Abstract
Bubbles adsorb and transport particulate matter both in industrial and marine systems. While methane-containing bubbles emitted from anoxic sediments are found extensively in aquatic ecosystems, relatively little attention has been paid to the possibility that such bubbles transport particle-associated chemical or biological material from sediments to surface waters of freshwater lakes. We quantified transport of particulate material from sediments to the surface by bubbles in Upper Mystic Lake, MA and in a 15 m tall experimental column. Vertical particle transport was positively correlated with the volume of gas bubbles released from the sediment. Particles transported by bubbles originated 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, resulting in daily fluxes as large as 0.18 mg of arsenic m-2 and 2 × 104 cyanobacterial cells m-2 in the strongly stratified Upper Mystic Lake. While bubble-facilitated arsenic transport currently appears to be a modest component of total arsenic cycling in this lake, bubble-facilitated cyanobacterial transport could comprise as much as 17% of recruitment in this lake and may thus be of particular importance in large, deep, stratified lakes.
Highlights
In 2004, 1.5 million lake acres in the US were impaired by metals such as lead, chromium, and arsenic (Environmental Protection Agency, 2004)
Bubbles could shed particles partway up the water column during rise
We did note that the first bubble column test conducted after repositioning the sediment source yielded a higher particle transport rate than those found in subsequent tests (Fig. 2), consistent with the intuitively reasonable possibility that mechanical sediment disturbance can affect particle loading on bubbles
Summary
Deterioration of water quality is widespread and expected to become more acute with increased urbanization and climate change (Zhang, 2016; Paerl et al, 2011). In a 2012 national assessment, 15.2 % of surveyed lakes in the US were categorized as “most disturbed” due to the concentration of cyanobacteria, a significant increase in lakes with this categorization (8.3 %, 95 % confidence intervals 4.0 %–12.5 %) over the 2007 assessment (U.S Environmental Protection Agency, 2016). Metals are important contaminants in freshwater systems because of their persistence and toxicity (Bronmark and Hansson, 2002). In 2004, 1.5 million lake acres in the US were impaired by metals such as lead, chromium, and arsenic (Environmental Protection Agency, 2004). Identifying the sources and mechanisms of transport of these substances within lake ecosystems can help predict the fate of contaminants and aid remediation efforts
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