Temporal records of δ13C and δ15N in North Pacific pinnipeds: inferences regarding environmental change and diet

Sea lion and seal populations in Alaskan waters underwent various degrees of decline during the latter half of the twentieth century and the cause(s) for the declines remain uncertain. The stable carbon (¹³C/¹²C) and nitrogen (¹⁵N/¹⁴N) isotope ratios in bone collagen from wild Steller sea lions (Eumetopias jubatus), northern fur seals (Callorhinus ursinus) and harbor seals (Phoca vitulina) from the Bering Sea and Gulf of Alaska were measured for the period 1951-1997 to test the hypothesis that a change in trophic level may have occurred during this interval and contributed to the population declines. A significant change in '¹⁵N in pinniped tissues over time would imply a marked change in trophic level. No significant change in bone collagen '¹⁵N was found for any of the three species during the past 47 years in either the Bering Sea or the Gulf of Alaska. However, the ¹⁵N in the Steller sea lion collagen was significantly higher than both northern fur seals and harbor seals. A significant decline in '¹³C (almost 2 ‰ over the 47 years) was evident in Steller sea lions, while a declining trend, though not significant, was evident in harbor seals and northern fur seals. Changes in foraging location, in combination with a trophic shift, may offer one possible explanation. Nevertheless, a decrease in '¹³C over time with no accompanying change in '¹⁵N suggests an environmental change affecting the base of the foodweb rather than a trophic level change due to prey switching. A decline in the seasonal primary production in the region, possibly resulting from decreased phytoplankton growth rates, would exhibit itself as a decline in '¹³C. Declining production could be an indication of a reduced carrying capacity in the North Pacific Ocean. Sufficient quantities of optimal prey species may have fallen below threshold sustaining densities for these pinnipeds, particularly for yearlings and subadults who have not yet developed adequate foraging skills.     

Shoot δ15N and δ13C values of non-host Brassica rapa change when exposed to ±Glomus etunicatum inoculum and three levels of phosphorus and nitrogen

Glasshouse experiments were conducted to study the response of non-host Brassica rapa and host Sorghum bicolor to inoculation with the arbuscular mycorrhizal fungus (AMF) Glomus etunicatum when given different levels of N (0.9 mmol kg^-1 sand, 2.7 mmol kg^-1 sand, 8.1 mmol kg^-1 sand) and P (3.6 µmol kg^-1 sand, 10.7 µmol kg^-1 sand, 32.0 µmol kg^-1 sand) fertiliser. On both plant species, the presence of G. etunicatum inoculum (+AMF) was associated with significant changes of shoot '¹⁵N values, with +AMF plants having larger average '¹⁵N values than uninoculated plants (-AMF). These values are the largest average differences in shoot '¹⁵N yet recorded for AMF and nutrient effects. B. rapa shoot '¹⁵N average differences ranged from 1.67‰ to 2.70‰, while for S. bicolor they range between 2.07‰ and 4.40‰. For shoot '¹³C only the non-host B. rapa responded to -AMF and added N. Although the harvested dry weight biomass (-35.2% B. rapa; +39.8% S. bicolor) of both plant species responded to AMF inoculation, no direct relationship was observed between isotopic discrimination and growth inhibition for the non-host B. rapa. In this paper we discuss some implications regarding AMF inocula on the basis of our findings and current literature.     

Stable isotope enrichment (δ15N and δ13C) in a generalist predator (Pardosa lugubris, Araneae: Lycosidae): effects of prey quality

