Data from: Phytoplankton responses to nitrogen enrichment in Pacific Northwest, USA mountain lakes

Limited information is available about threshold lake nitrogen concentrations necessary to stimulate phytoplankton species and biomass responses in remote nitrogen-limited mountain lakes. We conducted in situ enrichment bioassays in mountain lakes within Mount Rainier, North Cascades, and Olympic National Parks in Washington State, USA to characterize phytoplankton species and biomass responses to nitrogen enrichment, and associated dissolved inorganic nitrogen (DIN) concentration thresholds. Based on biomass and growth measurements, phytoplankton were nitrogen-limited or co-limited by nitrogen and phosphorus in the nine bioassay lakes. We identified 20 taxa that responded to nitrogen enrichment, and estimated response thresholds using nitrogen Monod half-saturation constants (K s) for 18 of these taxa. DIN thresholds in nitrogen-limited lakes were 13 μg N l−1 for any increase in chlorophyll a, and 25 μg N l−1, for an increase beyond typical inter-annual chlorophyll a variation. K s values ranged from 0.02 to 77 μg N l−1 across most N-responsive taxa, and diatom K s values were higher than those previously quantified in U.S. Rocky Mountain lakes. Approximately, 75% of sampled mountain lakes in the parks have summer dissolved inorganic nitrogen concentrations below biomass response thresholds. This finding suggests that phytoplankton in park mountain lakes are likely sensitive to future deposition-induced lake nitrogen enrichment.

目前针对偏远氮限制山地湖泊中，刺激浮游植物物种与生物量响应所需的湖泊氮浓度阈值的相关研究信息较为有限。本研究在美国华盛顿州雷尼尔山、北喀斯喀特与奥林匹克国家公园内的山地湖泊中开展原位富集生物测定实验，以表征浮游植物物种与生物量对氮富集的响应特征，以及对应的溶解态无机氮（dissolved inorganic nitrogen, DIN）浓度阈值。基于生物量与生长量测定结果，9个参与生物测定实验的湖泊中的浮游植物均受氮限制，或受氮与磷共同限制。本研究共鉴定出20个对氮富集产生响应的浮游植物类群，并针对其中18个类群，利用氮Monod半饱和常数（Monod half-saturation constants, K_s）估算了其响应阈值。在氮限制湖泊中，当叶绿素a（chlorophyll a）出现任意程度的提升时，DIN阈值为13 μg N·L⁻¹；若叶绿素a的提升幅度超出典型年际波动范围，则DIN阈值为25 μg N·L⁻¹。多数对氮响应的类群的K_s值介于0.02至77 μg N·L⁻¹之间，且硅藻的K_s值高于此前美国落基山地湖泊中测定得到的结果。该公园内约75%的采样山地湖泊夏季溶解态无机氮浓度低于生物量响应阈值。这一研究结果表明，公园山地湖泊中的浮游植物大概率对未来由大气沉降诱导的湖泊氮富集过程较为敏感。