The chemical exchange between sediments and overlying water can be a significant component of the total loading to or loss from the bottom water, particularly in shallow settings like the central basin of Lake Erie, since the areal fluxes are substantial on a volumetric basis.  For example, water column hypoxia is caused, in part, by the consumption of oxygen by microbial and chemical reactions in the sediment (sediment oxygen demand, SOD). The small volume of the hypolimnion in the central basin of Lake Erie does not contain enough oxygen to meet the SOD requirements during the entire summer stratified period and therefore the bottom waters gradually become depleted in oxygen.  Unfortunately, our knowledge of nutrient and oxygen exchange between sediment and the water column is not well developed, and there is considerable uncertainty in the predictive abilities of SOD and nutrient cycling in part because of a lack of quantitative knowledge of rates of reactions in sediments and understanding of the sediment-water chemical exchange process.  This project directly addresses this deficiency.

    Regular ship-board monitoring of oxygen in the hypolimnion of Lake Erie by the United States Environmental Protection Agency Great Lakes National Program Office (GLNPO) has been established to monitor the status of the lake and determine if the water quality is meeting the terms of the Great Lakes Water Quality Agreement (GLWQA). Their annual water quality monitoring reveals that in spite of reductions in phosphorus loadings over the past 20 years spring total phosphorus concentrations are as high as in the 1970s, Lake Erie bottom waters have gone anoxic in the late summer months and the areal extent of the anoxia has increased during recent years (Rockwell and Warren 2003). These observations can be termed ‘The Lake Erie Trophic Paradox’. The traditional eutrophication model predicts that a decrease in phosphorus loading will result in a decrease in phosphorus concentration which will result in a decrease in algal production (chlorophyll) which will result in less hypolimnion oxygen depletion.  This is exactly what was observed in Lake Erie during the 1970-1990 time period. However, since then these systems all appear to be disconnected.    Phosphorus loadings have remained at about the target loadings of 11,000 metric tons/year, phosphorus concentrations have increased to levels as high as in the 1970s while chlorophyll concentrations have dropped to the lowest values observed and oxygen depletion rates appear to have increased.  These findings indicate that there is a need to better understand nutrient cycling and oxygen depletion in Lake Erie. This project seeks to obtain an accurate, calibrated description of the SOD and sediment-water nutrient exchange in Lake Erie.  This will be accomplished by obtaining measurements of SOD and nutrient exchange by three techniques:  whole core incubations, calculations using microelectrode and pore water profiles, and from plug flow-through reactor measurements.

This work involved the collection of data that that will enable an improved quantitative understanding of sediment oxygen demand and sediment-water nutrient exchange in Lake Erie. The work is part of the PIs long-range plan to develop a calibrated model for SOD and nutrient loadings for Lake Erie.  This work is directly applicable to nutrient cycling and bioenergetic models, ecosystem models, and SOD monitoring.  These data and eventual calculations of calibrated models of nutrient cycling and anoxia forecasting can help reduce uncertainty by increasing an understanding of the conditions that lead to hypoxia and in establishing target nutrient loadings.  The results can be used by resource managers to assess management strategies and enable them to make informed decisions regarding hypoxia in Lake Erie. The project will help close the gaps between data generators, modelers, and resource managers.  The data created from this project will be useful in the future evaluation of target chemical loads for Lake Erie. The project addresses the Great Lakes Water Quality Agreement, the Ecosystems and Habitats Thematic area of Ohio Sea Grant's Strategic Plan, and the Ensure Sustainable Water Use objective of the Great Lakes Governor's Priorities.

The work plan involved conducting measurements and experiments to obtain a better understanding of the sediment oxygen demand and the internal chemical loading to Lake Erie and can be summarized as follows:

    •  collect box cores from EPA master monitoring stations in each of the three major basins in Lake Erie (EPA Station ER91M in the western basin, EPA Station ER43 in the central basin, and EPA Station ER15M in the eastern basin) and sub-sample for microelectrode measurements, for whole core incubations, for pore water analyses, and for plug flow-through reactor experiments;
    •  obtain vertical profiles in sub-cores from each sampling site of O2, SO42-, NO3-, and pH using microelectrodes;
    •  conduct whole core incubations to monitor O2 depletion (SOD) in sub-cores from each sampling site;
    •  extract vertical profiles of pore waters from sub-cores from each sampling site for analysis of alkalinity, ammonium, ferrous ion, manganous ion, total (leachable) iron, and total (leachable) manganese;
    •  conduct plug flow-through reaction experiments on sub-cores from each sampling site:
    - construct plug flow-through reactors;
    - conduct these experiments with four concentrations of O2(<1% O2, 1-2% O2, 10% O2, 21% O2);
    - conduct these experiments on five different core section depth intervals (0-1 cm, 1-2 cm, 2-3 cm, 3-4 cm, and 4-5 cm) to obtain the reaction rates as a function of depth;
    - conduct these experiments at two different temperatures (10oC and 20oC) to obtain the reaction rates as a function of temperature;
    - conduct these experiments with/without the sediment poisoned with sodium azide to prevent microbial activity;
    - monitor O2, nitrate, sulfate, sulfide, alkalinity, ammonium, pH, ferrous ion, and manganous ion in the effluent stream of the reactors to obtain rates of O2 consumption and byproduct regeneration;
    - measure total leachable iron and total leachable manganese in sediment solids from each depth interval;
    -    model data to obtain appropriate reaction and exchange rates;
    •  compare SODs calculated by all three methods and at all three stations;
    •  compare reaction and exchange rates to those reported from other settings (including marine systems);
    •    use reaction rates in future biogeochemical modeling of Lake Erie SOD and sediment-water nutrient exchange.