Norwegian Institute for Water Research, Gaustadalléen 21, 0349 Oslo, Norway.
Department of Bioscience, University of Oslo, Blindernveien 31, 0371 Oslo, Norway.
Sci Total Environ. 2019 Jul 15;674:9-18. doi: 10.1016/j.scitotenv.2019.03.446. Epub 2019 Mar 29.
Increases in terrestrial organic matter (tOM) transport from catchments to boreal lakes can affect methylmercury (MeHg) accumulation in aquatic biota both directly by increasing concentrations of aqueous MeHg, and indirectly through effects on MeHg bioavailability and on energy pathways in the lower food web. We carried out a detailed seasonal study of water chemistry, zooplankton diet, and MeHg accumulation in zooplankton in two lakes with contrasting tOM concentrations. Between-lake differences explained 51% of the variability in our water chemistry data, with no observed effect of season or sampling depth, contrary to our expectations. Higher tOM was correlated with higher aqueous Hg concentrations, lower areal pelagic primary productivity, and an increased contribution of terrestrial particles to pelagic particulate organic matter. Based on dietary marker analysis (δC, δN, and fatty acid [FA] composition), zooplankton diet was strongly linked to feeding mechanism, with dietary reliance on phytoplankton highest in the selective-feeding calanoid copepods, and lowest in filter feeding cladocerans. Zooplankton dietary reliance on phytoplankton and their concentrations of high-quality lipids, including polyunsaturated fatty acids, were higher in the clear-water lake than in the brown-water lake, where bacterial and terrestrial food sources were more prevalent. MeHg was highest in zooplankton from the brown-water lake, with highest concentrations in the 200-500 μm zooplankton size fraction for both lakes. Contrary to our expectations, there was no effect of season on zooplankton dietary markers or MeHg. Our results suggest that, overall, higher tOM results in higher MeHg concentrations in water and zooplankton, and reduces zooplankton dietary reliance on phytoplankton. Increased tOM thus leads to a decrease in the nutritional quality of zooplankton (i.e. higher MeHg concentrations, and lower concentrations of essential fatty acids), which may cascade up the food web with negative implications for higher trophic levels.
陆地有机质（tOM）向北方湖泊的输入增加会直接通过增加水相甲基汞（MeHg）浓度，以及间接通过影响 MeHg 的生物可利用性和底层食物网中的能量途径，影响水生生物群中甲基汞的积累。我们对两个具有不同 tOM 浓度的湖泊进行了水化学、浮游动物饮食和浮游动物中 MeHg 积累的详细季节性研究。湖泊间差异解释了我们水化学数据变异性的 51%，而季节或采样深度没有观察到影响，与我们的预期相反。较高的 tOM 与较高的水相 Hg 浓度、较低的面状浮游初级生产力以及更多的陆地颗粒对浮游颗粒有机物质的贡献有关。基于饮食标记分析（δC、δN 和脂肪酸 [FA] 组成），浮游动物的饮食与摄食机制密切相关，选择性摄食的哲水蚤对浮游植物的饮食依赖度最高，而滤食性的枝角类动物则最低。浮游动物对浮游植物的饮食依赖度及其高质量脂质（包括多不饱和脂肪酸）的浓度在清水湖中均高于在棕色水中，而棕色水中的细菌和陆地食物来源更为普遍。来自棕色水的浮游动物中 MeHg 含量最高，两个湖泊的 200-500 μm 浮游动物大小组的浓度最高。与我们的预期相反，季节对浮游动物饮食标记物或 MeHg 没有影响。我们的结果表明，总体而言，较高的 tOM 会导致水中和浮游动物中的 MeHg 浓度升高，并降低浮游动物对浮游植物的饮食依赖。增加的 tOM 会导致浮游动物营养质量下降（即 MeHg 浓度升高，必需脂肪酸浓度降低），这可能会对食物链产生负面影响，从而对更高营养级产生负面影响。
