Department of Civil and Environmental Engineering, University of Wisconsin-Madison, Madison, Wisconsin, United States of America.
PLoS One. 2013;8(2):e56103. doi: 10.1371/journal.pone.0056103. Epub 2013 Feb 6.
Toxic cyanobacterial blooms threaten freshwaters worldwide but have proven difficult to predict because the mechanisms of bloom formation and toxin production are unknown, especially on weekly time scales. Water quality management continues to focus on aggregated metrics, such as chlorophyll and total nutrients, which may not be sufficient to explain complex community changes and functions such as toxin production. For example, nitrogen (N) speciation and cycling play an important role, on daily time scales, in shaping cyanobacterial communities because declining N has been shown to select for N fixers. In addition, subsequent N pulses from N(2) fixation may stimulate and sustain toxic cyanobacterial growth. Herein, we describe how rapid early summer declines in N followed by bursts of N fixation have shaped cyanobacterial communities in a eutrophic lake (Lake Mendota, Wisconsin, USA), possibly driving toxic Microcystis blooms throughout the growing season. On weekly time scales in 2010 and 2011, we monitored the cyanobacterial community in a eutrophic lake using the phycocyanin intergenic spacer (PC-IGS) region to determine population dynamics. In parallel, we measured microcystin concentrations, N(2) fixation rates, and potential environmental drivers that contribute to structuring the community. In both years, cyanobacterial community change was strongly correlated with dissolved inorganic nitrogen (DIN) concentrations, and Aphanizomenon and Microcystis alternated dominance throughout the pre-toxic, toxic, and post-toxic phases of the lake. Microcystin concentrations increased a few days after the first significant N(2) fixation rates were observed. Then, following large early summer N(2) fixation events, Microcystis increased and became most abundant. Maximum microcystin concentrations coincided with Microcystis dominance. In both years, DIN concentrations dropped again in late summer, and N(2) fixation rates and Aphanizomenon abundance increased before the lake mixed in the fall. Estimated N inputs from N(2) fixation were large enough to supplement, or even support, the toxic Microcystis blooms.
有毒蓝藻水华威胁着全球淡水，但由于形成水华和产生毒素的机制尚不清楚，尤其是在每周的时间尺度上，因此很难预测。水质管理继续侧重于综合指标，如叶绿素和总养分，这些指标可能不足以解释复杂的群落变化和毒素产生等功能。例如，氮（N）形态和循环在塑造蓝藻群落方面起着重要作用，尤其是在每日的时间尺度上，因为氮的减少已被证明会选择固氮生物。此外，随后来自 N2 固定的 N 脉冲可能会刺激和维持有毒蓝藻的生长。本文描述了初夏快速下降的氮随后爆发固氮如何塑造了一个富营养化湖泊（美国威斯康星州门登多湖）中的蓝藻群落，这可能导致整个生长季节的有毒微囊藻水华。在 2010 年和 2011 年的每周时间尺度上，我们使用藻蓝蛋白基因间 spacer（PC-IGS）区域监测富营养化湖泊中的蓝藻群落，以确定种群动态。同时，我们测量了微囊藻毒素浓度、N2 固定率以及可能有助于构建群落的潜在环境驱动因素。在这两年中，蓝藻群落的变化与溶解无机氮（DIN）浓度密切相关，在非毒性、毒性和毒性后阶段，鱼腥藻和微囊藻交替占主导地位。在首次观察到显著的 N2 固定率几天后，微囊藻毒素浓度增加。然后，在初夏大量的 N2 固定事件之后，微囊藻增加并成为最丰富的物种。最大的微囊藻毒素浓度与微囊藻的优势相一致。在这两年中，DIN 浓度在夏末再次下降，在秋季湖泊混合之前，N2 固定率和鱼腥藻的丰度增加。N2 固定估计的氮输入量足以补充，甚至支持有毒的微囊藻水华。
