Effects of a transient oxic period on mineralization of organic matter to CH4 and CO2 in anoxic peat incubations

Rates of organic matter mineralization in peatlands, and hence production of the greenhouse gases CH₄ and CO₂, are highly dependent on the distribution of oxygen in the peat. Using laboratory incubations of peat, we investigated the sensitivity of the anoxic production of CH₄ and CO₂ to a transient oxic period of a few weeks’ duration. Production rates during 3 successive anoxic periods were compared with rates in samples incubated in the presence of oxygen during the second period. In surface peat (5–10‐cm depth), with an initially high level of CH₄ production, oxic conditions during period 2 did not result in a lower potential CH₄ production rate during period 3, although production was delayed ～1 week. In permanently anoxic, deep peat (50–55‐cm depth) with a comparatively low initial production of CH₄, oxic conditions during period 2 resulted in zero production of CH₄ during period 3. Thus, the methanogens in surface peal—but not in deep peat—remained viable after several weeks of oxic conditions. In contrast to CH₄ production, the oxic period had a negligible effect on anoxic CO₂ production during period 3, in surface as well as deep peat. In both surface and deep peat, CO₂ production was several times higher under oxic than under anoxic conditions. However, for the first 2 weeks of oxic conditions, CO₂ production in the deep peat was very low. Still, deep peat obviously contained facultative microorganisms that, after a relatively short period, were able to maintain a considerably higher rate of organic matter mineralization under oxic than under anoxic conditions.     

Zooplankton response to shading effects of free-floating plants in shallow warm temperate lakes: a field mesocosm experiment

Dense mats of free floating plants (FFP) often produce severe underwater light attenuation and strong oxygen depletion in the water column. In this study, we experimentally assessed the zooplankton response to artificial shading using field mesocosms. During 30 days, we simulated three different light scenarios by mimicking the persistence, absence, and fluctuation of FFP typically encountered in vegetated shallow subtropical lakes. We used dark meshes to simulate the abiotic effects engineered by FFP. Both in the permanently covered and fluctuating situations, anoxia impaired zooplankton development. Anoxia constituted a major driving force in shaping the zooplankton response, whereas the feeding resource availability (phytoplankton) seemed to play a minor role; no top down effect on phytoplankton occurred in anoxic situations. In the fluctuating cover regime (periodic darkness and anoxia), the temporal variation of nanophytoplankton was not affected by zooplankton; once again oxygen availability seemed the main force shaping the zooplankton dynamics. Either periodical or permanent shading, associated to anoxic conditions, impaired the success of small herbivores. Large herbivores and microphytoplankton were negatively affected only under persistent shade and anoxia. In contrast, when neither light nor oxygen limitation occurred, such as in the scenario without shading, top-down control occurred. This study highlights the importance that the oxygen dynamics driven by the presence of FFP exert on the structure and dynamics of zooplankton assemblages and on the top down cascading effects on phytoplankton in warm temperate or subtropical shallow lakes.     

Responses of Active Ammonia Oxidizers to Eutrophication and Oxygen Statuses in Taihu Freshwater Sediments

Abstract

Here, we employed DNA-based stable isotope probing (SIP) and molecular biology methods to investigate active ammonia oxidizer communities in suboxic sediments (0 to –2?cm) at the micromolar oxygen level and layers (–2 to –5?cm) at nanomolar oxygen concentrations from meso-eutrophic and light-eutrophic locations in Taihu Lake. The results revealed that ammonia-oxidizing archaea (AOA) were less active in the anoxic layer of meso-eutrophic sites, while ammonia-oxidizing bacteria (AOB) were less active in suboxic sediments of light-eutrophic sites after 8?weeks of incubation. The active AOA in the meso- and light-eutrophic sediments belonged to the Nitrosopumilus, Nitrosotalea, and Nitrososphaera clusters and the Nitrosopumilus and Nitrososphaera clusters, respectively, with Nitrosopumilus cluster as the predominant AOA, which took up a higher ratio in the light-eutrophic and suboxic layers than their counterparts. The advantageous active AOB were numerically predominated by the Nitrosomonas cluster in the suboxic layers, and the Nitrosospira cluster in the anoxic layers, respectively, both of which were distributed in diverse frequencies in different eutrophication statuses. The role and community composition diversities of active ammonia oxidizers in freshwater sediments were attributed to the different eutrophication (including nitrogen and organic carbon content) and oxygen statuses.     

