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1.
  • Adrian, Rita, et al. (author)
  • Lakes as sentinels of climate change
  • 2009
  • In: Limnology and Oceanography. - : Wiley. - 0024-3590 .- 1939-5590. ; 54:6(2), s. 2283-2297
  • Journal article (peer-reviewed)abstract
    • While there is a general sense that lakes can act as sentinels of climate change, their efficacy has not been thoroughly analyzed. We identified the key response variables within a lake that act as indicators of the effects of climate change on both the lake and the catchment. These variables reflect a wide range of physical, chemical, and biological responses to climate. However, the efficacy of the different indicators is affected by regional response to climate change, characteristics of the catchment, and lake mixing regimes. Thus, particular indicators or combinations of indicators are more effective for different lake types and geographic regions. The extraction of climate signals can be further complicated by the influence of other environmental changes, such as eutrophication or acidification, and the equivalent reverse phenomena, in addition to other land-use influences. In many cases, however, confounding factors can be addressed through analytical tools such as detrending or filtering. Lakes are effective sentinels for climate change because they are sensitive to climate, respond rapidly to change, and integrate information about changes in the catchment.
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2.
  • Ahlgren, Joakim, et al. (author)
  • Degradation of organic phosphorus compounds in anoxic Baltic Sea sediments : A P-31 nuclear magnetic resonance study
  • 2006
  • In: Limnology and Oceanography. - Uppsala Univ, Dept Ecol & Evolut, Evolutionary Biol Ctr, S-75236 Uppsala, Sweden. Uppsala Univ, Dept Chem, S-75124 Uppsala, Sweden. Univ So Denmark, Inst Biol, DK-5230 Odense, Denmark.. - 0024-3590 .- 1939-5590. ; 51:5, s. 2341-2348
  • Journal article (peer-reviewed)abstract
    • The composition and abundance of phosphorus extracted by NaOH-ethylenediaminetetraacetic acid from anoxic Northwest Baltic Sea sediment was characterized and quantified using solution P-31 nuclear magnetic resonance. Extracts from sediment depths down to 55 cm, representing 85 yr of deposition, contained 18.5 g m(-2) orthophosphate. Orthophosphate monoesters, teichoic acid P, microbial P lipids, DNA P, and pyrophosphate corresponded to 6.7, 0.3, 1.1, 3.0, and 0.03 g P m(-2), respectively. The degradability of these compound groups was estimated by their decline in concentration with sediment depth. Pyrophosphate had the shortest half-life (3 yr), followed by microbial P lipids with a half-life of 5 yr, DNA P (8 yr), and orthophosphate monoesters (16 yr). No decline in concentration with sediment depth was observed for orthophosphate or teichoic acid P.
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3.
  • Alling, Vanja, et al. (author)
  • Tracing terrestrial organic matter by delta S-34 and delta C-13 signatures in a subarctic estuary
  • 2008
  • In: Limnology and Oceanography. - 0024-3590 .- 1939-5590. ; 53:6, s. 2594-2602
  • Journal article (peer-reviewed)abstract
    • A key issue to understanding the transformations of terrestrial organic carbon in the ocean is to disentangle the latter from marine-produced organic matter. We applied a multiple stable isotope approach using delta S-34 and delta C-13 isotope signatures from estuarine dissolved organic matter (DOM), enabling us to constrain the contribution of terrestrial-derived DOM in an estuarine gradient of the northern Baltic Sea. The stable isotope signatures for dissolved organic sulfur (delta S-34(DOS)) have twice the range between terrestrial and marine end members compared to the stable isotope signatures for dissolved organic carbon (delta C-13(DOC)); hence, the share of terrestrial DOM in the total estuarine DOM can be calculated more precisely. DOM samples from the water column were collected using ultrafiltration on board the German RV Maria S Merian during a winter cruise, in the Bothnian Bay, Bothnian Sea, and Baltic proper. We calculated the terrestrial fraction of the estuarine DOC (DOCter) from both delta C-13(DOC) and delta S-34(DOS) signatures and applying fixed C: S ratios for riverine and marine end members to convert S isotope signatures into DOC concentrations. The delta S-34(DOS) signature of the riverine end member was +7.02 parts per thousand, and the mean signatures from Bothnian Bay, Bothnian Sea, and Baltic proper were +10.27, +12.51, and +13.67 parts per thousand, respectively, showing an increasing marine signal southwards (d34SDOS marine end member = 18.1 parts per thousand). These signatures indicate that 87%, 75%, and 67%, respectively, of the water column DOC is of terrestrial origin (DOCter) in these basins. Comparing the fractions of DOCter in each basin-that are still based on few winter values only-with the annual river input of DOC, it appears that the turnover time for DOCter in the Gulf of Bothnia is much shorter than the hydraulic turnover time, suggesting that high-latitude estuaries might be efficient sinks for DOCter.
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4.
  • Alling, Vanja, 1978-, et al. (author)
  • Tracing terrestrial organic matter by delta34S and delta13C signatures in a subarctic estuary
  • 2008
  • In: Limnology and Oceanography. - : Wiley. - 0024-3590 .- 1939-5590. ; 53:6, s. 2594-2602
  • Journal article (peer-reviewed)abstract
    • A key issue to understanding the transformations of terrestrial organic carbon in the ocean is to disentangle the latter from marine-produced organic matter. We applied a multiple stable isotope approach using 34S and 13C isotope signatures from estuarine dissolved organic matter (DOM), enabling us to constrain the contribution of terrestrial-derived DOM in an estuarine gradient of the northern Baltic Sea. The stable isotope signatures for dissolved organic sulfur (34SDOS) have twice the range between terrestrial and marine end members compared to the stable isotope signatures for dissolved organic carbon (13CDOC); hence, the share of terrestrial DOM in the total estuarine DOM can be calculated more precisely. DOM samples from the water column were collected using ultrafiltration on board the German RV Maria S Merian during a winter cruise, in the Bothnian Bay, Bothnian Sea, and Baltic proper. We calculated the terrestrial fraction of the estuarine DOC (DOCter) from both 13CDOC and 34SDOS signatures and applying fixed C: S ratios for riverine and marine end members to convert S isotope signatures into DOC concentrations. The 34SDOS signature of the riverine end member was +7.02‰, and the mean signatures from Bothnian Bay, Bothnian Sea, and Baltic proper were +10.27, +12.51, and +13.67‰, respectively, showing an increasing marine signal southwards (34SDOS marine end member 5 18.1‰). These signatures indicate that 87‰, 75‰, and 67‰, respectively, of the water column DOC is of terrestrial origin (DOCter) in these basins. Comparing the fractions of DOCter in each basin—that are still based on few winter values only—with the annual river input of DOC, it appears that the turnover time for DOCter in the Gulf of Bothnia is much shorter than the hydraulic turnover time, suggesting that high-latitude estuaries might be efficient sinks for DOCter.
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5.
