Beavers and climate change mitigation in freshwater systems

重新引入海狸是欧洲和北美重复的一个保护成功的故事。通过在小河流上筑坝并修建池塘和运河，海狸恢复了丧失的生态系统服务，例如：增强景观生物多样性(Nummi等人，2019；Willby等人，2018年)，减轻洪水和干旱，改善水质(Braier等人，2020年)。然而，这些生态系统的改变也可能在气候变化中发挥作用，这是因为河狸坝上游池塘中积累的大量沉积物，以及水生植物生物量的增加(Law等人，2016年)和构成河狸坝的大量枯木。因此，这些重新开垦的景观也可以作为碳汇，因此，在面对不断变化的气候时，它可能是创建弹性生态系统的潜在有价值的工具。然而，所有类型的池塘都同时是温室气体(GHGs)的不成比例的大来源，温室气体(GHGs)是全球变暖的主要驱动因素(Holgerson和Raymond，2016)，因为它们深度浅，混合频繁，营养负荷高，能量周转快。我们对池塘的温室气体排放，无论是在数量上还是在驱动过程方面都缺乏了解，在河狸池塘中更是如此，因为河狸池塘的基本物理化学特征与其他淡水系统不同(Cazzolla Gattiet al.，2018年)。目前缺乏评估河狸池塘全碳质量平衡的研究，没有英国规模的研究，因此我们无法量化重新引入海狸对碳循环的净影响。这限制了我们对海狸在气候变化方面的整体影响和影响的理解。在我们努力实现净零碳排放和开发基于自然的解决方案时，认识和量化生态系统和单个物种在碳循环中的作用是至关重要的。现在需要强有力的证据证明海狸在碳循环中的作用，以确定海狸池塘的沉积物、植被和组成水坝的枯木中储存的碳是否抵消了排放的碳。该博士项目的总体目标是量化海狸对碳循环的净影响，以确定它们对气候的影响。

The reintroduction of beavers has been a conservation success story repeated acrossEurope and North America. By damming small rivers and creating pools, ponds andcanals, beavers have restored lost ecosystem services such as: enhanced landscapebiodiversity (Nummi et al., 2019; Willby et al., 2018), flood and drought alleviation andimproved water quality (Brazier et al., 2020). Yet, these ecosystem modifications mayalso play a role in climate change as a function of the vast quantities of sediment thataccumulate in the ponds upstream of beaver dams, in addition to increased biomass ofaquatic plants (Law et al., 2016) and large volumes of dead wood comprising the beaverdams. These rewilded landscapes may therefore also act as carbon sinks and, hence, asa potentially valuable tool in creating resilient ecosystems in the face of a changingclimate.However, ponds of all types are simultaneously a disproportionately large source ofgreenhouse gases (GHGs), a major driver of global warming (Holgerson and Raymond,2016), due to their shallow depths, frequent mixing, high nutrient loading and fast energyturnover. Our understanding of GHG emissions from ponds, both in terms of magnitudeand driving processes, is lacking, even more so in beaver ponds which differ from otherfreshwater systems in their fundamental physico-chemical characteristics (Cazzolla Gattiet al., 2018). There is currently a dearth of studies that assess the full carbon massbalance of beaver ponds, with none at the UK scale and therefore we are unable toquantify the net effect of beaver reintroduction on carbon cycling. This limits ourunderstanding of the overall impact and influence of beavers in relation to climatechange.Recognising and quantifying the role of ecosystems and individual species in carboncycling is essential as we work towards net-zero carbon emissions and developingnature-based solutions. Robust evidence of the role of the beavers in carbon cycling isnow needed to identify whether carbon emitted is offset by carbon stored in beaverponds' sediments, vegetation and the dead wood comprising their dams. The overall aimof this PhD project is to quantify the net effect of beavers on carbon cycling, in order todetermine their climate impact.