Periphyton biomass,potential production and respiration in a shallow lake during winter and spring

Abundance, depth distribution, potential productivity and respiration of periphyton on short-time (1 month) and long-time incubated strips were followed monthly during the winter–spring (January–May) transition in a shallow eutrophic lake. A taxonomic shift occurred from dominance of diatoms under ice to chlorophyte dominance in spring communities on the long-time incubated strips, while diatoms dominated until May on the short-time incubated strips. Periphyton biomass accrual was low during the ice-covered winter months (November–January: 4 mg chl a m⁻² month⁻¹), but increased to a maximum of 112 mg chl a m⁻² month⁻¹ immediately after ice-out in February. During February–April, the biomass remained constant before declining in May. Periphyton on long-time incubated strips was equally distributed in the water column in winter (January–February), but was higher near the water surface in spring (March–May). Periphyton did not change with depth on the short-time incubated strips. The potential production to respiration ratio (P/R) was negatively correlated with periphyton biomass. Throughout the study, P/R was <1 for the short-time incubated periphyton, while this was only the case in March–April for the long-time incubations. This study showed a high productive capacity of winter periphyton, resulting in accumulation of a relatively high periphytic biomass early in the season. A massive periphyton density in eutrophic lakes already in winter–spring may potentially delay or prevent the establishment and re-occurrence of submerged macrophytes in the early oligotrophication phase following a reduction of the external nutrient loading. Handling editor: Luigi Naselli-Flores     

Biomass production and nutrient accumulation in <Emphasis Type="Italic">Sparganium emersum</Emphasis> Rehm. after sediment treatment with mineral and organic fertilisers in three mesocosm experiments

The response of the aquatic plant Sparganium emersum to different sediment nutrient levels was studied in three mesocosm experiments. The aim was to assess plant growth parameters and nutrient accumulation in the plant tissue under conditions relevant for habitats with sediments affected by anthropogenic nutrient enrichment. The experimental treatments were produced by fertilisation of the rooting medium (washed river sand) with differing doses of either NPK mineral fertiliser or digested sludge from solid pig slurry waste. Growth inhibition by high nutrient levels was not observed in any treatment (highest nutrient concentrations in the sediment with mineral fertiliser: N 250 mg kg⁻¹, P 50 mg kg⁻¹; organic fertiliser: N 6300 mg kg⁻¹, P 1800 mg kg⁻¹), which confirms the tolerance of S. emersum to high nutrient loads. The sediment nutrient concentration was best reflected in shoot dry mass. Nutrient contents in plant tissues were similar for most nutrient concentrations in the rooting media; only N increased significantly with N levels in the sediment in belowground parts. Nutrient standing stocks in plants, however, generally corresponded to the nutrient supply, and reached highest values (max. N 3.7 g m⁻², P 1.2 g m⁻²) in the richest treatments with organic fertiliser. The capability of S. emersum to use nutrients from high sediment concentrations and in organically polluted environments recommends this species for use in water quality management including tertiary wastewater treatment.     

Controls on chlorophyll-a in nutrient-rich agricultural streams in Illinois, USA

Nitrogen and phosphorus are the primary nutrients that affect water quality in streams in the midwestern USA and high concentrations of these nutrients tend to increase algal biomass. However, how nutrients interact with physical controls in regulating algal biomass is not well known in agricultural streams. Eighteen streams in east-central Illinois (USA) were sampled during June and September 2003 to analyze factors possibly regulating algal biomass. Additionally, two shaded and two non-shaded sites in the Embarras River in east-central Illinois were sampled intensively from June to December 2003. Both sestonic and periphytic chlorophyll-a (chl-a) were analyzed, and periphytic chl-a was assessed on natural substrata and unglazed ceramic tiles. Although high concentrations of nutrients were found in these streams (mean total P = 0.09–0.122 mg l⁻¹ and mean NO₃-N=4.4–8.4 mg l⁻¹), concentrations of sestonic chl-a were low among all sites and both sampling periods (<18 mg m⁻³, median values of 5 and 3 in June and September, respectively). Filamentous algae were an important component of the algal communities in streams with stable substrata. Periphytic chl-a was generally not related to the concentration of N or P in the water column, and in non-shaded streams periphyton appeared at times to be light-limited due to turbid water. Turbidity was found to be an important factor controlling chl-a on ceramic tiles across the 18 sites and for the Embarras River sites; chl-a decreased exponentially in concentration (132–0 mg m⁻²) as turbidity increased from 4 to 39 NTU (r ² = 0.80). In general, the interaction between hydrology and light (turbidity) likely controlled algal biomass in these nutrient-rich, agricultural streams.     

