Vertical transport of sediment-associated metals and cyanobacteria by ebullition in a stratified lake

Abstract

Bubbles adsorb and transport particulate matter in a variety of natural and engineered settings, including industrial, freshwater, and marine systems. While methanecontaining bubbles emitted from anoxic sediments are found widely in freshwater ecosystems, relatively little attention has been paid to the possibility that these bubbles transport particle-associated chemical or biological material from sediments to surface waters of freshwater lakes. We triggered ebullition and quantified transport of particulate material from sediments to the surface by bubbles in Upper Mystic Lake, MA, and in a 15m tall experimental column. Particle transport was positively correlated with the volume of gas bubbles released from the sediment, and particles transported by bubbles appear to originate almost entirely in the sediment, rather than being scavenged from the water column. Concentrations of arsenic, chromium, lead, and cyanobacterial cells in bubble-transported particulate material were similar to those of bulk sediment, and particles were transported from depths exceeding 15 m, implying the potential for daily average fluxes as large as 0.18 μg arsenicm-2 and 2×104 cyanobacteria cellsm-2 in the strongly stratified Upper Mystic Lake. Bubble-facilitated arsenic transport currently appears to be a modest component of total arsenic cycling in this lake. Although more work is needed to reduce uncertainty in budget estimates, bubble-facilitated cyanobacterial transport has the potential to contribute substantially to the cyanobacteria cell recruitment to the surface of this lake and may thus be of particular importance in large, deep, stratified lakes.