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  • 1. Bergström, Ann-Kristin
    et al.
    Jonsson, Anders
    Isles, Peter D. F.
    Creed, Irena F.
    Lau, Danny C. P.
    Changes in nutritional quality and nutrient limitation regimes of phytoplankton in response to declining N deposition in mountain lakes2020Ingår i: Aquatic Sciences, ISSN 1015-1621, E-ISSN 1420-9055, Vol. 82, nr 2, artikel-id 31Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Phytoplankton play a key role in supporting aquatic food webs. However, the effects of ongoing large-scale changes in the concentrations and stoichiometry of important biological compounds [dissolved inorganic N (DIN), total phosphorus (TP), dissolved organic carbon (DOC) and DIN:TP] on the development and nutritional quality of phytoplankton for higher trophic levels are unclear. We conducted lake studies and in situ bioassay experiments in two Swedish mountain regions [Abisko (north) and Jämtland (south)] with different N deposition and where lakes in each region were distributed along a similar gradient in lake DOC (2–7 mg L−1) to assess whether differences in nutrients, DOC and DIN:TP induced differences in phytoplankton quantity [chlorophyll a (Chl-a) and seston carbon (C)] and quality [seston C:N:P stoichiometry and fatty acid (FA) composition]. Using long-term monitoring data from lakes in these two mountain regions, we found declining long-term trends in N deposition and lake DIN and total TP concentrations, but not in lake DIN:TP. Lakes in Abisko received lower N deposition and had lower DIN:TP than those in Jämtland. Phytoplankton was N- to NP-limited in Abisko lakes but NP dual-limited in Jämtland lakes. The N fertilization effects induced by higher DIN:TP were weak on phytoplankton quantity but strong on phytoplankton quality. The phytoplankton had lower eicosapentaenoic acid (EPA) content and higher P content (lower seston C:P) in Abisko compared to in Jämtland. In addition, the quality of the DOC (as indicated by its aromaticity and SUVA) influenced not only the light conditions and the seston C:P ratios, but also the FA composition. We found higher bacteria FA concentrations in seston in Abisko than in Jämtland, despite lower amounts of FA of terrestrial origin in Abisko. Our findings suggest that declining N deposition and enhanced colored terrestrial C loadings leads to lower nutritional quality of basal resources for higher consumers in mountain lakes.

  • 2. Bergström, Ann-Kristin
    et al.
    Karlsson, Jan
    Karlsson, Daniel
    Vrede, Tobias
    Contrasting plankton stoichiometry and nutrient regeneration in northern arctic and boreal lakes2018Ingår i: Aquatic Sciences, ISSN 1015-1621, E-ISSN 1420-9055, Vol. 80, nr 24Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Contrasting carbon: nitrogen: phosphorus (C: N: P) stoichiometry between phytoplankton and zooplankton affect consumer growth and phytoplankton nutrient limitation via nutrient recycling by zooplankton. However, no study has assessed how regional differences in terrestrial loadings of organic matter affect plankton N: P stoichiometry and recycling in systems with low N deposition and N-limited phytoplankton. We address this question by using data from 14 unproductive headwater arctic and boreal lakes. We found that boreal lakes had higher lake water- and seston C, N and P concentrations than arctic lakes, whereas seston C: N, C: P and N: P ratios did not differ among regions. Boreal zooplankton were also richer in N and P relative to C, with lower somatic N: P ratios, compared to arctic lakes. Consequently, the estimated N: P imbalances between seston and zooplankton were negative in arctic lakes, indicating zooplankton feeding on phytoplankton of suboptimal N content, resulting in low consumer driven N: P recycling (medians arctic sub-mid and high altitude lakes: 11 and 13). In boreal lakes, estimated N: P imbalance did not differ from zero, with a seston N: P stoichiometry matching the N:P requirements of zooplankton, which resulted in higher consumer driven N: P recycling (median 18). Our results imply that regional climate induced catchment differences, through enhanced terrestrial nutrient inputs, affect plankton stoichiometry by raising consumer N: P recycling ratio and changing zooplankton from being mainly N- (arctic) to NP co-limited (boreal). Browning of lakes, in regions with low N deposition, may therefore promote large-scale regional changes in plankton nutrient limitation with potential feedbacks on pelagic food webs.

  • 3.
    Isles, Peter D. F.
    et al.
    Umeå universitet, Institutionen för ekologi, miljö och geovetenskap.
    Jonsson, Anders
    Umeå universitet, Institutionen för ekologi, miljö och geovetenskap.
    Creed, Irena F.
    Bergström, Ann-Kristin
    Umeå universitet, Institutionen för ekologi, miljö och geovetenskap.
    Does browning affect the identity of limiting nutrients in lakes?2020Ingår i: Aquatic Sciences, ISSN 1015-1621, E-ISSN 1420-9055, Vol. 82, nr 2, artikel-id 45Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Concentrations of dissolved organic carbon (DOC) have increased recently in many lakes at high latitudes in North America and Europe, but it is unclear what effect this will have on the identity of the limiting nutrient for phytoplankton [nitrogen (N) vs. phosphorus (P)]. Identifying the effect of changing DOC on phytoplankton nutrient limitation is complicated by spatial covariation between atmospheric N deposition and increasing DOC in areas where lake browning occurs. We conducted nutrient-limitation assays in 27 lakes from three sites along gradients of climate and atmospheric N deposition in Sweden. Within each site, lakes were selected to represent the range of DOC concentrations. We also conducted statistical analyses of large-scale lake survey data (n = 4768 lakes divided into 47 regions) to investigate relationships between DOC and nutrient stoichiometry while controlling for differences in N deposition. Our findings confirmed that most lakes were dual-limited by both N and P in the south, whereas northern lakes were primarily N-limited. Throughout Sweden the ratio of dissolved inorganic nitrogen (DIN) to total phosphorus (TP) declined with increasing DOC in most regions, suggesting that browner lakes are more likely to be N limited. These results were not supported by our nutrient limitation assays, which identified no relationship between DOC and relative strength of limitation by N or P. Increased DOC also resulted in significant increases in both total and inorganic N and P fractions, suggesting that other factors such as light limitation or increased top-down control become more important as DOC increases.

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