Abstract Boreal lakes typically have high levels of allochthonous dissolved organic matter (DOM), causing light limitation of photosynthetic CO2 consumption while stimulating CO2 production. They are therefore considered as important sources of atmospheric CO2. However, boreal lakes are also experiencing a marked expansion of bloom-forming motile phytoplankton organisms that can circumvent the shading effect of DOM by performing phototaxis and that thus might have an impact on the lakes’ CO2 balance. We tested this idea in a DOM-rich lake using the widespread raphidophyte flagellate Gonyostomum semen as model organism. Employing continuous field measurements, we found that G. semen can reduce the partial pressure of CO2 even at low algal densities. Periods with high algal densities were associated with CO2 undersaturation and invasion of atmospheric CO2. The mean daily net losses of CO2 to the atmosphere during and after a G. semen bloom were estimated at 12.9 and 70.4 mmol C m–2 day–1, respectively. G. semen caused steep pCO2 gradients in space and time, which make it difficult to unveil the species’ impact without employing continuous pCO2 vertical profiling. This suggests that the effect of G. semen on the CO2 balance of boreal lakes might have been overlooked in the past. Taken together, our data suggest that G. semen can significantly reduce the CO2 emissions from boreal lakes despite high concentrations of allochthonous DOM. G. semen and other motile phytoplankton species should therefore be considered when estimating CO2 emissions from boreal lakes, especially if these organisms continue their expansion.

中文翻译：

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Gonyostomum semen 等活动浮游植物物种可以显着减少北方湖泊的二氧化碳排放

摘要 北方湖泊通常具有高水平的外来溶解有机质 (DOM)，导致光合 CO2 消耗量受到光照限制，同时刺激 CO2 产生。因此，它们被认为是大气 CO2 的重要来源。然而，北方湖泊也正在经历形成水华的活动浮游植物生物的显着扩张，这些生物可以通过执行趋光性来规避 DOM 的遮荫效应，从而可能对湖泊的 CO2 平衡产生影响。我们在一个富含 DOM 的湖中使用广泛存在的鞭毛藻鞭毛 Gonyostomum 精液作为模式生物测试了这个想法。通过连续现场测量，我们发现 G. 精液即使在低藻类密度下也能降低 CO2 的分压。藻类密度高的时期与 CO2 不饱和和大气 CO2 侵入有关。在 G. 精液开花期间和之后，大气中 CO2 的平均每日净损失估计分别为 12.9 和 70.4 mmol C m-2 day-1。G. 精液在空间和时间上造成了陡峭的 pCO2 梯度，这使得在不采用连续 pCO2 垂直剖面分析的情况下很难揭示物种的影响。这表明 G. 精液对北方湖泊 CO2 平衡的影响在过去可能被忽视了。总之，我们的数据表明，尽管外来 DOM 浓度很高，但 G. semen 可以显着减少北方湖泊的 CO2 排放。因此，在估算来自北方湖泊的 CO2 排放量时，应考虑 G. 精液和其他活动的浮游植物物种，