothesis suggests that despite the principle of specificity of training, athletes may improve performance in one mode of exercise by training using another mode. To test this hypothesis we studied 30 well-trained individuals (10 men, 20 women) in a randomized longitudinal trail. Subjects were evaluated before and after 8 weeks of enhanced training (+10%/week), accomplished by adding either running (R) or swimming (XT) to baseline running, versus continued baseline running (C). Both R ( – 26.4s) and XT (– 13.2s) improved time trial (3.2 km) performance, whereas C did not (– 5.4s). There were no significant changes during treadmill running in maximum oxygen uptake (O_2peak; – 0.2, – 6.0, and + 2.7%), steady state submaximal O₂ at 2.68 m · s^–1 ( – 1.2, – 3.3 and + 0.2 ml · kg^–1 · min^–1), velocity at O_2peak (+0.05, +0.25 and +0.09 m · s^–1) or accumulated O₂ deficit (+ 11.2, – 6.1 and + 9.4%) in the R, XT or C groups, respectively. There was a significant increase in velocity associated with a blood lactate concentration of 4 mmol · l^–1 in R but not in XT or C ( + 0.32, + 0.07 and + 0.08 m · s^–1). There were significant changes in arm crank O_2peak ( + 5%) and arm crank O₂ at 4 mmol · l^–1 ( + 6.4%) in XT. There was no significant changes in arm crank O_2peak ( + 1.3 and – 7.7%) or arm crank O₂ at 4 mmol · l^–1 ( + 0.8 and + 0.4%) in R or C, respectively. The data suggest that muscularly non-similar XT may contribute to improved running performance but not to the same degree as increased specific tranining.     

Pygoscelid penguins in a swim canal

Summary In Antarctica, we investigated the energy consumption of Adélie (Pygoscelis adeliae), Gentoo (P. papua) and Chinstrap (P. antarctica) penguins while resting in the water (8.4 W-kg^–1) and swimming underwater at various speeds, using a 21m long canal filled with sea-water at 4°C in conjunction with respirometry. The birds swam at will and consumed 15.7, 16.1 and 10 W·kg^–1 at the speed where cost of transport was minimal (2.1, 2.3 and 2.5 m·s^–1 in Adélie, Gentoo and Chinstrap penguins, respectively). Thermal conductance in pygoscelid penguins was 3.3 W·°C^–1. m^–2 and energy expenditure (P_i, W·kg^–1) while resting in the water is given by P_j = -0.3 t_a+9.6, where t_a is water temperature in °C. During the breeding season, pygoscelid penguins spend 25–40% of their daily energy expenditure while foraging at sea. The importance of accurate estimates of at-sea activity and energy consumption is discussed.     

Gross and net primary production in the Scotia-Weddell Sea sector of the Southern Ocean during spring 1988

Summary Daily rates of gross and net primary production were calculated in the Scotia-Weddell Sea sector of the Southern Ocean during spring 1988 (EPOS, Leg 2) on the basis of kinetic experiments, which combine radiotracer technology and classic biochemical procedures, and by taking into account the light regime, the physical structure of the water column, the vertical distribution of chlorophyll a, and the protozoan grazing pressure. From these calculations, three distinct sub-areas were identified: the Closed Pack Ice Zone (CPIZ), characterized by the lowest average gross primary production (0.36 gC · m^–2 · day^–1); the Marginal Ice Zone (MIZ) with a maximum mean value of 1.76 gC · m^–2 · day^–1; and the Open Ocean Zone off the ice edge (OOZ) with an intermediate mean value of 0.87 gC · m^–2 · day^–1. Net primary production fluctuated nearly in the same proportions, averaging 0.55, 0.2 and 1.13 gC · m^–2 · day^–1 in the OOZ, CPIZ and MIZ respectively, representing 53% of the total photo-assimilated carbon under heavy ice cover (CPIZ) and 64% in the two other areas. Available light, strongly dependent on the ice cover, was shown to control the level of primary production in the sea ice associated sub-areas, whilst protozoa grazing on phytoplankton determined the moderate primary production level characteristic of the well illuminated OOZ area.Data presented here were collected during the European Polarstern Study (EPOS) sponsored by the European Science Foundation     

Phosphorus and nitrogen excretion rates of zooplankton from the eutrophic Loosdrecht lakes,with notes on other P sources for phytoplankton requirements