黑龙江省柳树河盆地古近系八虎力组孢粉组合及其时代和古环境意义

黑龙江省东部柳树河盆地古近系八虎力组为一套含油页岩矿床的地层。目前,对其时代的认定还没有确切的古生物学依据,尚存在不同的看法,同时,关于成矿环境的探讨也较薄弱。通过对该套地层开展系统的孢粉学研究,由下向上划分了4个孢粉组合,分别是Quercoidites-Retitricolpites-Retitricolporites组合、Retitricolpites-Retitricolporites组合、Polypodiaceaesporites-Retitricolpites-Retitricolporites组合和Pinuspollenites-Polypodiaceaesporites组合。根据孢粉组合特征,详细讨论了孢粉组合的地质时代,指出八虎力组时代为中始新世至晚始新世。根据孢粉组合的组成情况推测,八虎力组沉积时为亚热带-暖温带温暖湿润气候下的山间盆地环境。在山地高处生长有针、阔叶混交林植被,在低洼处生长有湿地水生植物、蕨类和苔藓类,从而有利于油页岩的形成和保存。温度向上可能有所降低,但幅度并不大。     

Contrasting resistance and resilience to light variation of the coupled oxic and anoxic components of an experimental microbial ecosystem

Understanding how microbial communities of aquatic ecosystems respond to environmental change remains a critical challenge in microbial ecology. In this study, we used light‐dependent oxic–anoxic micro‐ecosystems to understand how the functioning and diversity of aerobic and anaerobic lake analog communities are affected by a pulse light deprivation. Continuous measurements of oxygen concentration were made and a time series of full‐length 16S rRNA sequencing was used to quantify changes in alpha‐ and beta diversity. In the upper oxic layer, oxygen concentration decreased significantly under light reduction, but showed resilience in daily mean, minimum, and maximum after light conditions were restored to control level. Only the amplitude of diurnal fluctuations in oxygen concentrations did not recover fully, and instead tended to remain lower in treated ecosystems. Alpha diversity of the upper oxic layer communities showed a delayed increase after light conditions were restored, and was not resilient in the longer term. In contrast, alpha diversity of the anoxic lower layer communities increased during the light reduction, but was resilient in the longer term. Community composition changed significantly during light reduction, and showed resilience in the oxic layer and lack of resilience in the anoxic layer. Alpha diversity and the amplitude of daily oxygen fluctuations within and among treatments were strongly correlated, suggesting that higher diversity could lead to less variable oxygen concentrations, or vice versa. Our experiment showed that light deprivation induces multifaceted responses of community function (oxygen respiration) and structure, hence focusing on a single stability component could potentially be misleading.     

Growth and metabolism of Typha species in relation to cutting treatments

Container and field experiments, in which Typha latifolia L. and Typha angustifolia L. were cut either above or below the water level, were conducted to determine the physiological basis for reports that the latter treatment was more effective as a control measure. In containers, measurements of oxygen concentrations within the aerenchyma of the rhizome both with an oxygen electrode and by gas chromatography showed that oxygen could diffuse very readily to plant parts growing in an anoxic environment if there was a small amount of leaf or cut plant stem growing above the water level. When all shoots were cut below water, the oxygen in submersed plant parts was rapidly consumed and anaerobic respiration resulted in the production of ethanol. Lactate or elevated malate levels were not found. The below-water biomass decayed rapidly under these conditions and the plants had a much lower regenerative ability than plants cut above water where oxygen continued to reach the roots and rhizomes. In the field, three cuts during the growing season below water were sufficient to kill nearly all the underwater biomass; similar cuts above water reduced the total biomass compared with uncut plants, but much of the underwater biomass remained healthy and able to regenerate.     

