Determination%20of%20sediment%20phosphorus%20concentrations%20in%20St.%20Albans%20Bay,%20Lake%20Champlain:%20Assessment%20of%20internal%20loading%20and%20seasonal%20variations%20of%20phosphorus%20sediment-water%20column%20cycling

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Determination of sediment phosphorus
concentrations in St. Albans Bay, Lake Champlain
Assessment of internal loading and seasonal
variations of phosphorus sediment-water column
cycling

• Greg Druschel
• Department of Geology
• University of Vermont

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Acknowledgements

• Funding for this study through State of Vermont
  Clean Clear Program, administered by the
  Department of Environmental Conservation
• VT DEC, especially Eric Smeltzer and Pete
  Stengel, for providing logistical support
• St. Albans Bay Watershed association and town
  administrator Dan Lindley
• Students at UVM who assisted Aaron Hartmann,
  Rachel Lomonaco, Deb Schulman, and Ken Oldyryd
• Mary Watzin, Eric Smeltzer, Andrea Lini, Neil
  Kamman for thoughtful and insightful discussions

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Program Goals

• Determine the amount of phosphorus currently
  contained in St. Albans Bay sediments
• Determine how that phosphorus may be mobile
  within the sediments ? what controls P profiles
  and how P may be released from sediments into the
  water column, providing nutrients feeding algal
  blooms

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Redox Fronts

• Boundary between oxygen-rich (oxic) and more
  reduced (anoxic) waters
• Oxygen consumed by microbes which eat organic
  material
• When Oxygen is gone, there are species of
  microbes that can breathe oxidized forms of
  iron, manganese, and sulfur

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Methods

• Gravity Coring
• Seasonal site ? 10 separate samplings, late May
  through early October
• Early August collected 43 cores across the bay
  and Stevens Brook Wetland
• In-situ pore water measurments to determine redox
  front
• Chemical extractions of iron, manganese, and
  phosphorus in the sediment
• Inductively-coupled plasma optical emission
  spectroscopy (ICP-OES) to measure iron,
  manganese, and phosphorus from extractant

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Results Seasonal Work

• Sediments generally become more reduced as summer
  progresses
• Redox fronts move up and down in response to
  Temperature, wind, biological activity changes

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Summer 2004

• Summer 2004 was, in general, colder than average,
  windy, and rainy ? these factors would drive
  redox fronts higher in the sediment column
• Sediments at other locations (deeper, in the
  wetland) would be slightly different possibly
  having a lower redox front

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Seasonal Phosphorus mobility

• Ascorbic acid extractions of Fe, Mn, and P from
  10 sediment cores collected in summer 2004 show
  strong dependence between P and Mn or Fe
• Further, profiles show overall enrichment of all
  3 parameters in upper sections of sediment
• Fe and Mn would be primarily in the form of Fe
  and Mn oxyhydroxide minerals ? transformation of
  these minerals is key to P movement

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Sediment P mobility and water column

• Does the P mobilized in the sediment get into the
  water column and does it contribute to algal
  activity/ blooms?

• From our data, it seems that Fe, Mn mineral
  changes and P mobility in the sediments is linked
  to both P release and algal activity
• Our study methods were not specifically designed
  for this, but now that we know the processes
  involved, a better study of P transfer from
  sediment and use by algae is possible

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St. Albans Bay P Load

• 43 cores subdivided into 7 depth sections each,
  extracted 5 different ways analyzed in
  triplicate
• 1200 individual samples and 3000 individual
  analyses
• Results
• Top 4 cm contains 500 tons of P
• Top 10 cm of sediment contains 1200 tons of P

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• 1992 vs 1982 comparisons suggested P was being
  essentially flushed out of the sediments into the
  main lake

This study finds no evidence to support P
concentrations are substantially decreasing in
time i.e. ? the bay sediments will NOT clean
themselves in the near future
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Spatial distributions

• Profiles again reflect a general increase of P,
  Mn, and Fe in top few cm of these cores

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P Loading and sediment deposition

• Constantly moving redox fronts affect Fe and Mn
  minerals, mobilize P and turn ideal profile into
  what we actually see

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Summary of Findings

• There remains significant P in the sediments of
  St. Albans Bay, and overall rates of P loss from
  the sediments should be much lower than
  previously thought
• P mobility in the sediments is at least partly
  governed by Iron and Manganese mineralization
  which is strongly affected by redox fronts
• This mobility has changed the profile of P in the
  sediments by smoothing out any recorded changes
  in P loading.