The physical and chemical description of the limnetic sediments from Langford Lake, South Vancouver Island, British Columbia
Date
1987
Authors
McKean, C. J. P.
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Abstract
Langford Lake was chosen to observe the effects of climate, geology, and geomorphology on the physical appearance and chemical composition of lake sediments . Thirty-two surface sediment samples , and two cores (8 and 12 m) were collected from the hypolimnion (> 7 m water depth ) of Langford Lake . The results indicate that the analytical and within-sit e variability is sufficiently low that one sample or core per site is representative of that site .
The elements analysed from the surface sediments exhibited several different horizontal distributions . The concentrations of the elements associated with the organic fraction (C, Ca, and Kjeldahl nitrogen) decreased with increasing water depth. Longer settling times for seston with increasing water depth , and the redistribution of sediments during the process of sedi ment focusing , permits mor e complete decomposition of the organic fraction.
Fe and Mn concentrations also decreased with increasing water depth due to lower pH and redox potentials in the deeper waters. Al, and Mg concentrations appeared to be highest near the shores where the erosion of elastic materials would be highest . Phosphorus concentrations exhibits a similar distribution as the organic fraction . However , its horizontal distribution is more complicated because inorganic phosphorus can be bound in the sediments to the minerals of Fe , Al , and Mn.
The sediments of Langford Lake are primarily composed of autochthonous organics , inorganic diatom frustrules, and inorganic amprophous minerals, forming a homogeneous yellow-brown organic Limus detrituosus. Limus sediments dominated the surface sediments and most of the vertical stratigraphy . Based on physical appearance , the core stratigraphy could be divided into three organic layers, a layer of volcanic tephra from Mount Mazama, a zone of allochthonous elastics (clay) representing the influence of the Vashon glaciers, and a zone representing the interphase between the elastic and organic zones. The chemical analysis of the core indicates that Al and Mg are primarily associated with the elastics minerals, because their concentrations are highest in the clay zone. The concentrations of Fe, Mn, and Pare thought to be the result of the weathering and leaching of minerals from the drainage basin, because their concentrations are highest in the non-elastic (organic) sediments.
The organic C:TKN ratio of the core's organic fraction is approximately 1 0: 1 , indicating autochthonous organic production. The source of the carbon and nitrogen is the fixation of atmospheric carbon and nitrogen by plants and bacteria, respectively. In contrast, the source of inorganic elements is from the erosion and/or weathering of minerals from within the air- and watershed. The organic carbon content of the core sediments is positively correlated with phosphorus, indicating the weathering and leaching of phosphorus from the watershed limits the rate of autochthonous production and deposition (eutrophication) in Langford Lake.