Cadmium and lead toxicity on tropical freshwater periphyton communities under laboratory-based mesocosm experiments

Periphyton constitutes an important community that is useful for assessment of ecological conditions in lotic systems. The objective of this study was to assess the effects of different mixtures of Cd and Pb on periphyton growth as well as Cd and Pb mixtures toxicity to diatom assemblages in laboratory mesocosm experiments. A natural periphyton community sampled from the Monjolinho River (South of Brazil) was inoculated into five experimental systems containing clean glass substrates for periphyton colonization. The communities were exposed to mixtures of dissolved Cd and Pb concentrations of 0.01 and 0.1 mg l⁻¹ Cd and 0.033 and 0.1 mg l⁻¹ Pb. Periphyton ash-free dry weight, growth rate, diatom cell density and diatom community composition were analyzed on samples collected after 1, 2 and 3 weeks of colonization. High Cd concentration (0.1 mg l⁻¹) has negative effects on periphyton growth while high concentration of Pb (0.1 mg l⁻¹) decreased the toxic effects of Cd on periphyton growth. Shifts in species composition (development of more resistant species like Achnanthidium minutissimum and reduction of sensitive ones like Cymbopleura naviculiformis, Fragilaria capucina, Navicula cryptocephala, Encyonema silesiacum, Eunotia bilunaris, and Gomphonema parvulum), decreases in species diversity of diatom communities with increasing Cd and Pb concentrations and exposure duration have been demonstrated in this study making diatom communities appropriate monitors of metal mixtures in aquatic systems.     

Phytoplankton and primary production in clear-vegetated,inorganic-turbid,and algal-turbid shallow lakes from the pampa plain (Argentina)

Shallow lakes often alternate between two possible states: one clear with submerged macrophytes, and another one turbid, dominated by phytoplankton. A third type of shallow lakes, the inorganic turbid, result from high contents of suspended inorganic material, and is characterized by low phytoplankton biomass and macrophytes absence. In our survey, the structure and photosynthetic properties (based on ¹⁴C method) of phytoplankton were related to environmental conditions in these three types of lakes in the Pampa Plain. The underwater light climate was characterized. Clear-vegetated lakes were more transparent (K _d 4.5–7.7 m⁻¹), had high DOC concentrations (>45 mg l⁻¹), low phytoplankton Chl a (1.6–2.7 μg l⁻¹) dominated by nanoflagellates. Phytoplankton productivity and photosynthetic efficiency (α ~ 0.03 mgC mgChla ⁻¹ h⁻¹ W⁻¹ m²) were relatively low. Inorganic-turbid lakes showed highest K _d values (59.8–61.4 m⁻¹), lowest phytoplankton densities (dominated by Bacillariophyta), and Chl a ranged from 14.6 to 18.3 μg l⁻¹. Phytoplankton-turbid lakes showed, in general, high K _d (4.9–58.5 m⁻¹) due to their high phytoplankton abundances. These lakes exhibited the highest Chl a values (14.2–125.7 μg l⁻¹), and the highest productivities and efficiencies (maximum 0.56 mgC mgChla ⁻¹ h⁻¹ W⁻¹ m²). Autotrophic picoplankton abundance, dominated by ficocianine-rich picocyanobacteria, differed among the shallow lakes independently of their type (0.086 × 10⁵–41.7 × 10⁵ cells ml⁻¹). This article provides a complete characterization of phytoplankton structure (all size fractions), and primary production of the three types of lakes from the Pampa Plain, one of the richest areas in shallow lakes from South America. Handling editor: J. Padisak     

