Studies of tolerance to heavy metals in the flora of the rivers Ystwyth and Clarach, Wales

A survey of the river Ystwyth demonstrated that it was polluted with metals especially below the areas of old lead mines. Hormidium spp. Kütz in particular Hormidium rivulare Kütz were the most tolerant filamentous green algae present. Scapania undulata (L.) Dum. was a tolerant bryophyte on the Ystwyth and Clarach rivers and was found alone in polluted sites, but was less frequent in cleaner areas. Metal extracts of this bryophyte mirrored the variation in metal concentrations of the environment to some extent. In general, lower levels of Fe, Pb and Mn were found in Scapania as compared with the less tolerant Fontinalis squamosa Hëdw. When the latter was transplanted to polluted sites an increase in its Pb, Cu, Zn and Mn content occurred within 6 weeks and the plant started to die and decay after 18 weeks. Scapania survived when it was transplanted from its natural polluted habitat to a less polluted area, and there was no marked change in its metal content. Zn⁶⁵ tracing experiments, in the laboratory, demonstrated that both plants had similar uptake patterns for the metal, but that the amount taken up was slightly higher in the less tolerant Fontinalis. There was no change in the number of diatoms found upon Scapania when it was transplanted, but a reduction in numbers occurred on the transplanted Fontinalis when it started dying. The composition of these diatom floras was different in polluted sites as compared to that in the cleaner areas.     

Macroinvertebrate colonization of acrylic plates in a large river

From April 1977 through June 1978, scuba divers used a double-cup bar-clamp sampler to monitor colonization rates and standing crops of macroinvertebrates on acrylic plates near the rocky bottom of the Susquehanna River in northeastern Pennsylvania. Monthly, bimonthly, and cumulative plates were exposed for about 30, 60, and 124–362 days, respectively.The upper surface of monthly plates was colonized from June through September at rates ranging from 125 organisms/m²/day in June 1978 to 3,200 org/m²/day in August ({xx\-x} = 1,300 org/m²/day). Chironomids (Rheotanytarsus spp.) and hydropsychids (Hydropsyche phalerata and Cheumatopsyche spp.) composed 97% of the total. The lower surface of monthly plates was colonized from May through September at rates ranging from 110 org/m²/day in June 1977 to 760 org/m²/day in May 1977 ({xx\-x} = 260 org/m²/day). Naidids (Nais spp. and Chaetogaster spp.) and chironomids (Nanocladius spp.) composed 89% of the total.Macroinvertebrates remained on cumulative plates after colonization of monthly plates ceased. The upper surface of cumulative plates was inhabited from June through October at densities ranging from 5,800 org/m² in June 1977 to 78,000 org/m² in August ({xx\-x} = 40,000 org/m²). The lower surface of cumulative plates was inhabited year-round, at densities ranging from t,000 org/m² in December to 15,000 org/m² in June 1977 ({xx\-x} = 6,100 org/m²).This study was supported by Pennsylvania Power and Light CompanyThis study was supported by Pennsylvania Power and Light Company     

Long-term effects of PO4 fertilization on the distribution of bryophytes in an arctic river

1. Each year since 1983, H₃PO₄ has been added continuously during the ice-free season to a P-limited tundra river (Kuparuk River, North Slope, Alaska). Effects on epilithic metabolism, invertebrate community structure and fish production developed quickly. 2. In 1990, 7 years after fertilization began, we noted extensive coverage by bryophytes within the fertilized reach of the river, where very little had been noted before. Bryophyte biomass from a limited set of quadrats taken in 1990 and 1991 yielded 17 ± 9 (SE) g dry mass m^?2 in control reaches and 322 ± 96 g dry mass m^?2 in fertilized reaches. 3. An initial survey of macroalgal and bryophyte cover in 1991 suggested that the moss Schistidium (Grimmia) agassizii was distributed in both control and fertilized reaches of the river. No clear difference in coverage by this species was found in either reach. 4. In contrast, two species of Hygrohypnum (H. alpestre and H. ochraceum) were found almost exclusively in the fertilized reach. An extensive point transect survey done in 1992, above, within and below the fertilized reach, indicated that increased cover and biomass of Hygrohypnum spp. were confined to the fertilized reach of the river. Detrended correspondence analysis clearly separated the macrophyte and macroalgal communities in the fertilized reach from those in the control and downstream reaches. 5. A fourth bryophyte species (Fontinalis neomexicana) also occurred almost exclusively in the fertilized reach, but was much less abundant than the Hygrohypnum species. 6. Analysis of total N and P in the tissues of the Hygrohypnum spp., and estimates of average coverage (～15%) and biomass (～150g dry weight m^?2) over an 8km fertilized reach, suggest that these species alone may have removed two-thirds of the P added in the fertilizer experiment. The bryophyte community in this river is likely to be the dominant sink for P in the fertilized reach.     

