Warming and Inhibition of Salinization at the Ocean's Surface by Cyanobacteria

Abstract

This paper describes high‐resolution in situ observations of temperature and, for the first time, of salinity in the uppermost skin layer of the ocean, including the influence of large surface blooms of cyanobacteria on those skin properties. In the presence of the blooms, large anomalies of skin temperature and salinity of 0.95°C and −0.49 practical salinity unit were found, but a substantially cooler (−0.22°C) and saltier skin layer (0.19 practical salinity unit) was found in the absence of surface blooms. The results suggest that biologically controlled warming and inhibition of salinization of the ocean's surface occur. Less saline skin layers form during precipitation, but our observations also show that surface blooms of Trichodesmium sp. inhibit evaporation decreasing the salinity at the ocean's surface. This study has important implications in the assessment of precipitation over the ocean using remotely sensed salinity, but also for a better understanding of heat exchange and the hydrologic cycle on a regional scale.

The Ocean's Skin Layer in the Tropics

“…At Stations 1–3, a faulty setting of the cell constant of the conductivity sensor caused inaccuracy in the calibration and, therefore, in the measurements. At Station 4, we observed intense cyanobacterial blooms leading to unique sea surface features; this has been reported elsewhere (Wurl et al, ). At Station 12, weather conditions did not permit deployment of field equipment (see next section).…”

“…Consequently, fresh water lenses can potentially merge to form large and very warm pools under calm sea conditions. Surfactant films have been suggested to modify salinity anomalies (Δ S ; Yu, ) and recently shown by Wurl et al () in the presence of surface slicks. However, in this study Δ S correlated weakly with the concentrations of surfactants in the skin layer ( r = 0.266, 95% CI [0.089, 0.428], n = 83) but shows that surfactant concentrations as Triton‐X100 equivalent (Teq) below 100 μg Teq/L has no effect on Δ S (Figure S7).…”

“…However, in this study Δ S correlated weakly with the concentrations of surfactants in the skin layer ( r = 0.266, 95% CI [0.089, 0.428], n = 83) but shows that surfactant concentrations as Triton‐X100 equivalent (Teq) below 100 μg Teq/L has no effect on Δ S (Figure S7). Conclusive statements about inhibition of salinization at increasing concentrations of surface‐active substances, as observed at Station 4 (reported by Wurl et al, ), cannot be made due to the limited number of observations in the presence of higher concentrations.…”

Using Ship-Deployed High-Endurance Unmanned Aerial Vehicles for the Study of Ocean Surface and Atmospheric Boundary Layer Processes

“…Figure 3 shows a map of the focus region north of Papua New Guinea and several UAV flight missions, including one flight track coded with surface brightness temperature. These operations demonstrated the utility of the ship-deployed UAVs using the HQ-60 from the R/V Falkor (Rahlff et al, 2018;Wurl et al, 2018). We demonstrated the impact of scientific UAV usage extends into the realm of upper-ocean biological processes.…”

“…The sea surface microlayer (SML; the upper 40-100 µm of the ocean surface) is a region of dynamic biological, chemical and physical activity, is a challenging environment to observe (Kurata et al, 2016;Engel et al, 2017;Ribas-Ribas et al, 2017). Through high resolution thermal and hyperspectral imaging of the sea surface, one is able to investigate the idea that biogenic slicks, of Phytoplankton and other sea surface microlayer constituents will affect the transfer of heat into the water column and thereby significantly alter the surface heat budget and the response of the mixed layer (Wurl et al, 2018).…”