  • Ask, Jenny, 1976-, et al. (author)
  • Terrestrial organic matter and light penetration : Effects on bacterial and primary production in lakes
  • 2009
  • In: Limnology and Oceanography. - : American Society of Limnology and Oceanography, Inc.. - 0024-3590 .- 1939-5590. ; 54:6, s. 2034-2040
  • Journal article (peer-reviewed)abstract
    • We investigated productivity at the basal trophic level in 15 unproductive lakes in a gradient ranging from clear-water to brown-water (humic) lakes in northern Sweden. Primary production and bacterial production in benthic and pelagic habitats were measured to estimate the variation in energy mobilization from external energy sources (primary production plus bacterial production on allochthonous organic carbon) along the gradient. Clear-water lakes were dominated by autotrophic energy mobilization in the benthic habitat, whereas humic lakes were dominated by heterotrophic energy mobilization in the pelagic habitat. Whole-lake (benthic + pelagic) energy mobilization was negatively correlated to the light-extinction coefficient, which was determined by colored terrestrial organic matter in the lake water. Thus, variation in the concentration of terrestrial organic matter and its light-absorbing characteristics exerts strong control on the magnitude, as well as on the processes and pathways, of energy mobilization in unproductive lakes. We suggest that unproductive lakes in general are sensitive to input of terrestrial organic matter because of its effects on basal energy mobilization in both benthic and pelagic habitats.
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6.
  • Baltar, Federico, et al. (author)
  • Evidence of prokaryotic metabolism on suspended particulate organic matter in the dark waters of the (sub)tropical North Atlantic
  • 2009
  • In: Limnology and Oceanography. - 0024-3590 .- 1939-5590. ; 54:1, s. 182-193
  • Journal article (peer-reviewed)abstract
    • The distribution of prokaryotic abundance (PA), respiratory activity (ETS), heterotrophic production (PHP), and suspended particulate (POM) and dissolved (DOM) organic matter was determined in the meso- and bathypelagic waters of the (sub) tropical North Atlantic. PA decreased by one order of magnitude from the lower euphotic zone to the bathypelagic waters, while ETS decreased by two and PHP by three orders of magnitude. On a section following the Mid-Atlantic Ridge from 35 degrees N to 5 degrees N, ETS below 1000-m depth increased southwards up to three-fold. This latitudinal gradient in the deep waters was paralleled by a six-fold increase in Particulate Organic Carbon (POC), whereas no trend was apparent in the DOM distribution. Significant correlations between POM and ETS were obtained in the water masses between 1000-m and 3000-m depth, the Antarctic Intermediate Water and the North East Atlantic Deep Water. A strong imbalance in the dark ocean was found between prokaryotic carbon demand (estimated through two different approaches) and the carbon sinking flux derived from sediment-trap records corrected with Th-230. The imbalance was greater when deeper in the water column, suggesting that the suspended carbon pool must account for most of the carbon deficit. Our results, together with other recent findings discussed in this paper, indicate that microbial life in the dark ocean is likely more dependent on slowly sinking or buoyant, laterally advected suspended particles than hitherto assumed. 
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7.
  • Berggren, Martin, 1981-, et al. (author)
  • Aging of allochthonous organic carbon regulates bacterial production in unproductive boreal lakes
  • 2009
  • In: Limnology and Oceanography. - : Wiley. - 0024-3590 .- 1939-5590. ; 54:4, s. 1333-1342
  • Journal article (peer-reviewed)abstract
    • We calculated average aquatic dissolved organic carbon (DOC) age (the time span from soil discharge to observation) in water from the inlets and outlets of two unproductive Swedish lakes at different times during an annual cycle. Bacterial production (BP) and bacterial growth efficiency (BGE) determined during 7-d bioassays decreased with increasing average aquatic DOC age. Parallel to the declines in BP and BGE there was a rise in specific ultraviolet absorbance at the wavelength of 254 nm (SUVA254), which indicates that decreasing BP and BGE were connected to a shift to a more aromatic and recalcitrant DOC pool. The relationships between bacterial metabolism and DOC age were stronger after a Q10 correction of the DOC age, showing that temperature affected rates of DOC quality changes over time and should be taken into account when relating lake bacterial growth to substrate aging in natural environments. We propose that hydrological variability in combination with lake size (water renewal time) have a large influence on pelagic BP in lakes with high input of terrigenous DOC.
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8.
  • Clarke, A. L., et al. (author)
  • Long-Term Trends in Eutrophication and Nutrients in the Coastal Zone
  • 2006
  • In: Limnology and Oceanography. - 0024-3590 .- 1939-5590. ; 51:1, s. 385-397
  • Journal article (peer-reviewed)abstract
    • We used high-resolution paleoecological records of environmental change to study the rate and magnitude of eutrophication over the last century in two contrasting coastal ecosystems. A multiproxy approach using geochemical and biological indicators and diatom-based transfer functions provides a long-term perspective on changes in nutrient concentrations and the corresponding biological and sedimentary responses. In Roskilde Fjord, Denmark, total nitrogen (TN) increased 85% during the last century, with the most rapid increase occurring after the 1950s, corresponding to the postwar increase in N fertilizer use. In Laajalahti Bay, an urban embayment near Helsinki, Finland, total dissolved nitrogen (TDN) increased with growing wastewater inputs and decreased with the remedial actions taken to reduce these discharges. These changes are small relative to the order of magnitude increases in nutrient loading that have occurred in northwestern Europe, where the dissolved inorganic nitrogen (DIN) load has increased more than threefold in certain areas.
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9.
  • Dokulil, Martin T., et al. (author)
  • Twenty years of spatially coherent deepwater warming in lakes across Europe related to the North Atlantic Oscillation
  • 2006
  • In: Limnology and Oceanography. - 0024-3590 .- 1939-5590. ; 51:6, s. 2787-2793
  • Journal article (peer-reviewed)abstract
    • Twenty to fifty years of annual mean deepwater (hypolimnetic) temperature data from twelve deep lakes spaced across Europe (2 degrees 95'W to 14 degrees 0'E, 46 degrees 27' to 59 degrees 00'N) show a high degree of coherence among lakes, particularly within geographic regions. Hypolimnetic temperatures vary between years but increased consistently in all lakes by about 0.1 - 0.2 degrees C per decade. The observed increase was related to the weather generated by large-scale climatic processes over the Atlantic. To be effective, the climatic signal from the North Atlantic Oscillation (NAO) must affect deep lakes in spring before the onset of thermal stratification. The most consistent predictor of hypolimnetic temperature is the mean NAO index for January-May (NAO(J-M)), which explains 22-63% of the interannual variation in deepwater temperature in 10 of the 12 lakes. The two exceptions are remote, less wind-exposed alpine valley lakes. In four of the deepest lakes, the climate signal fades with depth. The projected hypolimnetic temperature increase of approximately 1 degrees C in 100 yr, obtained using a conservative approach, seems small. Effects on mixing conditions, thermal stability, or the replenishment of oxygen to deep waters result in accumulation of nutrients, which in turn will affect the trophic status and the food web.
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10.
