A 39-cm sediment core from Castle Lake, California (USA) spans the last ~ 450 years and was analyzed for diatoms and organic geochemistry (δ15N, δ13C, and C:N), with the goal of determining sensitivity to natural climate variation and twentieth century anthropogenic effects. Castle Lake is a subalpine, nitrogen-limited lake with ~ 5 months of annual ice cover. Human impacts include light recreational use, past fish stocking, and experimental use by the Castle Lake Research Station. The base of the core (below 32 cm; pre mid-1700s) represents the period of maximum ice cover. In contrast, the end of the Little Ice Age (mid 1700s–early 1800s) is dominated by cyclotelloids (mostly Discostella stelligera), indicating significant open-water periods, a condition that persisted into the early 1900s. Cyclotelloids began to decline in the 1960s and were replaced by the Fragilaria tenera grp. (peak in 1970s), succeeded by Asterionella formosa (peak ~ 2010), and accompanied by a reduction in δ15N values and a decrease in C:N that may represent increased atmospheric nitrogen deposition. Another anthropogenic signal was discerned in the core and was interpreted to be the result of an ammonium nitrate fertilization experiment of the epilimnion that was conducted in 1980 and 1981. This signal was manifested in the core largely by a negative excursion in δ15N, possibly caused by fractionation during denitrification in surface sediment. A phytoplankton monitoring dataset collected by the Castle Lake Research Station from 1967 to 1984 corroborates the timing of increased araphid euplanktonic species in the 1970s, and increases in two benthic diatoms (Staurosirella pinnata and Tabellaria fenestrata), entrained in the phytoplankton tows during the experimentation years. Both ice cover and nitrogen addition appear to be strong drivers that affected the lake diatoms, although additional drivers, such as fish stocking and associated cascade effects need further exploration. These data will be helpful for interpreting longer core records from Castle Lake, should the opportunity arise, as well as cores from similar systems in the region.

中文翻译：

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根据硅藻和有机地球化学推断，美国加利福尼亚州城堡湖 450 年的环境变化记录

来自加利福尼亚州（美国）城堡湖的 39 厘米沉积物核心跨越了过去约 450 年，并分析了硅藻和有机地球化学（δ15N、δ13C 和 C:N），目的是确定对自然气候变化的敏感性和二十世纪的人为影响。城堡湖是一个亚高山、氮含量有限的湖泊，每年有约 5 个月的冰盖。人类影响包括轻度娱乐用途、过去的鱼类放养以及城堡湖研究站的实验用途。核心的底部（低于 32 厘米；1700 年代中期之前）代表最大冰盖时期。相比之下，小冰河时代的末期（1700 年代中期至 1800 年代初期）以小环藻（主要是 Discostella stelligera）为主，表明明显的开放水域时期，这种情况一直持续到 1900 年代初期。小环藻在 1960 年代开始下降，并被 Fragilaria tenera grp 所取代。（峰值在 1970 年代），继之以 Asterionella formosa（峰值 ~ 2010 年），并伴随着 δ15N 值的减少和 C:N 的减少，这可能代表大气氮沉积增加。在地核中发现了另一个人为信号，并被解释为 1980 年和 1981 年对表皮层进行硝酸铵施肥实验的结果。该信号在地核中主要表现为 δ15N 的负偏移，可能是由表层沉积物中反硝化过程中的分馏。城堡湖研究站 1967 年至 1984 年收集的浮游植物监测数据集证实了 1970 年代真浮游蚜虫物种增加的时间，并且在实验期间浮游植物拖带中夹带的两种底栖硅藻（Staurosirella pinnata 和 Tabellaria fenestrata）增加。冰盖和氮添加似乎是影响湖泊硅藻的强大驱动因素，但其他驱动因素，如鱼类放养和相关联的级联效应需要进一步探索。如果有机会，这些数据将有助于解释来自 Castle Lake 的更长岩心记录，以及来自该地区类似系统的岩心。