Rising bubbles enhance the gelatinous nature of the air–sea interface

Abstract

Bubbles rising through the water column are known to scavenge organic material and microorganisms, and transport them through the air–sea interface after bursting. This mechanism has important implications for air–sea exchange processes. However, little is known about how bubbles influence the chemical and biological properties of the sea‐surface microlayer (SML), a gelatinous film at the air–sea interface. We used floating mesocosms in the coastal Baltic Sea and a laboratory tank filled with seawater from the North Sea to study the effect of bubbling on the gelatinous nature of the SML. Bubbling was found to always increase concentrations of transparent exopolymer particles (TEP) in the SML. In the field, TEP in the SML already increased after 2 min (53% ± 63%) and 10 min (19% ± 12%) bubbling, respectively. During the tank experiment, TEP enriched in the SML by 312% (± 244%) after > 3 h of bubbling. Therefore, bubbling is a highly efficient mechanism for TEP enrichment in the SML. Bubbling caused enrichment and depletion of microbial abundances (prokaryotes, flagellates, eukaryotes) in the SML. However, the incorporation of 3H‐thymidine (i.e., bacterial carbon production) was consistently stimulated after 10 min of bubbling in the field experiment, indicating a bubble‐induced import of unstressed bacteria and fresh organic substrates into the SML. Overall, our results suggest that the gelatinous matrix of the SML is re‐formed within min after disruption by bursting bubbles, and, thus, highlights the importance of biogeochemical interactions within the air–sea interface.

Depth is Relative: The Importance of Depth on TEP in the Near Surface Environment

“…This supports previous studies which found 280 enrichment of material even at wind speeds >8 m s -1 (Kuznetsova et al, 2004;Reinthaler et al, 2008), including the enrichment of TEP in the SML at moderate wind speeds (Wurl et al, 2009). Breaking waves from moderate wind regimes can create bubble plumes in the near surface water (Blanchard and Woodcock, 1957;Deane and Stokes, 2002), and this bubbling has proven to be an effective transport mechanism for TEP and DOM (Robinson et al, 2019;Zhou et al, 1998) to the SML. Thus, bubbling and turbulence at moderate wind speeds can induce more 285 complex enrichment processes and subdue any direct correlation with wind speed.…”

“…Further supporting that while phytoplankton are the main source for TEP production, the transport mechanisms for TEP and 270 phytoplankton differ. In large part due to the motility of phytoplankton species, which are known to have vertical migration patterns (Bollens et al, 2010;Schuech and Menden-Deuer, 2014) and can have motility responses to physical changes like turbulence (Sengupta et al, 2017).…”

“…We suggest that this is due to the abundance of precursor material as well as the increased formation of TEP 305 via the abiotic pathway. A tank experiment with the same setup as used by (Robinson et al, 2019), with different techniques to bubble the water, was also employed in Cape Verde, using water from the same near shore area as field samples. The unfiltered water was bubbled using the waterfall technique (Cipriano and Blanchard, 1981;Haines and Johnson, 1995) and via bubbling, large abundances of TEP were created (supplementary Figure S2).…”

Depth is relative: the importance of depth for transparent exopolymer particles in the near-surface environment

“…Baltic Sea (EMB 184) Station 3 4 5 8 9 10 11 12 Variance 3 × 10 4 2 × 10 0 2 × 10 3 1 × 10 3 1 × 10 6 4 × 10 5 1 × 10 5 7 × 10 5 Variance 2 × 10 5 2 × 10 3 1 × 10 3 3 × 10 3 6 × 10 2 2 × 10 3 7 × 10 3 2 × 10 3 1 × 10 3 2 × 10 3 2 × 10 3 7 × 10 3 2 × 10 3 (Robinson et al, 2019b).…”

Riding the Plumes: Characterizing Bubble Scavenging Conditions for the Enrichment of the Sea-Surface Microlayer by Transparent Exopolymer Particles

“…The accumulation and enrichment of TEP and attached OM in the SML have been found to be greatly enhanced by bubble-associated scavenging [22,23]. Enrichment of TEP in the SML can alter the sea surface hydrodynamics and act as a physicochemical barrier, suppressing the air-sea exchange of carbon dioxide and other trace gasses [1,3,24].…”