  • Downing, J. A., et al. (author)
  • The global abundance and size distribution of lakes, ponds, and impoundments
  • 2006
  • In: Limnology and Oceanography. - 0024-3590 .- 1939-5590. ; 51:5, s. 2388-2397
  • Journal article (peer-reviewed)abstract
    • One of the major impediments to the integration of lentic ecosystems into global environmental analyses has been fragmentary data on the extent and size distribution of lakes, ponds, and impoundments. We use new data sources, enhanced spatial resolution, and new analytical approaches to provide new estimates of the global abundance of surface-water bodies. A global model based on the Pareto distribution shows that the global extent of natural lakes is twice as large as previously known (304 million lakes; 4.2 million km(2) in area) and is dominated in area by millions of water bodies smaller than 1 km(2). Similar analyses of impoundments based on inventories of large, engineered dams show that impounded waters cover approximately 0.26 million km(2). However, construction of low-tech farm impoundments is estimated to be between 0.1% and 6% of farm area worldwide, dependent upon precipitation, and represents > 77,000 km(2) globally, at present. Overall, about 4.6 million km(2) of the earth's continental "land" surface (> 3%) is covered by water. These analyses underscore the importance of explicitly considering lakes, ponds, and impoundments, especially small ones, in global analyses of rates and processes.
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13.
  • Erlandsson, Carina P., et al. (author)
  • The sensitivity of minimum oxygen concentrations in a fjord to changes in biotic and abiotic external forcing
  • 2006
  • In: Limnology and Oceanography. - 0024-3590. ; 51:1, part 2, s. 631-638
  • Journal article (peer-reviewed)abstract
    • We investigated the possible biotic and abiotic causes of the observed long-term decrease of oxygen minimum concentrations in the deep water of Gullmar Fjord. Physical factors explained about 40% of the decreased minimum oxygen concentrations since the 1950s. The North Atlantic Oscillation and more regional climate variations account for an important part of this change. The effect of the climate variations was mainly on the timing of the renewal of the basin water. The consumption rate of oxygen in the basin water has increased by 50% since the 1950s. This biotic effect explained about 60% of the decrease in minimum oxygen concentrations. The likely dominating source of oxygen-consuming matter was (remote) production in the Skagerrak, suggesting that there has been a long-term increase of particulate organic carbon in the Skagerrak water.
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14.
  • Gorokhova, Elena, et al. (author)
  • Molecular evidence for the occurrence of Mertensia ovum in the northern Baltic Sea and implications for the status of Mnemiopsis leidyi invasion.
  • 2009
  • In: Limnology and Oceanography. - 0024-3590 .- 1939-5590. ; 54:6, s. 2025-2033
  • Journal article (peer-reviewed)abstract
    • Nucleotide sequence analysis of 18S ribosomal RNA gene ( rRNA), internal transcribed spacer, and 5.8S rRNA was used for taxonomic identification of ctenophores collected in the northern Baltic Sea, where invasive Mnemiopsis leidyi and native Pleurobrachia pileus have been reported to occur. Contrary to previous reports, sequence analysis of 53 randomly selected specimens from seven stations revealed that none of them were M. leidyi or P. pileus. The 18S rRNA and 5.8S rRNA sequences were 100% identical to those of Mertensia ovum, a ctenophore with a broad Arctic and circumboreal distribution, which has never been reported to occur in the Baltic Sea. Polymerase chain reaction screening with primers designed to amplify all three species, and using ctenophores collected by vertically stratified sampling, confirmed that all ctenophores collected in this survey were M. ovum. The ctenophore abundance was high, up to 4500 individuals m(-2), positively correlating with salinity. Our findings emphasize the utility of applying molecular tools to biological surveys and the importance of rigorous species identification. They also indicate that M. leidyi, which is a threat to the southern Baltic ecosystem, does not occur in the northern part of the sea, and call for a pan-Baltic survey to establish current distributions of ctenophores, both native and invasive.
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15.
  • Gälman, Veronika, et al. (author)
  • Carbon and nitrogen loss rates during aging of lake sediment : Changes over 27 years studied in varved lake sediment
  • 2008
  • In: Limnology and Oceanography. - Waco, Tex. : American Society of Limnology and Oceanography. - 0024-3590 .- 1939-5590. ; 53:3, s. 1076-1082
  • Journal article (peer-reviewed)abstract
    • We used a collection of ten freeze cores of annually laminated (varved) lake sediment from Nylandssjön in northern Sweden collected from 1979 to 2007 to follow the long-term loss of carbon (C) and nitrogen (N) due to processes that occur in the lake bottom as sediment ages. We compared specific years in the different cores. For example, the loss of C from the surface varve of the 1979 core (sediment deposited during 1978) was followed in the cores from 1980, 1985, 1989, and so on until 2006. The C concentration of the sediment decreased by 20% and N decreased by 30% within the first five years after deposition, and after 27 yr in the sediment, there was a 23% loss of C and 35% loss of N. Because the relative loss of C with time was smaller than loss of N, the C:N ratio increased with increasing age of the sediment; the surface varves start with a ratio of ~10, which then increases to ~12.
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16.
  • Gälman, Veronika, et al. (author)
  • Decadal diagenetic effects on d13C and d15N studied in varved lake sediment
  • 2009
  • In: Limnology and Oceanography. - : Association for the Sciences of Limnology and Oceanography. - 0024-3590 .- 1939-5590. ; 54:3, s. 917-924
  • Journal article (peer-reviewed)abstract
    • To assess the long-term (27 yr) effects of sediment aging on stable carbon and nitrogen isotope values (δ13C and δ15N), we used a collection of eight freeze cores of annually laminated (varved) lake sediment collected from 1979 to 2007 in Nylandssjon (northern Sweden). Previous research has shown that 20–23% of carbon and 35% of nitrogen is lost in 27 yr. Material from specific years was compared in the cores, e.g., δ13C and δ15N of the surface varve of the 1979 core was followed in cores retrieved in 1980, 1989, 1993, 2002, 2004, and 2006. δ13C increased by 0.4–1.5% during the first 5 yr. After this initial increase, only minor fluctuations were recorded. There is a good correlation between the magnitude in δ13C changes and the initial carbon and nitrogen concentrations, indicating that the initial sediment composition is important for the 13C fractionation. δ15N gradually decreased by 0.3–0.7% over the entire 27-yr period. The lack of correlation with the initial sediment composition and the gradual decrease in δ15N indicates a microbial control on δ15N change. The diagenetic changes in the stable isotope values that occur in Nylandssjon are small, but of the same magnitude as the down-core variation in the varves deposited 1950–2006. Diagenetic effects should be considered when δ13C and δ15N are used to study organic matter sources or paleoproductivity, especially when dealing with recent trends or small changes. Based on our findings, diagenetic effects for δ13C are observed during the first 5–10 yr, whereas no delimitation can be recommended for δ15N.
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17.
