Date of Award

Winter 2014

Project Type

Dissertation

Program or Major

Earth and Environmental Sciences

Degree Name

Doctor of Philosophy

First Advisor

Wilfred M Wollheim

Second Advisor

William H McDowell

Third Advisor

Julia G Bryce

Abstract

A five-fold global urban population increase over the last century is driving global land use change and causing coastal eutrophication and dead zones. This dissertation was designed to better understand how suburbanization in the Ipswich and Parker River watersheds, the two major rivers draining to the Plum Island Estuary, affects nutrient loading the Plum Island Estuary. I used an almost 17-year long-term nutrient record to test whether ongoing watershed suburbanization has affected net fluxes of nutrients to the Plum Island Estuary over the study period. Additionally, I paired this long-term study with spatial sampling of headwater watersheds across a range of suburbanization and sampling at small-scales along a number of longitudinal reaches to understand variability in inputs and stream nutrient concentrations. Lastly, I conducted a number of field experiments in seven reaches to better understand the controls nutrient uptake.

Despite ongoing land use change in the mainstem watersheds, Chapter 1 demonstrates there was not a detectable increase in dissolved inorganic nitrogen (DIN) or PO4 export over the time period. Climate and runoff variability overwhelmed any incremental increase of DIN and PO4 export from the mainstem watersheds. Mainstem watershed exports also diverged from headwater exports the most during summer, low-flow periods, suggesting retention of DIN and PO4 and the importance of buffering by the river network.

While the long-term study shows the importance of buffering at the network-scale, the sampling and experiments show catchment- and reach-scale buffering also have substantial influence on nutrient exports to downstream ecosystems. Using boron (B) as a tracer of P, Chapter 2 demonstrates the importance of catchment processes and in-stream uptake in controlling anthropogenic P. Meanwhile, the experiments in Chapter 3 show that despite the many impacts to streams from catchment suburbanization, these changes at the catchment scale do not consistently impact ecosystem processes.

Overall, catchment- and reach-scale processes have significant influence on nutrient cycling and fluxes from headwater ecosystems. Combined with nutrient retention within the river network, these processes have buffered the higher nutrient loading from the ongoing suburbanization within the PIE watersheds over the study period.

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