Similarity between contemporary vegetation and plant remains in the surface sediment in Mediterranean lakes

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The role of cladocerans in tracking long-term change in shallow lake trophic status

Shallow lakes have been affected by a variety of human activities profoundly altering their ecological structure and function. Cladocerans have been used to track change resulting from a variety of drivers at a number of time scales. Aquatic macrophytes are well recognised as reflecting the ecological condition of a lake. Here, we compare the plant macrofossils with the sub-fossil cladoceran assemblages from 20 dated sediment cores. Co-correspondence analysis was used to determine the degree of commonality of change in community composition of the two biological groups through time. This analysis revealed very high levels of agreement in the nature and timing of change at all the sites examined with very high correlation coefficients between the axis 1 scores for macrofossils and cladocerans. Furthermore, at all sites a high proportion of the variance (min 20%, max 54%) in the macrofossil data was explained by the change in the cladoceran assemblage. Sub-fossil macrofossil and cladoceran assemblages, from at least from 1700 AD onwards, were examined in more detail at three sites: Ormesby Great Broad, Felbrigg Lake and Lake Søbygaard. There was very good accord in the main shifts of the cladoceran and macrofossil assemblages at all three sites. This may reflect the long-term shift in the principal focus of primary production from the benthic to the pelagic habitat. We suggest that the combination of their central position in the food-web and the presence of both pelagic and benthic taxa make cladocerans a strong candidate as the single best indicator of (palaeo) ecological condition related to changing trophic status and alteration in food-web structure in shallow lakes.     

Contraction-induced skeletal muscle FAT/CD36 trafficking and FA uptake is AMPK independent

The aim of this study was to investigate the molecular mechanisms regulating FA translocase CD36 (FAT/CD36) translocation and FA uptake in skeletal muscle during contractions. In one model, wild-type (WT) and AMP-dependent protein kinase kinase dead (AMPK KD) mice were exercised or extensor digitorum longus (EDL) and soleus (SOL) muscles were contracted, ex vivo. In separate studies, FAT/CD36 translocation and FA uptake in response to muscle contractions were investigated in the perfused rat hindlimb. Exercise induced a similar increase in skeletal muscle cell surface membrane FAT/CD36 content in WT (+34%) and AMPK KD (+37%) mice. In contrast, 5-aminoimidazole-4-carboxamide ribonucleoside only induced an increase in cell surface FAT/CD36 content in WT (+29%) mice. Furthermore, in the perfused rat hindlimb, muscle contraction induced a rapid (1 min, +15%) and sustained (10 min, +24%) FAT/CD36 relocation to cell surface membranes. The increase in cell surface FAT/CD36 protein content with muscle contractions was associated with increased FA uptake, both in EDL and SOL muscle from WT and AMPK KD mice and in the perfused rat hindlimb. This suggests that AMPK is not essential in regulation of FAT/CD36 translocation and FA uptake in skeletal muscle during contractions. However, AMPK could be important in regulation of FAT/CD36 distribution in other physiological situations.     

Low-copy-number human transgene is recognized as an X inactivation center in mouse ES cells, but fails to induce cis-inactivation in chimeric mice

X chromosome inactivation is initiated from a segment of the mammalian X chromosome called the X inactivation center. Transgenes from this region of the murine X chromosome are providing the means to identify the DNA needed for cis inactivation in mice. We recently showed that chimeric mice carrying transgenes from the human X inactivation center (XIC) region also provide a functional assay for human XIC activity; approximately 6 copies of a 480-kb human transgene (ES-10) were sufficient to initiate random X inactivation in cells of male chimeric mice (Migeon et al., 1999, Genomics, 59, 113-121). Now, we report studies of another human transgene (ES-5), which contains less than 300 kb of the human XIC region on Xq13.2 including an intact XIST locus and which has inserted in one or two copies into mouse chromosome 6. The ES-5 transgene is recognized as an X inactivation center in mouse embryonic stem cells, but is not sufficient to induce random X inactivation in somatic cells of highly chimeric mice. Human transgenes in chimeric mice provide a means to uncouple the key steps in this complex pathway and facilitate the search for essential components of the human XIC region.     

Decrease in Vitamin D Status in the Greenlandic Adult Population from 1987–2010

Background

Low vitamin D status may be pronounced in Arctic populations due to limited sun exposure and decreasing intake of traditional food.

Objective

To investigate serum 25(OH)D3 as a measure of vitamin D status among adult Inuit in Greenland, predictors of low serum 25(OH)D3 concentrations and the trend from 1987 to 2005–2010.

Design

A total of 2877 randomly selected Inuit (≥18 years) from the Inuit Health in Transition study were included. A sub-sample (n = 330) donated a blood sample in 1987 which allowed assessment of time trends in vitamin D status.