  • Gälman, Veronika, et al. (author)
  • Decadal diagenetic effects on δ13C and δ15N studied in varved lake sediment
  • 2009
  • In: Limnology and Oceanography. - : Association for the Sciences of Limnology and Oceanography. - 0024-3590 .- 1939-5590. ; 54:3, s. 905-916
  • Journal article (peer-reviewed)abstract
    • The Livingstone and Imboden (1996) model for the oxygen depletion rate in lake hypolimnia was evaluated with field results in 32 lakes. The volumetric oxygen consumption rate (JV) was strongly related to lake trophic state, as represented by the growing season mean chlorophyll a (Chl a) and annual mean total phosphorus concentration, and might reach a maximum of 0.23–0.24 g O2 m-3 d-1 in eutrophic lakes. The areal oxygen consumption rate (JA) was not related to trophic state and was lower than the value estimated by the sediment core method, particularly in eutrophic lakes; the mean was 0.0816 6 0.0150 g O2 m-2 whole sediment d-1. We modified the model and it was used to confirm that the field JA, the average for the hypolimnion for the deoxygenation period, is influenced by JV and the range of sediment area to water volume ratio, (Z) (Z is water depth), in the hypolimnion, as well as the oxygen consumption rate in whole sediment (R). It was suggested that this is the reason for the difference between field and sediment core JA values. The R in 10 lakes varied from 7 to 460 g O2 m-3 whole sediment d-1. Overall, the Livingstone and Imboden model with constant JA and with a JV value derived from an empirical relationship with either measure of trophic state was considered to be a suitable condensed model for oxygen depletion in lake hypolimnia.
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18.
  • Hall, Per, 1954, et al. (author)
  • Dissolved organic matter in abyssal sediments: Core recovery artifacts
  • 2007
  • In: Limnology and Oceanography. - 0024-3590. ; 52:1, s. 19-31
  • Journal article (peer-reviewed)abstract
    • We report measurements of pore-water dissolved organic carbon (DOC), dissolved organic nitrogen, total dissolved carbohydrates, dissolved free monosaccharides, and ammonium in recovered deep-sea sediments from the Porcupine Abyssal Plain (PAP), Northeast Atlantic. There were distinct maxima close to the sediment–water interface of these constituents at all times of the year. The very high diffusive effluxes calculated from these porewater distributions were not compatible with simultaneous sediment trap measurements of particulate organic carbon, nitrogen, and carbohydrate fluxes toward the seafloor. Effluxes calculated from pore-water DOC distributions in recovered cores from another Atlantic deep-sea site, showing almost identical maxima as those at PAP, were more than an order of magnitude greater than simultaneous in situ chamber DOC flux measurements. We suggest that the dissolved organic matter maxima are predominantly artifacts induced by lysis of, or leakage from, mainly bacterial biomass resulting from decompression and/or warming during recovery of the sediment cores from the abyssal seafloor. Temperature elevation during core recovery from the abyss gives a N2 saturation of about 150%, and the combined effect of warming and decompression results in a CO2 saturation of about 135%, which together plausibly are associated with bubble formation creating cell bursting. Previous estimates of microbial biomass in abyssal sediments may be underestimates because of the difficulty of counting lysed bacterial cells. Since exoenzymes are inducible, previous measurements of their activities in recovered abyssal sediments may be overestimates.
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19.
  • Hansson, Lars Johan, et al. (author)
  • Prey-specific encounter rates and handling efficiencies as causes of prey selectivity in ambush-feeding hydromedusae
  • 2006
  • In: Limnology and Oceanography. - 0024-3590. ; 51:4, s. 1849-1858
  • Journal article (peer-reviewed)abstract
    • We examined the foraging process in the jellyfish Sarsia tubulosa feeding on three types of prey: cirripede nauplii, cypris larvae, and Acartia tonsa copepodites. Clearance rate was used as measure of prey selectivity. To estimate maximal clearance rate (F-max), we used a predictive encounter model with input parameters quantified from video observations. Both encounter rate and handling efficiency were important in determining F-max. Encounter volume rate was three times higher for cirripede nauplii than for copepodites, but sequential handling was 10 times more efficient for copepodites than for cirripede larvae. Two critical steps in the postcapture feeding process-capture of encountered prey with the tentacle, and mouth attachment to the captured prey-created a clear selectivity for copepods over barnacle larvae. Predicted values were close to laboratory measurements of F-max, and for cirripede nauplii also to field-estimated F-max. We suggest that species-specific handling efficiency is the main factor creating trophic niche separation in the large functional group of ambush-feeding hydromedusae.
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20.
  • Hylander, Samuel, et al. (author)
  • Complementary UV protective compounds in zooplankton
  • 2009
  • In: Limnology and Oceanography. - : Wiley. - 0024-3590 .- 1939-5590. ; 54:6, s. 1883-1893
  • Journal article (peer-reviewed)abstract
    • Zooplankton accumulate several groups of photoprotective compounds to shield against damaging ultraviolet radiation (UV). One of these groups, the carotenoids, makes the animals more conspicuous to visually hunting predators, whereas others, such as the mycosporine-like amino acids (MAAs) may not. The blend of photoprotective compounds is therefore important for the UV defense but also for the ability to escape predation through crypsis. Here we assess laboratory and field data from different latitudes to examine how UV, predation threat, and pigment availability ( in food) affects the mixture of UV-protective compounds in copepods. Overall, the blend of MAAs and carotenoids was partly explained by the availability of MAAs in the food, the UV-threat, and the presence of predators. Copepods upregulated their MAA content when UV threat was increasing (i.e., if MAAs were abundant in food), and in field data this accumulation only occurred at high levels of predation threat. If MAAs were scarce, copepods instead compensated with higher carotenoid accumulation. However, when there was a high predation threat this carotenoid compensatory effect was disadvantageous, and low concentrations of both MAAs and carotenoids at high UV-threat resulted in lower reproduction. In all, these results showed that carotenoids and MAAs are complementary substances, i.e., one is high when the other is low, and copepods are, hence, able to adjust their blend of different UV-protective compounds to optimize their defenses to the threats of UV and predation. These defense systems may buffer against direct food-web interactions and help the zooplankton to survive in environments with high UV threat.
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21.
  • Hylander, Samuel, et al. (author)
  • Zooplankton vertical migration and plasticity of pigmentation arising from simultaneous UV and predation threats
  • 2009
  • In: Limnology and Oceanography. - : Wiley. - 0024-3590 .- 1939-5590. ; 54:2, s. 483-491
  • Journal article (peer-reviewed)abstract
    • We assessed how zooplankton (copepods) handle the simultaneous threats of predators and ultraviolet (UV) radiation and whether they respond with changes in pigmentation, vertical migration, or both. We found weak vertical migration among copepods in response to UV stress, and this response was not apparently influenced by predation risk. Exposure to high levels of UV radiation caused copepods to retain pigments in the absence of a predation threat. When exposed to predation threat, they reduced their pigmentation regardless of UV level. Thus, they ranked predation as a threat more severe than UV radiation. Reducing the protective pigment level in response to predation in a situation in which UV radiation is high may, however, lead to higher mortality.
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22.