Analysis of the natural variations in stable isotope ratios in animal tissue may be a powerful tool to reveal the trophic position and feeding preferences of generalist predators such as lycosid spiders. In the present study, changes in ¹⁵N/¹⁴N and ¹³C/¹²C ratios in the lycosid spider Pardosa lugubris (Araneae: Lycosidae), fed with prey of different quality, were investigated. Experimental food chains included three trophic levels: prey media, prey organism and predator. In order to analyse the time course of stable isotope enrichment, different life stages of P. lugubris feeding on Drosophila melanogaster were studied. The ¹⁵N content of hatchlings of P. lugubris was significantly lower than that of their mothers, indicating the existence of nitrogen pools with different ¹⁵N signatures in female P. lugubris. With duration of feeding and progressing development, age and body weight, the ¹⁵N content in spiderlings increased. Starvation resulted in ¹⁵N and ¹³C enrichment in P. lugubris. Being fed with prey of different quality resulted in varying patterns of stable isotope enrichment. ¹⁵N but not ¹³C content consistently increased when fed on high quality prey (Heteromurus nitidus, D. melanogaster and a mixed diet consisting of H. nitidus and D. melanogaster). In contrast, low quality prey (Rhopalosiphum padi, Folsomia candida and a mixed diet consisting of H. nitidus, D. melanogaster and F. candida) resulted in deviations from postulated patterns of stable isotope enrichment with ¹⁵N content being similar to that of starving spiderlings. When fed on high quality prey, ¹⁵N enrichment of P. lugubris was close to ca. 3‰. It is concluded that the analysis of variations in the natural abundance of stable isotopes is particularly helpful in revealing the trophic structure of terrestrial food webs in which polyphagous feeders predominate, as is the case in litter and soil.     

Nitrogen and carbon isotope fractionation between mothers, neonates, and nursing offspring

Stable isotope signatures of lactating females and their nursing offspring were measured on 11 species, including herbivores, carnivores, hibernators, and non-hibernators. We hypothesized that: (1) nursing offspring would have stable isotope signatures that were a trophic level higher than their mothers, and (2) this pattern would be species-independent. The plasma of adult females had a '¹⁵N enrichment over their diets of 4.1ǂ.7‰, but offspring plasma had a mean '¹⁵N enrichment over maternal plasma of 0.9ǂ.8‰ and no C enrichment (0.0ǂ.6‰). The trophic level enrichment did not occur between mother and offspring because milk was depleted in both '¹⁵N (1.0ǂ.5‰) and '¹³C (2.1ǂ.9‰) relative to maternal plasma. Milk to offspring plasma enrichment was relatively small ('¹⁵N enrichment of 1.9ǂ.7‰ and '¹³C enrichment of 1.9ǂ.8‰) compared to the trophic level enrichment between the adults and their diets. While some species did have significant differences between the isotope signatures of mother and offspring, the differences were not related to whether they were hibernators or non-hibernators, carnivores or herbivores. Investigators wanting to use stable isotopes to quantify weaning or other lactation processes or diets of predators when both adults and nursing offspring are consumed must first establish the parameters that apply to a particular species/environment/diet combination.     

13C content of ecosystem respiration is linked to precipitation and vapor pressure deficit

Variation in the carbon isotopic composition of ecosystem respiration ('¹³C_R) was studied for 3 years along a precipitation gradient in western Oregon, USA, using the Keeling plot approach. Study sites included six coniferous forests, dominated by Picea sitchensis, Tsuga heterophylla, Pseudotsuga menziesii, Pinus ponderosa, and Juniperus occidentalis, and ranged in location from the Pacific coast to the eastern side of the Cascade Mountains (a 250-km transect). Mean annual precipitation across these sites ranged from 227 to 2,760 mm. Overall '¹³C_R varied from -23.1 to -33.1‰, and within a single forest, it varied in magnitude by 3.5-8.5‰. Mean annual '¹³C_R differed significantly in the forests and was strongly correlated with mean annual precipitation. The carbon isotope ratio of carbon stocks (leaves, fine roots, litter, and soil organic matter) varied similarly with mean precipitation (more positive at the drier sites). There was a strong link between '¹³C_R and the vapor saturation deficit of air (vpd) 5-10 days earlier, both across and within sites. This relationship is consistent with stomatal regulation of gas exchange and associated changes in photosynthetic carbon isotope discrimination. Recent freeze events caused significant deviation from the '¹³C_R versus vpd relationship, resulting in higher than expected '¹³C_R values.     