Different competitive outcomes among four species of cladocerans under different alga combinations of colonial <Emphasis Type="Italic">Microcystis</Emphasis> spp. and green alga <Emphasis Type="Italic">Scenedesmus obliquus</Emphasis>

Cyanobacteria blooms (especially Microcystis spp.) are thought to alter dominance of large-sized daphnids into small-sized metazoan zooplankton. However, several field investigations show different phenomena. Laboratory experiments were conducted based on the hypothesis that different Microcystis spp. concentrations would influence competitive outcomes using two algal combinations of different concentrations and four species of cladocerans. In the algal combination of 50 mg l⁻¹ colonial Microcystis spp. + 1 mg l⁻¹ Scenedesmus obliquus (fresh weight), Daphnia carinata was absent during the experiment in competition with other cladocerans. Decreasing colonial Microcystis spp. concentration (10 mg l⁻¹) resulted in a shift from dominance by small-sized cladocerans to dominance by D. carinata. No significant effects of different concentrations of colonial Microcystis spp. on competitive outcomes were shown among three small-sized cladocerans. These results support the idea that cyanobacteria concentration affects the dominance status of large-bodied daphnid.     

Winter phytoplankton community structure in three shallow temperate lakes during ice cover

The general model of seasonal phytoplankton succession in temperate lakes suggests that winter phytoplankton growth is minimal under ice-cover. However, some studies have found diverse phytoplankton communities during winter. The primary objectives of this study were to determine the species composition and the changes in the winter phytoplankton community structure under the ice. For 2 consecutive winters, phytoplankton samples were collected under ice-cover at 4 sites on 3 lakes in Arrowwood National Wildlife Refuge (ANWR), near Pingree, North Dakota. Ninety taxa were identified and enumerated. Densities of several of these taxa frequently exceeded 10⁶ cells l^–1. The winter phytoplankton communities in these lakes were dominated by flagellates, principally cryptomonads, a synurophyte (Synura uvella), small chrysophytes (Chrysococcus spp., Kephyrion spp.) and a dinoflagellate (Peridinium aciculiferum), as well as non-flagellate microchlorophytes (Monoraphidium spp., Ankistrodesmus spp., and Pseudodictyosphaerium sp.), a cyanobacterium (Gloeocapsa aeruginosa) and centric diatoms (Stephanodiscus minutulus, S. parvus and Cyclotella meneghiniana).     

Litterfall of epiphytic macrolichens in Nothofagus forests of northern Patagonia,Argentina: Relation to stand age and precipitation

Abstract: The objective of this study was to analyse how stand age and precipitation influence abundance and diversity of epiphytic macrolichens in southern beech Nothofagus forests, estimated by lichen litter sampling. Five sites of Nothofagus dombeyi (Mirbel) Oersted were selected in Nahuel Huapi National Park, Argentina. At each site, lichen fragments from the forest floor were collected at 12.5 m² plots in pairs of young and mature N. dombeyi forest. Additionally, two sites with multi‐aged subalpine Nothofagus pumilio (Poepp. et Endl.) Krasser forest were investigated in a similar manner. Average litterfall biomass per stand varied from less than 1 kg ha^?1 in a young low‐precipitation stand to a maximum of 20 kg ha^?1 in a mature high‐precipitation stand. In places with higher precipitation, litterfall biomass in N. dombeyi forest was considerably higher in old stands as compared with young ones. In places with less than 2000 mm of precipitation, differences in biomass were less pronounced. Old humid stands contained about twice as many taxa in the litter as old low‐precipitation stands and young stands in general. Mature stands in low‐precipitation sites only contained 17% of the litter biomass as compared with mature stands in high‐precipitation sites. Epiphytic lichen composition changed from predominating fruticose lichens (Usnea spp. and Protousnea spp.) in low‐precipitation stands to Pseudocyphellaria spp., Nephroma spp. and other foliose lichens, in the high‐precipitation stands. There were no clear differences in the proportion of fruticose and foliose lichens between young and old stands. Fruticose lichens dominated litter biomass in both N. pumilio sites.     

