Beavers and climate change mitigation in freshwater systems

在欧洲和北美，重新引入海狸一直是保护动物的成功案例。通过在小河流上筑坝和建造水池、池塘和运河，海狸恢复了失去的生态系统服务，如：增强景观生物多样性（Nummi等人，2019；Willby等人，2018），减轻洪涝和干旱，改善水质（Brazier等人，2020）。然而，除了水生植物生物量增加（Law et al., 2016）和构成海狸坝的大量枯木外，海狸坝上游池塘中积聚的大量沉积物也可能在气候变化中发挥作用。因此，这些被重新开垦的景观也可能起到碳汇的作用，因此，在面对气候变化的情况下，它们是创造有弹性的生态系统的潜在有价值的工具。然而，所有类型的池塘同时也是不成比例的温室气体（ghg）的大来源，这是全球变暖的主要驱动因素（Holgerson和Raymond，2016），因为它们的深度浅，频繁混合，养分负荷高，能量周转快。我们对池塘温室气体排放的理解，无论是在量级还是在驱动过程方面，都是缺乏的，在海狸池塘中更是如此，因为海狸池塘在基本的物理化学特征上不同于其他淡水系统（Cazzolla Gattiet al., 2018）。目前缺乏评估海狸池塘全碳质量平衡的研究，没有一个是在英国规模的，因此我们无法量化海狸重新引入对碳循环的净影响。这限制了我们对海狸与气候变化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的过程中，认识和量化生态系统和单个物种在碳循环中的作用至关重要。现在需要确凿的证据来证明海狸在碳循环中的作用，以确定排放的碳是否被储存在海狸池沉积物、植被和构成水坝的枯木中的碳所抵消。这个博士项目的总体目标是量化海狸对碳循环的净效应，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.