Mapping of a thermo-sensitive earliness <Emphasis Type="Italic">per se</Emphasis> gene on <Emphasis Type="Italic">Triticum monococcum</Emphasis> chromosome 1A<Superscript>m</Superscript>

An earliness per se gene, designated Eps-A^m1, was mapped in diploid wheat in F₂ and single-seed descent mapping populations from the cross between cultivated (DV92) and wild (G3116) Triticum monococcum accessions. A QTL with a peak on RFLP loci Xcdo393 and Xwg241, the most distal markers on the long arm of chromosome 1A^m, explained 47% of the variation in heading date (LOD score 8.3). Progeny tests for the two F_2:3 families with critical recombination events between Xcdo393 and Xwg241 showed that the gene was distal to Xcdo393 and linked to Xwg241. Progeny tests and replicated experiments with line #3 suggested that Eps-A^m1 was distal to Xwg241. This gene showed a large effect on heading date in the controlled environment experiments, and a smaller, but significant, effect under natural conditions. Eps-A^m1 showed significant epistatic interactions with photoperiod and vernalization treatments, suggesting that the different classes of genes affecting heading date interact as part of a complex network that controls the timing of flowering induction. Besides its interactions with other genes affecting heading date, Eps-A^m1 showed a significant interaction with temperature. The effect of temperature was larger in plants carrying the DV92 allele for late flowering than in those carrying the G3116 allele for early flowering. Average differences in heading date between the experiments performed at 16 °C and 23 °C were approximately 11 days (P < 0.001) for the lines carrying the Eps-A^m1 allele for early flowering but approximately 50 days (P < 0.0001) for the lines carrying the allele for late flowering. The large differences in heading time (average 80 days) observed between plants carrying the G3116 and DV92 alleles when grown at 16 °C, suggest that it would be possible to produce very detailed maps for this gene to facilitate its future positional cloning.     

Carbon isotope discrimination in photosynthetic bark

We developed and tested a theoretical model describing carbon isotope discrimination during photosynthesis in tree bark. Bark photosynthesis reduces losses of respired CO₂ from the underlying stem. As a consequence, the isotopic composition of source CO₂ and the CO₂ concentration around the chloroplasts are quite different from those of photosynthesizing leaves. We found three lines of evidence that bark photosynthesis discriminates against ¹³C. First, in bark of Populus tremuloides, the '¹³C of CO₂ efflux increased from -24.2‰ in darkness to -15.8‰ in the light. In Pinus monticola, the '¹³C of CO₂ efflux increased from -27.7‰ in darkness to -10.2‰ in the light. Observed increases in '¹³C were generally in good agreement with predictions from the theoretical model. Second, we found that '¹³C of dark-respired CO₂ decreased following 2-3 h of illumination (P<0.01 for Populus tremuloides, P<0.001 for Pinus monticola). These decreases suggest that refixed photosynthate rapidly mixes into the respiratory substrate pool. Third, a field experiment demonstrated that bark photosynthesis influenced whole-tissue '¹³C. Long-term light exclusion caused a localized increase in the '¹³C of whole bark and current-year wood in branches of P. monticola (P<0.001 and P<0.0001, respectively). Thus bark photosynthesis was shown to discriminate against ¹³C and create a pool of photosynthate isotopically lighter than the dark respiratory pool in all three experiments. Failure to account for discrimination during bark photosynthesis could interfere with interpretation of the '¹³C in woody tissues or in woody-tissue respiration.     

A basal aquatic-terrestrial trophic link in rivers: algal subsidies via shore-dwelling grasshoppers

Rivers provide important resources for riparian consumers, especially in arid or seasonally arid biomes. Pygmy grasshoppers (Paratettix aztecus and P. mexicanus; Tetrigidae) graze river algae stranded along shorelines of the South Fork Eel River in northern California (39°44'N, 123°39'W) as the river recedes during the summer drought. Densities of tetrigids during the mid to late summer were highest (1 individual/m² in July) within 1 m of the river margin, and declined to near zero at 4 m from the margin, especially during peak temperatures in the afternoon. These observations suggested that the distribution of tetrigids was determined by the availability of algae, water, or both. We manipulated the presence/absence of water and beached algae (Cladophora glomerata) in a 2Ƕ factorial design. All treatments were positioned 2 m upslope from the river's edge (about 30 cm above the water table), where the cobble bar was naturally dry and devoid of algae and densities of tetrigids were lower than at the river margin (0.4 individuals/m² in July). Tetrigids responded only to the wet Cladophora treatment, which had 302 higher densities than other treatments. Stable isotopic signatures ('¹³C) of tetrigids (-19.7‰) collected from the same cobble bars were more similar to those of epilithic algae (-20.4‰) than terrestrial plants (-28.2‰), and higher than those of acridid grasshoppers (-27.9‰) from the same habitats. Mixing models suggest that 88-100% of the C in tetrigid grasshoppers at our study site is derived from riverine algae. A preliminary analysis suggests that tetrigids ingested sufficient quantities of algae to easily meet their energetic demands during the summer. This study supports the idea that algae, produced in stream systems, can determine the distribution and relative abundance of a common terrestrial scavenger and provide an additional pathway for energy exchange between rivers and riparian food webs.     

