Identifier

etd-10312013-120652

Degree

Doctor of Philosophy (PhD)

Department

Oceanography and Coastal Sciences

Document Type

Dissertation

Abstract

Humans are responsible for global-scale alteration of nitrogen (N) and phosphorus (P) biogeochemical cycles to support food production. Increases in N and P inputs into soils and waste-streams has resulted in excessive nutrient loading to surface waters, including the Mississippi River, leading to eutrophication. Here I investigated N and P dynamics occurring in the Lake Pontchartrain Estuary and Basin. I measured two biogeochemical processes using intact sediment core incubations and quantified their importance in the context of nutrient-rich Mississippi River flood diversions through the Bonnet Carré Spillway. I show that diffusion of nitrate-N into sediments accounts for a substantial magnitude of nitrate loss from the water column during diversions, but plays a relatively minor role in the transformation of the large amount of nitrate received. Diffusive flux of P from sediments is a significant source of dissolved inorganic P to the water column and may be an important contributor to summertime blooms of N-fixing harmful algae. I present a comparative analysis of ecosystem response in Lake Pontchartrain during the three most recent Bonnet Carré Spillway openings (1997, 2008, and 2011). Nutrients in the Mississippi River diversion plume are rapidly depleted during summertime, after which sediment P loading restores N-limited conditions. Several interrelated chemical and physical parameters influence ecosystem response during diversion events and there is no simple stimulus-response relationship between N loading and harmful cyanobacterial blooms. In 2011, cyanobacteria were likely suppressed by hydraulic flushing. Accounting for P and improving human P use efficiency are critical tasks given the finite global supply of phosphate rock. I used material flow analysis to examine anthropogenic P cycling in the Upper Pontchartrain Basin for 2001-2005 and 2006-2010. Mass balances encompassed human-mediated P fluxes in food production and consumption subsystems across agricultural, developed, and forested landscapes. Increases in fertilizer and oil prices were correlated to drastic reductions (78%) in purchased inorganic P fertilizer. The dominant source of P input shifted from food production to the consumption subsystem between periods. Leakage to the Pontchartrain Estuary and the Mississippi River represented 18-24% of total P input, while the vast majority accumulated within soils, wastewater systems, and landfills.

Date

2013

Document Availability at the Time of Submission

Student has submitted appropriate documentation to restrict access to LSU for 365 days after which the document will be released for worldwide access.

Committee Chair

White, John R

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