Phytoplankton and phytobenthos pigment strategies: implications for algal survival in the changing Arctic

We compared phytoplankton and phytobenthos pigment strategies in 17 shallow lakes and ponds from northern Canada and Alaska, sampled during mid to late summer. Benthic chlorophyll a concentrations (8–261 mg m⁻²) greatly exceeded those of the phytoplankton (0.008–1.4 mg m⁻²) in all sites. Cyanobacteria dominated the phytobenthos, while green algae and fucoxanthin-groups characterized the plankton. Both communities had higher photoprotection in cold, UV-transparent, high latitude waters. Phytoplankton had higher concentrations of photoprotective carotenoids per unit chlorophyll a than the phytobenthos. The planktonic photoprotective pigments were positively correlated with UV-penetration, and inversely correlated with temperature and coloured dissolved organic matter. A partial redundancy analysis showed that the benthic pigments were related to latitude, area and temperature. The UV-screening compound scytonemin occurred in high concentrations in the phytobenthos and was inversely related to temperature, while benthic carotenoids per unit chlorophyll a showed much lower variability among sites. These differing pigment strategies imply divergent responses to environmental change between the phytobenthos and phytoplankton in high latitude lakes.     

Biofiltering efficiency, uptake and assimilation rates of Ulva clathrata (Roth) J. Agardh (Clorophyceae) cultivated in shrimp aquaculture waste water

The growth, biofiltering efficiency and uptake rates of Ulva clathrata were studied in a series of outdoor tanks, receiving waste water directly from a shrimp (Litopenaeus vannamei) aquaculture pond, under constant aeration and two different water regimes: (1) continuous flow, with 1 volume exchange a day (VE day^-1) and (2) static regime, with 1 VE after 4 days. Water temperature, salinity, pH, dissolved inorganic nitrogen (DIN), phosphate (PO₄), chlorophyll-a (chl-a), total suspended solids (TSS), macroalgal biomass (fresh weight) and tissue nutrient assimilation were monitored over 12 days. Ulva clathrata was highly efficient in removing the main inorganic nutrients from effluent water, stripping 70–82% of the total ammonium nitrogen (TAN) and 50% PO₄ within 15 h. Reductions in control tanks were much lower (Tukey HSD, P < 0.05). After 3 days, the mean uptake rates by the seaweed biomass under continuous flow were 3.09 mg DIN g DW day⁻¹ (383 mg DIN m⁻² day⁻¹) and 0.13 mg PO₄ g DW day⁻¹ (99 mg PO₄ m⁻² day⁻¹), being significantly higher than in the static regime (Tukey HSD, P < 0.05). The chl-a decreased in seaweed tanks, suggesting that U. clathrata inhibited phytoplankton growth. Correlations between the cumulative values of DIN removed from the water and total nitrogen assimilated into the seaweed biomass (r = 0.7 and 0.8, P < 0.05), suggest that nutrient removal by U. clathrata dominated over other processes such as phytoplankton and bacterial assimilation, ammonia volatilization and nutrient precipitation.     

Somatic embryogenesis and plant regeneration in two wild cotton species belong to G genome

The present work describes the plant regeneration via somatic embryogenesis in two wild cotton species belonging to G genome: Gossypium nelsonii Fryx and Gossypium australe F Muell. The role of plant hormones and carbohydrates was also evaluated for somatic embryogenesis and somatic embryo development. Normal plants were obtained from G. nelsonii Fryx; abnormal plants and somatic embryos were obtained from G. australe F Muell. The best medium for callus induction for these G genome wild cotton species was MSB₅ supplemented with 0.1 mg L⁻¹ KT and 0.1 mg L⁻¹ 2,4-D. For embryogenic callus proliferation, the best medium used was MSB₅ supplemented with 0.2 mg L⁻¹ KT and 0.5 mg L⁻¹ IBA. The medium MSB₅ supplemented with 0.15 mg L⁻¹ KT and 0.5 mg L⁻¹ NAA was used successfully for root initiation and plant growth. In addition, adding CuSO₄ and AgNO₃ in the callus-inducing and proliferation medium resulted in a number of somatic embryos. Glucose and maltose, the carbon sources in somatic culture, were used for callus induction, but maltose worked even better than glucose for proliferation of embryogenic callus and development of somatic embryos.     