Long-term dynamics of phytoplankton assemblages: Microcystis-domination in Lake Taihu, a large shallow lake in China

Long-term phytoplankton assemblages in a large shallow Chineselake, Lake Taihu, were presented using the monthly monitoringdata from October 1991 to December 1999. Earlier research results(1960, 1981 and 1988) were applied to discuss the differenttrophic stages of the lake. The species composition in the lakewas more closely related to eutrophication level than to lake-size,shallowness, or turbidity. Each summer, a single peak of phytoplanktonbiovolume appeared in Meiliang Bay. The results of principalcomponents analysis showed a distinct temporal shift in speciescomposition between summer and winter. A clear spatial differencein phytoplankton occurred between Meiliang Bay and the lakecentre. Wind speed and direction affected the horizontal distributionof phytoplankton, especially Microcystis, in the lake. Temperature,underwater light climate, nutrients and grazing by zooplanktonand by fish were discussed to explain the overwhelming dominanceof Microcystis. Four nutrient-phytoplankton stages were identifiedin the lake: an oligo-mesotrophic stage with low algal biomassuntil 1981, a eutrophic situation with blooms of Microcystisduring 1988–1995, hypertrophic conditions with the dominanceof Planctonema and total phosphorus up to 200 mg m^-3 from 1996to 1997 and the restoration period after 1997. The wax and waneof the phytoplankton assemblages were mainly controlled by temperature,wind and turbidity while long-term biomass dynamics were influencedby the level of nutrients.     

Evidence of enhanced freezing damage in treeline plants during six years of CO2 enrichment and soil warming

Climate change and elevated atmospheric CO₂ levels could increase the vulnerability of plants to freezing. We analyzed tissue damage resulting from naturally occurring freezing events in plants from a long–term in situ CO₂ enrichment (+ 200 ppm, 2001–2009) and soil warming (+ 4°C since 2007) experiment at treeline in the Swiss Alps (Stillberg, Davos). Summer freezing events caused damage in several abundant subalpine and alpine plant species in four out of six years between 2005 and 2010. Most freezing damage occurred when temperatures dropped below –1.5°C two to three weeks after snow melt. The tree Larix decidua and the dwarf shrubs Vaccinium myrtillus and Empetrum hermaphroditum showed more freezing damage under experimentally elevated CO₂ and/or temperatures than under control conditions. Soil warming induced a 50% die‐back of E. hermaphroditum during a single freezing event due to melting of the protective snow cover. Although we could not identify a clear mechanism, we relate greater freezing susceptibility to a combination of advanced plant phenology in spring and changes in plant physiology. The climate record since 1975 at the treeline site indicated a summer warming by 0.58°C/decade and a 3.5 days/decade earlier snow melt, but no significant decrease in freezing events during the vegetation period. Therefore, in a warmer climate with higher CO₂ levels but constant likelihood of extreme weather events, subalpine and alpine plants may be more susceptible to freezing events, which may partially offset expected enhanced growth with global change. Hence, freezing damage should be considered when predicting changes in growth of alpine plants or changes in community composition under future atmospheric and climate conditions.     

Oligochaetes and eutrophication; an experiment over four years in outdoor mesocosms

Eight experimental ditch mesocosms were used to study the effect of eutrophication over four years. The experimental ditches had a sand or clay bottom. The ditches were treated with additions of phosphorus, phosphorus and nitrogen, or without additions (controls). Oligochaetes were sampled by deploying trays with substratum for colonization over twenty weeks. Both the important variables phosphorus, nitrogen and oxygen as well as the oligochaete species and numbers are presented. The effects of nutrient additions on phosphorus, nitrogen and oxygen concentrations were described together with changes in oligochaete species composition and numbers. The results were further analyzed by redundancy analysis (RDA). In the clay-lined ditches nutrient addition coincided with fluctuation in oxygen concentration. The higher the nutrient addition levels the longer the period of oxygen depletion became. During oxygen depletion the number of oligochaetes was strongly reduced or even became zero. The low nutrient status of the sandy bed in the sand-lined ditches slowed down the rate of colonization. Only a few tubificids were collected. Eutrophication effects were only observed at the highest nutrient addition level. Considerable variation is attributed to stochastic factors in the sand-lined ditches. Whether oligochaete species were present was related to the length of the colonization period. The substratum composition and food together with oxygen regime decided whether they become more or less abundant in ditches. Large-scale mesocosm experiments require time to develop. Only after the first colonization period variables of species presences and abundances can be employed to detect changes associated with eutrophication. Oligochaetes can be used to measure colonization as well as eutrophication processes.     