  • Jonsson, Per R., 1957, et al. (author)
  • Particle depletion above experimental bivalve beds: In situ measurements and numerical modeling of bivalve filtration in the boundary layer
  • 2005
  • In: Limnology and Oceanography. - 0024-3590. ; 50:6, s. 1989-1998
  • Journal article (peer-reviewed)abstract
    • Suspension feeders may deplete the near-bed layer of food particles, limiting growth of downstream, individuals. In a field experiment, we examined food depletion above a bed with bivalves (Cerastoderma edule) compared to beds devoid of Suspension feeders and how depletion depended on boundary-layer flow. Water above the test plots was sampled with an array of artificial siphons mimicking bivalve inhalant flow. Along the 3-m, bed with bivalves, chlorophyll a (Chl a) in the near-bed layer was depleted by 5-30%. Contrary to expectations from turbulent mixing, Chl a depletion increased with friction velocity. To explore the possibility that the bending of the exhalant jet in a strong boundary-layer flow Could lead to this depletion, we studied (lie advection and turbulent diffusion of exhalant water by the injection Of fluorescent dye through artificial siphons. The plume of fluorescent dye indicated that the interaction between the exhalant jet and horizontal water flow strongly affected the near-bed mixing of depleted water. At high ratios between jet and friction velocities (VR), the vertical momentum of the exhalant jet reduced the proportion of exhalant water reaching downstream neighbors. A hydrodynamic model incorporating inhalant and exhalant flows in the boundary layer predicted that exhalant jet flow lines reach the bed immediately downstream when the VR ratio is <20, potentially increasing refiltration at higher flow speeds due to jet bending. However, the model could not reproduce the observed increase in refiltration with increasing, friction velocity in simulations of aggregated filtration in a bed of bivalves.
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23.
  • Jäger, Christoph G., et al. (author)
  • Influence of water-column depth and mixing on phytoplankton biomass, community composition, and nutrients
  • 2008
  • In: Limnology and Oceanography. - 0024-3590 .- 1939-5590. ; 53:6, s. 2361-2373
  • Journal article (peer-reviewed)abstract
    • We independently manipulated mixing intensity (strong artificial mixing vs. background turbulence) andwater-column depth (2 m, 4 m, 8 m, and 12 m) in order to explore their separate and combined effects in a fieldenclosure experiment. To accentuate the vertical light gradient, enclosures had black walls, resulting in a euphoticdepth of only 3.7 m. All enclosures were placed in a well-mixed water bath to equalize temperature acrosstreatments. Phytoplankton responded to an initial phosphorus pulse with a transient increase in biomass, whichwas highest in the shallowest, least light-limited water columns where dissolved mineral phosphorus subsequentlybecame strongly limiting. As a consequence, the depth-averaged mineral phosphorus concentration increased andthe seston carbon (C) : phosphorous (P) ratio decreased with increasing water-column depth. Low turbulenceenclosures became quickly dominated by motile taxa (flagellates) in the upper water column, whereas mixedenclosures became gradually dominated by pennate diatoms, which resulted in higher average sedimentation ratesin the mixed enclosures over the 35-d experimental period. Low turbulence enclosures showed pronouncedvertical structure in water columns .4 m, where diversity was higher than in mixed enclosures, suggesting verticalniche partitioning. This interpretation is supported by a primary production assay, where phytoplanktonoriginating from different water depths in low-turbulence treatments had the relatively highest primaryproductivity when incubated at their respective depths of origin.
  •  
24.
  • Karlson, Agnes, 1981-, et al. (author)
  • Incorporation and burial of carbon from settling cyanobacterial blooms by deposit-feeding macrofauna
  • 2008
  • In: Limnology and Oceanography. - : American Society of Limnology and Oceanography, Inc.. - 0024-3590 .- 1939-5590. ; 53:6, s. 2754-2758
  • Journal article (peer-reviewed)abstract
    • Summer blooms of filamentous, nitrogen-fixing cyanobacteria are typical of the Baltic Sea, and recent findings indicate that cyanobacteria may be an important food source for the benthos below the euphotic zone. In a 2-week laboratory experiment, we measured incorporation of cyanobacterial carbon by the deposit-feeding amphipod Monoporeia affinis when exposed to 14C-radiolabeled, natural blooms of cyanobacteria dominated by either the toxic Nodularia spumigena or non-toxic Aphanizomenon sp. Carbon from both cyanobacterial blooms was used, with greater incorporation from Aphanizomenon-dominated bloom material than from N. spumigena, indicating that the latter is less suitable as food. However, neither cyanobacterium supported significant amphipod growth. Also, less cyanobacterial carbon was mixed down in the sediment in the N. spumigena treatment, indicating lower bioturbation activity in this treatment. Long-term effects on feeding and survival remain to be studied, especially for the toxic N. spumigena.
  •  
25.
  • Karlsson, Jan, 1969-, et al. (author)
  • Winter respiration of allochthonous and autochthonous organic carbon in a subarctic clear-water lake
  • 2008
  • In: Limnology and Oceanography. - : American Society of Limnology and Oceanography. - 0024-3590 .- 1939-5590. ; 53:3, s. 948-954
  • Journal article (peer-reviewed)abstract
    • We studied a small subarctic lake to assess the magnitude of winter respiration and the organic carbon (OC) source for this respiration. The concentration and stable isotopic composition (d13C) of dissolved inorganic carbon (DIC) accumulating in the lake water under ice was analyzed over one winter (7 months). The DIC concentration increased and the d13C of DIC decreased over time, with the greatest changes at the lake bottom. Winter respiration was 26% of annual respiration in the lake. Keeling plot analysis demonstrated that the d13C of respired DIC varied spatially, high d13C values occurring at shallow (2.5 m, 21.7‰) compared with intermediate (4 m, 25.1‰) and deep (6 m, 27.8‰) locations in the lake. The variation in the d13C of respired DIC was related to the variation in the d13C of the sediments between locations, suggesting that sediment OC supported much of the winter respiration and that the dominant OC source for respiration was OC from benthic algae at shallow locations and settled OC, of predominately terrestrial origin, at deep locations. The respiration of OC from benthic algae constituted 55% of the winter respiration, equaling 54% of the primary production by benthic algae the previous summer. The study indicates the importance of temporal and spatial variation in respiration for the metabolism and net DIC production in unproductive high-latitude lakes; both allochthonous and autochthonous carbon can contribute to winter DIC accumulation and, consequently, to spring CO2 emissions from lakes.
  •  
26.
  • Kisand, Veljo, et al. (author)
  • Bacterial freshwater species successfully immigrate to the brackish water environment in the northern Baltic
  • 2005
  • In: Limnology and Oceanography. - : American Society of Limnology and Oceanography. - 0024-3590 .- 1939-5590. ; 50:3, s. 945-956
  • Journal article (peer-reviewed)abstract
    • We studied the distribution and seasonal dynamics of five species from the genus Flavobacterium and one species from the genus Marinomonas over the course of a year along a northern Baltic Sea river-marine transect. All of the species had been previously demonstrated as important consumers of riverine dissolved organic carbon. Quantitative DNA-DNA hybridization data showed that two of the Flavobacterium spp. and the Marinomonas sp. had highest abundance in the river water (maximum 20,000 cells ml-1), with maximum relative abundance of 0.5-2.5% of the bacterial community. These species declined in abundance from the river to the estuary and the offshore site. Abundance and dynamics in the estuarine environment suggested successful immigration of freshwater bacteria, accompanied by growth in the brackish water environment. Two of the three abundant species showed high cell numbers also during late autumn to early spring in the estuary, indicating a selective advantage when riverine dissolved organic carbon was the main carbon source. The remaining three species showed more episodic abundance close to the detection limit of the method, providing weaker evidence of occurrence in the freshwater environment. Some bacterioplankton consuming riverine organic carbon in the brackish water environment in the northern Baltic are therefore freshwater species, with a selective advantage during winter.