Changes in bacterial and ciliate densities with trophic status in Mediterranean shallow lakes

Ciliate and bacterial densities and their link with eutrophication were studied in fourteen shallow lakes in northwest Spain. Total phosphorus (TP) in these lakes varied between 30 μg l⁻¹ and 925 μg l⁻¹ and chlorophyll a concentration (chla) between 0.5 μg l⁻¹ and 107 μg l⁻¹. Bacterial abundance ranged from 1 × 10⁶ to 14 × 10⁶ cells ml⁻¹, while ciliate abundance ranged from 0.6 cells ml⁻¹ to 229 cells ml⁻¹. Lakes were classified into three trophic types from their TP and chla concentrations. Bacterial abundance was significantly correlated with trophic type, as well as with TP and with chla separately, whereas ciliate abundance was only correlated with chla. No significant relationship could be established between bacterial and ciliate abundance across the trophic gradient. A general pattern was observed in the ratios of bacterial abundance to TP and chla concentrations, of decreasing ratios with increases in the nutrient loading. This pattern was not found for ciliates. The dominant zooplankton group in 13 of the 14 lakes studied was Rotifera, which accounted for a mean of 71% of total zooplankton abundance (41% of zooplankton biomass). The positive correlation between bacteria and ciliates with this group, and the absence of any relationship with Cladocera suggest that top down control by cladocerans was weaker in our lakes than previously shown in northern European shallow lakes. Rotifers could be important predators of bacteria in the high-nutrient lakes of our study. Higher slopes of regressions on bacterial abundance towards the hypertrophic range indicate that top-down control was weaker in our lakes than in northern European shallow lakes.     

Combination effect of lactoalbumin hydrolysate and methyl jasmonate on ginsenoside and polysaccharide production in <Emphasis Type="Italic">Panax quinquefolium</Emphasis> L. cells cultures

Lactoalbumin hydrolysate (LH) at 100 mg L⁻¹ with methyl jasmonate (MJ) at 2 mg L⁻¹ synergistically stimulated ginsenoside accumulation in Panax quinquefolium cells compared with 100 mg L⁻¹ LH. Combination elicitors led to higher ginsenoside productivity (45.93 mg L⁻¹) than single treatment of 100 mg L⁻¹ LH (31.37 mg L⁻¹). This present result will be helpful in providing a tool for enhancing the productivity of ginsenoside by Panax quinquefolium cell cultures on a commercial scale.     

Estimation of periphytic microalgae gross primary production with DCMU-fluorescence method in Yenisei River (Siberia, Russia)

Periphyton (epilithon) gross primary production (GPP) was estimated using the DCMU-fluorescence method in the Yenisei River. In the unshaded littoral zone, chlorophyll a concentration (Chl a) and GPP value varied from 0.83 to 973.74 mg m⁻²and 2–304,425 O₂ m⁻² day⁻¹ (0.64–95 133 mg C m⁻² day⁻¹), respectively. Positive significant correlation (r = 0.8) between daily GPP and periphyton Chl a was found. Average ratio GPP:Chl a for periphyton was 36.36 mg C mg Chl a m⁻² day⁻¹. The obtained GPP values for the Yenisei River have a high significant correlation with values predicted by a conventional empirical model for stream periphyton. We concluded that the DCMU-fluorescence method can be successfully used for measuring of gross primary production of stream phytoperiphyton at least as another useful tool for such studies.     

