Changes in the physical and geochemical properties of suspended sediment delivered to the headwaters of LOIS river basins over the last 100 years: a preliminary analysis of lake and reservoir bottom sediments

Bottom sediments from a single core retrieved from nine lakes and/or reservoir sites have been analysed for particle size, loss on ignition and the total concentration of Ca, Mg, Na, K, Fe, Al, Mn and P. Site differences reflect the relative significance of autochthonous and allochthonous contributions to the sedimentary record and are related to changes in sedimentation rate through time as determined from a 210 Pb and 137 Cs chronology determined for each core. Whilst changing sedimentation rates reflect both sediment influx from the contributing catchment and focusing within the lake basin, the patterns generally reveal changes associated with catchment disturbance over the last century. Calcium-rich sediments are found in two lakes draining catchments underlain by Oolitic limestones. Organic matter contents, as determined by low temperature loss on ignition, range from 13 to 25%. High organic matter levels are found in bottom sediments either where the contributing catchment has extensive blanket peat accumulations or in lakes with low sediment accumulation rates, where the organic matter content is more likely to be controlled by lake productivity. In general, organic matter content is inversely related to sedimentation rate and to sediment yield. Greatest down-core variations in sediment physical and geochemical signatures are found in lake sediment records where catchments have experienced significant increases in sediment influx from the catchment. The average sediment yield determined for each of the nine lake-catchments is inversely related to average organic matter and P content of the sediments, but an increase in sediment yield is associated with better sorting and a decrease in the proportion of coarse and medium silt sized particles. Six of the nine sites show evidence for significant increases in the total P load in their near-surface sediments, either resulting from higher than average sediment yields or as a result of significant increases in sediment-P concentrations.

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