Effects of Anthropogenic Urea on Lakes of the North American Great Plains: Ecological Implications Based on Experimental and Observational Studies

Abstract

Urea consumption has increased dramatically since the 1960s and now comprises over 50% of the nitrogen (N)-based fertilizer used globally. Currently, only 30-50% of N-fertilizers applied to cropland are effectively used by crops, while the remainder is lost to the environment. Urea is also a component of livestock and human wastes. In principal,export of urea to lakes as a consequence of human activities may exacerbate ecological problems associated with eutrophication (e.g. increased productivity of aquatic fauna, deep water anoxia, biodiversity loss, fish kills, etc.), especially in phosphorus (P)-rich aquatic ecosystems. Biweekly measurements of urea content and limnological variables (water chemistry, hydrology, algae, zooplankton) during two summers in a chain of seven productive lakes in central Canada were used to quantify human and environmental influences on temporal and spatial patterns of urea occurrence. Mean (+ SD) urea concentrations varied between 28.7 + 14.0 and 131.7 + 64.9 μg N L-1, increased from headwater to downstream sites, and represented 10-50% of bio-available N. Principal components analysis demonstrated that urea concentrations were elevated in agriculturally-impacted lakes with abundant dissolved organic and inorganic nutrients (N, P, C) and low O2 concentrations, and were inconsistently correlated with plankton abundance and community composition. In contrast, urea concentrations were elevated more than two-fold in lakes receiving N from cities, despite low concentrations of urea in tertiary-treated urban effluent (~50% of lake values). Furthermore, dissolved organic N accounted for ~90% of total dissolved N in a survey of 69 closed basin lakes, suggesting that urea is ubiquitous in regional lakes. These findings suggest a new model for the regulation of the urea in lakes in which land use practices regulate lotic influx, stimulate regeneration from lake sediments, and influence the balance between planktonic consumption and release of urea. Differential effects of urea pollution on phytoplankton and heterotrophic bacteria were quantified in three mesocosm experiments conducted in P-rich, hypereutrophic Wascana Lake, Saskatchewan. Urea was added weekly at 0, 1, 3, 8, and 18 mg N L-1 to mesocosms (~3000-L) for 21-days each during July, August, and September of 2009. Repeated-measures analysis of variance (RM-ANOVA) revealed all urea concentrations stimulated increases in phytoplankton biomass and productivity to a stable plateau by day 7, afterwards light and P may have limited future autotrophic responses. The magnitude of algal response generally increased with urea loads up to 3 – 5 mg N L-1, but additions beyond that level had little effect on algal abundance, and actually reduced primary production relative to maximum values. In contrast, bacterial abundance and production responded more slowly in a linear fashion to urea amendments, such that bacterial activity was sufficient to deplete oxygen by day 21 in trials with > 8 mg N L-1. These findings suggest that urea pollution at concentrations < 3 mg N L-1 may rapidly enhance net autotrophy, while urea additions > 5 mg N L-1 decreasingly favor net autotrophy. Together, these results suggest that the expected 100 million metric ton increase in urea use by ~2050 is likely to alter global N biogeochemistry, ecosystem metabolism,and accelerate water quality degradation in eutrophic aquatic ecosystems across the planet. This thesis concludes with a brief consideration of management strategies which may reduce urea influx to surface waters and favor maintenance of the ecological integrity of aquatic communities.