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- Guo, Junwen, et al.
(författare)
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Organic vs. inorganic nitrogen as promoting nutrient in aquatic microplankton food webs
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- In aquatic microplankton food webs, the relative availability of dissolved inorganic nitrogen (DIN) and organic nitrogen (DON) can shape trophic interactions, food web structure and the stoichiometry of the organisms. To evaluate the importance of nitrogen forms, i.e., whether microplankton have access to organic or inorganic forms of nitrogen, we performed a short-term microcosm study of a coastal microplankton food web (organism size < 50 µm). In this experiment, the microplankton community was exposed to two different carbon (C) and nitrogen (N) sources: C and N were either associated in a single organic molecule or dissociated in two different molecules. The results showed that the different nitrogen forms had a strong impact on the food web composition, which resulted in different nitrogen food web use efficiency. The entire microplankton food web benefited from the association of C and N in a single DON molecule. The association or dissociation of C and N input had marked effects on all trophic levels, most probably through its effect on bacteria-phytoplankton interaction, which switched from mutualism to competition. Hence, the degree of association between N and C is an important factor to be considered in microplanktonic food web dynamics, at least for the short-term food web response.
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- Guo, Junwen, et al.
(författare)
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Stoichiometric changes in planktonic food web upon environmental fluctuations in northern oligotrophic coastal ecosystem
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- Climate-driven changes, e.g. increasing precipitation, will increase terrestrial runoff and freshwater inputs to coastal ecosystems in the future. Consequently, we expect that altered availability of inorganic and organic forms of carbon (C), nitrogen (N) and phosphorus (P) will lead to spatial and temporal variation in the elemental composition of coastal organisms. To identify the major spatiotemporal patterns in seston and zooplankton elemental (C, N and P) composition, we sampled four bays of northern Baltic Sea from May to September 2018. This sampling design covers spatial and temporal variation in freshwater input, as it includes the spring flood driven by snowmelt in May, and bays receiving varying catchment areas. We analyzed CNP composition for different size fractions of seston, and dominant zooplankton taxa. Our results showed that the stoichiometric composition of individual seston size fractions varied more through time than in space. The concentrations of dissolved organic carbon (DOC) and dissolved organic nitrogen (DON) in water were the major explanatory variables for seston stoichiometry. We also found that zooplankton stoichiometry was relatively similar between taxa within specific months, although the dominant zooplankton taxa differed over time and among sites. This could partly be explained by the similar stoichiometric requirements and homeostasis among zooplankton taxa. Our findings imply that the spatiotemporal variation in physicochemical characteristics of coastal ecosystems will alter the quality of seston and zooplankton. Yet, further investigations on the role of zooplankton in stoichiometry related nutrient cycling in the Baltic Sea coastal ecosystems are needed.
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- Skrobonja, Aleksandra, et al.
(författare)
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Multiple impacts of humic-rich dissolved organic carbon on methylmercury accumulation in heterotrophic pelagic food webs
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- In this study we investigate how humic-rich dissolved organic carbon (DOC) impacts the bioaccumulation of methylmercury (MeHg) in coastal pelagic lower food webs. Mesocosm scale ecosystems with four levels of humic-rich DOC (ranging from 4 mg L-1 – no added DOC - up to 8.0 mg L-1) were run for 36 days and the bioaccumulation factor (BAF) of MeHg was determined in four seston size fractions at the end of the experiment. The pelagic food web was highly heterotrophic in all treatments with 72%−88% bacteria production of the total basal production. Increases in humic-rich DOC in coastal areas, manifested in our study as a shift towards more heterotrophic systems, can impact the bioaccumulation of MeHg through multiple processes. We found an increase in dissolved MeHg concentration with increased DOC loading, indicating a shift in partitioning from the particulate to the dissolved phase. However, a separate experiment showed that the presence of the humic-rich DOC lowered the bioavailability of dissolved MeHg with 40% across our study range, supporting previous results that the amount of MeHg incorporated at the base of the food web is not simply determined by the total concentration of dissolved MeHg. Our determined BAFs from the experiment supported this conclusion and we also calculated a corrected BAF’ value that took into account the change in MeHg bioavailability to isolate the impact on biomagnification processes. The range in MeHg log BAF’ values was 4.5−4.7 and 5.8−6.0 for 20-50 μm and >200 μm seston size fractions, respectively. Higher BAF’ values were observed in the highest DOC treatment and in our reference for some seston size classes. The high BAF’ at the highest DOC level may be explained by the large fraction of bacteria production observed for this treatment. The high BAF in the reference system could be explained by a slightly larger proportion of energy going through the autotrophic food web causing a higher trophic transfer efficiency of MeHg at the low trophic levels in the web. Taken together our results demonstrate a complex impact of humic DOC and pelagic food web heterotrophy on MeHg bioaccumulation in seston with partly counteracting processes. A detailed understanding of the factors controlling these multiple processes are important for accurate predictions of the net impact on MeHg bioaccumulation in coastal food webs following different environmental change scenarios.
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