Societal phosphorus metabolism in future coastal environments: Insights from recent trends in Louisiana, USA

Successful adaptation to global environmental change will require confronting multiple unfolding challenges in concert. Coastal regions vulnerable to sea level rise and tropical storms will likely also be influenced by resource limitation in an uncertain future. In this paper, we explore the interrelated dynamics of coastal population migration, economic instability, and anthropogenic phosphorus (P) flows. Accounting for P flows and improving human P use efficiency are critical tasks given the finite global supply of phosphate rock and widespread eutrophication. We use material flow analysis to examine societal P metabolism in the Upper Pontchartrain Basin in coastal Louisiana, USA for two 5-y time periods (2001-2005 and 2006-2010) to capture the effects of fertilizer economics and population growth partially driven by the impact of Hurricane Katrina in the lower basin in 2005. Mass balances encompass human-mediated P fluxes in food production and consumption subsystems across agricultural, developed, and forested landscapes. Drastic reductions in locally purchased inorganic P fertilizer (78% decline between periods) were correlated to increases in fertilizer prices. Total P input to the study region decreased from 5452 to 3268 Mg P y−1 between periods. Changes in P flows were primarily driven by fertilizer economics, declining dairy production, and the influx of new residents, which has been characterized by decentralized settlement that limits P recycling. Societal P metabolic efficiency increased from 22% to 32% due largely to reduced fertilizer inputs. Leakage to the Pontchartrain Estuary and the Mississippi River represented 17-23% of total system P input, while the vast majority of P accumulated within soils, wastewater systems, and landfills. We discuss basin trends and management implications. A historic opportunity exists to encourage future coastal development characterized by synergies between local agriculture and human habitation to promote energy efficient nutrient recycling. The effect would be a decreased vulnerability to future fertilizer price spikes, along with the mitigation of current and future eutrophication.