Abstract
Diurnally-migrating Chaoborus spp. reach populations of up to 130,000 individuals m−2 in lakes up to 70 meters deep on all continents except Antarctica. Linked to eutrophication, migrating Chaoborus spp. dwell in the anoxic sediment during daytime and feed in the oxic surface layer at night. Our experiments show that by burrowing into the sediment, Chaoborus spp. utilize the high dissolved gas partial pressure of sediment methane to inflate their tracheal sacs. This mechanism provides a significant energetic advantage that allows the larvae to migrate via passive buoyancy rather than more energy-costly swimming. The Chaoborus spp. larvae, in addition to potentially releasing sediment methane bubbles twice a day by entering and leaving the sediment, also transport porewater methane within their gas vesicles into the water column, resulting in a flux of 0.01–2 mol m−2 yr−1 depending on population density and water depth. Chaoborus spp. emerging annually as flies also result in 0.1–6 mol m−2 yr−1 of carbon export from the system. Finding the tipping point in lake eutrophication enabling this methane-powered migration mechanism is crucial for ultimately reconstructing the geographical expansion of Chaoborus spp., and the corresponding shifts in the lake’s biogeochemistry, carbon cycling and food web structure.
Highlights
Metabolic expenditure by staying in the often-colder hypolimnion[7]
Our experiments confirmed that Chaoborus spp. initially took up the dissolved methane when exposed to methane-saturated water (~1.5 mmol L−1)
After the initial exposure to the methane-saturated water and subsequent transferal to the experimental “methane-free” flask (~3 nmol L−1), methane in their gas sacs was redissolved into the surrounding water and into the head space as indicated by the increase in head-space CH4 concentration measured by the gas analyzer (Fig. 2a)
Summary
Metabolic expenditure by staying in the often-colder hypolimnion[7]. The conservation of energy is critical, as lower energy requirements allow migrating Chaoborus spp. to expand their habitat to deeper and less productive (less prey) lakes. We suggest that Chaoborus spp. utilize the high partial-pressure of dissolved gases in sediment porewater as a transport mechanism to allow them to migrate via buoyancy (Fig. 1). Combined with dissolved nitrogen (N2), the other most abundant sparingly-soluble gas in the sediment porewater, the two gases (CH4 and N2) lead to high enough partial pressures (matching or exceeding the ambient hydrostatic pressure) to effortlessly inflate the Chaoborus spp. gas sacs when in contact with the porewater. This provides an energetic advantage for migration through passive buoyancy rather than active swimming. As important side-effects, migrating Chaoborus spp. transport porewater methane to the water column directly in their gas sacs and bioturbate methane-rich sediments, allowing methane to bypass diffusive and oxidation limitations at the sediment-water interface
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