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.

目前关于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十分有限。我们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的山地湖泊中开展了原位富集生物测定（in situ enrichment bioassays），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，本次9个生物测定湖泊中的浮游植物均受氮限制，或受氮与磷共同限制。我们共鉴定出20种对氮富集产生响应的浮游植物分类群，并针对其中18类群利用莫诺半饱和常数（Monod half-saturation constants, K_s）估算了其响应阈值。在氮限制湖泊中，叶绿素a（chlorophyll a）出现任何提升时的DIN阈值为13 μg N·l⁻¹，而当叶绿素a提升幅度超出年度间典型波动范围时，DIN阈值为25 μg N·l⁻¹。多数对氮响应的分类群的K_s值介于0.02~77 μg N·l⁻¹之间，且硅藻（diatom）的K_s值高于此前在美国落基山湖泊中测得的数值。该公园山地湖泊中约75%的采样点夏季DIN浓度低于生物量响应阈值。这一结果表明，公园山地湖泊中的浮游植物很可能对未来由沉降诱导的湖泊氮富集较为敏感。