  •  
27.
  •  
28.
  • Langenheder, Silke, et al. (author)
  • Weak coupling between community composition and functioning of aquatic bacteria
  • 2005
  • In: Limnology and Oceanography. - : Wiley. - 0024-3590 .- 1939-5590. ; 50:3, s. 957-967
  • Journal article (peer-reviewed)abstract
    • We performed a batch culture experiment with a factorial design in which sterile water from four lakes and bacterial assemblages (size-fractionated lake water) from the same lakes were set up in all possible combinations. The functional performance (biomass yield, respiration, growth rates, and growth efficiency) of bacterial communities growing in the cultures depended primarily on the type of the medium and to a much lesser extent on the origin of the bacterial assemblage. Functional changes were only partly paralleled by changes in community composition, as indicated by terminal restriction fragment length polymorphism analysis. Similar bacterial communities developed in different cultures as a result of receiving either the same medium or the same inoculum, indicating that bacterial communities are comprised of populations of generalists that can grow under most conditions as well as populations with the life strategy of specialists. However, bacteria originating from a slightly acidic polyhumic lake failed to grow, grew unsteadily, or exhibited an extended lag phase when exposed to media originating from other lakes, indicating that the bacterial community in the polyhumic lake was not able to adapt rapidly to changes in environmental conditions.
  •  
29.
  •  
30.
  • Leavitt, Peter R., et al. (author)
  • Paleolimnological evidence of the effects on lakes of energy and mass transfer from climate and humans
  • 2009
  • In: Limnology and Oceanography. - 0024-3590 .- 1939-5590. ; 54:6, s. 2330-2348
  • Research review (peer-reviewed)abstract
    • The premise of this article is that climate effects on lakes can be quantified most effectively by the integration of process-oriented limnological studies with paleolimnological research, particularly when both disciplines operate within a common conceptual framework. To this end, the energy (E)-mass (m) flux framework (Em flux) is developed and applied to selected retrospective studies to demonstrate that climate variability regulates lake structure and function over diverse temporal and spatial scales through four main pathways: rapid direct transfer of E to the lake surface by irradiance, heat, and wind; slow indirect effects of E via changes in terrestrial development and subsequent m subsidies to lakes; direct influx of m as precipitation, particles, and solutes from the atmosphere; and indirect influx of water, suspended particles, and dissolved substances from the catchment. Sedimentary analyses are used to illustrate the unique effects of each pathway on lakes but suggest that interactions among mechanisms are complex and depend on the landscape position of lakes, catchment characteristics, the range of temporal variation of individual pathways, ontogenetic changes in lake basins, and the selective effects of humans on m transfers. In particular, preliminary synthesis suggests that m influx can overwhelm the direct effects of E transfer to lakes, especially when anthropogenic activities alter m subsidies from catchments.
  •  
31.
  • Naddafi, Rahmat, et al. (author)
  • Effects of the zebra mussel, an exotic freshwater species, on seston stoichiometry
  • 2008
  • In: Limnology and Oceanography. - : American Society of Limnology and Oceanography. - 0024-3590 .- 1939-5590. ; 53:5, s. 1973-1987
  • Journal article (peer-reviewed)abstract
    • We examined the effect of the zebra mussel, Dreissena polymorpha, an exotic species, on seston stoichiometry by conducting laboratory experiments in which we varied nutrient composition of seston and mussels over time. Zebra mussels altered the stoichiometry of seston through removal of particulate organic nutrients and changed the stoichiometry of the dissolved nutrient pool through nutrient excretion. Grazers had stronger effects on carbon : phosphorus (C : P) and nitrogen (N) : P ratios than on the C:N ratio of seston. Elemental residence time in tissue and high mass-specific nutrient excretion by small mussels caused small mussels to be more efficient nutrient recyclers than larger mussels. Zebra mussels reduced P availability through enhancing C: P and N: P molar ratios of seston during the period extending from June to August, when P was limited in the lake, and increased the C:N molar ratio of seston in June, when N was at the minimum level in the lake. C: P and N: P molar ratios for zebra mussel tissue were higher in August and somewhat in September than in all other months. N was retained more efficiently than P in Dreissena tissue. Nutrient mass-specific uptake rate was higher than excretion rate by zebra mussels.
  •  
32.
  • Nascimento, F.J.A., et al. (author)
  • Settling blooms of filamentous cyanobacteria as food for meiofauna assemblages.
  • 2008
  • In: Limnology and Oceanography. - 0024-3590 .- 1939-5590. ; 53:6, s. 2636-2643
  • Journal article (peer-reviewed)abstract
    • Summer blooms of filamentous nitrogen-fixing cyanobacteria in the Baltic Sea are normally dominated by Aphanizomenon sp. and the toxin-producing Nodularia spumigena. In a 2-week laboratory experiment, we followed the uptake by representative benthic meiofauna species of C-14-labeled organic carbon from blooms, each dominated by one of these cyanobacteria. Natural bloom material was collected and labeled by incubation with (NaHCO3)-C-14. Uptake of cyanobacterial carbon was recorded for the major meiofauna taxa living in the first-centimeter layer, namely ostracods, harpacticoids, and nematodes. The uptake rates were within the range found for diatoms in other studies, indicating that cyanobacteria may be an important food resource for the meiobenthos. The uptake of cyanobacterial carbon varied significantly among species, even within the same class. The ostracod Candona neglecta showed the highest uptake values, whereas two other ostracod species took up very little of the label. There was no significant difference in utilization of carbon from Aphanizomenon sp. and N. spumigena and no reduction in the abundance of the meiofaunal taxa analyzed compared to unexposed controls, indicating that Baltic meiofaunal assemblages in general experience no mortality when exposed to settled cyanobacteria, even the hepatotoxic N. spumigena.
  •  
33.
  • Rosen, P., et al. (author)
  • Effects of climate on organic carbon and the ratio of planktonic to benthic primary producers in a subarctic lake during the past 45 years
  • 2009
  • In: Limnology and Oceanography. - 0024-3590 .- 1939-5590. ; 54:5, s. 1723-1732
  • Journal article (peer-reviewed)abstract
    • The effects of climatic variables on lake-water total organic carbon (TOC) concentrations and benthic and pelagic primary producers during the past 45 yr were assessed using the sediment records of two subarctic lakes, one with mires and one without mires connected to the lake. The lake with a mire showed large and synchronous changes in the planktonic to benthic (P : B) ratio of diatoms and concentrations of TOC inferred from near-infrared spectroscopy. During periods of warm temperatures, high precipitation, and long ice-free conditions, we inferred high TOC in the lake, and the diatom community was dominated by planktonic species. The stable carbon isotopic (delta C-13) values of sediment organic matter were negatively correlated with inferred TOC concentration and P : B ratio. We suggest that the changes in TOC and P : B ratio were a result of changing climate, permafrost degradation, and related changes in the catchment. Terrestrial organic matter, by its strong effect on the penetration of light through the lake water, possibly affected the habitats available for benthic photosynthesis and thus the delta C-13 of the sediment organic matter. The large changes in recent times may also be because of unusually long ice-free periods, warmer temperatures, and other associated limnological changes. The lake with no mire next to the lake showed only minor changes in lake-water TOC during the same period and P : B ratio remained almost constant until the past 5 yr, when the P : B ratio increased rapidly. The observed changes in P : B ratio within this lake may be because of complex interactions of several climate-related variables
  •  
34.
