Date of Award

8-1-2016

Degree Type

Thesis

Degree Name

Master of Science

Department

Freshwater Sciences and Technology

First Advisor

Laodong Guo

Committee Members

Harvey Bootsma, James Whaples

Keywords

Chemistry, Dissolved Organic Matter, Freshwater, Nitrogen, Nutrients, Phosphorus

Abstract

Physical, chemical and biological processes directly influence the transport, composition, and fluxes of dissolved organic matter (DOM) in river watersheds. Changes in the abundance and composition of DOM and nutrients (P&N) in the watershed should reflect changes in hydrological cycle, effluent discharge, land-use and land-cover, and anthropogenic activities in the river basin, especially in rivers that run through metropolitan areas such as the Milwaukee River. Despite the importance of DOM to ecosystem health and function, a literature search to date finds no comprehensive accounting of DOM in the Milwaukee River. To examine DOM dynamics, monthly water samples were collected between February 2014 and April 2015 for the measurements of hydrographic parameters, bulk dissolved organic carbon (DOC), chromophoric-DOM and fluorescent-DOM to determine temporal variations in source and composition of DOM in the lower Milwaukee River and fluxes of DOM to Lake Michigan, as well as influence of human activities in the river basin.

Concentrations of DOC varied from 336 µM-C during the winter under ice cover to 1,146 µM-C during the snowmelt in early spring, with an average of 726±224 µM-C. DOC abundance in the Milwaukee River was found to be relatively high in comparison to other world rivers. Absorption coefficients at 254 nm (a254) averaged 71±31 m-1, and show a significant correlation with DOC (R2=0.8370). Non-chromophoric DOC represented approximately 33% of the bulk DOC in the river, and majority of DOC (67%) was optically active. Specific ultra violet absorbance at 254 nm (SUVA254) and spectral slope (S275-295) were inversely correlated with an average of 3.6±0.7 L mg-C-1 m-1 and 0.0170±0.0034 nm-1 respectively. SUVA254 and S275-295 were mostly controlled by hydrological, biological and degradation processes. Applications of parallel factor (PARAFAC) modeling on EEMs data identified three major fluorescent DOM components (C1, C2 and C3) in the river waters, including two terrestrial humic-like components (C1 and C2) and one protein-like component (C3).

In addition to DOM, variations in abundance, chemical speciation, and export fluxes of nutrients (P&N) from the lower Milwaukee River, as well as the influence of hydrology and anthropogenic activities, also have been studied. Nutrient species including nitrate (NO3-), dissolved organic nitrogen (DON), dissolved inorganic phosphorus (DIP) or phosphate, dissolved organic phosphorus (DOP), particulate inorganic phosphorus (PIP) and particulate organic phosphorus (POP) were measured. Overall, NO3- was the predominant dissolved N species and DON contributed up to ~30% of the total dissolved N transported in the river. Concentrations of NO3- ranged from 62 μM-N during the warmer season in spring and summer to 259 μM-N in the colder months in winter with an average of 125±60 μM-N, suggesting that NO3- was predominately controlled by biological uptake and hydrology. Average NO3- concentration during 2014-2015 was among the highest in comparison with major world rivers, reflecting the anthropogenic impact on nutrient abundances in the Milwaukee River potentially through agricultural activities. Similar to DOC seasonal variations, DON varied from 2 μM-N under ice in winter to 254 μM-N during the spring snowmelt with an average of 54±50 μM-N. Within the total dissolved phosphorus (TDP) pool, almost 50% were organic phosphorus (DOP) and the other 50% were inorganic (DIP), indicating a potential role for DOP in nutrient cycling. Phosphorus exported from the Milwaukee River to Lake Michigan coastal zone was predominately in the dissolved form (DIP+DOP represented 62% of total P). DIP exhibited large seasonal variability ranging from 0.114 μM-P in January 2015 under the ice to as high as 2.75 μM-P during April 2015 storm event, with an average of 0.970 ± 0.791 μM-P. DOP varied from 0.080 μM-P when discharge was high to 4.687 μM-P during the summer when discharge is low and primary productivity is intense, with an average of 1.120±0.990 μM-P. PIP and POP abundances were mainly controlled by hydrology. They were low under ice and very high during the April 2015 storm event and uniquely much lower than those of other world rivers. PIP varied from 0.045 to 2.229 μM-P with an average of 0.473±0.483 μM-P and POP varied from 0.224 to 3.791 μM-P with an average of 0.809±0.648 μM-P. The average N/P ratios in inorganic and organic nutrient pools are 151±84 and 80±67 respectively, indicating a N- enriched but P-depleted ecosystem in the lower Milwaukee River. Using the interpolation method of regression, the annual export fluxes of DIP, DOP, PIP, POP, NO3-, DON and DOC were 63, 14, 47, 64, 1325, 200 and 6710 (103 kg-P, N or C) respectively and the yields were 28, 6, 21, 28, 581, 88 and 2943 (kg-P, N or C) km-2 respectively.

Available for download on Wednesday, December 05, 2018

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