Effects of copper on species composition of periphyton in a Sierra Nevada, California, stream

SUMMARY. 1. Changes in species composition of the periphyton of an oligotrophic. Sierra Nevada stream continuously dosed for 1 year at three concentrations of copper (2.5, 5 and 10 μg 1^?1 Cu_T; approximately 12, 25 and 50 ng 1^?1 Cu²⁺) were determined. 2. The numerically most abundant taxa were Bacillariophyceae (Achnanthes minutissima, Cocconeis placentula, Cymbella microcephala, C. sinuata, Fragilaria conslruens, F. crotonensis. Navicula spp., Synedra acus and S. rumpens), and the Cyanophyta Lyngbya spp., a co-dominant during spring and summer. 3. Population densities of Lyngbya spp. were markedly reduced at all test concentrations of copper. Population densities of the principal Chlorophyta (Spirogyra spp. and Cladophora spp.) and the diatom Amphipleura pellucida were reduced at 5 μg 1^?1 CU_T. Of the twenty-two most abundant taxa, sixteen were reduced in abundance by continuous exposure to 10 μg 1^?1 Cu_T. 4. There was no commensurate reduction in standing crop (total number of individuals of all taxa). Achnanthes minutissima, a co-dominant in the control, was the primary replacement species. Other taxa that were more abundant at 5 μg1^?1 Cu_T than in the control were Ceratoneis arcus, Cocconeis placentula, Navicula spp. and Synedra rumpens. Only A. minutissima and Calothrix spp. were more abundant at 10 μg 1^?1 than in the control. 5. Three resemblance measures (Canberra metric, Bray-Curtis and Dice) and diversity (Brillouin's) were evaluated for detecting differences in species composition among experimental stream sections. The Canberra metric, an index sensitive to proportional rather than absolute differences, was the most informative of these indices.     

Fusarium root rot of coneflower seedlings and integrated control using <Emphasis Type="Italic">Trichoderma</Emphasis> and fungicides

Fusarium root rot (Fusarium spp.) is one of the most important seedling diseases of coneflower (Echinacea spp.) in Alberta greenhouses. Effects of microbial antagonists (Trichoderma spp.) and fungicides, including difenoconazole, fludioxonil, and a mixture of fludioxonil, metalaxyl and difenoconazole, on the management of this disease, were investigated in Alberta. Twenty Trichoderma isolates demonstrated antagonistic activity to Fusarium in agar plate bioassays, with inhibition rates ranging from 44 to 65%. Some Trichoderma isolates significantly ( p < 0.05) reduced disease incidence and severity on seedlings in greenhouse experiments. An in vitro bioassay indicated that difenoconazole and the mixture equally inhibited the growth of both Fusarium and Trichoderma, but, while fludioxonil strongly inhibited the growth of Fusarium, it had little effect on Trichoderma, according to the dose--response models developed ( p < 0.01, R²= 0.902-0.998). Two Trichoderma isolates, T1 and T13 were applied singly or in combination with a low rate of fludioxonil in greenhouse evaluations. The results suggested that fludioxonil and Trichoderma could be integrated into a disease management program for fusarium root rot in coneflower.     

