Sediment accumulation and Pb burdens in submerged macrophyte beds
Limnol. Oceanogr., 44(4), 1999, 1081-1090 | DOI: 10.4319/lo.19188.8.131.521
ABSTRACT: This study quantifies the role of lake morphometry and submerged macrophyte beds on the accumulation of sediments in the littoral zone. Stable Pb, a historical marker of lacustrine sediments in southern Quebec, was used to date the sediments (~110 yr) and to calculate three long-term sediment accumulation rates (SARs) in Lake Memphremagog (located in Quebec and Vermont). The anthropogenic Pb burden in the littoral zone of Lake Memphremagog was found to be two to eight times greater (per m2) than the Pb burdens in the profundal zone of surrounding eastern township lakes. Pb concentrations were nearly fourfold higher than background Pb concentrations (115 and 31 ug g-1, respectively), providing a reliable marker for littoral sediment core analysis. Lake morphometry is related to the three accumulation rates measured by providing distinct threshold limits (i.e., littoral slope, >10%; exposure, >10 km2) where sediments are unable to accumulate. Macrophyte beds are shown to disproportionately accumulate sediments at rates 2 to 20 times greater (per m2) than in the profundal zone. Linear regression models show that both the total SAR (mean, 1.7 mm yr-1) and organic SAR (mean, 83.1 g m-2 yr-1) are best predicted by the biomass density of the macrophytes, closely followed by plant mean biomass. Bulk SAR, principally representing the larger, inorganic fraction of sediments, is least predictable. Biomass density, but not mean biomass, was related to the accumulation rate of stable Pb (mean, 37.7 mg m-2 yr-1), supporting empirical models that show growth form to be an important determinant of both sediment and plant tissue elemental concen-trations. The variety of macrophyte communities sampled across Lake Memphremagog demonstrates that the conclusions drawn are not restricted to monospecific stands but to assemblages as a whole.