Widespread long-term increases in dissolved organic carbon (DOC) concentrations (i.e., “browning”) have been observed in many lakes, but the ecological consequences are poorly understood. Some studies suggest a unimodal relationship between DOC and primary productivity, with peak productivity at intermediate DOC concentrations. This peak is hypothesized to result from the tradeoff between light absorbing properties of DOC, and increases in limiting nutrients with browning. Nevertheless, it is unclear whether nutrient stoichiometry is constant as lakes brown. Across both regional and national surveys, we found a positive linear relationship between DOC and both total and organic forms of nitrogen and phosphorus. However, long-term data from a suite of browning lakes indicates that total nutrients do not increase as DOC increases through time. Our results show that DOC and limiting nutrients are coupled spatially, but not temporally, and that this temporal mismatch challenges previous conceptualizations of the long-term effects of browning on productivity.

中文翻译：

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湖泊褐变导致溶解的有机碳和限制性营养物质之间的时空不匹配

已在许多湖泊中观察到溶解有机碳 (DOC) 浓度长期广泛增加（即“褐变”），但对生态后果知之甚少。一些研究表明 DOC 和初级生产力之间存在单峰关系，在中等 DOC 浓度下具有峰值生产力。这个峰值被假设是由于 DOC 的光吸收特性和随着褐变而增加的限制营养素之间的权衡造成的。然而，尚不清楚营养化学计量是否随着湖泊变褐而恒定。在区域和国家调查中，我们发现 DOC 与氮和磷的总形式和有机形式之间存在正线性关系。然而，一组褐化湖泊的长期数据表明，总养分不会随着 DOC 的增加而增加。