Survival and Recovery of Methanotrophic Bacteria Starved under Oxic and Anoxic Conditions

The effects of carbon deprivation on survival of methanotrophic bacteria were compared in cultures incubated in the presence and absence of oxygen in the starvation medium. Survival and recovery of the examined methanotrophs were generally highest for cultures starved under anoxic conditions as indicated by poststarvation measurements of methane oxidation, tetrazolium salt reduction, plate counts, and protein synthesis. Methylosinus trichosporium OB3b survived up to 6 weeks of carbon deprivation under anoxic conditions while maintaining a physiological state that allowed relatively rapid (hours) methane oxidation after substrate addition. A small fraction of cells starved under oxic and anoxic conditions (4 and 10%, respectively) survived more than 10 weeks but required several days for recovery on plates and in liquid medium. A non-spore-forming methanotroph, strain WP 12, displayed 36 to 118% of its initial methane oxidation capacity after 5 days of carbon deprivation. Oxidation rates varied with growth history prior to the experiments as well as with starvation conditions. Strain WP 12 starved under anoxic conditions showed up to 90% higher methane oxidation activity and 46% higher protein production after starvation than did cultures starved under oxic conditions. Only minor changes in biomass and morphology were seen for methanotrophic bacteria starved under anoxic conditions. In contrast, starvation under oxic conditions resulted in morphology changes and an initial 28 to 35% loss of cell protein. These data suggest that methanotrophic bacteria can survive carbon deprivation under anoxic conditions by using maintenance energy derived solely from an anaerobic endogenous metabolism. This capability could partly explain a significant potential for methane oxidation in environments not continuously supporting aerobic methanotrophic growth.     

Modeling underwater light climate in relation to sedimentation,resuspension, water quality and autotrophic growth

The underwater light climate ultimately determines the depth distribution, abundance and primary production of autotrophs suspended within and rooted beneath the water column. This paper addresses the underwater light climate, with reference to effects of suspended solids and growth responses of autotrophs with emphasis on phytoplankton.Effects of the most important factors contributing to the absorption and scattering of light in surface waters were described. A comparison between spectral and scalar approaches to underwater light climate modeling was made and examples of linear approximations to light attenuation equations were presented. It was demonstrated that spectral and scalar photosynthesis models may converge to similar values in spectral-flat, high photon flux environments, but that scalar PAR models may overestimate biomass-specific production by 70%. Such differences can lead to serious overestimates of habitat suitability for the growth and survival of submersed macrophytes, particularly in relatively turbid, coastal waters.Relationships between physical and optical properties of suspended sediments were described theoretically, and illustrated with modeling examples and measurements. It was found that the slowly settling particulate fraction contributed substantially to the suspended solids concentration, and greatly to light attenuation within the water column. It was concluded that distinguishing particles by fall velocity and concomitant light attenuation properties in the modeling of underwater light conditions allowed the establishment of useful, although not simply linear, relationships.In eutrophic, shallow lakes, the largest contribution to light attenuation often originates from phytoplankton on a seasonal basis (months–years), but from suspended solids behavior on a shorter time scale (days–weeks), particularly when water bodies are wind-exposed. Temporal and spatial variabilities in wave height, suspended solids concentrations, and light attenuation within the water column, and their importance for autotrophic growth were described, and illustrated with a case study pertaining to Markermeer, The Netherlands. The influence of underwater light conditions on phytoplankton succession was briefly discussed and illustrated with a case study pertaining to Lake Veluwe, The Netherlands. It was concluded that modeling the underwater light climate in a water body on a few sites only can indicate how important various components are for the attenuation of light, but based on the current state of the art, it can not be expected that this will provide accurate predictions of the underwater light climate, and of phytoplankton and submersed macrophyte growth.     