  • Rosén, Peter, et al. (author)
  • Effects of climate on organic carbon and the ratio of planktonic to benthic primary producers in a subarctic lake during the past 45 years
  • 2009
  • In: Limnology and Oceanography. - : American Society of Limnology and Oceanography. - 0024-3590 .- 1939-5590. ; 54:5, s. 1723-1732
  • Journal article (peer-reviewed)abstract
    • The effects of climatic variables on lake-water total organic carbon (TOC) concentrations and benthic and pelagic primary producers during the past 45 yr were assessed using the sediment records of two subarctic lakes, one with mires and one without mires connected to the lake. The lake with a mire showed large and synchronous changes in the planktonic to benthic (P : B) ratio of diatoms and concentrations of TOC inferred from nearinfrared spectroscopy. During periods of warm temperatures, high precipitation, and long ice-free conditions, we inferred high TOC in the lake, and the diatom community was dominated by planktonic species. The stable carbon isotopic (d13C) values of sediment organic matter were negatively correlated with inferred TOC concentration and P :B ratio. We suggest that the changes in TOC and P : B ratio were a result of changing climate, permafrost degradation, and related changes in the catchment. Terrestrial organic matter, by its strong effect on the penetration of light through the lake water, possibly affected the habitats available for benthic photosynthesis and thus the d13C of the sediment organic matter. The large changes in recent times may also be because of unusually long ice-free periods, warmer temperatures, and other associated limnological changes. The lake with no mire next to the lake showed only minor changes in lake-water TOC during the same period and P :B ratio remained almost constant until the past 5 yr, when the P :B ratio increased rapidly. The observed changes in P :B ratio within this lake may be because of complex interactions of several climate-related variables.
  •  
35.
  • Sobek, Sebastian, et al. (author)
  • Patterns and regulation of dissolved organic carbon : An analysis of 7,500 widely distributed lakes
  • 2007
  • In: Limnology and Oceanography. - 0024-3590 .- 1939-5590. ; 52:3, s. 1208-1219
  • Journal article (peer-reviewed)abstract
    • Dissolved organic carbon (DOC) is a key parameter in lakes that can affect numerous features, including microbial metabolism, light climate, acidity, and primary production. In an attempt to understand the factors that regulate DOC in lakes, we assembled a large database (7,514 lakes from 6 continents) of DOC concentrations and other parameters that characterize the conditions in the lakes, the catchment, the soil, and the climate. DOC concentrations were in the range 0.1-332 mg L-1, and the median was 5.71 mg L-1. A partial least squares regression explained 48% of the variability in lake DOC and showed that altitude, mean annual runoff, and precipitation were negatively correlated with lake DOC, while conductivity, soil carbon density, and soil C:N ratio were positively related with lake DOC. A multiple linear regression using altitude, mean annual runoff, and soil carbon density as predictors explained 40% of the variability in lake DOC. While lake area and drainage ratio (catchment:lake area) were not correlated to lake DOC in the global data set, these two factors explained significant variation of the residuals of the multiple linear regression model in several regional subsets of data. These results suggest a hierarchical regulation of DOC in lakes, where climatic and topographic characteristics set the possible range of DOC concentrations of a certain region, and catchment and lake properties then regulate the DOC concentration in each individual lake.
  •  
36.
  • Tranvik, Lars J., et al. (author)
  • Lakes and reservoirs as regulators of carbon cycling and climate
  • 2009
  • In: Limnology and Oceanography. - : Wiley. - 0024-3590 .- 1939-5590. ; 54:6:2, s. 2298-2314
  • Research review (peer-reviewed)abstract
    • We explore the role of lakes in carbon cycling and global climate, examine the mechanisms influencing carbon pools and transformations in lakes, and discuss how the metabolism of carbon in the inland waters is likely to change in response to climate. Furthermore, we project changes as global climate change in the abundance and spatial distribution of lakes in the biosphere, and we revise the estimate for the global extent of carbon transformation in inland waters. This synthesis demonstrates that the global annual emissions of carbon dioxide from inland waters to the atmosphere are similar in magnitude to the carbon dioxide uptake by the oceans and that the global burial of organic carbon in inland water sediments exceeds organic carbon sequestration on the ocean floor. The role of inland waters in global carbon cycling and climate forcing may be changed by human activities, including construction of impoundments, which accumulate large amounts of carbon in sediments and emit large amounts of methane to the atmosphere. Methane emissions are also expected from lakes on melting permafrost. The synthesis presented here indicates that (1) inland waters constitute a significant component of the global carbon cycle, (2) their contribution to this cycle has significantly changed as a result of human activities, and (3) they will continue to change in response to future climate change causing decreased as well as increased abundance of lakes as well as increases in the number of aquatic impoundments.
  •  
37.
  • von Wachenfeldt, Eddie, et al. (author)
  • Linking allochthonous dissolved organic matter and boreal lake sediment carbon sequestration : The role of light-mediated flocculation
  • 2008
  • In: Limnology and Oceanography. - : Wiley. - 0024-3590 .- 1939-5590. ; 53:6, s. 2416-2426
  • Journal article (peer-reviewed)abstract
    • We measured flocculation of dissolved organic carbon (DOC) in the water from a humic lake (DOC = 14.9 mg C L-1) and from an adjacent mire (DOC = 25.7 mg C L-1), in in situ enclosure experiments with different light regimes. Light stimulated the formation of organic particles in both waters, and organic particle formation was observed at all incubation depths, even in the dark controls. Production of phytoplankton biomass was negligible, and allochthonous DOC was the most important precursor of the sinking particles. 8-22% and 25-60% of the loss of DOC in lake and mire water, respectively, could be accounted for by flocculation. Depth-integrated flocculation based on the enclosure experiments was 14.7 mg C m(-2) d(-1). Lake-water DOC concentration and water color has been increasing during the last decade, and sediment trap studies show that gross sedimentation of organic carbon also increased. Thus flocculation of allochthonous DOC, stimulated by light, constitutes a pathway for the sequestration of carbon in lake sediments.
  •  
38.
  • von Wachenfeldt, Eddie, et al. (author)
  • Microbially induced flocculation of allochthonous dissolved organic carbon in lakes
  • 2009
  • In: Limnology and Oceanography. - : Wiley. - 0024-3590 .- 1939-5590. ; 54:5, s. 1811-1818
  • Journal article (peer-reviewed)abstract
    • The flocculation of allochthonous dissolved organic carbon (DOC) in lakes was investigated in a series of laboratory experiments. Flocculation was enhanced by increased temperatures, and addition of glucose increased the flocculation further. A change in pH within the range of 3.3 to 7.3 resulted in decreased flocculation with increasing deviation from the original pH. Flocculation was similar under oxic and anoxic conditions. For all treatments (i.e., temperature and glucose, pH, and O2 regime), occulation was positively correlated to bacterial respiration. However, bacterial biomass made up a negligible fraction of the formed flocs, suggesting that the formation of detrital particles was a result of bacterial activity, but there was no significant contribution of bacteria to the mass of the particles formed. In all experiments, both the concentration of DOC and concentration of colored dissolved organic matter (CDOM) decreased concomitantly with flocculation, suggesting that CDOM is the precursor of the flocs. Bacteria mediate a translocation of DOC in the water column into particles prone to gravitational settling.
