Student Investigator
Bjorn Lake,
Doctoral Candidate
Civil and Environmental Engineering Department, UMaine
Project Synopsis
Phosphorus (P) is often the limiting macronutrient in freshwater ecosystems. Eutrophication is a problem affecting many Maine lakes, especially in the Belgrade Lakes region. Bjorn is focusing his dissertation on P cycling between the sediment and water column of Salmon and Lovejoy ponds. He is using a passive sampling technique involving hydrogels to measure the P porewater concentration gradients in both ponds. Each pond develops large P concentration gradients in the hypolimnion during the summer stratification period. Preceding the release of P from the sediment to the water column, Bjorn’s hypothesis is the P porewater concentration depth profiles change dramatically and characteristically in each pond based on sediment properties. He is measuring the porewater in both ponds at four times during the winter, spring and summer. Based on his literature search, this will be the first time P porewater profiles will be examined temporally using hydrogel technology. This research will augment his knowledge of these systems and provide the data needed to complete the final chapter of his dissertation.
Introduction
Sediments can be a source or a sink of phosphorus (P) in a lake. Vertical concentration gradients in the sediment porewater can lead to a substantial release of P under certain physiochemical conditions. Phosphorus concentrations in sediment porewater are often controlled by solid-solution reactions involving iron, aluminum and manganese oxides. Examples of these reactions are redox, sorption, dissolution, and precipitation. Most of these processes are mediated by microbial activity in the sediment. Microbes may also contribute directly to P porewater concentrations by mineralization of organic P compounds in the sediment. Since P is of great concern with respect to the eutrophication of lakes, understanding the dynamics of P porewater concentration profiles and associative metals is essential for effective lake management strategies.
Hydrogels were first developed in the field of biochemistry for electrophoresis studies of DNA. Recently, hydrogels have been synthesized for use in environmental studies as passive samplers. The gels consist of acrylamide and bisacrylamide cross-linker to form a polymer that is approximately 90% water. These hydrogels can then be physically manipulated, placed in deployment paddles, and covered with a permeable membrane to measure in-situ solute concentrations by molecular diffusion into the gel. Depending on the thickness of the gel, equilibration with the surrounding porewater can be reached within hours to days. After retrieval, the gels are eluted with dilute acid and measured for the analyte of interest.
Both Salmon and Lovejoy ponds experience mild to severe algal blooms during the summer months which correspond with large vertical concentration gradients of P in the hypolimnion. This suggests that P is being released from the sediment and contributes to the productivity of each lake. Sediment chemical extractions of both ponds have been analyzed by standard methods and nuclear magnetic resonance. These data have shown that indeed the sediment is a major source of P to the water column. However, the mass balance of solid phase P released in the sediment to the P accumulated in the water column has not matched, though the trends are convincing. In Salmon Pond, the amount of P released from the sediment solid phase is much larger than the accumulated P in the hypolimnion and, in Lovejoy Pond, the amount of P accumulated in the hypolimnion is much larger than the P released from the sediment solid phase. To explain these mass balance discrepancies, accurate P porewater concentration profiles are needed to model the flux of P upward into the water column and downward into the sediment from the concentration maxima.
Objectives
- Accurately measure the concentrations of P and metals in the porewater of Salmon and Lovejoy pond in the winter, spring and summer.
- Use the collected data to model the flux of P at different times during the year.
- Compare the results to already published values.
- Suggest possible mechanisms of P release using the porewater results and possible remediation strategies for mesotrophic to eutrophic ponds.
Methods Outline
The methods of this project have already been thoroughly scrutinized and tested. A benthic lander has been built to accommodate the sampler paddles. Each lander is equipped with 4 paddles for each deployment, two with a cell spacing of 1.5 cm and two with a cell spacing of 0.75 cm. One paddle will be used to measure the pH of the porewater, 2 paddles will be for P analysis, and the remaining paddle for metals. The benthic lander will be deployed in the profundal zone of the lake for 2 full days to reach equilibrium with the sediment porewater. Before retrieval, an Aquavision underwater viewer will be utilized to inspect the positioning of the benthic lander in the sediment. Immediately after retrieval, the pH paddle will be analyzed at the site with a portable pH probe to minimize carbon dioxide degassing. Hydrogen activity will be used to calculate metal and P speciation and solubility. The remaining paddles will be returned to the lab, disassembled, and the gels placed in 25 mL of 0.25M sulfuric acid for P analysis and 10 mL of 0.1M nitric acid for metals analysis. P analysis is done using standard colorimetric methods and the metals will be analyzed with an inductively-coupled plasma optical emission spectrophotometer (ICP-OES).
Impact of Project
The results of this project will immediately be submitted for peer-reviewed publication. This will be the first time that the porewater profiles will be characterized temporally using hydrogels. The method developed here can easily be duplicated by state and other monitoring agencies to measure the release of P from sediments in eutrophication studies.
Expected Deliverables
- A simple and relatively inexpensive method to measure porewater chemical profiles in lake sediment
- Phosphorus flux modeling from the sediment to the overlying water in both Salmon and Lovejoy ponds.
- A chapter of Bjorn’s dissertation
- A peer-reviewed publication
- An annual report submitted to the Mitchell Center
- A poster presentation at the 2010 Maine Water Conference
Investigators Qualifications
A former USGS employee in the Water Resources Division at Menlo Park, CA, Bjorn Lake has been pursuing his doctorate degree studying lake biogeochemistry since January, 2005. He is an expert in experimental design, sampling, analysis and data synthesis.