Results

The geometric mean serum 25(OH)D3 (25[OH]D2 concentrations were negligible and not reported) in 2005–2010 was lowest among the 18–29 year old individuals (30.7 nmol/L; 95% CI: 29.7; 31.7) and increased with age. In all age-groups it decreased from 1987 to 2005–2010 (32%–58%). Low 25(OH)D3 concentrations (<50 nmol/L) were present in 77% of the 18–29 year old and decreased with age. A characteristic seasonal variation in 25(OH)D3 concentrations was observed (range 33.2–57.1 nmol/L, p<0.001), with the highest concentrations in August to October. Age (2.0% per year increase; CI: 1.7, 2.2), female gender (7.1%; CI: 2.0; 12.5), alcohol intake (0.2% per increase in drinks/week; 0.0; 0.4), and traditional diet (10.0% per 100 g/d increase; CI: 7.9; 12.1) were associated with increased serum 25(OH)D3, whereas smoking (−11.6%; CI: −16.2; −6.9), BMI (−0.6%; CI: −1.1; −0.2) and latitude (−0.7% per degree increase; CI: −1.3; −0.2) were associated with decreased concentrations.

Conclusion

We identified a remarkable decrease in vitamin D status from 1987 to 2005–2010 and a presently low vitamin D status among Inuit in Greenland. A change away from a traditional diet may well explain the observed decline. The study argues for the need of increased dietary intake of vitamin D and supplementation might be considered.     

Nitrogen Loss and Denitrification as Studied in Relation to Reductions in Nitrogen Loading in a Shallow,Hypertrophic Lake (Lake Søbygård,Denmark)

A detailed mass balance on nitrogen was carried out in shallow and hypertrophic Lake Søbygård during 4.5 years before through 2.5 years after a 36 % reduction in nitrogen loading. Annual mean loss rate of nitrogen was 159–229 mg N m⁻² d⁻¹ before the loading reduction and 125 mg N m⁻² d⁻¹ after. In spite of a short hydraulic retention time (18–27 days) the proportion of nitrogen loading lost in the lake was high (38–53 %) and not affected by changes in loading. Calculated denitrification accounted for 86–93% of the loss rate, while 7–14% was permanently buried. Marked seasonal variations in the loss percentage were found during the season, ranging from 23 % in first quarter to 65 % in third quarter. The seasonal variation in the loss percentage of nitrogen showed a hysteresis like relationship to temperature, with a high percentage in fourth quarter. This suggests that the amount of available substrate, which mainly consists of sedimentated phytoplankton, accumulated during summer, is an important regulating factor. The ability of various published input-output models to predict the observed changes in in-lake nitrogen concentration in Lake Søbygård was tested. This study has further confirmed that small lakes with short retention and high nitrogen loading may significantly reduce the nitrogen loading of downstream aquatic environments.     

Fish manipulation as a lake restoration tool in shallow,eutrophic, temperate lakes 2: threshold levels,long-term stability and conclusions

In order to evaluate short-term and long-term effects of fish manipulation in shallow, eutrophic lakes, empirical studies on relationships between lake water concentration of total phosphorus (P) and the occurrence of phytoplankton, submerged macrophytes and fish in Danish lakes are combined with results from three whole-lake fish manipulation experiments. After removal of less than 80 per cent of the planktivorous fish stock a short-term trophic cascade was obtained in the nutrient regimes, where large cyanobacteria were not strongly dominant and persistent. In shallow Danish lakes cyanobacteria were the most often dominating phytoplankton class in the P-range between 200 and 1 000μg P l⁻¹. Long-term effects are suggested to be closely related to the ability of the lake to establish a permanent and wide distribution of submerged macrophytes and to create self-perpetuating increases in the ratio of piscivorous to planktivorous fish. The maximum depth at which submerged macrophytes occurred, decreased exponentially with increasing P concentration. Submerged macrophytes were absent in lakes>10 ha and with P levels above 250–300μg P l⁻¹, but still abundant in some lakes<3 ha at 650μg P l⁻¹. Lakes with high cover of submerged macrophytes showed higher transparencies than lakes with low cover aboveca. 50μg P l⁻¹. These results support the alternative stable state hypothesis (clear or turbid water stages). Planktivorous fish>10 cm numerically contributed more than 80 per cent of the total planktivorous and piscivorous fish (>10 cm) in the pelagical of lakes with concentrations above 100μg P l⁻¹. Below this threshold level the proportion of planktivores decreased markedly toca. 50 per cent at 22μg P l⁻¹. The extent of the shift in depth colonization of submerged macrophytes and fish stock composition in the three whole-lake fish manipulations follows closely the predictions from the relationships derived from the empirical study. We conclude that a long-term effect of a reduction in the density of planktivorous fish can be expected only when the external phosphorus loading is reduced to below 0.5–2.0 g m⁻² y⁻¹. This loading is equivalent to an in-lake summer concentration below 80–150μg P l⁻¹. Furthermore, fish manipulation as a restoration tool seems most efficient in shallow lakes.     