  •  
39.
  • Vuorio, K., et al. (author)
  • Natural isotopic composition of carbon (delta C-13) correlates with colony size in the planktonic cyanobacterium Gloeotrichia echinulata
  • 2009
  • In: Limnology and Oceanography. - : Wiley. - 0024-3590 .- 1939-5590. ; 54:3, s. 925-929
  • Journal article (peer-reviewed)abstract
    • To assess variability in carbon isotope signatures (delta C-13) between and within populations under natural conditions, with a particular emphasis on colony size, we repeatedly collected planktonic colonies of a freshwater cyanobacterium Gloeotrichia echinulata in two lakes, Pyhajarvi (southwest Finland) and Erken (southeast Sweden). Despite substantial differences in the average delta C-13 signature of Gloeotrichia between lakes (-6.9 parts per thousand in Pyhajarvi and -20.7 parts per thousand in Erken), a similar, systematic increase in delta C-13 with colony size was observed in both lakes (of 2-3 parts per thousand in Pyhajarvi and 3-5 parts per thousand in Erken). This suggests declining isotope fractionation with increasing colony size, probably related to diffusion limitation of carbon availability. Temporal variation explained a minor fraction of total subsample variability (range delta C-13 similar to 4 parts per thousand in Pyhajarvi and similar to 6 parts per thousand in Erken). Isotopic C-13 fractionation in Gloeotrichia was likely affected both by carbon source and by colony size.
  •  
40.
  • Waite, A. M., et al. (author)
  • Linking ecosystem dynamics and biogeochemistry: Sinking fractionation of organic carbon in a Swedish fjord
  • 2005
  • In: Limnology and Oceanography. - 0024-3590. ; 50:2, s. 658-671
  • Journal article (peer-reviewed)abstract
    • We studied the growth and sedimentation of a phytoplankton bloom in the Gullmar Fjord in spring 2001. Sinking fractionation, as measured by differential sinking of components having varied carbon isotope signals, was an important process in determining carbon fluxes within this complex coastal ecosystem. Large, rapidly sinking diatoms aggregated with their own carbon exudates. This exudate material was the single largest contributor to vertical carbon flux in the fjord over the study period, and amounted to an order of magnitude greater carbon flux than the live biomass reaching the sediment traps. We estimate that diatoms' primary production contributed 75% of the total integrated production, similar to 100% of new production and similar to 100% of the total sedimentary flux during the primary sedimentation event, despite the fact that the surface bloom biomass was dominated by high concentrations of the flagellate Chattonella sp. Diatoms carried a heavy carbon isotope signature (delta(13)C = -19 parts per thousand) to depth; this moved downward as a layer through the water column, distinct from other particular organic carbon (POC) at the surface having a very light signature (-23 to -26 parts per thousand). This light delta(13)C signature at the surface coincided with a surface peak in particulate and dissolved organic carbon (POC/DOC) and transparent exopolymer particles (TEP), and material carrying the low delta(11)C signature had a negligible sinking rate. This distinctive surface signature probably contained small POC typical of the microbial loop and its products, a component of which was the very low -32 parts per thousand of the organic colloids. Measurable aggregation of these colloids occurred, and they may have contributed up to -20% of the vertical flux very early in the study. Sinking fractionation of POC caused isotope composition shifts on the order of 3-7 parts per thousand.
  •  
41.
  • Weyhenmeyer, Gesa A., et al. (author)
  • Nitrate-depleted conditions on the increase in shallow northern European lakes
  • 2007
  • In: Limnology and Oceanography. - 0024-3590 .- 1939-5590. ; 52:4, s. 1346-1353
  • Journal article (peer-reviewed)abstract
    • We determined relative nitrate-nitrogen (NO3- N) loss rates in 100 north-mid-European lakes from late spring to summer by using the exponential function N-2 5 N-1e(-k)( (t)(2) - (t)(2)), where N-1 and N-2 are NO3- N concentrations at the beginning (t(1)) and the end (t(2)) of the time interval, respectively, and k is the specific NO3- N loss rate. We found that k decreased with increasing lake depth. Adjusting k to the lake depth (k(adj)), we observed that k(adj) was positively related to spring NO3-N concentrations, but this relationship became insignificant at mean lake depths exceeding 12.5 m. A relationship between k(adj) and spring NO3- N concentrations in lakes shallower than 12.5 m implies that changes in spring NO3-N concentrations influence the NO3- N loss rate and thereby summer NO3- N concentrations. Time series from one Estonian, one German, and 14 Swedish lakes shallower than 12.5 m since 1988 revealed that May to August NO3-N concentrations have decreased over time everywhere, and the number of time periods exhibiting a NO3-N depleted condition, i.e., NO3-N levels below 10 mu g L-1, in these lakes has tripled since 1988. We explained the decreasing NO3-N concentrations by a reduction in external nitrogen loading including atmospheric deposition, and by changes in climate. The observed prolongation of NO3- N depleted conditions might be one possible explanation for the increasing occurrence of nitrogen- fixing cyanobacteria in a variety of lake ecosystems.
  •  
42.
  • Weyhenmeyer, Gesa A., et al. (author)
  • Nonlinear response of dissolved organic carbon concentrations in boreal lakes to increasing temperatures
  • 2009
  • In: Limnology and Oceanography. - : Wiley. - 0024-3590 .- 1939-5590. ; 54:6, part 2, s. 2513-2519
  • Journal article (peer-reviewed)abstract
    •  Recent increases in concentrations of dissolved organic carbon (DOC) in lakes and rivers over large regions have been related to both changes in the climate and in atmospheric deposition chemistry. Using a data set of 1041 boreal lakes along a 13° latitudinal gradient, sampled in 1995, 2000, and 2005, and an additional data set of 90 lakes along a 1000-m altitudinal gradient at 68°N, we show that DOC concentrations increase in a nonlinear way along a latitudinal and altitudinal temperature gradient. The nonlinear relation of DOC to increasing temperatures was consistent over space and time. Out of 14 meteorological, catchment, morphometric, and atmospheric deposition variables tested, the variable best explaining this kind of nonlinear pattern was the number of days when air temperatures exceeded 0°C, i.e., the duration of the main growing and runoff season (DT>0). Using DT>0 as an input variable, we were able to predict the nonlinear temperature response of DOC concentrations, both spatially (R2 = 0.90, p < 0.0001) and temporally (R2 = 0.90, p < 0.0001). DT>0 has an advantage over other variables because it includes the time factor, which is decisive for the duration that biogeochemical processes can take place. We suggest that DOC concentrations in lakes are influenced by climate change and that present temperature increases over Sweden result in an accelerated DOC increase toward warmer geographical regions. 
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