Microbiological Community Structure of the Biofilm of a Methanol-Fed,Marine Denitrification System,and Identification of the Methanol-Utilizing Microorganisms

We demonstrated in a previous study that the biofilm of the methanol-fed fluidized marine denitrification reactor at the Montreal Biodome was composed of at least 15 bacterial phylotypes. Among those were 16S ribosomal RNA (rDNA) gene sequences affiliated to Hyphomicrobium spp., and Methylophaga spp.; the latter made up 70% of a clone library. By using fluorescent in situ hybridization (FISH), we investigated the structure of the biofilm during the colonization process in the denitrification reactor by targeting most of the bacterial families that the 16S rDNA gene library suggested would occur in the biofilm. Our results revealed that gamma-Proteobacteria (mostly Methylophaga spp.) accounted for up to 79% of the bacterial population, confirming the abundance of Methylophaga spp. within the biofilm. alpha-Proteobacteria represented 27–57% of the population, which included Hyphomicrobium spp. that appeared after 20 days of colonization and represented 7–8% of the population. We noticed a great abundance and diversity of eukaryotic cells, which made up 20% of the biomass at the beginning of the colonization but decreased to 3–5% in the mature biofilm. We then used FISH combined with microautoradiography (MAR–FISH) to identify the methylotrophs in the biofilm. The results showed that alpha-Proteobacteria used ¹⁴C methanol in the presence of nitrate, suggesting their involvement in denitrification. Despite their abundance, Methylophaga spp. did not assimilate methanol under those conditions.     

Survival and energy metabolism in an oxygen deficient environment. Field and laboratory studies on the bottom fauna from the profundal zone of Lake Esrom,Denmark

The three macroinvertebrate taxa, Potamothrix hammoniensis, Chironomus anthracinus and Pisidium spp. are permanent inhabitants of the regularly microxic/anoxic profundal zone in Lake Esrom. In situ and laboratory studies (10 °C) of metabolism (aerobic and anaerobic) and anaerobic survival in P. hammoniensis and Pisidium spp. are compared with previous results from C. anthracinus. The late summer microxic conditions in the lake lasts 2–2 months, during which the three taxa display metabolic and behavioral strategies in order to survive. All three are respiratory oxy-regulators with critical oxygen levels at 1 mg O₂ l^–1 (P. hammoniensis and Pisidium spp.) or 2–3 mg O₂ l^–1 (C. anthracinus). The lethal time (LD₅₀) in experimental anoxia follows a similar trend, with 150–170 days of survival in P. hammoniensis and Pisidium spp., compared to 2–5 weeks in C. anthracinus. The glycogen stores are almost (C. anthracinus) or fully exploited (P. hammoniensis and Pisidium spp.) during anaerobis and the animals finally enter a state of quiescence or dormancy. During the late phase of anoxia, their metabolism is down at (C. anthracinus) or below (P. hammoniensis and Pisidium spp.) 1% of normoxic metabolism. The populations in the lake behave rather similar in so far that the energy gain from anaerobic degradation of glycogen maximizes 1% of normoxic conditions regardless of species. Also, in Pisidium this appears to be the only energy source during dormancy. However, as previously presented in case of C. anthracinus, P. hammoniensis maintain a partly aerobic metabolism constituting 44% of normoxia during the microxic period, compared to the 12–19% obtained by C. anthracinus. It is thus demonstrated that P. hammoniensis and Pisidium spp. possess a remarkable ability to survive in situ severe oxygen depletion. P. hammoniensis can benefit from the presence of merely traces of oxygen, whereas C. anthracinus with poorer anaerobic survival is strongly dependent on minute oxygen supplies.     

