Ecology and Genetics Research Unit, University of Oulu, Oulu, Finland.
Natural Resources Institute Finland (Luke), Paltamo, Finland.
Glob Chang Biol. 2022 Sep;28(17):5159-5171. doi: 10.1111/gcb.16279. Epub 2022 Jun 15.
Concentrations of terrestrial-derived dissolved organic carbon (DOC) in freshwater ecosystems have increased consistently, causing freshwater browning. The mechanisms behind browning are complex, but in forestry-intensive regions browning is accelerated by land drainage. Forestry actions in streamside riparian forests alter canopy shading, which together with browning is expected to exert a complex and largely unpredictable control over key ecosystem functions. We conducted a stream mesocosm experiment with three levels of browning (ambient vs. moderate vs. high, with 2.7 and 5.5-fold increase, respectively, in absorbance) crossed with two levels of riparian shading (70% light reduction vs. open canopy) to explore the individual and combined effects of browning and loss of shading on the quantity (algal biomass) and nutritional quality (polyunsaturated fatty acid and sterol content) of the periphytic biofilm. We also conducted a field survey of differently colored (4.7 to 26.2 mg DOC L ) streams to provide a 'reality check' for our experimental findings. Browning reduced greatly the algal biomass, suppressed the availability of essential polyunsaturated fatty acids, especially eicosapentaenoic acid (EPA), and sterols, but increased the availability of terrestrial-derived long-chain saturated fatty acids (LSAFA). In contrast, loss of shading increased primary productivity, which resulted in elevated sterol and EPA contents of the biofilm. The field survey largely repeated the same pattern: biofilm nutritional quality decreased significantly with increasing DOC, as indicated particularly by a decrease of the ω-3:ω-6 ratio and increase in LSAFA content. Algal biomass, in contrast, was mainly controlled by dissolved inorganic nitrogen (DIN) concentration, while DOC concentration was of minor importance. The ongoing browning process is inducing a dramatic reduction in the nutritional quality of the stream biofilm. Such degradation of the major high-quality food source available for stream consumers may reduce the trophic transfer efficiency in stream ecosystems, potentially extending across the stream-forest ecotone.
陆地衍生溶解有机碳（DOC）在淡水生态系统中的浓度持续增加，导致淡水变褐。变褐的机制很复杂，但在林业密集地区，土地排水会加速变褐。溪流沿岸林缘森林中的林业活动改变了树冠遮荫，加上变褐，预计将对关键生态系统功能产生复杂且在很大程度上不可预测的控制。我们进行了一个溪流中观实验，有三个变褐水平（对照、中度和高度，吸光度分别增加 2.7 倍和 5.5 倍），并与两个林缘遮荫水平（70%的光减少和开阔树冠）交叉，以探索变褐和失去遮荫对周丛生物膜数量（藻类生物量）和营养质量（多不饱和脂肪酸和甾醇含量）的单独和综合影响。我们还对不同颜色（4.7 至 26.2 mg DOC L ）的溪流进行了实地调查，为我们的实验结果提供了“现实检查”。变褐大大减少了藻类生物量，抑制了必需多不饱和脂肪酸，特别是二十碳五烯酸（EPA）和甾醇的供应，但增加了陆地衍生的长链饱和脂肪酸（LSAFA）的供应。相比之下，失去遮荫增加了初级生产力，导致生物膜中甾醇和 EPA 含量升高。实地调查在很大程度上重复了相同的模式：生物膜的营养质量随着 DOC 的增加而显著下降，特别是 ω-3：ω-6 比值下降和 LSAFA 含量增加。相比之下，藻类生物量主要受溶解无机氮（DIN）浓度的控制，而 DOC 浓度的重要性较小。正在进行的变褐过程正在导致溪流生物膜的营养质量急剧下降。这种溪流消费者可用的主要高质量食物源的退化可能会降低溪流生态系统中的营养转移效率，潜在地延伸到溪流-森林交错带。
