Department of Environmental Science and Engineering, Ewha Womans University, Seoul 03760, South Korea.
Department of Environmental Science and Engineering, Ewha Womans University, Seoul 03760, South Korea.
Water Res. 2021 Sep 15;203:117510. doi: 10.1016/j.watres.2021.117510. Epub 2021 Aug 3.
Long-term trends in riverine nutrient availability have rarely been linked to both phytoplankton composition and functioning. To explore how the changing availability of N and P affects not only phytoplankton abundance and composition but also the resource use efficiency of N, P, and CO, a 25-year time series of water quality in the lower Han River, Korea, was combined with additional measurements of riverine dissolved organic carbon (DOC) and CO. Despite persistent eutrophication, recent decreases in P relative to N have been steep in the lowest reach, increasing the annual mean mass ratio of N to P (N/P) from 24 (1994-2015) to 65 (2016-2018). While Chl a and cyanobacterial abundance exhibited overall positive and inverse relationships with P concentrations and N/P, respectively, severe harmful algal blooms (HABs) concurred with short-term increases in P and temperature. Microcystis often dominated HABs at low N/P that usually favors N-fixing cyanobacteria such as Anabaena. In the middle and lower reaches, phytoplanktonic P use efficiency was typically lower at low N/P. V-shaped relationships between N/P and CO concentrations, together with longitudinal upward shifts in the inverse relationship between Chl a and CO, implied that eutrophication-enhanced phytoplankton biomass could turn into a significant source of CO after passing a threshold. The combined results suggest that cyanobacterial dominance co-limited by P availability and temperature can lower planktonic P use efficiency, while enhancing riverine CO emissions at low N/P ratios.
长期以来，河流中营养物质的可利用性很少与浮游植物的组成和功能联系起来。为了探讨氮（N）和磷（P）的可利用性变化不仅如何影响浮游植物的丰度和组成，还如何影响 N、P 和 CO 的资源利用效率，本研究结合韩国汉江下游 25 年的水质数据以及对河流溶解有机碳（DOC）和 CO 的额外测量，对其进行了研究。尽管持续富营养化，但在最低流域，P 相对于 N 的减少幅度很大，导致每年 N 与 P 的平均质量比（N/P）从 24（1994-2015 年）增加到 65（2016-2018 年）。虽然 Chl a 和蓝藻的丰度与 P 浓度呈正相关，与 N/P 呈负相关，但严重的有害藻华（HAB）与 P 和温度的短期增加同时发生。低 N/P 时，微囊藻通常占 HAB 的主导地位，而 N/P 通常有利于固氮蓝藻，如鱼腥藻。在中下游，低 N/P 时浮游植物 P 利用效率通常较低。N/P 与 CO 浓度之间的 V 形关系，以及 Chl a 与 CO 之间的负相关关系的纵向向上转移，表明富营养化增强的浮游植物生物量在超过阈值后可能成为 CO 的重要来源。综合结果表明，P 可用性和温度共同限制下的蓝藻优势可以降低浮游植物的 P 利用效率，同时在低 N/P 比下增加河流 CO 的排放量。
