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Phosphorus Relese from Oxic PHOSPHORUS RELEASE FROM SEDIMENTS IN SHAWANO LAKE, WISCONSIN by DARRIN HOVERSON A thesis submitted in partial fulfillment of the requirements for the degree of MASTER OF SCIENCE IN NATURAL RESOURCES (WATER RESOURCES) COLLEGE OF NATURAL RESOURCES UNIVERSITY OF WISCONSIN STEVENS POINT, WISCONSIN FEBRUARY 2008 ABSTRACT The release of phosphorus (P) from lake sediments to the water column is important to lake water quality. Previous research on sediment P release has largely been in deeper, stratified lakes where hypolimnetic anoxia can lead to very high sediment P release rates. Recent studies suggest that sediment P release may also be important in large shallow lakes. Sediment P release in shallow lakes is poorly understood, and it is important that lake managers have a better understanding of how it influences lake nutrient budgets. This research developed a P budget for Shawano Lake, a large shallow lake in north central Wisconsin, and used the sediment P release to explain the difference between measured loads and summer P increases in the lake. Laboratory derived P release rates from intact sediment cores taken from littoral areas of Shawano Lake exhibit mean P release rates that are high under anoxic conditions (1.25 mg m-2 d-1) and lower, although still significant, under oxic conditions (0.25 mg m-2 d-1). When compared to the other components of the summer P budget, internal sediment P load accounted for 71% (46% to 81% range) of the summer P budget. This P release could be explained with an oxic P release rate of 0.31 mg m-2 d-1 (0.10 to 0.49 mg m-2 d-1 range). Sediment P release was clearly important to P concentrations in Shawano Lake as it accounted for the largest source of P in the monitored summers. This internal P source is a vital P pathway to the lake and should be considered when attempting to manage P cycling in Shawano Lake and other shallow lakes. iii TABLE OF CONTENTS TABLE OF CONTENTS...................................................................................iv LIST OF FIGURES............................................................................................. v LIST OF TABLES..............................................................................................vi LIST OF APPENDICES ...................................................................................vii 1.0 Introduction.................................................................................................... 1 2.0 Literature Review .......................................................................................... 3 3.0 Methods and Material.................................................................................... 8 3.1 Study location and description: Shawano Lake, Wisconsin .................. 8 3.2 Shawano Lake and watershed water quality evaluation ........................ 9 3.3 Phosphorus Release Experiments ......................................................... 11 3.4 Core Sediment Analysis........................................................................ 15 4.0 Results and Discussion................................................................................ 17 4.1 Shawano Lake Phosphorus Budget ...................................................... 17 4.2 Quantifying Sediment P Release Rates ................................................ 29 4.3 Role of pH in Sediment P Release........................................................ 36 4.4 Sediment Characteristics....................................................................... 39 4.5 Summary................................................................................................ 44 5.0 Recommendations and further research...................................................... 45 REFERENCES .................................................................................................. 46 iv LIST OF FIGURES Figure 1. Shawano Lake, Shawano County, Wisconsin and core sampling locations...........8 Figure 2. Locations of the two in-lake water quality sampling locations (east and west), fourteen aquatic macrophyte bed sampling locations, and USACOE water quality unit deployment location ...............................................................................10 Figure 3. Environmental chamber used to house sediment cores, a mixing / hydration system, and a controlled environment. ......................................................................12 Figure 4. Sediment cores taken in Shawano Lake.................................................................12 Figure 5 Sediment agitation apparatus...................................................................................16 Figure 6. P mass (kg) in the epilimnion of Shawano Lake over time in 2006......................18 Figure 7. TP concentrations (µg L-1) collected during early morning sampling near dense aquatic plant beds. An increase in late June follows the senescence of curly-leaf pondweed...................................................................................................21 Figure 8. 2006 hypolimnion and epilimnion P concentrations (µg L-1)................................24 Figure 9 Temperature profiles in Shawano Lake in 2006 collected at east location............25 Figure 10. Dissolved oxygen (in mg/l) profiles in Shawano Lake in 2006 collected at east location................................................................................................................25 Figure 11. Typical P release pattern in anoxic (a) and oxic (b) sediment cores. Linear regression line shows time period used for calculation of release rates. Different symbols are different cores. .......................................................................................30 Figure 12. Relationship between pH and P release rate under oxic conditions for Shawano Lake and compared to other lakes where oxic P release rates were measured.....................................................................................................................38 Figure 13. P fractions of surficial sediment sampled at Site 5. .............................................40 v LIST OF TABLES Table 1. Examples of oxic P release rates reported for nine shallow lakes. ..........................7 Table 2. P Budget for the 2006 summer (July-September)...................................................28 Table 3. P release rates for anoxic and oxic sediment cores from Shawano Lake. ..............31 Table 4. P release rates for oxic and oxic, low pH sediment cores from Shawano Lake.....37 Table 5. Sediment P fractions determined by sequential extraction. ....................................40 Table 6. Elemental composition and other characteristics of sediment................................41 Table 7. Pearson correlation coefficients (r) for oxic P release rates (mg m-2 d-1), sediment P fractions (mg/g dry weight), and calcium, iron, and P (mg/g dry weight). Asterisks indicate significant correlations at the 0.05 level or less............42 vi LIST OF APPENDICES Appendix 1 - Laboratory derived P release rates, pH, and upper and lower bounds 95% confidence interval, site 3 and 4 ................................................................................54 Appendix 2 - Sediment characteristics at site 5.....................................................................56 Appendix 3 - P fractions as mg P g-1 sediment......................................................................57 vii 1.0 Introduction The transfer of phosphorus (P) across the sediment-water interface may be important to water quality. Where P moves from the water to the sediment it reduces the P available for algae and other aquatic organisms. When P from the sediment is released back to the water, it becomes readily available for these same organisms, particularly the phytoplankton community, which can increase in productivity (Sondegaard 2001 and 2003, Welch 2005). Because P sedimentation removes particulate and detrital P and P release is largely soluble P, the sedimentation and release is converting P to a more algal available form. P release from the anoxic sediment in deep lakes has been the subject of research for many years, but less is known about the importance of P release from oxic sediments in shallow lakes. Understanding P transfer across the sediment-water interface has become more important as management efforts to reduce external P loads have not led to the anticipated improvement in water quality in many shallow lakes. In some cases, this delayed lake response has been attributed to P release from the sediment. In those lakes, the P that accumulates in the sediment during periods of high external load may provide a source of phosphorus to the lake that continues after external loading has been reduced. This P can be released to the lake slowly over time and take many years to deplete. It is important that lake managers develop a better understanding of sediment P release, its impact on lake nutrient budgets, and its importance to improving water quality. It is hoped that the research on sediment P 1 release described in this thesis will assist lake managers better understand the role of sediment P transfer in lake systems. 2 2.0 Literature Review P release from sediment is influenced by many factors. For example, the release of P from the sediment increases with warmer water temperature, lowered dissolved oxygen, high or low pH, and a lower redox potential. The influence of these different P release mechanisms may vary over time and between lakes. Increased summer
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