In vitro Decline in Plant Cultures: Detection of a Legion of Covert Bacteria as the Cause for Degeneration of Long-term Micropropagated Triploid Watermelon Cultures

In vitro cultures of triploid seedless watermelon cv. Arka Manik displayed a decline in shoot and root growth after 4–5 years of active culturing. Visibly clean cultures upon indexing on enriched media showed covert bacteria, and a significant improvement in proliferation and rooting in response to surface sterilization. The bacteria however survived endophytically. Low pH and reduced clarity of agar gelled medium were found to mask the expression of bacteria in the tissue culture medium. Gentamycin, streptomycin or broad-spectrum bactericide cefazolin provided as a 2 ml overlay in the medium in factorial combination at 0 or 50 mg l^–1 resulted in selective suppression of some bacteria depending on the treatment and eight bacterial clones comprising of four Gram-positive (Bacillus spp.) and four-Gram negative (3 × Pseudomonas spp. and 1 × Aeromonas sp.) strains were isolated from the cultures. Provision of 50 mg l^–1 gentamycin in 2 ml overlay in the multiplication or rooting medium coupled with occasional decontamination of cultures helped in circumventing the decline problem. The plants established in the field after 6 years of active in vitro culturing appeared normal and fertile suggesting the feasibility of keeping cultures for long periods, thus saving time and other resources. Freeing the cultures from covert bacteria was complicated by the presence of different bacterial types and this will be addressed later.     

The longitudinal distribution and community dynamics of zooplankton in a regulated large river: a case study of the Nakdong River (Korea)

The longitudinal distribution and seasonal dynamics of zooplankton were examined along a 200-km section of the middle to lower Nakdong River, Korea. Zooplankton was sampled twice a month from January 1995 to December 1997 at five sites in the main river channel. There was considerable longitudinal variation in total zooplankton abundance (ANOVA, p < 0.001). All major zooplankton groups (rotifers, cladocerans, copepodids and nauplii) increased significantly with distance downstream along the river. There also were statistically significant seasonal differences in zooplankton abundance at the sampling sites (ANOVA, p < 0.01). Zooplankton abundance was high in spring and fall and low in summer and winter. The seasonal pattern of rotifers was similar to that of total zooplankton. This reflected the fact that rotifers (Brachionus calyciflorus, B. rubens, Keratella cochlearis and Polyarthra spp.) strongly dominated the zooplankton community at all locations. Among the macrozooplankton, small-bodied cladocerans (e.g. Bosmina spp.) dominated; the abundance of large-bodied cladocerans (e.g. Daphnia) was negligible (0–5 ind. l^–1). Among the environmental variables considered, partial residence time seemed to play the most important role in determining characteristics of the river zooplankton community.     

Pollen Morphology of the Family Magnoliaceae

The pollen morphology of the following genera is described: Manglietia (1 sp.), Magnolia (10 spp.), Talauma (2 spp.), Alcimandra (1 sp.), Michelia (5 spp.) and Liriodendron (2 spp.). It is shown that from a pollen morphological point of view the Magnoliaceae are a homogeneous, natural group.     

Natural sway frequencies and damping ratios of trees: concepts,review and synthesis of previous studies

Previous studies that measured the natural frequencies and damping ratios of conifer trees were reviewed and results synthesized. Analysis of natural frequency measurements from 602 trees, belonging to eight different species, showed that natural frequency was strongly and linearly related to the ratio of diameter at breast height to total tree height squared (i.e., DBH/H ²). After accounting for their size, pines (Pinus spp.) were found to have a significantly lower natural frequency than both spruce (Picea spp.) and Douglas-fir (Pseudotsuga spp.). Natural sway frequencies of de-branched trees were significantly higher than those of the same trees with the branches intact, and the difference increased with an increasing ratio of DBH/H ². Damping mechanisms were discussed and methods for measuring damping ratio were presented. Analysis of available data suggested that internal damping ratios were typically less than 0.05 and were not related to tree diameter. External damping was mainly due to aerodynamic drag on the foliage and contact between the crowns of adjacent trees. Analysis of data from previous wind-tunnel studies indicated that damping due to aerodynamic drag is a nonlinear function of velocity. Damping due to crown contact has been suggested by a previous author to be a function of both the distance to and the size of adjacent trees. Therefore, in uniformly spaced stands it may be possible to model crown contact damping as a function of stand density index (SDI), a common forestry measure which incorporates both of these variables.     