In vitro hormone-regulated growth and floral induction of <Emphasis Type="Italic">Cuscuta reflexa</Emphasis>: a parasitic angiosperm

The role of different growth regulators in callus induction, shoot regeneration, floral induction and chlorophyll content of the obligatory parasitic plant Cuscuta reflexa has been studied. Callus development was excellent from the nodal part of the shoot explants in modified Murashige and Skoog (MMS) media supplemented with 2 mg L⁻¹ benzyl adenine (MMS1c). Supplementation of 2 mg L⁻¹ naphthalene acetic acid (NAA) along with MMS1c (MMS2c) was responsible for estimable shoot induction and development in callus. 2,4-Dichloro acetic acid (2,4-D) played a crucial role in the floral induction of C. reflexa in vitro. MMS supplemented with 2 mg L⁻¹ NAA and 2 mg L⁻¹ 2,4-D (MMS3b) supported floral induction after shooting in vitro. MMS supplemented with 3 mg L⁻¹ 2,4-D (MMS4a) rapidly induced flower directly from the stem explants without showing any elongation of shoot. MMS1c along with MMS3b (MMS5a) showed callus proliferation followed by shoot elongation and floral induction. In vitro MMS5a grown plants show a sharp increase in the chlorophyll contents. Cytokinin treatment further increases the chlorophyll level of the plant.     

Partial quantification of pigments extracted from the zooxanthellate octocoral <Emphasis Type="Italic">Sinularia flexibilis</Emphasis> at varying irradiances

Chlorophyll-a (chl-a) and carotenoid pigments of the zooxanthellate octocoral Sinularia flexibilis were analyzed using high performance liquid chromatography following exposure to three light intensities for over 30 days. From the coral fragments located at different light intensities, a total carotenoid of >41 μg g⁻¹ dry weight, including peridinin, xanthophylls (likely diadinoxanthin + diatoxanthin), and chl-a as the most abundant pigments, with minor contents of astaxantin and β-carotene were detected. The whole content of chl-a weighed 5 μg g⁻¹ dry weight in all coral colonies. Chl-a and carotenoids contributed 11.2% and 88.2%, respectively, to all pigments detected, and together accounted for 99.4% of the total pigments present. The highest contents of carotenoids and chl-a was observed in the coral grafts placed in an irradiance of 100 μmol quanta m⁻² s⁻¹; they showed lower ratios of total carotenoids: chl-a compared to those exposed to 400 μmol quanta m⁻² s⁻¹ after >30 days of incubation. The ratios of peridinin and xanthophylls with respect to chl-a from the colonies at 400 μmol quanta m⁻² s⁻¹ were approximately double those observed at irradiances of 100 and 200 μmol quanta m⁻² s⁻¹. Partial quantification of pigments in this study showed that the carotenoids of S. flexibilis showed a decrease at irradiances above 100 μmol quanta m⁻² s⁻¹, with the exception of an increase in β-carotene at 200 μmol quanta m⁻² s⁻¹.     

Seasonal modulation of bacterioplankton community at a temperate eutrophic shallow lake

At the present time, there is still a lack of information about environmental parameters modulating variations on bacterial diversity in temperate lakes, particularly from Portugal. Fermentelos Lake (Central Portugal) is a shallow water body that sustains an important wetland area. The strong nutrient inputs from agriculture and industrial runoffs have led to its current eutrophic status. The present work aimed to understand which factors modulate the seasonal bacterioplankton diversity at this lake using 16S rRNA PCR-denaturing gradient gel electrophoresis (DGGE) and multivariate analysis. Environmental data demonstrated eutrophic features throughout all samples with nitrate concentrations reaching 12.0 mg N (NO₃ ⁻) l⁻¹ in March 2006, while the highest conductivity (609 μS cm⁻¹), soluble reactive phosphorus (0.37 mg l⁻¹), total suspended solids (87.2 mg l⁻¹) and chlorophyll a (286.6 μg l⁻¹) levels were recorded in August 2007. Over the past two decades there was a general increase in nitrate, pH and conductivity levels at this lake, suggesting the eutrophication process is still in progress. Multivariate analysis showed that summer versus winter DGGE patterns could be established for bacterial assemblages and were mainly defined by water temperature and chlorophyll a. Actinobacteria were dominant throughout the study period although a general preference for higher temperature, pH, total suspended solids, conductivity, soluble reactive phosphorus (SRP) and chlorophyll a levels was observed. The highest concentrations of nitrogen sources were related to Bacteroidetes and phototrophic eukaryote (cryptophycean) dominance. The expansion of Betaproteobacteria, Alphaproteobacteria and Cyanobacteria phylotypes was generally associated to high temperature, pH, conductivity and SRP values.     