Water availability and carbon isotope discrimination in conifers

The stable C isotope composition ('¹³C) of leaf and wood tissue has been used as an index of water availability at both the species and landscape level. However, the generality of this relationship across species has received little attention. We compiled literature data for a range of conifers and examined relationships among landscape and environmental variables (altitude, precipitation, evaporation) and '¹³C. A significant component of the variation in '¹³C was related to altitude (discrimination decreased with altitude in stemwood, 2.53‰ km^-1 altitude, r²=0.49, and in foliage, 1.91‰ km^-1, r²=0.42), as has been noted previously. The decrease in discrimination with altitude was such that the gradient in CO₂ partial pressure into the leaf (P_a-P_i) and altitude were generally unrelated. The ratio of precipitation to evaporation (P/E) explained significant variation in P_a-P_i of stemwood (r²=0.45) and foliage (r²=0.27), but only at low (<0.8) P/E. At greater P/E there was little or no relationship, and other influences on '¹³C probably dominated the effect of water availability. We also examined the relationship between plant drought stress (O) and '¹³C within annual rings of stemwood from Pinus radiata and Pinus pinaster in south-western Australia. Differential thinning and fertiliser application produced large differences in the availability of water, nutrients and light to individual trees. At a density of 750 stems ha^-1, O and '¹³C were less (more negative) than at 250 stems ha^-1 indicating greater drought stress and less efficient water use, contrary to what was expected in light of the general relationship between discrimination and P/E. The greater '¹³C of trees from heavily thinned plots may well be related to an increased interception of radiation by individual trees and greater concentrations of nutrients in foliage - attributes that increase rates of photosynthesis, reduce P_i and increase '¹³C. '¹³C was thus modified to a greater extent by interception of radiation and by nutrient concentrations than by water availability and the '¹³C-O relationship varied between thinning treatments. Within treatments, the relationship between '¹³C and O was strong (0.38<r²<0.58). We conclude that '¹³C may well be a useful indicator of water availability or drought stress, but only in seasonally dry climates (P/E<1) and where variation in other environmental factors can be accounted for.     

The influence of UV-B radiation on light-dependent photosynthetic performance in <Emphasis Type="Italic">Sanionia uncinata </Emphasis>(Hedw.) Loeske in Antarctica

Photosynthetic activity of the moss Sanionia uncinata (Hedw.) Loeske was investigated on Léonie Island (67°35'S, 68°20'W, Antarctic Peninsula) in response to short-term changes of UV-B radiation. The UV-environment of natural mat formations dominated by S. uncinata was altered using filter screens. Two filter experiments were conducted in the Antarctic summers 1998 and 1999. A third filter experiment was conducted during springtime ozone depletion in October 1998. Photosynthetic activity of S. uncinata was mainly determined by photosynthetically active photon flux density (PPFD). Light response of relative electron transport rate through photosystem II (rel ETR=jF/Fm'2PPFD) remained unaffected by ambient summer levels of UV-B radiation. The same was found for net photosynthesis and dark respiration. In October 1998, S. uncinata was mainly metabolically inactive due to low temperatures. No significant levels of DNA-damage measured as cyclobutane pyrimidine dimers (CPDs) were induced by ambient summer levels of UV-B. Artificially enhanced UV-B radiation supplying a Setlow-DNA-dose of 8.7 kJ m^ф day^у UV-B led to formation of 7Dž CPD (10⁶ nucleotides)^-1. It is concluded that current ambient summer levels of UV-B radiation do not affect photosynthetic activity in S. uncinata.     