Microbial Community Structure and Metabolic Potential of the Hyporheic Zone of a Large Mid-Stream Channel Bar

Abstract

To develop a greater understanding of hyporheic zone microbial biogeochemistry, we sampled pore fluids from a piezometer array associated with the McCarran Ranch channel bar (MRCB); a partially submerged cobble island in the Truckee River, NV, USA. Flowing surface water and pumped pore fluids were characterized by prokaryotic community structure, metabolic potential, and aqueous physicochemistry. Concentrations of potential respiratory electron acceptors were highest in surface water and riverbed porewater and sequentially depleted in porewaters along the inferred flowpath (O₂, then NO₃^?, then SO₄^2?). Correspondingly, cultivable nitrate reducers/denitrifiers were most abundant in surface water and riverbed porewater, despite oxic conditions. Cultivable sulfate reducers were overall most abundant in surface water. Prokaryotic community reconstruction from 16S rRNA gene sequences indicates that the surface water community was less diverse than that of porewater and supports a shift in metabolic strategy, from aerobic heterotrophy in surface water (e.g., Comamonadaceae and Sporichthyaceae) to chemolithotrophy and anaerobic metabolisms (e.g., Hydrogenophaga spp., Ferribacterium spp., Methanobacterium spp.) along the hyporheic flow path. These data indicate that prokaryotic communities within the MRCB are phylogenetically and metabolically diverse and contribute to biogeochemical cycling in this common yet relatively understudied habitat.     

Stable isotope evidence indicates the incorporation into Japanese catchments of marine‐derived nutrients transported by spawning Pacific Salmon

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Superficial and hyporheic oligochaete communities as indicators of pollution and water exchange in the River Moselle, France

Benthic oligochaetes were sampled on three occasions (June, August and October 1992) in the upper (0–10 cm) and hyporheic (35–45 cm depths) sediments at five sites of the River Moselle, from upstream of the town of Epinal to Velle-sur-Moselle. The first site (upstream from Epinal) is considered as unpolluted and the four remaining sites are polluted by industrial effluents. The most polluted stations were generally dominated by the pollution tolerant taxon Limnodrilus. Numbers of individuals of this taxon decreased at the less polluted last site in recovery zone, and were also scarce in the first unpolluted site. It is noteworthy that these tendencies were observed in both superficial and hyporheic substrates and to the greatest degree in hyporheic ones. At the unpolluted site, the hyporheic habitat is dominated by the groundwater species Propappus volki, Pristina spp., Pristinella spp. At the less polluted site (last site), the deep sediments are dominated by groundwater species and the Tubificidae without hair setae decrease from June to October. As a result of water exchange between superficial and subterranean waters, superficial substrates of the first and the last stations tend to be colonised by a high proportion of hyporheic species that suggests that flow is primarily from subterranean to superficial waters. The contrary is the case at other polluted stations which are characterised by the invasion of hyporheic substrates by the pollution tolerant superficial taxa Limnodrilus. This suggests that water flows from the river to the deeper groundwater. These two stations are located near drinking water plants which utilise groundwater, thus increasing the vulnerability of groundwater to surface contaminants.     

Structure of the diatom community of the River Adige (North-Eastern Italy) along a hydrological gradient

Physical constrains such as water discharge, suspended solids and turbidity act as dominant factors in driving the planktonic diatom assemblages of the River Adige (North-Eastern Italy). Two sampling stations, characterised by different hydromorphological features (Cortina all’Adige and Boara Pisani, with torrential and more potamal characteristics, respectively) were sampled fortnightly following an integrated approach encompassing physical, chemical and biological measurements and aiming at identifying the dominant factors controlling the temporal development of the community. A morpho-functional approach was used to classify the diatom assemblages where Morpho-Functional Diatom Groups (MFDG) were defined for diatom genera, according to their morphology, habitat selection and modality of adhesion to river substrate. In the two sampling points, algal growth was never limited by nutrients or zooplankton. The irregular development of MFDG was determined by the stochastic hydrological events and changes in variables related to water discharge (suspended solids and light attenuation). Tychoplanktonic, benthic and drifted taxa (such as Diatoma spp., Encyonema spp., Navicula spp. and Nitzschia spp.) were dominant in the torrential station (Cortina all’Adige), while the contribution of euplanktonic unicellular centric taxa (such as Cyclotella spp., and Stephanodiscus spp.) was higher in the potamal station (Boara Pisani).     

