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  • 1. Alling, Vanja
    et al.
    Humborg, Christoph
    Morth, Carl-Magnus
    Rahm, Lars
    Pollehne, Falk
    Tracing terrestrial organic matter by delta(34)S and delta(13)C signatures in a subarctic estuary2008In: Limnology and Oceanography, ISSN 0024-3590, E-ISSN 1939-5590, Vol. 53, no 6, p. 2594-2602Article in journal (Refereed)
    Abstract [en]

    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(34)S and delta(13)C 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(34)S(DOS)) have twice the range between terrestrial and marine end members compared to the stable isotope signatures for dissolved organic carbon (delta(13)C(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 (DOC(ter)) from both delta(13)C(DOC) and delta(34)S(DOS) signatures and applying fixed C: S ratios for riverine and marine end members to convert S isotope signatures into DOC concentrations. The delta(34)S(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 (DOC(ter)) in these basins. Comparing the fractions of DOC(ter) 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 DOC(ter) in the Gulf of Bothnia is much shorter than the hydraulic turnover time, suggesting that high-latitude estuaries might be efficient sinks for DOC(ter).

  • 2. Berggren, M.
    et al.
    Bengtson, P.
    Soares, A. R. A.
    Karlsson, J.
    Terrestrial support of zooplankton biomass in northern rivers2018In: Limnology and Oceanography, ISSN 0024-3590, E-ISSN 1939-5590, Vol. 63, no 6, p. 2479-2492Article in journal (Refereed)
    Abstract [en]

    Abstract The contribution of terrestrially derived carbon to micro-crustacean zooplankton biomass (i.e., allochthony) has been previously studied in lakes, reservoirs, and estuaries, but little is known about zooplankton allochthony in rivers. In lacustrine environments, allochthony is regulated by distinct selective feeding behavior of different taxa. However, we hypothesized that restricted possibility for selective grazing in turbulent environments such as rivers would decouple zooplankton from specific microbial and algal food resources, such that their allochthony would mirror the terrestrial contribution to the surrounding bulk particle pool. We tested this idea by analyzing allochthony in 13 widely distributed Swedish rivers, using a dual-isotope mixing model. Zooplankton biomasses were generally low, and allochthony in different micro-crustacean groups (Cladocera, Cyclopoida, Calanoida) varied from 2% to 77%. As predicted, there were no correlations between allochthony and variables indicating the supply of algal and microbial food resources, such as chlorophyll a and bacterial production. Instead, the allochthony was generally similar to the share allochthonous contribution in bulk particulate organic matter, with relationships close to the 1 : 1 line. The zooplankton community allochthony was strongly regulated by the ecosystem metabolic balance between production and respiration, which in turn was dependent upon the ratio between total autochthonous organic carbon concentrations and water color. Our study for the first time shows that micro-crustacean allochthony is regulated differently in rivers compared to in lacustrine systems, and points to inefficient support of zooplankton biomass by algal resources in turbulent waters.

  • 3.
    Jansson, Mats
    et al.
    Umeå universitet, Institutionen för ekologi, miljö och geovetenskap.
    Karlsson, Jan
    Umeå universitet, Institutionen för ekologi, miljö och geovetenskap.
    Blomqvist, P
    Allochthonous organic carbon decreases pelagic energy mobilization in lakes2003In: Limnology and Oceanography, ISSN 0024-3590, E-ISSN 1939-5590, Vol. 48, no 4, p. 1711-1716Article in journal (Refereed)
    Abstract [en]

    Over the past decade, it has been Shown that unproductive lakes worldwide are net heterotrophic because bacterial respiration of allochthonous. organic carbon (AOC) makes community respiration exceed primary production. Net heterotrophy means that aquatic systems are net sources of CO2 to the atmosphere but also that bacterial utilization of AOC increases bacterioplankton production (BP) and bacterial uptake of limiting inorganic nutrients at the expense of phytoplankton production (PP). We studied 15 unproductive lakes in northern Sweden with dissolved organic carbon concentrations between 3 and 22 mg L-1. We found a highly significant negative relationship between the degree of heterotrophy and total pelagic energy mobilization (PP + BP based on AOC) per unit of limiting nutrient. We suggest that this is because the high cell phosphorous (P) requirement of bacteria makes energy mobilization per P unit considerably lower in bacterioplankton than in phytoplankton. We also suggest that the productivity of the entire pelagic ecosystem is determined by the availability of inorganic nutrients and AOC and by whether nutrients are allocated to BP or PP.

  • 4. Karlsson, J
    et al.
    Jonsson, A
    Meili, M
    Jansson, M
    Control of zooplankton dependence on allochthonous organic carbon in humic and clear-water lakes in northern Sweden2003In: Limnology and Oceanography, ISSN 0024-3590, E-ISSN 1939-5590, Vol. 48, no 1, p. 269-276Article in journal (Refereed)
    Abstract [en]

    We compared the stable carbon isotopic composition (delta(13)C) of crustacean zooplankton with that of potential carbon sources in 15 lakes in northern Sweden with different dissolved organic carbon (DOC) concentrations (2-9 mg L-1) to test the hypothesis that zooplankton depended more on allochthonous carbon in humic lakes than in clear-water lakes. Based on delta(13)C signature, we found that the pool of organic matter in the lakes was dominated by carbon of allochthonous origin over the whole DOC gradient. Zooplankton were generally depleted in C-13 compared to organic matter in the catchment, particulate organic matter in the lake water, and shallow surface sediment. However, the isotopic composition of zooplankton could not be explained without a significant contribution from both allochthonous and autochthonous carbon sources in all lakes. The relative importance of these two carbon sources did not relate to the concentration of, or proportion between, allochthonous and autochthonous organic carbon in the water. Instead, the proportion between allochthonous and autochthonous carbon in the crustacean zooplankton was consistent with a rather conservative use of the energy mobilized by bacterioplankton and phytoplankton in the lakes.

  • 5. Rosén, Peter
    et al.
    Cunningham, Laura
    Vonk, Jorien
    Karlsson, Jan
    Effects of climate on organic carbon and the ratio of planktonic to benthic primary producers in a subarctic lake during the past 45 years2009In: Limnology and Oceanography, ISSN 0024-3590, E-ISSN 1939-5590, Vol. 54, no 5, p. 1723-1732Article in journal (Refereed)
    Abstract [en]

    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 (Δ13C) 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 Δ13C 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.

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