The influence of wetlands in regulating water quality in the Seronera River,Serengeti National Park,Tanzania

The distribution of temperature, salinity, visibility and dissolved oxygen was sampled from 1996 to 2002 at sites along the Seronera River. The minimum temperature decreased with distance upstream. The salinity increased up-river where occasionally hypersaline conditions prevailed. Dissolved oxygen was highly variable spatially and temporally, depending on both the level of eutrophication by animal dung and the presence of wetlands that help filter the excess nutrients. During the study period, fringing, freshwater wetlands have generally been degraded and in some cases destroyed, and this has been accompanied by significantly decreased oxygen levels, sometimes nearing anoxic conditions. Also during this period, saltwater wetlands have increased, and since wildlife impacted these wetlands little, dissolved oxygen levels remained high throughout. Visibility was highest in areas fringed by wetlands.This revised version wa published online in March 2005 with corrections to the issue cover date.     

淡水湖泊浮游藻类对富营养化和气候变暖的响应

水体富营养化和气候变暖是淡水生态系统面临的两大威胁。文章分别阐述了富营养化和气候变暖对淡水湖泊浮游藻类直接和间接效应, 并总结气候变暖可能通过影响水体理化性质、水生植物组成、食物链结构从而直接或间接改变浮游藻类生物量或群落结构。作者重点分析了气候变暖下湖泊生态系统蓝藻水华暴发机制, 比较了不同湖泊蓝藻对气候变暖和富营养化响应的异同点, 发现气候变暖和富营养化对湖泊生态系统影响存在相似性, 表现在均促进湖泊由清水-浊水稳态转变、增加蓝藻水华发生频率和强度。然而二者对湖泊浮游藻类影响的相对重要性取决于分层型湖泊和混合型湖泊的差异性、不同营养型湖泊和不同类群蓝藻组成差异性。作者认为, 开展气候变暖和富营养化下, 湖泊浮游藻类功能群响应研究亟待进行。     

Fermentation metabolism in roots of wheat seedlings after hypoxic pre-treatment in different anoxic incubation systems

A hypoxic pre-treatment (HPT) can improve the anoxic survival of flooding sensitive plants. Here, we tested whether a 4-d HPT of wheat plants (Triticum aestivum L.) would improve their anoxic resistance, and if so, why. We found that the metabolic adjustment during prolonged HPT involved an increased lactate excretion rate, the up-regulation of glycolytic and fermentative enzymes as well as the accumulation of various sugars. Therefore, HPT wheat roots could sustain a 3 times higher ethanolic fermentation rate during an anoxic period compared to non-pre-treated (NHPT) roots. Nevertheless, the enhanced fermentation rate provided temporary relief to the energy crisis only, and both NHPT and HPT plants died after 5d of anoxia in illumination. Comparison of different low oxygen incubation systems using excised roots or roots of intact plants revealed striking differences. The benefits of intact shoots, oxygen transport as well as additional sugar supply enabled a more stable energy supply of anoxia-treated NHPT and HPT roots. However, the height of the fermentation rate was correlated with a high ATP content during dark anoxic incubation, but not in illumination.     

Light acclimation, CO2 response and long-term capacity of underwater photosynthesis in three terrestrial plant species

To characterize underwater photosynthetic performance in some terrestrial plants, we determined (i) underwater light acclimation (ii) underwater photosynthetic response to dissolved CO₂, and (iii) underwater photosynthetic capacity during prolonged submergence in three species that differ in submergence tolerance: Phalaris arundinacea, Rumex crispus (both submergence-tolerant) and Arrhenatherum elatius (submergence-intolerant). None of the species had adjusted to low irradiance after 1 week of submergence. Under non-submerged (control) conditions, only R. crispus displayed shade acclimation. Submergence increased the apparent quantum yield in this species, presumably because of the enhanced CO₂ affinity of the elongated leaves. In control plants of the grass species P. arundinacea and A. elatius, CO₂ affinities were higher than for R. crispus. The underwater photosynthetic capacity of R. crispus increased during 1 month of submergence. In P. arundinacea photosynthesis remained constant during 1 month of submergence at normal irradiance; at low irradiance a reduction in photosynthetic capacity was observed after 2 weeks, although there was no tissue degeneration. In contrast, underwater photosynthesis of the submergence-intolerant species A. elatius collapsed rapidly under both irradiances, and this was accompanied by leaf decay. To describe photosynthesis versus irradiance curves, four models were evaluated. The hyperbolic tangent produced the best goodness-of-fit, whereas the rectangular hyperbola (Michaelis-Menten model) gave relatively poor results.     

