Silicon cyling along the land-ocean continuum
Carey, Joanna C.
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The alteration of the global environment by human activities is so widespread that scientists argue we've entered a new geologic epoch known as the Anthropocene. This dissertation examines the impact of human activities on biogeochemical cycling at the land-sea interface. I focus primarily on the role of land use/land cover (LULC) and coastal nutrient enrichment on silicon (Si) cycling in New England rivers and salt marshes. On land, Si is taken up by vegetation, improving plant fitness and protecting plants from a variety of environmental stressors. In aquatic systems, diatoms, the dominant type of phytoplankton in coastal temperate waters, require Si to survive. My research demonstrates that LULC is an important driver of Si export to coastal systems, accounting for 40-70% of the variability of riverine fluxes. Developed watersheds export significantly (p=0.03) more Si than their forested counterparts, which I hypothesize is due to less vegetated cover, a known Si sink, in developed watersheds. Building on this, I calculated the amount of Si fixed by land plants globally (84 Tmol yr-1) and the percent (55%) of global terrestrial net primary production that can be attributed to active Si-accumulating organisms. Next, I created the first complete salt marsh Si budget by quantifying tidal creek fluxes and net Si accumulation in a relatively undisturbed low-nutrient salt marsh. Further, comparing this Si accumulation to that of a high-nutrient marsh revealed that the high-nutrient marsh contained significantly (p<0.05) more Si within the sediments, roots, and porewater. Combining my original data from six New England salt marshes with published values, I quantify the mode of Si accumulation (rejective, passive, or active) by Spartina grasses and the environmental controls on such accumulation. Finally, using radionuclide tracers 137Cs and 210Pb, I calculated vertical accretion rates of five salt marshes and compared these values to historical measurements. I found that accretion rates have slowed and this deceleration is driven, in part, by a decrease in organic matter accumulation. Together, this dissertation improves our knowledge of Si cycling in terrestrial and aquatic ecosystems, and identifies previously unrecognized ways in which humans are perturbing biogeochemical cycles at the land-sea interface.