Seasonal Impacts and Regulation of Nitrogen Pollution in the Northern Great Plains Insights from Microcosm, Mesocosm, and Mensurative Scale Studies
Abstract
The value of controlling nitrogen (N) and its impact on water quality are frequently
contested, partly due to inconsistencies in the effects of N pollution depending on the
form, amount, and seasonal conditions in which it is released to freshwater ecosystems.
This thesis aims to shed light on some of those differential effects, and advance our
understanding of the seasonal variation in the impacts and drivers of N pollution to
prairie lentic and lotic environments.
In my first chapter, I evaluate the seasonal and differential effects (i.e.,
suppression vs stimulation) of NH4
+ on phytoplankton abundance, using 16 years of
nutrient bioassay experiments, conducted bi-weekly during the open water season.
Phytoplankton biomass was significantly affected by NH4
+ amendment in 44.8% of the
experiments, and generalized additive models (GAMs) demonstrated that the the seasonal
patterns of phytoplankton response showed a marked rise in the occurrence of both spring
suppression and summer stimulation over the study period. Binomial logit GAMs
demonstrated that the likelihood of NH4
+ suppression of phytoplankton growth increased
with abundance of siliceous algae, cryptophytes, and unicellular cyanobacteria, when
water temperatures and soluble reactive phosphorus (SRP) concentrations were low,
while stimulation of phytoplankton growth was more likely when chlorophytes and non-
N2-fixing cyanobacteria were abundant, and temperatures and SRP concentrations were
high.
In my second chapter, I present the results of a series of 22 factorial urea and
phosphorus (P) fertilization experiments, conducted monthly from ice-off to ice-formation
in 3000-L mesocosms. These mesocosm experiments were also run in tandem
with NH4+ bioassays to compare the seasonal effects of urea pollution with those of NH4+.
Results showed that addition of P alone had no significant impact on either
phytoplankton abundance or community composition, but that urea, alone or concert with
P, consistently increased abundance of cryptophytes, chlorophytes, and non-diazotrophic
cyanobacteria in spring, and abundances of chlorophytes and non-diazotrophic
cyanobacteria in the summer and early fall. Comparison of urea mesocosms with NH4
+
bioassays demonstrated that urea lacked the inherent toxicity of NH4
+ in cool waters.
In my final chapter, I identify the temporal and spatial patterns in urea export
along a 250-km lotic continuum in the NGP, using three years of data collected bi-weekly
from May-September, and investigate the hydrologic, land use, and instream drivers of
urea concentration. I found that urea concentrations were elevated compared to
previously studied lentic systems in this region, and ranged from 5.2 – 792.1 μg N L-1.
Significant differences in longitudinal patterns of urea concentration emerged over the
course of the open water period (F(6, 290) = 8.183, p < 0.001), and were mediated by
interactions between hydrology, land use inputs, and internal processing. Contrary to
expectations, I detected no significant effect of wastewater effluent discharge on instream
urea concentrations during low and moderate discharge rates. Instead, average porewater
urea concentrations ̅ = 528.5 μg N L-1, SD = 229.8 μg N L-1) were over five times
higher than instream concentrations, emphasizing the importance of hyporheic sources.
Together, these three studies provide a comprehensive assessment of how the risk
of N pollution to water quality in the NGP varies across the open water season, and
informs a series of recommendations, provided in Chapter 5, for future water quality
management.