Diatom Si deposition, spring warming duration, and spring mixing duration of sediment core Tiefer-See

Monitoring during three meteorologically different spring seasons in 2012, 2013, and 2014 revealed that temperature increase in spring, which influences spring lake mixing duration, markedly affected nutrient availability and diatom deposition in a sediment trap close to the bottom of deep Tiefer See, NE Germany. Deposition of Stephanodiscus taxa and small Cyclotella taxa was much higher after late ice out and a deep, short lake mixing period in spring 2013, compared to that after gradual warming and lengthy lake mixing periods in spring 2012 and 2014, when only brief or marginal ice cover occurred. Availability of dissolved Si and P was 33 and 20 % higher, respectively, in 2013 compared to 2014. The observed relation between high (low) diatom deposition and short (lengthy) mixing duration in spring was applied to varved sediments deposited between AD 1924 and 2008. Low detrital Si content in trapped material and a sediment core enabled use of µXRF-counts of Si as a proxy for diatom silica. The spring mixing duration for 1951-2008 was derived from FLake-model calculations. The spring warming duration related to lake mixing was approximated from air temperatures for 1924-2008 using the dates when daily mean air temperature exceeded 5 °C (start) and 10 °C (end). Diatom silica deposition showed a significant (p < 0.0001) inverse linear relationship with the modeled spring mixing duration (R**2 = 0.36) and the spring warming duration (R**2 = 0.28). In both cases, the relationship is strengthened when data from the period of low diatom production (1987-2005) is excluded (R**2 = 0.59 and R**2 = 0.35). Part of this low diatom production is related to external nutrient supply that favored growth of cyanobacteria at the expense of diatoms. This approach shows that diatom Si deposition was strongly influenced by the availability of light and nutrients, related to the duration of lake mixing and warming in spring, during most of the studied period. The remaining unexplained variability, however, indicates that additional factors influence Si deposition. Further tests in other deep, temperate lakes are necessary to verify if this relation is a common feature and consequently, if diatom Si can be used as a proxy for spring mixing duration in such lakes.

对2012、2013、2014年三个气象特征迥异的春季开展监测后发现，春季气温升高会同时调控湖泊春季混合时长，且显著影响德国东北部深水蒂费尔湖（deep Tiefer See）近底沉积物捕集器中的营养盐有效性与硅藻沉积过程。与2012、2014年春季的情况相比——这两年春季升温平缓、湖泊混合时长偏长，且仅存在短暂或边缘性冰盖——2013年春季在晚冰消与深水短时长混合事件后，星盘藻属（Stephanodiscus）类群与小型小环藻属（Cyclotella）类群的沉积量显著更高。2013年溶解态硅（Si）与磷（P）的有效性分别较2014年高出33%与20%。将春季硅藻沉积量高（低）与湖泊混合时长短（长）之间的观测关联，应用于公元1924年至2008年沉积的纹泥沉积物（varved sediments）中。捕集物质与沉积物岩芯中低碎屑硅（detrital Si）含量，使得可利用硅的微区X射线荧光光谱（µXRF）计数作为硅藻硅质（diatom silica）的代用指标。1951年至2008年的春季湖泊混合时长通过FLake模型（FLake-model）计算得到。针对1924年至2008年，以日平均气温超过5℃（起始日）与10℃（终止日）的日期为依据，估算得到与湖泊混合相关的春季升温时长。硅藻硅质沉积量与模拟得到的春季混合时长（决定系数R²=0.36）、春季升温时长（R²=0.28）均呈现显著的负线性相关关系（p<0.0001）。剔除1987年至2005年硅藻生产力偏低时段的数据后，上述两种相关关系均得到增强（R²分别为0.59与0.35）。该时段硅藻生产力偏低的部分原因，在于外源营养盐供给更利于蓝藻（cyanobacteria）生长，而以硅藻生长为代价。该研究方法表明，在研究时段的大部分周期内，硅藻硅质沉积量受到光照与营养盐有效性的强烈调控，而光照与营养盐有效性又与春季湖泊混合与升温时长相关。但仍存在部分未得到解释的变异，表明尚有其他因素影响硅质沉积。需在其他深水温带湖泊中开展进一步测试，以验证该关联是否具有普遍性，进而确认硅藻硅质是否可作为此类湖泊春季混合时长的代用指标。