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

尽管湖泊只占地球陆地表面的约3%，但它是生产力的热点，在景观和全球尺度上对碳的生物固定、矿化和埋藏(集体循环)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)，这尤其是个问题。考虑到碳循环的复杂性和湖泊水域和沉积物中有机化合物的不稳定，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，是湖泊中重要的初级生物量生产者，在其贝壳(或‘阀门’)的化学特征上可能提供了一种可能的解决方案，这些贝壳由二氧化硅(天然玻璃)制成，在许多淡水沉积物中保存完好。硅藻瓣膜中硅的稳定同位素比率的分析(三角洲-30Siditom)为追踪湖泊过去的初级生产力提供了一种潜在的替代手段，特别是提供了气候和水文变化的独立证据，这些证据也来自硅藻硅藻(三角洲-18O)中发现的氧的稳定同位素。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。在硅藻硅化(阀门建造)过程中，较轻的硅同位素(28Si)优先被吸收，导致较重的同位素30Si在剩余水中的富集度与硅藻种群的增长成正比，这反映在增量-30Siarium的增加上。虽然Delta-30Si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，硅的生物吸收和湖内硅的循环(因为它确实慢慢地溶解在水中和湖底的沉积物中)对湖泊中最终的Delta-30Si信号的相对影响是关键的，但人们了解得很少。硅藻氧同位素分析(德尔塔-18O)受到了更大的关注，实验室培养实验中硅藻硅化过程中这两种同位素的分馏是一名共同研究人员正在进行的NERC联谊会的目标。然而，实地研究很少，考虑到湖泊中硅循环的复杂性，对于淡水系统来说，仍然有一个令人兴奋的机会和迫切需要开发这种方法。揭开湖泊中的硅循环及其Delta-30Siditom指纹的第一步，需要对精心挑选和具有代表性的地点进行详细的案例研究。为了解决这些问题，我们建议在两个春季(硅藻盛开)期间，每月监测切郡Rostherne Mel的硅藻Delta-30和Delta-18O的硅循环，采用每周至每两周一次的自动沉积物捕获和水样采集的综合方法，结合水柱剖面、流入和地下水的季节性测量，以及年度大宗沉积物陷阱的季节性测量。该项目解决了与Delta-30Si生物地球化学有关的基本问题，并将为理解如何利用湖泊沉积物中的Delta-30Si硅藻告诉我们湖泊过去的状况(功能和生产力)提供重要的一步，确立这项技术作为水生系统生产力示踪剂的潜力，并提供对营养循环的洞察(对环境管理人员很有价值)。了解湖泊在过去在不同的气候和人类活动条件下的运行方式，可以帮助我们预测和管理我们今天看到的和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.