It takes two to tango but three is a tangle: mutualists and cheaters on the carnivorous plant <Emphasis Type="Italic">Roridula</Emphasis>

Roridula dentata is associated with hemipterans, which facilitate nitrogen assimmilation from insects. R. dentata is also associated with spiders and their role in digestion is unknown. We quantify approximately how much nitrogen Roridula assimilates from insects through "indirect digestion." Using '¹⁵N we then determine whether nitrogen absorption from hemipteran insects differs with varying spider densities. In this way, we are able to determine their nutritional role. At low spider densities, indirect digestion of prey accounts for approximately 70% of plant nitrogen. These values are comparable to methods of direct prey digestion found in other carnivorous plants. However spiders decrease the numbers of hemipteran individuals inhabiting Roridula plants and also decrease efficiency of indirect prey digestion by up to 30%. We deduce that spiders are cheaters as they exploit plant rewards without offering any rewards in return. However, indirect carnivory is still efficient enough when hemipteran densities are at their lowest, ensuring that the mutualism does not break down.     

Influence of groundwater depth on the seasonal sources of water accessed by <Emphasis Type="Italic">Banksia</Emphasis> tree species on a shallow,sandy coastal aquifer

In Mediterranean ecosystems vegetation overlying shallow, transient aquifers is often dominated by woody phreatophytes, trees and shrubs that have been shown to be dependent on groundwater for their water requirements. Natural and anthropogenic alterations of groundwater tables (abstraction) are of clear importance to phreatophytic vegetation as reduction of water tables may sever these plants from their natural water sources. Seasonal water sources were determined for species growing on a coastal dune system that overlies a shallow sandy aquifer in south-western Australia. The plants studied grew over groundwater that ranged in depth from 2.5 to 30 m. The naturally occurring stable isotope of hydrogen (deuterium, '²H) was used to distinguish potential water sources. Isotopic ratios from vascular water of the dominant species of the study area (Banksia ilicifolia R. Br. and Banksia attenuata R. Br. trees) were compared with those of potential sources of precipitation, soil moisture and groundwater. A relatively shallow-rooted perennial shrub, Hibbertia hypericoides Benth., was also included as an isotopic reference. The results suggest that both B. attenuata and B. ilicifolia are phreatophytic as they derived some of their water from groundwater throughout the dry-wet cycle, with the exception of B. attenuata at the site of greatest depth to groundwater (30 m) which did not use groundwater. A high proportion (>50%) of groundwater use was not maintained throughout all seasons. With the onset of the hot Mediterranean summer, progressive drying of the surface soils resulted in increased use of groundwater and deep soil moisture. During the wet winter plants used proportionately more water from the upper layers of the soil profile. The degree to which groundwater was utilised by the study species was dependent on the proximity of groundwater, availability of moisture in shallower horizons of the soil profile, root system distribution and maximum root depth.     

Plants feed ants: food bodies of myrmecophytic Piper and their significance for the interaction with Pheidole bicornis ants

Several species of Piper (Piperaceae) live in symbiosis with Pheidole bicornis (Formicidae-Myrmicinae) on the southern Pacific slope of Costa Rica. These plants produce small single-celled food bodies (FBs) in leaf domatia, formed by the petiole bases and roofing leaf sheaths. In the present study the dependency of ants on FBs of Piper fimbriulatum as a food source was analysed by comparing the natural abundance of ¹³C and ¹⁵N in ants and FBs. Both '¹³C and '¹⁵N values were very similar between FBs and Pheidole bicornis ants but differed substantially between the plant and other ant species. Therefore we suggest that FBs are a main food source for Pheidole bicornis ants. To strengthen this suggestion, the chemical composition of FBs of four myrmecophytic Piper species was analysed, with special emphasis on the nutritional requirements of inhabiting Pheidole bicornis ants. Standard chemical methods were modified and combined to a novel analysis scheme by which all major FB constituents could be quantified from minute [3-10 mg dry mass (DM)] quantities. Piper FBs mainly consisted of lipids (41-48% of DM) and proteins (17-24% of DM). Soluble carbohydrates and amino acids proved to be quantitatively unimportant. N was predominantly stored as soluble protein and, thus, was easily available to the ants. FBs proved to be a high-energy food source (up to 23 kJ g^-1 DM), with a chemical composition that meets well the nutritional needs of the inhabiting ants.     

