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Phytoplankton biomass and production regulates key aspects of freshwater ecosystems yet its variability and subsequent predictability is poorly understood. We estimated within‐lake variation in biomass using high‐frequency chlorophyll fluorescence data from 18 globally distributed lakes. We tested how variation in fluorescence at monthly, daily, and hourly scales was related to high‐frequency variability of wind, water temperature, and radiation within lakes as well as productivity and physical attributes among lakes. Within lakes, monthly variation dominated, but combined daily and hourly variation were equivalent to that expressed monthly. Among lakes, biomass variability increased with trophic status while, within‐lake biomass variation increased with increasing variability in wind speed. Our results highlight the benefits of high‐frequency chlorophyll monitoring and suggest that predicted changes associated with climate, as well as ongoing cultural eutrophication, are likely to substantially increase the temporal variability of algal biomass and thus the predictability of the services it provides. [ABSTRACT FROM AUTHOR]

Diadenosine polyphosphates are present intracellularly and in extracellular fluid due to release from secretory vesicles in platelets, chromaffin cells and other cells. This study investigates effects of diadenosine pentaphosphate (AP5A) on heart muscle function.Contractile force amplitude and action potential duration at 90% repolarization (APD90) were measured after challenge with AP5A 50 μmor isoproterenol 50–70 nM in guinea pig papillary muscles. Isoproterenol was given immediately after AP5A-exposure or after 45 min washout. AP5A was combined with antagonists to the purinergic P2 receptor (suramin 100 μm), the dinucleotide receptor [diinosine pentaphosphate 30 μm(IP5I)] or adenosine receptors [8-(P-sulfophenyl) theophylline 50 μm(8-SPT)].Results are %-change (mean ± SEM) from value before exposure. AP5A increased contractile force by 22 ± 3%* (* P < 0.05), and IP5I abolished this. AP5A prolonged APD90 by 7 ± 2%*. AP5A significantly reduced response to isoproterenol acutely from 31 ± 4* (controls) to 9 ± 4% and after 45 min washout from 61 ± 14* (controls) to 16 ± 5%. 8-SPT abolished the sustained effect. Increase in contractile force by AP5A was confirmed in human atria trabecula preparations.AP5A increased contractile force and prolonged APD90. Contractile force increased by stimulation of the dinucleotide receptor in guinea pig myocardium. The sustained anti-β-adrenergic effect of AP5A was due to adenosine receptor stimulation. [ABSTRACT FROM AUTHOR]

ABSTRACT A sustained anti-β -adrenergic effect of adenosine has been reported. This study was initiated to investigate this topic and especially elucidate the role of protein kinase C (PKC). Contractile force amplitude and action potential duration at 90% repolarization (APD90 ) were measured in guinea-pig papillary muscles before and after 5 min challenge with 5 nm isoproterenol. Protocols contained 30 min exposure to the test agents adenosine 33 μ m (ado), adenosine + PKC-inhibitor bisindolylmaleimide 20 nM (ado + BIM), PKC-activator 1,2-dioctanoyl-sn-glycerol 10 μ m (DOG) and α -agonist phenylephrine 5 μ m (phe). Isoproterenol was given at the end of test exposure and after 15 min washout. Results are mean ± SEM of percentage-change, P ≤ 0.05 considered significant and labelled *. The first isoproterenol challenge significantly increased contractile force (27 ± 7%*) in the control group. Responses in the test groups were 2 ± 4 (ado), 1 ± 5 (ado + BIM), 14 ± 4* (DOG), 0 ± 2% (phe). After washout of adenosine, DOG and phenylephrine, isoproterenol induced 3 ± 8 (ado), 23 ± 5* (ado + BIM), 13 ± 5* (DOG), 15 ± 7% (phe) increase in test groups compared with 22 ± 5%* increase in contractile force in the control group. After 45 min washout of adenosine the inotropic response was still significantly reduced compared with control (29 ± 4 vs. 79 ± 8%*). Isoproterenol stimulation shortened APD90 in controls at both time points (5 ± 1%* and 4 ± 1%*), with no significant shortening in test groups. Adenosine induces sustained anti-β -adrenergic effects on contractile force as well as APD90 . A role for PKC in signal transduction is supported with respect to contractile force. [ABSTRACT FROM AUTHOR]

Carbon stable isotopic composition was determined for zooplankton, POM, DOM and, in two cases, phytoplankton from 12 small forest lakes in southern Finland during summer 1997. The lakes were selected to provide a wide range of water colour (12-365 mg Pt l-1) but minimum variability in other limnological characteristics. POM and DOMshowed similar stable isotope ratios (delta13C around -28, comparable with values commonly reported for terrestrial C3 vegetation) and these showed no relationship to water colour. Zooplankton were consistently depleted in 13C relative to the bulk POM on which they might feed. This relative 13C depletion increased significantly with water colour. Therefore the zooplankton must have been feeding selectively on isotopically light food sources and doing so to a greater extent in the more humic lakes. Although phytoplankton were 13C-depleted relative to POM, their abundance and their variability between lakes did not appear to explain adequately the observed trend in zooplankton stable isotope ratios. We propose that some zooplankton were grazing on methanotrophic bacteria which utilize isotopically light methane derived from allochthonous organic matter decomposed under anaerobic conditions. This process would be more pronounced in colored lakes with a higher loadingof allochthonous organic matter and greater development of hypolimnetic anoxia. [ABSTRACT FROM AUTHOR]

Vertical distribution of primary production and phytoplankton was studied in a polyhumic brownwater lake and in an oligo-mesohumic lake. During summer both lakes were thermally, chemically and biologically stratified. In the brownwater lake primary production was restricted to the uppermost layer of 1-1.5 m of epilimnion. In the oligo-mesohumic lake noticeable primary production was detected down to depths of 2-3 m. The ice-free period primary production was about 20% higher in the oligo-mesohumic lake, though occasionally the surface production was 2-3 times higher in the brownwater lake. Epilimnetic total phosphorus and total nitrogen concentrations were higher in the brownwater lake, while nitrate-nitrite, ammonium and phosphate concentrations were very low in both lakes. Phytoplankton was confined to the uppermost productive layer in the brownwater lake. In the oligo-mesohumic lake phytoplankton was distributed more evenly, though the mean maximum biomass was at the depth of 3-4 m. Below the oxic water layer biomass decreased abruptly in both lakes. In the oligo-mesohumic lake chlorophyll concentration was extremely high (max. 320 mg chl a m -3) in the anoxic hypolimnion, due to green sulphus bacteria. Flagellated chlorophytes and thrysophytes dominated in the brownwater lake; in spring Chlamydomonas species, followed by Mallomonas caudata. In the oligo-mosohumic lake small coccal green algae, such as Oocystis. Scenedesmus and Westella-like species, dominated in mid-summer, and chrysophytes and cryptomonads in autumn. [ABSTRACT FROM AUTHOR]

An abstract of the study "Sustained anti-beta-adrenergic effect of opioids and bradykinin in guinea pig papillary muscle," by L. Arvola, G. Bertelsen and K. Ytrehus, is presented.