Phosphorus and nitrogen excretion rates by zooplankton communities from two eutrophic and shallow Dutch lakes were measured in laboratory. The variations in excretion rates in the lakes (May–October) were caused mainly by fluctuation in zooplankton biomass. Mean summer excretion rates (June–September) were 2.4 and 0.9 µg PO₄P·1^–1·d^–1 in Lake Loosdercht and Lake Breukeleveen, respectively. This difference between the lakes was caused mainly by the lower zooplankton biomass in Lake Breukeleveen. The excretion of 2.4 µg PO₄P·1^–1·d^– compared with the calculated P-demand of phytoplankton of 8.0 µg PO₄P·1^–1·d^–1 is substantial in the summer (June–September) and far more important than the external P-supply of 0.4 µg P·1^–1·d^–1 and sediment release of 0.5 µg P·1^–1·d^–1. Both temperature and composition of zooplankton affected the weight specific excretion rates of the zooplankton community. The weight specific community excretion rates of P and N increased with temperature (exponential model); 1–8 g PO₄P·mg^–1 zooplankton-C·d^–1 and 5–42 µg NH₃N·mg^–1 zooplankton-C·d^–1 (10°C–20°C).     

The importance of avian-contributed nitrogen (N) and phosphorus (P) to Lake Grand-Lieu,France

The largest natural lake in France, Grand-Lieu, has suffered eutrophication. The objective of the study was to estimate the annual input of nutrients (N, P) resulting from avian excrement, deposited by birds feeding out of the lake and returning to its waters for breeding or roosting, as compared to the input by the rivers that enter in the lake. Two years are compared: 1981–82 and 1990–91. About 1600–2000 breeding herons and cormorants, 20 000–33 000 wintering ducks, gulls and cormorants and 1–2.4 million starlings deposited about 5800 kg total N in 1981–82 and 7640 kg in 1990–91. Respectively, 2000 and 2530 kg total P were deposited over the same time periods. These represent 0.7% and 0.4% of the total N input of the lake and 2.4 and 6.6% of the total P input in 1981–82 and 1990–91. Starlings account for 74% of the N and mallards most of the rest. P input by starlings (36% in 1981–82, 41% in 1990–91), and by mallards and herons (35% and 27% in 1981–82 and 22% and 24% in 1990–91 respectively) plays an appreciable role among birds. During the plant growing period (April–September), the contribution by birds can increase to 37% of total P input of the lake. Piscivorous bird colonies concentrate Phosphorus 42 times more within the colony than outside the colony. Overall, the role birds play in total N and P input is relatively small due to very high inputs from human sewage and agriculture run off. The monthly mean concentration of the water of the two rivers reaches currently 10 mg l^–1 of N (to 23 mg during peak floods) and 394 mg m^–3 of P (to 468 mg during peak floods). Earlier, for example in the 1960's, water in Brittany only contained 0.1 to 1.1 mg 1^–1 of N and 1 to 5 mg m^–3 of P during the maximum flow period. At this time, birds could probably have represented annually up to 37% of the N input and up to 95% of the P input to the lake.     

Nitrogen fixation in subarctic streams influenced by beaver (Castor canadensis)

Nitrogen fixation was measured in four subarctic streams substantially modified by beaver (Castor canadensis) in Quebec. Acetylene-ethylene (C₂H₂ C₂H₄) reduction techniques were used during the 1982 ice-free period (May–October) to estimate nitrogen fixation by microorganisms colonizing wood and sediment. Mean seasonal fixation rates were low and patchy, ranging from zero to 2.3 × 10^–3 µmol C₂H₄ · cm^–2 · h^–1 for wood, and from zero to 7.0 × 10^–3 µmol C₂H₄ · g AFDM^–1 · h^–1 for sediment; 77% of all wood and 63% of all sediment measurements showed no C₂H₂ reduction. Nonparametric statistical tests were unable to show a significant difference (p > 0.05) in C₂H₂ reduction rates between or within sites for wood species or by sediment depth.Nitrogen contributed by microorganisms colonizing wood in riffles of beaver influenced watersheds was small (e.g., 0.207 g N · m^–2 · y^–1) but greater than that for wood in beaver ponds (e.g., 0.008 g N · m^–2 · y^–1) or for streams without beaver (e.g., 0.003 g N · m^–2 · y^–1). Although mass specific nitrogen fixation rates did not change significantly as beaver transform riffles into ponds, the nitrogen fixed by organisms colonizing sediment in pond areas (e.g., 5.1 g N · m^–2 · y^–1) was greater than that in riffles (e.g., 0.42 g N · m^–2 · y^–1). The annual nitrogen contribution is proportional to the amount of sediment available for microbial colonization. We estimate that total nitrogen accumulation in sediment, per unit area, is enhanced 9 to 44 fold by beaver damming a section of stream.