Plant frequency,stem and root characteristics,and CO<Subscript>2</Subscript> uptake for <Emphasis Type="Italic">Opuntia acanthocarpa</Emphasis>: elevational correlates in the northwestern Sonoran Desert

A common cylindropuntia in the northwestern Sonoran Desert, Opuntia acanthocarpa, was investigated for the following hypotheses: its lower elevational limit is set by high temperatures, so its seedlings require nurse plants; its upper elevational limit is set by freezing; spine shading is the least at intermediate elevations; and changes in plant size and frequency with elevation reflect net CO₂ uptake ability. For four elevations ranging from 230 m to 1,050 m, the mean height of O. acanthocarpa approximately doubled and its frequency increased 14-fold. Nurse plants were associated with only 4% of O. acanthocarpa less than 20 cm tall at the two lower elevations compared with 57% at 1,050 m, where putative freezing damage was especially noticeable, suggesting that nurse plants protect from low temperature damage. Spine shading of the stem doubled from the lowest to the highest elevation. Net CO₂ uptake, which followed a Crassulacean acid metabolism pattern, was maximal at day/night air temperatures of 25/15°C and was halved by 4 weeks of drought and by reducing the photosynthetic photon flux from 30 to 12 mol m^-2 day^-1. The root system of O. acanthocarpa was shallow, with a mean depth of only 9 cm for the largest plants. Root growth was substantial and similar for plants at 25/15°C and 35/25°C, decreasing over 70-fold at 15/5°C and 45/35°C. Based on cellular uptake of the vital stain neutral red, neither roots nor stems tolerated tissue temperatures below -5°C for 1 h while both showed substantial high temperature acclimation, roots tolerating 1 h at 61°C and stems 1 h at 70°C for plants grown at 35/25°C. The increase in height and frequency of O. acanthocarpa with elevation apparently reflected both a greater ability for net CO₂ uptake and greater root growth and hence water uptake. This species achieves its greatest ecological success at elevations where it becomes vulnerable to low temperature damage.     

Physiological energetics of the protobranch bivalve <Emphasis Type="Italic">Yoldia hyperborea</Emphasis> in a cold ocean environment

Yoldia hyperborea is a deposit-feeding circumpolar protobranch that also inhabits muddy sediments of the cold water boreal system of Conception Bay, Newfoundland, Canada. Little is known about this species, despite its wide distribution and frequent high density in the benthos. The present work deals with oxygen consumption and ammonia excretion under cold ambient conditions. Y. hyperborea showed low basal metabolism [0.051 ml O₂ h^у·(g dry weight)^у, T=у°C] and low ammonia excretion rates [4.212 µg·NH₄-N·h^у·(g dry weight)^у, T=у°C]. Low metabolic activity could prove a useful strategy during periods of low food availability. In addition, Y. hyperborea was able to regulate its O₂ consumption rate at very low pO₂ levels, which may be advantageous for a species that may experience periods of hypoxia.     

Fatty acid composition of lipid-rich myctophids and mackerel icefish (<Emphasis Type="Italic">Champsocephalus gunnari</Emphasis>) – Southern Ocean food-web implications