Underwater photosynthesis in flooded terrestrial plants: a matter of leaf plasticity

BACKGROUND: Flooding causes substantial stress for terrestrial plants, particularly if the floodwater completely submerges the shoot. The main problems during submergence are shortage of oxygen due to the slow diffusion rates of gases in water, and depletion of carbohydrates, which is the substrate for respiration. These two factors together lead to loss of biomass and eventually death of the submerged plants. Although conditions under water are unfavourable with respect to light and carbon dioxide supply, photosynthesis may provide both oxygen and carbohydrates, resulting in continuation of aerobic respiration. SCOPE: This review focuses on evidence in the literature that photosynthesis contributes to survival of terrestrial plants during complete submergence. Furthermore, we discuss relevant morphological and physiological responses of the shoot of terrestrial plant species that enable the positive effects of light on underwater plant performance. CONCLUSIONS: Light increases the survival of terrestrial plants under water, indicating that photosynthesis commonly occurs under these submerged conditions. Such underwater photosynthesis increases both internal oxygen concentrations and carbohydrate contents, compared with plants submerged in the dark, and thereby alleviates the adverse effects of flooding. Additionally, several terrestrial species show high plasticity with respect to their leaf development. In a number of species, leaf morphology changes in response to submergence, probably to facilitate underwater gas exchange. Such increased gas exchange may result in higher assimilation rates, and lower carbon dioxide compensation points under water, which is particularly important at the low carbon dioxide concentrations observed in the field. As a result of higher internal carbon dioxide concentrations in submergence-acclimated plants, underwater photorespiration rates are expected to be lower than in non-acclimated plants. Furthermore, the regulatory mechanisms that induce the switch from terrestrial to submergence-acclimated leaves may be controlled by the same pathways as described for heterophyllous aquatic plants.     

Redox Fluctuations Frame Microbial Community Impacts on N-cycling Rates in a Humid Tropical Forest Soil

Fluctuating soil redox regimes may facilitate the co-occurrence of microbial nitrogen transformations with significantly different sensitivities to soil oxygen availability. In an upland humid tropical forest, we explored the impact of fluctuating redox regimes on gross nitrogen cycling rates and microbial community composition. Our results suggest that the rapidly fluctuating redox conditions that characterize these upland soils allow anoxic and oxic N processing to co-occur. Gross nitrogen mineralization was insensitive to soil redox fluctuations. In contrast, nitrifiers in this soil were directly affected by low redox periods, yet retained some activity even after 3–6 weeks of anoxia. Dissimilatory nitrate reduction to ammonium (DNRA) was less sensitive to oxygen exposure than expected, indicating that the organisms mediating this reductive process were also tolerant of unfavorable (oxic) conditions. Denitrification was a stronger sink for NO₃⁻ in consistently anoxic soils than in variable redox soils. Microbial biomass and community composition were maintained with redox fluctuation, but biomass decreased and composition changed under static oxic and anoxic soil regimes. Bacterial community structure was significantly correlated with rates of nitrification, denitrification and DNRA, suggesting that redox-control of soil microbial community structure was an important determinant of soil N-cycling rates. Specific nitrogen cycling functional groups in this environment (such as nitrifiers, DNRA organisms, and denitrifiers) appear to have adapted to nutrient resources that are spatially and temporally variable. In soils where oxygen is frequently depleted and re-supplied, characteristics of microbial tolerance and resilience can frame N cycling patterns.     

