Dilution of 210Pb by organic sedimentation in lakes of different trophic states, and application to studies of sediment-water interactions’

Lake sediments reflect conditions in the water column and can be used for rapid, integrative measurements of limnological variables. Examination of 2LoPb-dated cores from 12 Florida lakes of widely differing trophic state (expressed as Carlson’s trophic state index: TSI) shows that net accumulation rate of organic matter is related to primary productivity in the water column. In 26 other lakes the activity of unsupported *l”Pb g-l organic matter in surficial sediments is inversely related to trophic state and, therefore, to organic accumulation rate. From this observation we develop a new method that uses fallout *l”Pb as a dilution tracer to calculate net sedimentary accumulation rates of any material in surface mud. We demonstrate strong relationships between net loss rates of biologically important materials (C, N, P, and pigments) and their respective water concentrations (expressed as TSI). Multiple regression models incorporating net sediment accumulation rates of all four variables explain up to 70% of the lake-to-lake variation of TSI. The 210Pb-dilution method has applications for studies of material cycling, paleolimnology, and sediment accumulation processes. Loss of materials from the water column and their subsequent accumulation in the sediment is a poorly understood process. Until recently, there has been no direct and accurate method for measuring net flux of materials across the mud-water interface. Traps tend to misestimate sedimentation (Bloesch and Burns 1980; Bloesch and Evans 1982; Blomqvist and Hakanson 198 1). Losses to sediments calculated by difference between inputs to the lake and outflow, or calculated by regression techniques, are subject to equilibrium assumptions and limited to allochthonous materials (Bachmann 1984; Jones and Bachmann 1976; Chapra and Reckhow 1983a; Reckhow and Chapra 1983). The resulting uncertainties have contributed to controversies over which particular approach is most appropriate for examining factors that govern such things as primary productivity (Edmondson and Lehman 198 1; Chapra and Reckhow 1983b; Lehman and Edmondson 1983). The 210Pb dating of sediment cores provides a direct measurement of accumulation rates. However, the procedure is tedious and dependent on the often subjective application of various dating models. ReL This work supported by NSF grants DAR 79-248 12, EAR 8214308, and DEB 8211380. cent net accumulation rates have not been reported for timespans less than the last 30 years. In certain respects 210Pb dating is an art, with little standardization among laboratories and large uncertainties in details of applying the different models. We propose using the 210Pb content of the upper layer of sediment of a given lake as a measure of the modern rate of accumulation. Once the accumulation rate of the bulk sediment is known, then the rates of any constituent can be calculated. In the rest of this paper we develop the rationale explaining the relationship between sediment accumulation rate and 2 ‘OPb concentration, demonstrate that the relationship occurs in a number of lakes in Florida with known sediment accumulation rates, and apply the method to a different suite of lakes to measure their present rate of sediment accumulation. This application is different from the usual method of calculating accumulation rates from sediment cores in two ways. First, dates and accumulation rates calculated from 210Pb assay depend on the particular model used to describe the relationship between the rate of supply, the initial concentration, and the decay rate of 210Pb (Robbins 1978; Appleby and Oldfield 1978, 1983). Our model considers only the rate of supply, re-