High-resolution variability of the enrichment of fluorescence dissolved organic matter in the sea surface microlayer of an upwelling region

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.