State Key Laboratory of Lake Science and Environment, Nanjing Institute of Geography and Limnology, Chinese Academy of Sciences, Nanjing 210008, P. R. China.
State Key Laboratory of Lake Science and Environment, Nanjing Institute of Geography and Limnology, Chinese Academy of Sciences, Nanjing 210008, P. R. China.
Water Res. 2022 Jun 30;218:118493. doi: 10.1016/j.watres.2022.118493. Epub 2022 Apr 23.
Response of aquatic organisms to eutrophication have been well reported, while less studies are available for the recovery of eutrophic lakes following a reduction in the external loading, especially for systems where nitrogen is reduced but the phosphorus concentration is maintained high due to internal loading. Diatoms are nitrate (NO-N) opportunists but can also use ammonium (NH-N). They may, therefore, be more sensitive to nitrogen reduction than other algae that typically prefer NH-N. We document the variations of nutrients and diatoms in subtropical, eutrophic Lake Taihu over 28 yr during which a reduction of the external loading resulted from lake management. According to the results of change point analysis, data on environmental variables were divided into two periods (P1: 1992-2006; P2: 2007-2019) with two different seasons (WS: Winter-Spring; SA: Summer-Autumn), respectively. Compared with P1-WS, the concentration of NH-N decreased significantly whereas NO-N showed no significant change in P2-WS. In contrast, NH-N concentrations were low and showed no significant changes in P1-SA and P2-SA and NO-N decreased significantly in the latter period. Accordingly, NO-N: NH-N mass ratios in P1-SA and P2-WS were all significantly higher than those in P2-SA and P1-WS, respectively. The biomass of WS diatom increased significantly and the timing of the peak biomass shifted from P1-SA to P2-WS since 2007. The SEM analysis showed that NO-N was retained as a statistically significant predictor for diatom biomass in P1-SA and significant effects of windspeed, zooplankton and NH-N on diatom biomass in P2-WS. Windspeed and zooplankton have further changed the biomass of diatoms in the case of declining inorganic nitrogen. We conclude that the magnitude of vernal suppression or stimulation of diatom assemblages has increased, concomitant with the variations of NH-N and NO-N: NH-N mass ratios. Diatoms response to NH-N or NO-N is apparently changing in response to water temperature in this eutrophic shallow lake. Thus, parallel reductions in external nitrogen loading, along with variations in dominant inorganic nitrogen, will stimulate the growth of diatom and therefore increase the total biomass of phytoplankton in still high internal phosphorus loading, which is should be regarded as a good sign of restoration measures.
水生生物对富营养化的反应已有大量报道，而对于外部负荷减少后富营养湖泊的恢复，尤其是在由于内部负荷导致氮减少但磷浓度保持高位的系统中，相关研究则较少。硅藻是硝酸盐（NO-N）机会主义者，但也可以利用铵（NH-N）。因此，它们可能比其他通常更喜欢 NH-N 的藻类对氮减少更敏感。我们记录了亚热带富营养化太湖在 28 年期间的营养物质和硅藻的变化，在此期间，湖泊管理导致外部负荷减少。根据变化点分析的结果，环境变量的数据分别分为两个时期（P1：1992-2006；P2：2007-2019）和两个不同的季节（WS：冬春；SA：夏秋）。与 P1-WS 相比，P2-WS 中 NH-N 的浓度显著降低，而 NO-N 没有显著变化。相反，NH-N 浓度在 P1-SA 和 P2-SA 中较低且没有显著变化，而后者在 P2-SA 中显著降低。因此，P1-SA 和 P2-WS 中 NO-N:NH-N 质量比均显著高于 P2-SA 和 P1-WS。WS 硅藻的生物量显著增加，且峰值生物量的时间从 2007 年的 P1-SA 转移到 P2-WS。SEM 分析表明，NO-N 是 P1-SA 中硅藻生物量的一个具有统计学意义的预测因子，风速、浮游动物和 NH-N 对 P2-WS 中硅藻生物量有显著影响。在无机氮下降的情况下，风速和浮游动物进一步改变了硅藻的生物量。我们得出结论，随着 NH-N 和 NO-N:NH-N 质量比的变化，春季硅藻组合的抑制或刺激程度增加了。在这个富营养化的浅湖中，硅藻对 NH-N 或 NO-N 的反应显然随着水温的变化而变化。因此，外部氮负荷的平行减少，以及主要无机氮的变化，将刺激硅藻的生长，从而在仍然较高的内部磷负荷下增加浮游植物的总生物量，这应被视为恢复措施的一个好迹象。