The lipid content, fatty acid composition and calorific value of seven species of mesopelagic deep-sea fish of the family Myctophidae and the mackerel icefish, Champsocephalus gunnari, important in the diet of Southern Ocean marine predators, are presented. Fish were sampled at the Kerguelen Plateau (KP) and Macquarie Ridge (MR) in the Indian and Pacific sectors of the Southern Ocean respectively, to examine geographic variation in lipid compositon. All species of myctophid from KP and Electrona antarctica from MR were high in lipid content (6-18% wet mass), particularly Gymnoscopelus nicholsi (18%) and E. antarctica (15%). The mackerel icefish, and G. fraseri and Protomyctophum tenisoni from MR were generally lower in lipid content (3-5%) and varied significantly in fatty acid composition from KP species. KP myctophids were high in calorific content (9.3 kJ g^-1 wet mass) when compared with icefish (5.4 kJ g^-1 wet mass) and other published values for prey items of marine predators such as squid (1.7-4.5 kJ g^-1). KP myctophids were distinguished from each other and from C. gunnari and MR specimens by cluster and discriminant function analysis using six fatty acids (16:0, 18:1̩, 20:1̩, 22:1Ὃ, 20:5̣, 22:6̣). Findings presented here highlight trophic links between high-latitude fish and their prey and emphasise the importance of myctophids as a significant energy source for marine predators foraging in the Polar Frontal Zone.     

Foraging ecology of the endangered wood stork recorded in the stable isotope signature of feathers

Down feathers and regurgitant were collected from nestling wood storks (Mycteria americana) from two inland and two coastal breeding colonies in Georgia. The stable isotopic ratios of carbon (¹³C/¹²C) and nitrogen (¹⁵N/¹⁴N) in these materials were analyzed to gain insights into the natal origins of juvenile storks and the foraging activities of adults. Down feathers differed in '¹³C between inland and coastal colonies, having average isotopic values that reflected the sources of carbon fixed in biomass at the base of the food web. Feathers from the inland colonies differed between colonies in '¹⁵N, while those from the coastal colonies did not. These patterns primarily reflected the foraging activities of parent storks, with individuals capturing differing percentages of prey of distinct trophic status at each colony. Collectively, the carbon and nitrogen isotopic signatures of feather keratin were used to distinguish nestlings from each colony, except for instances where storks from different colonies foraged in common wetlands. The stable isotopic composition of food items in regurgitant was used to reconstruct the trophic structure of the ecosystems in which wood storks foraged. Predicted foraging activities based on the isotopic composition of keratin were generally consistent with the percentage of prey types (freshwater vs. saltwater and lower trophic level vs. upper trophic level consumer) observed in regurgitant, except for the coastal colony at St. Simons Island, where the '¹³C of feathers strongly suggested that freshwater prey were a significant component of the diet. This inconsistency was resolved by aerial tracking of adults during foraging excursions using a fixed-wing aircraft. Observed foraging activities supported interpretations based on the stable isotope content of feathers, suggesting that the latter provided a better record of overall foraging activity than regurgitant analysis alone. Observed foraging patterns were compared to the predictions of a statistical model that determined habitat utilization based on habitat availability using a geographic information system (GIS) database. Observed foraging activities and those predicted from feathers both suggested that some adult storks preferred to feed their young freshwater prey, even when saltwater resources were more accessible in the local environment. This conclusion supports the contention that wood stork populations are sensitive to changes in the distribution of freshwater habitats along the southeastern coastal plain of the United States.     

Symbiotic N2 fixation of Alnus incana ssp. rugosa in shrub wetlands of the Adirondack Mountains, New York, USA

Surface waters in forested watersheds in the Adirondack Mountains and northern New York State are susceptible to nitrogen (N) saturation. Atmospheric deposition of N to watersheds in this region has been measured but the extent of internal N inputs from symbiotic N₂ fixation in alder-dominated wetlands is not known. We estimated N₂ fixation by speckled alder in these wetlands by the ¹⁵N natural abundance method and by acetylene reduction using a flow-through system. Foliar N derived from fixation (%N_dfa) was estimated for five wetlands. The '¹⁵N of speckled alder foliage from four of the five sites did not differ significantly (PА.05) from that of nodulated speckled alders grown in N-free water culture (-1.2ǂ.1‰). Estimates from the ¹⁵N natural abundance method indicated that alders at these sites derive 85-100% of their foliar N from N₂ fixation. At one of the sites, we also measured biomass and N content and estimated that the alder foliage contained 43 kg N ha^-1 of fixed N in 1997. This estimate was based on a foliar N content of 55.4lj kg N ha^-1 (mean-SE), 86dž%N_dfa, and an assumption that 10% of foliar N was derived from reserves in woody tissues. At this site, we further estimated via acetylene reduction that 37ᆞ kg N ha^-1 was fixed by speckled alders in 1998. This estimate used the theoretical 4:1 C₂H₂ reduction to N₂ fixation ratio and assumed no night-time fixation late in the season. Nitrogen inputs in wet and dry deposition at this site are approximately 8 kg N ha^-1 year^-1. We conclude that speckled alder in wetlands of northern New York State relies heavily on N₂ fixation to meet N demands, and symbiotic N₂ fixation in speckled alders adds substantial amounts of N to alder-dominated wetlands in the Adirondack Mountains. These additions may be important for watershed N budgets, where alder-dominated wetlands occupy a large proportion of watershed area.     