Evaluation of green fluorescent protein as a reporter gene and phosphinothricin as the selective agent for achieving a higher recovery of transformants in cucumber (<Emphasis Type="Italic">Cucumis sativus</Emphasis> L. cv. Poinsett76) via <Emphasis Type="Italic">Agrobacterium tumefaciens</Emphasis>

Efficient Agrobacterium tumefaciens-mediated transformation and a higher recovery of transformed plants of cucumber cv. Poinsett76 were achieved via direct organogenesis from cotyledon explants. Stable transformants were obtained by inoculating explants with A. tumefaciens strains EHA105 or LBA4404, both harboring the binary vector pME508, which contains the neomycin phosphotransferase II (nptII) and phosphinothricin resistance genes (bar) conferring resistance to kanamycin and PPT, respectively, as selectable markers and the sgfp-tyg gene for the green fluorescent protein (GFP) as a visual marker driven by the constitutive CaMV35S promoter in the presence of acetosyringone (50 μM). Transformed shoots were obtained on MS Murashige and Skoog (Plant Physiol. 15: 473–497, 1962) medium supplemented with 1 mg L⁻¹ benzyladenine (BA), 20 mg L⁻¹ l-glutamine and 2 mg L⁻¹ phosphinothricin (PPT) or 100 mg L⁻¹ kanamycin. The regenerated shoots were examined in vivo using a hand-held long wave UV lamp for GFP expression. The GFP screening helped identify escapes and chimeric shoots at regular intervals to increase the growth of transformed shoots on cotyledon explants. Elongation and rooting of putative transformants were achieved on PPT (2 mg L⁻¹) containing MS media with 0.5 mg L⁻¹ gibberellic acid (GA₃) and 0.6 mg L⁻¹ indole butyric acid (IBA), respectively. PCR and Southern analyses confirmed the integration of the sgfp gene into the genome of T₀ and the progenies. T₁ segregation of transgenic progeny exhibited Mendelian inheritance of the transgene. The use of EHA105 resulted in 21% transformation efficiency compared to 8.5% when LBA4404 was used. This higher rate was greatly facilitated by PPT selection coupled with effective screening of transformants for GFP expression, thus making the protocol highly useful for the recovery of a higher number of transgenic cucumber plants.     

