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River Influence on the Nearshore Ecosystem of Western Lake Superior Joshua Delvaux University of Wisconsin-Milwaukee

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River Influence on the Nearshore Ecosystem of Western Lake Superior Joshua Delvaux University of Wisconsin-Milwaukee University of Wisconsin Milwaukee UWM Digital Commons Theses and Dissertations August 2017 River Influence on the Nearshore Ecosystem of Western Lake Superior Joshua Delvaux University of Wisconsin-Milwaukee Follow this and additional works at: https://dc.uwm.edu/etd Part of the Aquaculture and Fisheries Commons Recommended Citation Delvaux, Joshua, "River Influence on the Nearshore Ecosystem of Western Lake Superior" (2017). Theses and Dissertations. 1602. https://dc.uwm.edu/etd/1602 This Thesis is brought to you for free and open access by UWM Digital Commons. It has been accepted for inclusion in Theses and Dissertations by an authorized administrator of UWM Digital Commons. For more information, please contact [email protected]. RIVER INFLUENCE ON THE NEARSHORE ECOSYSTEM OF WESTERN LAKE SUPERIOR by Joshua M. Delvaux A Thesis Submitted in Partial Fulfillment of the Requirements for the Degree of Master of Science in Freshwater Sciences and Technology at The University of Wisconsin-Milwaukee August 2017 ABSTRACT RIVER INFLUENCE ON THE NEARSHORE ECOSYSTEM OF WESTERN LAKE SUPERIOR by Joshua M. Delvaux The University of Wisconsin-Milwaukee, 2017 Under the Supervision of Professor Harvey A. Bootsma As the interface between the terrestrial landscape and the open lake, nearshore areas of the Great Lakes play an important role in modulating whole-lake response to inputs of nutrients and energy from the watershed. These inputs occur primarily via tributary loading, and so it is critical to understand the dynamics of river plumes and the fate of organic carbon and nutrients delivered in the plumes. To assess the influence of river plumes on the biogeochemistry and metabolism of the Lake Superior nearshore zone, the spatial and temporal distribution of turbidity, nutrients, phytoplankton, dissolved oxygen, and dissolved carbon dioxide were measured in the western arm of Lake Superior and select tributaries from June 2016 through October 2016. This study focused on the nearshore ecosystem response to a large storm even in July, showing how the nearshore zone transitioned from a highly turbid, low productivity system immediately following the storm to high phytoplankton productivity after a one-month lag. A steady decrease in surface pCO2 in the month following suggests that increased water clarity and nutrient concentration following the plume event drive nearshore primary production. The shift towards net heterotrophy immediately following the storm event appears to be more so due to decreased water clarity and associated suppressed phytoplankton primary production rather than increased biological breakdown of dissolved organic carbon. ii © Copyright by Joshua M. Delvaux, 2017 All Rights Reserved iii TABLE OF CONTENTS ABSTRACT ..................................................................................................................................... i LIST OF FIGURES ....................................................................................................................... vi LIST OF TABLES ........................................................................................................................ vii LIST OF EQUATIONS ................................................................................................................ vii LIST OF SYMBOLS ................................................................................................................... viii ACKNOWLEDGEMENTS ........................................................................................................... ix Chapter 1: Introduction and Background .........................................................................................1 Chapter 2: Physical, Chemical, and Biological Response to a Large Storm Event in the Nearshore Zone of Lake Superior .......................................................................13 Introduction .............................................................................................................................13 Methods....................................................................................................................................19 Sampling Frequency & Spatial Coverage ....................................................................19 Horizontal Distribution of River Inflow .........................................................................25 Vertical Distribution of River Inflow .............................................................................30 Total Phosphorus & Organic Carbon Dynamics ..........................................................36 Phytoplankton Spatial & Temporal Variability .............................................................39 Results .....................................................................................................................................42 Horizontal Distribution of River Inflow .........................................................................42 Vertical Distribution of River Inflow .............................................................................49 Total Phosphorus & Organic Carbon Dynamics ..........................................................61 Phytoplankton Spatial & Temporal Variability .............................................................69 Discussion ................................................................................................................................73 Chapter 3: Metabolic Response of the Nearshore Zone of Lake Superior to an Episodic Inflow Event ..................................................................................................81 Introduction .............................................................................................................................81 Methods....................................................................................................................................85 Sampling Frequency & Spatial Coverage .....................................................................85 Spatiotemporal Variability in pCO2 & O2 % Saturation ................................................85 Lake-Atmosphere CO2 & O2 Flux ..................................................................................93 Temperature-Biological Effect on pCO2 ........................................................................96 Results ......................................................................................................................................97 iv Spatiotemporal Variability in pCO2 & O2 % Saturation ...............................................97 Lake-Atmosphere CO2 & O2 Flux ................................................................................104 Temperature-Biological Effect on pCO2 ......................................................................107 Discussion ..............................................................................................................................110 Chapter 4: General Conclusions ..................................................................................................115 REFERENCES ............................................................................................................................118 APPENDIX A: Oxygen Percent Saturation Function .................................................................128 v LIST OF FIGURES Figure Abbreviated Name Page 2.1 True color image of a Lake Superior surface, river plume 18 2.2 Western Lake Superior and associated tributary study sites 20 2.3 Spatial distribution of vertical profiles and discrete water samples 24 2.4 Relationship between measured and estimated discharge 27 2.5 Relationship between total suspended solids and turbidity (NTU) 34 2.6 Relationship between Secchi depth and turbidity (NTU) 35 2.7 Comparison of methods for determining chlorophyll a concentration 41 2.8 Hydrographs of study rivers from June 21 – October 26, 2016 45 2.9 Time-lapse images of surface plume dynamics using Rrs 645 nm 46 2.10 Depth-averaged current velocities and directionality 48 2.11 Vertical structure of riverine interflow on June 23, 2016 54 2.12 Vertical structure of riverine interflow on July 7, 2016 55 2.13 Vertical structure of riverine interflow on July 28, 2016 56 2.14 Vertical structure of riverine interflow on August 17, 2016 57 2.15 Vertical structure of riverine interflow on September 21, 2016 58 2.16 Vertical structure of riverine interflow on October 25, 2016 59 2.17 Spatiotemporal variability in relative proportion of river to lake water 60 2.18 Contour plots of vertical and horizontal distribution of SRP 65 2.19 Contour plots of vertical and horizontal distribution of TDP 66 2.20 Contour plots of vertical and horizontal distribution of DOC 67 2.21 Comparison of daily mean chlorophyll a along the nearshore-offshore transect 70 2.22 Chlorophyll a spatial variability over the field season 71 2.23 Estimated euphotic depth over the course of summer 72 3.1 Layout of the components and processes of the CO2 field system 91 3.2 Comparison of methods for determining pCO2 92 3.3 Vertical profiles of pCO2 before and after an episodic storm event 101 3.4 Vertical profiles of O2 % saturation before and after an episodic storm event 102 3.5 Nearshore Lake Superior surface pCO2 values 103 3.6 Lake – atmosphere CO2 and O2 flux biplot 106 3.7 Temperature and biological effect on observed pCO2 108 3.8 Regression between biological and temperature components and observed pCO2 109 vi LIST OF TABLES Table Abbreviated Name Page 2.1 Geographic
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