The role of phosphorus availability on nitrogen cycling: from Great Lakes to urban lakes

“Phosphorus is the master element that controls the availability of the others.” This idea, first proposed by AC Redfield, has been explored and advanced by Bob Sterner throughout his career. Our previous work with Sterner helped elucidate how differences in phosphorus (P) availability across the Laurentian Great Lakes influenced rates of nitrification and denitrification, allowing for a century-long increase in nitrate in Lake Superior. More broadly, this work suggested that management-driven reductions in P availability can lead to increased accumulation and export of nitrogen (N) in large lakes. Here, we explore how this idea applies to smaller urban lakes, many of which have been actively managed for P at the watershed level (e.g., by lawn P fertilizer restrictions and green stormwater infrastructure) and within lakes (e.g., through alum treatments). We examined temporal trends in urban lakes in the Minneapolis-Saint Paul metropolitan area. Many of these managed lakes have experienced declines in both total P and, to a lesser extent, total Kjeldahl N, resulting in increased water column N:P ratios. Potential ecosystem effects of reduced P availability may include increased downstream N export and decreased N-fixation. Changes in urban lake water nutrient chemistry are mediated by complex interactions between specific management activities, physical properties of the lake and watershed, and climate, but in aggregate, these urban lakes provide a unique laboratory to test hypotheses around ecological stoichiometry and its ecosystem scale impacts in human dominated lakes.     

Primary Presenter: Gaston Small, University of St. Thomas (gaston.small@stthomas.edu)

Authors:

Gaston Small, University of St. Thomas  (gaston.small@stthomas.edu)

Jacques Finlay, University of Minnesota  (jfinlay@umn.edu)

Catherine Polik, University of Minnesota  (polik020@umn.edu)