Pathways of assimilation and transfer of fixed nitrogen in coralloid roots of cycad-Nostoc symbioses

Freshly detached coralloid roots of several cycad species were found to bleed spontaneously from xylem, permitting identification of products of nitrogen transfer from symbiotic organ to host. Structural features relevant to the export of fixed N were described for Macrozamia riedlei (Fisch. ex Gaud.) Gardn. the principal species studied. Citrulline (Cit), glutamine (Gln) and glutamic acid (Glu), the latter usually in a lesser amount, were the principal translocated solutes in Macrozamia (5 spp.), Encephalartos (4 spp.) and Lepidozamia (1 sp.), while Gln and a smaller amount of Glu, but no Cit were present in xylem sap of Bowenia (1 sp.),and Cycas (2 spp.). Time-course studies of ¹⁵N enrichment of the different tissue zones and the xylem sap of ¹⁵N₂-pulse-fed coralloid roots of M. riedlei showed earlier ¹⁵N incorporation into Gln than into Cit, and a subsequent net decline in the ¹⁵N of Gln of the coralloid-root tissues, whereas Cit labeling continued to increase in inner cortex and stele and in the xylem sap. Hydrolysis of the ¹⁵N-labeled Cit and Gln consistently demonstrated much more intense labeling of the respective carbamyl and amide groups than of the other N-atoms. Coralloid roots of M. riedlei pulse-fed ¹⁴CO₂ in darkness showed ¹⁴C labeling of aspartic acid (Asp) and Cit in all tissue zones and of Cit of xylem bleeding sap. Lateral roots and uninfected apogeotropic roots of M. riedlei and M. moorei also incorporated ¹⁴CO₂ into Cit. The ¹⁴C of Cit was restricted to the carbamyl-C. Comparable ¹⁵N₂ and CO₂-feeding studies on corallid roots of Cycas revoluta showed Gln to be the dominant product of N₂ fixation, with Asp and alanine as other major ¹⁴C-labeled amino compounds, but a total absence of Cit in labeled or unlabeled form.Abbreviations Ala alanine - Asp aspartic acid - Cit citrulline - Gln glutamine - Glu glutamic acid - Orn ornithine     

Spatial and temporal patterns in the fish assemblages of individual pools in a midwestern stream (U.S.A.)

Synopsis The composition and consistency of fish assemblages in 14 adjacent pools (6–120 m long) of a clear-water, limestone and gravel creek in midwestern U.S.A. were quantified in eight snorkeling surveys over 19 months, to establish a baseline of natural variation in the system at this scale. The fauna of the stream was dominated numerically by minnows (Cyprinidae), sunfish and black bass (Centrarchidae), and topminnows (Fundulidae). The pool fish fauna of the total 1 km reach (including all 14 pools) was highly consistent throughout the study, despite two major floods. Assemblages in individual pools generally were consistent, but there was more variation within pools than at the scale of the entire reach. Throughout the study, most individual pools remained within discrete subsets of the total occupied multivariate space in a principal components analysis based on fish species abundances. Sunfishes (Lepomis spp.) and bass (Micropterus spp.) were more consistent in their distribution among pools than were minnows (Cyprinidae) or a topminnow (Fundulus). There were 25 significant correlations in occurrence of species pairs among stream pools, out of 91 possible comparisons of the 14 most abundant taxa in the reach. Many pools contained assemblages either dominated by large centrarchids or by abundant cyprinids and juvenile centrarchids, but intermediate assemblages also were observed. The dynamics of distribution of fish species and fish assemblages among individual stream pools are likely influenced by a combination of species-specific behaviors and habitat selection, predator constraints on use of individual pools by small fishes, riffles as size-selective barriers to fish movements between pools, dispersal of young-of-the-year fishes, and abiotic phenomena like floods. Individual stream pools appear to be discrete habitat units for fishes, and do represent an appropriate scale for biologically meaningful studies of fish assemblages or their effects on streams.Department of Zoology, University of Oklahoma