Epipelic and pelagic primary production in Alaskan Arctic lakes of varying depth

We compared on eight dates during the ice-free period physicochemical properties and rates of phytoplankton and epipelic primary production in six arctic lakes dominated by soft bottom substrate. Lakes were classified as shallow ( < 2.5 m), intermediate in depth (2.5 m <  < 4.5 m), and deep ( > 4.5 m), with each depth category represented by two lakes. Although shallow lakes circulated freely and intermediate and deep lakes stratified thermally for the entire summer, dissolved oxygen concentrations were always >70% of saturation values. Soluble reactive phosphorus and dissolved inorganic nitrogen (DIN = NO₃ ⁻–N + NH₄ ⁺–N) were consistently below the detection limit (0.05 μmol l⁻¹) in five lakes. However, one lake shallow lake (GTH 99) periodically showed elevated values of DIN (17 μmol l⁻¹), total-P (0.29 μmol l⁻¹), and total-N (33 μmol l⁻¹), suggesting wind-generated sediment resuspension. Due to increased nutrient availability or entrainment of microphytobenthos, GTH 99 showed the highest average volume-based values of phytoplankton chlorophyll a (chl a) and primary production, which for the six lakes ranged from 1.0 to 2.9 μg l⁻¹ and 0.7–3.8 μmol C l⁻¹ day⁻¹. Overall, however, increased resulted in increased area-based values of phytoplankton chl a and primary production, with mean values for the three lake classes ranging from 3.6 to 6.1 mg chl a m⁻² and 3.2–5.8 mmol C m⁻² day⁻¹. Average values of epipelic chl a ranged from 131 to 549 mg m⁻² for the three depth classes, but levels were not significantly different due to high spatial variability. However, average epipelic primary production was significantly higher in shallow lakes (12.2 mmol C m⁻² day⁻¹) than in intermediate and deep lakes (3.4 and 2.4 mmol C m⁻² day⁻¹). Total primary production (6.7–15.4 mmol C m⁻² day⁻¹) and percent contribution of the epipelon (31–66%) were inversely related to mean depth, such that values for both variables were significantly higher in shallow lakes than in intermediate or deep lakes. Handling editor: L. Naselli-Flores     

The influence of large water level fluctuations and hurricanes on periphyton and associated nutrient storage in subtropical Lake Okeechobee,USA

The abundance, community structure and nutrient content of periphyton, and the host plant taxa Chara, Hydrilla, Potamogeton, Vallisneria and Scirpus were studied in Lake Okeechobee, USA. Water levels were generally high during the study period (August 2002–January 2006), but substantial fluctuations occurred. All host plant biomass was seasonally variable but only Vallisneria biomass was spatially variable. All submerged plant beds disappeared after the passage of two hurricanes in September 2004, and a third hurricane passed over the lake in October 2005. Periphyton assemblages were statistically separated most by substrate and then by season. Prior to the hurricanes, annual maxima of periphyton biovolumes and those of summer submerged plant coverage coincided. During all study years, the diatom taxa dominated periphyton total biovolumes. Periphyton biomass was generally highest during the summer or prior to the hurricanes (in the case of epiphytes) and was spatially variable in the case of both Scirpus and Vallisneria. Epiphytic nutrient contents within each host plant group seasonally varied except for nitrogen and carbon in the Vallisneria epiphytes. Epipelic nutrient contents were spatially variable and seasonally variable for carbon. Nutrient contents in epipelon were significantly higher than that in Scirpus epiphytes and were similar but lower among all epiphytic communities. The total annual areal potential epiphytic phosphorus storage extrapolated during this study (2.0 × 10⁻⁴ metric tons ha⁻¹ year⁻¹) was underestimated because storage estimates for epipelon, Chara and Hydrilla-associated epiphytes were omitted. The Chara and Hydrilla-associated epiphytic nutrient storage values were omitted because of limited data, whereas the epipelic data may have not been spatially representative. For periphyton biovolume, host substrate type, water level fluctuation and hurricane impacts on host substrates appear to be more important than seasonal variation in such factors as temperature and nutrients. Epiphytic nutrient storage appears to be influenced most by water level fluctuation and hurricane-related impacts, while the host substrate type appears to be a less important factor than it is for periphyton biovolume. Maximum periphyton biomass and high nutrient storage in shallow subtropical and tropical eutrophic lakes may only occur at consistently lower water levels and during infrequent periods of disturbance, which enhance host substrate colonizable area.     

