Photo-chemical reactivity of dissolved organic carbon along a gradient of subarctic lakes with different color

Lake-atmosphere carbon exchanges can be significantly affected by photochemical dissolved organic matter (DOM) mineralization. However, there is an incomplete understanding of how shifting optical characteristics of DOM due to increasing allochthonous organic carbon input affect the photo-reactivity of the DOM per unit of absorbed incoming light. Here, we measured the absorption of ultraviolet (UV) light and subsequent photochemical DOM decay in 148 lakes within the subarctic region of Abisko. These lakes range from brown-water lakes with allochthonous inputs connected to some mires to tundra clear-water lakes with relatively more autochthonous inputs. Fluorescence excitation-emission matrix analysis was used to assess the chemical composition of the DOM. The aim was to see how increasing colored DOM (CDOM) affects the photo-mineralization. We found that the photo decay rates in absolute values were positively correlated to CDOM. However, the photo decay per unit of absorbed light energy did not increase with increasing CDOM; rather, it showed a weak decreasing trend. Fluorescence analyses helped explain these patterns, as humic-like fluorescent DOM of presumable terrestrial origin was associated with high absolute photo decay rates, but not generally with higher photo-reactivity per unit of absorbed light energy than other types of DOM. The results suggest that even though increasing inputs of terrestrial substances mean a higher abundance of photo-degradable materials, it does not necessarily mean that CO₂ emissions increase in lakes where browning limits the ability of light to penetrate deeper water.

湖泊-大气碳交换可显著受光化学溶解态有机质（dissolved organic matter, DOM）矿化作用的影响。然而，当前学界对外源有机碳输入增加所引发的DOM光学特征变化，如何调控单位吸收入射光的DOM光反应活性，仍缺乏完整认知。本研究对阿比斯库（Abisko）亚北极区域的148个湖泊，开展了紫外（ultraviolet, UV）光吸收及后续光化学DOM降解过程的测定。所涉湖泊涵盖两类：一类是与部分沼泽湿地相连、接收外源输入的棕水湖泊，另一类是内源输入占比相对更高的苔原清水湖泊。研究采用荧光激发-发射矩阵光谱分析，对DOM的化学组成进行表征，旨在明确有色溶解态有机质（colored DOM, CDOM）丰度增加对光矿化作用的影响。结果显示：绝对数值下的光降解速率与CDOM呈显著正相关；但单位吸收光能的光降解速率并未随CDOM升高而提升，反而呈现微弱下降趋势。荧光分析结果可对上述规律作出解释：推测为陆源来源的类腐殖质荧光DOM，与较高的绝对光降解速率相关，但通常并不比其他类型DOM具备更高的单位吸收光光反应活性。本研究结果表明，即便陆源物质输入增加意味着光可降解物质的丰度提升，在湖体褐变限制光线穿透深层水体的湖泊中，这并不必然导致二氧化碳（CO₂）排放增加。