Eutrophication,recovery and temperature in Lake Mjøsa: detecting trends with monitoring data and sediment records

1. How climate warming may interact with other pressures on aquatic ecosystems is an important issue for research and management. We combined lake monitoring data with a palaeolimnological study to explore the combined effects of eutrophication and subsequent oligotrophication with a long‐term temperature increase in epilimnetic waters. Our goals were (i) to evaluate how well sediment‐based reconstructions reflect the instrumental observations, (ii) to use the palaeo‐record to characterise a reference state for the lake and (iii) to explore whether data from the sediment record can aid in separating the effects of nutrient load and temperature in a large and deep lake. 2. Lake Mjøsa is a large and deep lake in south‐eastern Norway. Eutrophication symptoms peaked in the 1970s, which led to extensive measures to reduce the phosphorus load. A monitoring programme has run continuously from 1972. Monitoring has documented a marked decrease in phosphorus load and algal biomass and also revealed an increase in epilimnetic temperature and extended summer stratification. 3. Records of algal pigments and diatoms were extracted from sediment cores taken from 236 m depth. The pigment record documented dramatic changes in lake production consistent with the monitoring record. The diatom record reflected well the eutrophication history of the lake and also demonstrated that the assemblage of the recent recovery stage differs from that of the pre‐eutrophication period. 4. Ordination of diatom assemblages over time constrained by proxies for nutrient load and temperature indicated that the diatom assemblage correlated with both factors, which together accounted for 60% of the variation in diatom composition. No interaction was detected between these factors. The results suggest that the diatom assemblage has responded to varying nutrient loads as well as to changes in temperature and/or factors that correlate with temperature. 5. Reconstructions of algal biomass and total phosphorus content mirrored known changes through the monitoring period, although the absolute phosphorus estimates were too high relative to the instrumental record. The sediment record from Lake Mjøsa provides a baseline for lake production in terms of algal pigments and organic contents, and for the diatom assemblage composition in a pristine stage.     

Chemical criteria for marine eutrophication with special reference to the Oslofjord,Norway

Results of a chemical and geochemical study of the Oslofjord system are presented to explore the possibility of qualitatively identifying a eutrophication gradient. Nutrient concentration and dissolved oxygen may indicate such a gradient in areas of similar hydrographies. Uncertainties due to minor differences in hydrography may be resolved by considering sedimentary organic carbon, C:N ratios and the accumulation of mineralisation products in the sediment pore water. The reliability of the sedimentary organic matter and the pore water composition as gradient indicators can be impaired by input of terrestrial organics, dispersal of mineralisation products by the benthic fauna or the apparent high levels of organic carbon and pore water nutrients in the absence of the benthic fauna under anoxic conditions.     

The PsbQ protein is required in Arabidopsis for photosystem II assembly/stability and photoautotrophy under low light conditions

RNA interference was used to simultaneously suppress the expression of the two genes that encode the PsbQ proteins of Photosystem II (PS II) in Arabidopsis thaliana, psbQ-1 (At4g21280) and psbQ-2 (At4g05180). Two independent PsbQ-deficient plant lines were examined. These plant lines produced little detectable PsbQ protein. Under normal growth light conditions, the wild type and mutant plants were visually indistinguishable. Additionally, analysis of steady state oxygen evolution rates and chlorophyll fluorescence characteristics indicated little alteration of photosynthetic capacity in the mutant plants. No loss of other PS II proteins was evident. Interestingly, flash oxygen yield analysis performed on thylakoid membranes isolated from the mutant and wild type plants indicated that the oxygen-evolving complex was quite unstable in the mutants. Furthermore, the lifetime of the S2 state of the oxygen-evolving complex appeared to be increased in these plants. Incubation of the wild type and mutant plants under low light growth conditions led to a significantly stronger observed phenotype in the mutants. The mutant plants progressively yellowed (after 2 weeks) and eventually died (after 3-4 weeks). The wild type plants exhibited only slight yellowing after 4 weeks under low light conditions. The mutant plants exhibited a large loss of a number of PS II components, including CP47 and the D2 protein, under low light conditions. Additionally, significant alterations of their fluorescence characteristics were observed, including an increased FO and decreased FV, yielding a large loss in PS II quantum efficiency (FV/FM). Analysis of QA- decay kinetics in the absence of 3-(3,4-dichlorophenyl)-1,1-dimethyl urea indicated a defect in electron transfer from QA- to QB, whereas experiments performed in the presence of this herbicide indicated that the recombination rate between QA- and the S2 state was strongly retarded. These results indicate that the loss of the PsbQ protein induces significant changes in Photosystem II function, particularly in low light-grown plants, and that the PsbQ protein is required for photoautotrophic growth under low light conditions.