Perturbations to nutrient and carbon cycles by river damming
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The damming of rivers represents one of the most far-reaching human modifications to the flows of water and associated matter from land to sea. Globally there are over 70 000 large dams whose reservoirs store more than seven times as much water as natural rivers. Due to increasing demands for energy, irrigation, drinking water, and flood control, the construction of dams will continue into the foreseeable future. Indeed, there is currently an ongoing boom in dam construction, particularly focused in emerging economies, which is expected to double the fragmentation of rivers on Earth. Essential nutrient elements such as phosphorus (P), nitrogen (N), silicon (Si), and carbon (C) are transported and transformed along the land-ocean aquatic continuum (LOAC), forming the basis for freshwater food webs in lakes, rivers, wetlands, reservoirs, and floodplains, and ultimately for marine food webs in estuarine and coastal environments. The dam-driven fragmentation of the rivers along the LOAC will significantly modify global nutrient and C fluxes via elimination from the water column in reservoirs. In this thesis, I quantify in-reservoir elimination and transformation fluxes for phosphorus (P), silicon (Si), and organic carbon (OC), with the goal of determining (1) how much Si, P, and organic C (OC) are retained or eliminated globally due to river damming, (2) how damming modifies the balance of productivity (heterotrophy vs. autotrophy) in river systems worldwide, (3) to what extent damming changes nutrient speciation or reactivity along the LOAC, and (4) if reservoirs retain or eliminate certain nutrients more efficiently than others, and if so, how this decoupling changes nutrient ratios delivered to coastal zones. I address these research questions at the reservoir scale, by quantifying nutrient elimination in Lake Diefenbaker, Saskatchewan, and through the development of spatially explicit global nutrient and carbon models. In Chapter 2, I present a reservoir-scale field study of reactive silicon dynamics in Lake Diefenbaker, a reservoir in Canada’s central prairie province of Saskatchewan. I use a year-round dataset of surface water samples and sediment cores to construct a Si budget for the reservoir, including an estimation of the amount of Si buried in the reservoir annually. I use this study to illustrate the differences in retention of Si relative to N and P, and put forth the hypothesis that river damming results in a decoupling of nutrient cycling. This study acts as an introduction to the concept of differential nutrient retention in reservoirs, which I go on to show at the global scale for Si, P, and C in reservoirs in Chapters 3, 4, and 5. Following Chapter 2, I address my research questions by developing a mechanistic approach to global scale biogeochemical modelling. This approach yields spatially explicit results, which allows for the quantification of regional watershed and coastal trends, as well as lumped continental changes. In Chapter 3, the modelling approach itself is introduced, through application to the Si cycle. I show, via a meta-analysis comparing the distribution of physical and chemical parameters of published reservoir Si budgets to reservoirs worldwide, that the existing literature Si budgets are severely limited in their ability to represent the dataset of global reservoirs. I then introduce the mechanistic approach by developing a biogeochemical box model representing Si dynamics in reservoirs. I assign rate expressions to transformation fluxes and input/output fluxes, which are constrained as uniform distributions between limits that encapsulate possible global ranges. Using a Monte Carlo approach, I allow the model to randomly select each rate constant independently for 6000 iterations, generating a database of hypothetical Si dynamics in reservoirs worldwide. I use this generated dataset to establish expressions relating Si retention to water residence time, which I apply to an existing database of global reservoirs. Ultimately I develop a global estimate of dissolved and reactive Si burial in reservoirs for year 2000. Chapters 4 and 5 use the same modelling approach presented in Chapter 3, but applied to riverine P and organic carbon (OC) fluxes. Because the cycles of P and OC have been studied in more detail than Si in the literature, it is possible to constrain higher order probability density functions (PDFs) for many rate constants. In the case of OC, it also becomes possible to use a statistically significant semi-empirical approach to calculate a number of fluxes, as expressions to predict OC dynamics have been established from globally applicable datasets. Using the upstream-catchment area-normalized Global-NEWS model’s watershed yields as input to each reservoir, I use the 1970, 2000, 2030 and 2050 model predictions to estimate historical and predict future P and OC elimination by dams. In Chapter 4, I show that damming retains 12% of the global total P load to watersheds in year 2000, potentially rising to 17% by 2030. In Chapter 5, I show that global OC mineralization in reservoirs exceeds carbon fixation (P<R); the global P/R ratio, however, varies significantly, from 0.20 to 0.58 because of the changing age distribution of dams. I further estimate that at the start of the 21st Century, in-reservoir burial plus mineralization eliminated 4.0 ± 0.9 Tmol yr-1 (48 ± 11 Tg C yr-1) or 13% of total OC carried by rivers to the oceans. Because of the ongoing boom in dam building, in particular in emerging economies, this value could rise to 6.9 ± 1.5 Tmol yr-1 (83 ± 18 Tg C yr-1) or 19% by 2030. Chapter 6 ties the previous global scale P and Si model together, plus a global scale N model (Akbarzadeh et al., in preparation), to predict changes to nutrient ratios delivered by rivers to the coastal zones. I use this analysis, in combination with anthropogenic nutrient loading data, to contextualize the role of river damming as a driver of changing nutrient limitation in the coastal shelf zones of the world. Results indicate that dams preferentially eliminate P over Si, and Si over N, from the water column. I show that while damming drives riverine N:P ratios up, anthropogenic nutrient loading is shifting these ratios down, increasing the prominence of N-limitation in river water discharged to the coasts. Because of the preferential elimination of Si over N, the net rise in N-limitation increases the prominence of Si-limitation in coastal river discharge, potentially creating conditions suitable for harmful algal blooms to develop. My results show that damming is driving a severe reorganization of global nutrient cycles along the entire LOAC. By quantifying the changes to multiple nutrient cycles, I show that a multi-nutrient management approach is needed in heavily dammed watersheds, as deliberate reduction of one nutrient species flux can have unintended consequences on other nutrient elements. These alterations persist from the reservoir to the river’s discharge into coastal zones. The effects of damming on nutrient cycling, in combination with other human pressures and management strategies, therefore have the potential to affect ecosystems worldwide.
Cite this work
Taylor Maavara (2017). Perturbations to nutrient and carbon cycles by river damming. UWSpace. http://hdl.handle.net/10012/12043