Interactions between sediment and water in a shallow and hypertrophic lake: a study on phytoplankton collapses in Lake Søbygård,Denmark

Short-term changes in phytoplankton and zooplankton biomass have occurred 1–3 times every summer for the past 5 years in the shallow and hypertrophic Lake Søbygård, Denmark. These changes markedly affected lake water characteristics as well as the sediment/water interaction. Thus during a collapse of the phytoplankton biomass in 1985, lasting for about 2 weeks, the lake water became almost anoxic, followed by rapid increase in nitrogen and phosphorus at rates of 100–400 mg N M^–2 day^–1 and 100–200 mg P m^–1 day^–1. Average external loading during this period was about 350 mg N m^–2 day^–1 and 5 mg P m^–2 day^–1, respectively.Due to high phytoplankton biomass and subsequently a high sedimentation and recycling of nutrients, gross release rates of phosphorus and nitrogen were several times higher than net release rates. The net summer sediment release of phosphorus was usually about 40 mg P m^–2 day^–1, corresponding to a 2–3 fold increase in the net phosphorus release during the collapse. The nitrogen and phosphorus increase during the collapse is considered to be due primarily to a decreased sedimentation because of low algal biomass. The nutrient interactions between sediment and lake water during phytoplankton collapse, therefore, were changed from being dominated by both a large input and a large sedimentation of nutrients to a dominance of only a large input. Nitrogen was derived from both the inlet and sediment, whereas phosphorus was preferentially derived from the sediment. Different temperature levels may be a main reason for the different release rates from year to year.     

Can Simple Empirical Equations Describe the Seasonal Dynamics of Bacterioplankton in Lakes: An Eight-Year Study in Shallow Hypertrophic and Biologically Highly Dynamic Lake Søbygård,Denmark

Abstract Seasonal variation in bacterioplankton abundance, biomass, and bacterioplankton production was studied over eight years in hypertrophic Lake S?byg?rd. Biologically, the lake is highly variable; this is due mainly to large interannual variation in fish recruitment. Bacterioplankton production was low during winter, typically 1–3 × 10⁷ cells l⁻¹ h⁻¹, and high during summer, albeit greatly fluctuating with maximum rates typically ranging from 60 to 90 × 10⁷ cells l⁻¹ h⁻¹ (or 0.4 to 0.6 mg C l⁻¹ day⁻¹). Less pronounced variations were found in bacterioplankton abundance, which typically ranged from 3–8 × 10⁹ cells l⁻¹ in winter to 15–30 × 10⁹ cells l⁻¹ during summer. The specific growth rate of bacterioplankton varied from 0.02–0.2 d⁻¹ in winter to 0.5–2.3 day⁻¹ during summer. Interpolated mean bacterioplankton production, in terms of carbon, ranged from 0.08 to 0.16 mg C l⁻¹ day⁻¹, corresponding to 1.6–5.5% of the phytoplankton production, while biomass ranged from 0.28 to 0.36 mg C l⁻¹, corresponding to 1.9–4.6% of the phytoplankton biomass. We conducted regression analysis, relating the bacterioplankton variables to a number of environmental variables, and evaluated the interannual parameter variability. Chlorophyll a and phytoplankton production contributed less to the variation in the bacterioplankton variables than in most previous analyses using data from less eutrophic systems. We suggest that the proportion of phytoplankton production that is channelized through bacterioplankton in lakes decreases with increasing trophic state and decreasing mean depth. This probably reflects a concurrent increase in fish predation on macrozooplankton and loss by sedimentation. An important part of the residual variation in the equations hitherto proposed in the literature could be explained by variation in macrozooplankton biomass and pH > 10.2. A negative effect of high pH on bacterioplankton production was confirmed by laboratory experiments. The impact of different zooplankton varies considerably, with Daphnia seeming to have a negative impact on bacterioplankton abundance and, thereby, indirectly on bacterioplankton production, while Bosmina, rotifers, and cyclopoid copepods seem to stimulate both abundance and production. Bosmina apparently also stimulate the bacterioplankton specific growth rate. Received: 8 February 1996; Accepted: 16 July 1996     

Environment not dispersal limitation drives clonal composition of Arctic Daphnia in a recently deglaciated area

One of the most prominent manifestations of the ongoing climate warming is the retreat of glaciers and ice sheets around the world. Retreating glaciers result in the formation of new ponds and lakes, which are available for colonization. The gradual appearance of these new habitat patches allows us to determine to what extent the composition of asexual Daphnia (water flea) populations is affected by environmental drivers vs. dispersal limitation. Here, we used a landscape genetics approach to assess the processes structuring the clonal composition of species in the D. pulex species complex that have colonized periglacial habitats created by ice‐sheet retreat in western Greenland. We analysed 61 populations from a young (<50 years) and an old cluster (>150 years) of lakes and ponds. We identified 42 asexual clones that varied widely in spatial distribution. Beta‐diversity was higher among older than among younger systems. Lineage sorting by the environment explained 14% of the variation in clonal composition whereas the pure effect of geographical distance was very small and statistically insignificant ( = 0.010, P = 0.085). Dispersal limitation did not seem important, even among young habitat patches. The observation of several tens of clones colonizing the area combined with environmentally driven clonal composition of populations illustrates that population assembly of asexual species in the Arctic is structured by environmental gradients reflecting differences in the ecology of clones.