Evidence for active microbial nitrogen transformations in sea ice (Gulf of Bothnia, Baltic Sea) in midwinter

Nutrient concentrations, chlorophyll-a, bacterial biomass and relative activity of denitrifying organisms were investigated from ice-core, brine and underlying water samples in February 1998 in the Gulf of Bothnia, Baltic Sea. Examined sea ice was typical for the Baltic Sea; ice bulk salinity varied from 0.1 to 1.6 psu, and in underlying water salinity was from 4.2 to 4.7 psu. In 2- to 3-months-old sea ice (thickness 0.4–0.6 m), sea-ice communities were at the winter stage; chl-a concentrations were generally below 1 mg m⁻³ and heterotrophic organisms composed 7–20% of organism assemblage. In 1-month-old ice (thickness 0.2–0.25 m), an ice spring bloom was already developing and chl-a concentrations were up to 5.6 mg m⁻³. In relation to low salinity, high concentrations of NH⁺ ₄, NO⁻ ₂, PO³⁺ ₄ and SiOH₄ were found in the ice column. The results suggest that the upper part of ice accumulates atmospheric nutrient load during the ice season, and nutrients in the upper 10–20 cm of ice are mainly of atmospheric origin. The most important biological processes controlling the sea-ice nutrient status are nutrient regeneration, nutrient uptake and nitrogen transformations. Nutrient regeneration is specially active in the middle parts of the 50- to 60-cm-thick ice and subsequent accumulation of nutrients probably enhances the ice spring bloom. Nitrite accumulation and denitrifying activity were located in the same ice layers with nutrient regeneration, which together with the observed significant correlation between the concentrations of nitrogenous nutrients points to active nitrogen transformations occurring in the interior layers of sea ice in the Baltic Sea. Accepted: 12 June 2000     

Characteristics of γ-hexachlorocyclohexane biodegradation by a nitrogen-fixing cyanobacterium, Anabaena azotica

Biodegradation of γ-hexachlorocyclohexane (lindane) by a nitrogen-fixing cyanobacterium isolated from Chinese paddy soils, Anabaena azotica 118, was investigated. Lindane with an initial concentration of 0.2 mg L⁻¹ in the cultures had no negative effect on the chlorophyll a concentration of A. azotica after 5 d exposure. The tolerance of this cyanobacterium to lindane indicates that it has the potential to biodegrade lindane. The degradation experiments show that the percentage of lindane removal efficiency by A. azotica was 48.8% after 5 d, at an initial lindane concentration of 0.2 mg L⁻¹ and initial A. azotica chlorophyll a concentration of 50 mg L⁻¹. The calculated half-life was 4.78 d. Elevated culture temperature, irradiation, and usage of nitrate as the nitrogen source in the cultures could increase the biodegradation efficiency of lindane. γ-Pentachlorocyclohexene was detected as a metabolite of lindane. The ability of A. azotica to biodegrade lindane has potential use in the bioremediation for this organochlorine pesticide.     

Micropropagation and plant regeneration from embryogenic callus of Miscanthus sinensis

Miscanthus sinensis (Poaceae) is a typical perennial giant grass of East Asia. Due to its high photosynthetic efficiency, low input requirements, and high biomass production, M. sinensis shows outstanding potential as a biofuel feedstock. However, the lack of an efficient tissue culture system may limit its utilization potential. Different explants of M. sinensis were evaluated to develop an efficient tissue culture system. Shoot apices from in vitro-germinated seedling explants were tested for adventitious bud proliferation. The highest level of proliferation (multiplication coefficient 6.69) was obtained when shoot apices were cultured on Murashige and Skoog (MS) medium supplemented with 1.0 mg L⁻¹ 6-benzyladenine (BA), 2.0 mg L⁻¹ kinetin, 0.05 mg L⁻¹ α-naphthalene acetic acid (NAA), 3% sucrose, and 0.8% agar. The highest rooting percentage (95.4%) was obtained when adventitious buds were cultured on half-strength MS medium supplemented with 0.2 mg L⁻¹ NAA, 3% sucrose, and 0.8% agar. Significant differences were found in the formation of embryogenic callus among different explant types. The embryogenic callus derived from epicotyls had the highest regeneration capacity when cultured on a medium supplemented with 2.0 mg L⁻¹ 2,4-dichlorophenoxyacetic acid, 0.5 mg L⁻¹ BA, and 0.1 mg L⁻¹ thiamine. Under these conditions, the callus induction percentage was 82%.