Critically testing the role of delta-30 Si[diatom] as a novel productivity signal in temperate lakes

尽管湖泊仅占地球陆地面积的约3%，但它们是生产力热点，在景观和全球范围内在碳的生物固定、矿化和埋藏（集体循环）方面发挥着重要作用。湖泊对环境变化很敏感，但人们对湖泊生态系统的反应方式（就其整体结构和湖泊生产力等过程而言）知之甚少。出于这个原因，我们也不确定湖泊系统中碳的命运（就其循环而言），考虑到未来气候变化的不确定性（这可能会改变湖泊未来碳循环的方式），这尤其成问题。由于碳循环的复杂性以及湖泊沃茨和沉积物中有机化合物的不稳定性，从湖泊沉积物中储存的碳的地层剖面中找出过去的湖泊生产力是有问题的。硅藻是单细胞藻类，也是湖泊中生物质的主要生产者的重要群体，并且可能在其外壳（或“阀”）的化学特征中提供可能的解决方案，这些外壳由二氧化硅（天然玻璃）制成，并在许多淡水沉积物中保存良好。硅藻阀中硅（Si）的稳定同位素（δ-30Sidiatom）的比率分析提供了一种潜在的替代手段来追踪湖泊过去的初级生产力，特别是与来自硅藻硅（δ-18O）中也发现的稳定氧同位素的气候和水文变化的独立证据一起。分析方法的最新进展现在允许从同一样品中测量硅和氧同位素信号。硅藻硅化（阀建设）过程中优先吸收的较轻的硅同位素（28硅），导致富集的较重的同位素30硅内的残留水成比例的硅藻人口增长，反映了增加三角洲-30硅硅藻。虽然delta-30 Si在海洋研究中的应用正在迅速扩大，但湖泊研究的潜力尚未得到评估。特别是，硅的生物吸收和硅在湖泊内的回收（因为它慢慢溶解在水中，并从湖底的沉积物）对最终的三角洲-30硅信号的相对影响是至关重要的，但知之甚少。硅藻氧同位素分析（δ-18 O）受到了更大的关注，实验室培养实验中硅藻硅化过程中两种同位素的分馏是一位共同研究者正在进行的NERC研究金的目标。然而，实地研究是稀缺的，并考虑到硅循环在湖泊中的复杂性，仍然有一个令人兴奋的机会和迫切需要开发这种方法，关键是淡水系统。解开湖泊中硅循环的第一步，及其三角洲-30Sidiatom指纹，需要精心选择和代表性的网站的详细案例研究。我们建议解决这些问题，每月监测硅循环结合三角洲-30 Sidiatom和三角洲-18 O在罗斯赫恩梅尔，柴郡在两个春天（硅藻水华）期间，使用一种综合的方法，每周到每两周自动沉积物捕获和水样，结合季节性测量水柱剖面，流入和地下水，每年散装沉积物陷阱。该项目解决了有关三角洲-30Si生物地球化学的基本问题，并将为理解我们如何使用湖泊沉积物中的三角洲-30Si硅藻来告诉我们湖泊的过去状况（功能和生产力）提供重要的一步，建立这种技术作为水生系统生产力示踪剂的潜力，并提供对营养物循环的见解（对环境管理者有价值）。了解过去湖泊在不同气候和人类活动下的运作方式，可以帮助我们预测和管理我们今天看到的以及未来可能预期的变化。

Despite occupying only ~3% of the earth's land surface, lakes are productivity hotspots and play an important role in the biological fixing, mineralisation and burial (collectively cycling) of carbon, at both a landscape and global scale. Lakes are sensitive to environmental change, yet the ways that lake ecosystems respond (in terms of their overall structure, and processes such as lake productivity) are poorly understood. For this reason, we are also uncertain as to the fate of carbon within the lake system (in terms of its cycling), and this is especially problematic given the uncertainty with future climate change (which may change the way that lakes cycle carbon in the future). Given the complexity of the carbon cycle and instability of organic compounds in lake waters and sediments, finding out past lake productivity from stratigraphic profiles of carbon stored in lake sediments is problematic. Diatoms are unicellular algae, and an important group of primary producers of biomass in lakes, and may provide a possible solution in the chemical signature of their shells (or 'valves'), which are made of silica (natural glass) and preserve well in many freshwater sediments. Analysis of the ratio of stable isotopes of silicon (Si) in diatom valves (delta-30Sidiatom) offers a potential alternative means to trace past primary productivity in lakes, particularly alongside independent evidence for climate and hydrological change derived from stable isotopes of oxygen also found in diatom silica (delta-18O). Recent advances in the analytical methods now allows both silicon and oxygen isotope signals to be measured from the same sample. Preferential uptake of the lighter silicon isotope (28Si) during diatom silicification (valve building), leads to an enrichment of the heavier isotope 30Si within residual water proportional to diatom population growth, reflected by increasing delta-30Sidiatom. While the use of delta-30Si is rapidly expanding in marine studies, the potential for lake studies has yet to be assessed. In particular, the relative influence of biogenic uptake of silicon and the recycling of silicon within the lake (as it does slowly dissolve in water and from sediment on the lake floor) on the eventual delta-30Si signal in lakes is critical, yet poorly understood. Diatom oxygen isotope analysis (delta-18O) has received greater attention and the fractionation of both isotopes during silicification by diatoms in laboratory culture experiments is the aim of an ongoing NERC Fellowship by one of the co-investigators. Yet, field studies are scarce and considering the complex nature of Si cycling in lakes, there remains an exciting opportunity and urgent need to develop this approach critically for freshwater systems. The first steps towards unravelling the Si cycle in lakes, and its delta-30Sidiatom fingerprint, requires detailed case studies of well-chosen and representative sites. We propose to address these issues by monthly monitoring Si cycling in conjunction with delta-30Sidiatom and delta-18O at Rostherne Mere, Cheshire over two spring (diatom bloom) periods, using an integrated approach with weekly to fortnightly automatic sediment trapping and water sampling, in combination with seasonal measurements from water column profiles, inflows and groundwater, and annual bulk sediment traps. The project addresses fundamental questions concerning delta-30Si biogeochemistry and will provide an important step towards understanding how we can use delta-30Sidiatom from lake sediments to tell us about the past condition (function and productivity) of lakes, establishing the potential for this technique as a tracer for productivity in aquatic systems, and providing insight into nutrient recycling (valuable for environmental managers). Knowledge of the way lakes functioned in the past, under different regimes of climate and human activity, can help us predict and manage change that we see today and might expect in the future.