<Emphasis Type="Italic">Agrobacterium</Emphasis>-mediated production of transgenic plants of<Emphasis Type="Italic"> Muscari armeniacum</Emphasis> Leichtl. ex Bak.

A system for the production of transgenic plants has been developed for the Liliaceous ornamental plant Muscari armeniacum Leichtl. ex Bak via Agrobacterium-mediated transformation of embryogenic cultures. Leaf-derived embryogenic cultures were co-cultivated with each of three A. tumefaciens strains, all of which harbored the binary vector carrying the neomycin phosphotransferase II (nptII), hygromycin phosphotransferase (hpt) and intron-containing #-glucuronidase (gus-intron) genes in the T-DNA region. Following co-cultivation, the embryogenic cultures were cultured on a medium containing 500 mg l^-1 cefotaxime for 1 week followed by a medium containing 75 mg l^-1 hygromycin in addition to cefotaxime. After 4-5 weeks, several hygromycin-resistant (Hyg^r) cell clusters were produced from the co-cultivated embryogenic cultures. The highest efficiency of production of Hyg^r cell clusters was obtained when embryogenic cultures were inoculated with A. tumefaciens EHA101/pIG121Hm in the presence of 100 µM acetosyringone (AS) and 0.1% (v/v) of a surfactant (Tween20) followed by co-cultivation in the presence of 100 µM AS. Hyg^r embryogenic cultures developed into complete plants via somatic embryogenesis, and most of them were verified to be transgenic by GUS histochemical assay and polymerase chain reaction analysis. Southern blot analysis revealed the integration of one to five copies of the transgene into the genome of transgenic plants, but most of them had one or two copies.     

Short-term variability in the flux of rapidly sinking particles in the Antarctic marginal ice zone

A sediment trap deployment was made at a station (64°42'S, 139°59'E) at five depths (537, 796, 1,259, 1,722, 2,727 m) in the marginal ice zone (MIZ) of the Antarctic Ocean during a summer productive period from 26 December 1994 to 20 January 1995. This aim of the study was to reveal a possible occurrence of a sporadic bloom in surface layers and to evaluate the role of fast-sinking particles in transportation processes of bloom-derived material down to mesopelagic and bathypelagic layers. During the observation, a marked flux increase (70.5 mg C m^-2 day^-1, 7.7 mg N m^-2 day^-1) was observed at the depth of 537 m on 7-9 January. The increased flux at 537 m decreased with depth and time. The same mass of sinking particles forming the flux maximum at each depth sank down from the shallowest trap (537 m) to the deepest trap (2,727 m) within 4-11 days, indicating that these particles were transported downward to the bottom with the sinking rate of >200 m day^-1. Collected particles were composed of two major particle fractions; one was dominated by fecal pellets of macrozooplankton (mainly Euphausia superba) with relatively fast sinking rates (FSP; fast-sinking particles) and the other by minute diatoms of Fragilariopsis curta with slow sinking rates (SSP; slowly sinking particles). According to the comparison of time depth changes of these two fractions, the SSP had unexpectedly faster sinking rates comparable with the FSP during the periods of maximum fluxes, probably indicating the SSP were transformed from the FSP during sinking. The present result strongly suggests that a local bloom of F. curta and intensified zooplankton grazing activities occurred in surface layers in a few days in the MIZ, and then the egested fecal pellets were rapidly transported downward with fragmentation processes into small-sized minute particles in mesopelagic and bathypelagic layers.