Abstract
Enrichment of fluorescence dissolved organic matter (FDOM) in the sea surface microlayer (SML) provides insights into biogeochemical processes occurring at the sea surface, including cycling of organic matter, photochemistry, and air-sea gas exchange. We present data concerning the variability of FDOM enrichment in the SML during upwelling events in the Baltic Sea (Cruise M117). Our results show that FDOM is frequently enriched (75% of all samples) and that enrichment factors are significantly higher in SMLs located in regions with upwelling (pooled median = 1.4) compared to a non-upwelling region (median = 1.1). The enrichment factor of FDOM showed short time-scale variability, changing by 6% within ten-minute intervals. Larger variabilities (standard deviation up to ±0.14 µg L–1 compared to background of ±0.01 µg L–1) occurred when fronts were present and when the SML was mixed with underlying bulk water. Small-scale patchiness, indicated by changes in the variability of FDOM enrichment in SML, was a common feature of the sea surface. Wind speed played a potential role in controlling the enrichment of FDOM in the SML, but the effects of solar radiation on photochemical processes, mixing and upwelling of water masses, and biological processes as a source of FDOM also influence enrichment at this critical interface between ocean and atmosphere.
Highlights
The uppermost part of the ocean, covering approximately 70% of the surface is defined as the sea surface microlayer (SML) (Liss and Duce, 2005)
Mean Fluorescence dissolved organic matter (FDOM) concentration in the bulk water did not correlate with bulk salinity (p = 0.7500, r = 0.4000, n = 4; Figure S1a) recorded by the ship’s salinometer, but the number of stations was low and the sampled water masses had similar salinities
Date Latitude (°E) Longitude (°N) FDOM, SML FDOM, bulk water FDOM, enrichment factor (EF) high-precision temperatures (HPT) (°C), 2-cm depth HPT (°C), 15-cm depth Salinity, SML Salinity, bulk water Photosynthetic yield (Fv/Fm) Wind speed (m s–1) Solar radiation (W m–2) UV index Chlorophyll a (Chl a), bulk water Surfactants, SML Surfactants, bulk water a Reported as average ± standard deviation. b From Ribas-Ribas et al (2017)
Summary
The uppermost part of the ocean, covering approximately 70% of the surface is defined as the sea surface microlayer (SML) (Liss and Duce, 2005). Due to its unique position between the atmosphere and ocean, the SML plays an important role in biogeochemical processes, including the marine carbon cycle, photochemistry and air-sea exchange of climate-relevant gases. All materials exchanged between the ocean and atmosphere, including gases, particulate organic matter, and sea salt, have to pass through the SML (Liss and Duce, 2005). Biological and physical processes in the water column, such as primary productivity, diffusion, buoyancy flux, and rising bubbles, are the main factors determining the enrichment of chemical compounds and microbes in the SML (Wurl et al, 2009; Stolle et al, 2010). FDOM is often used as an indicator for humic-like marine DOM (Yamashita and Tanoue, 2009) and frequently enriched in the SML (Zhang and Yang, 2013) due to its hydrophobic properties
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