Spatiotemporal Variability in the Surface Microlayer of Delaware Bay

Nicole R R Coffey1, Jessica Irene Czarnecki1, Alina M Ebling2 and Andrew S Wozniak1, (1)University of Delaware, School of Marine Science and Policy, Newark, DE, United States, (2)University of Delaware, School of Marine Science and Policy, Lewes, DE, United States
The surface microlayer (SML) is a 10-100 μm thick layer between bodies of water and the atmosphere which serves to control fluxes of gases and aerosols across the air-sea interface. The SML’s enrichment in organic matter (OM) is well documented and thought to be a contributor to SML surfactant-like properties that moderate materials exchange across the air-sea interface, but the mechanisms behind this enrichment are still debated. This complicates prediction of SML properties at a given point in space and time. By comparing subsurface water samples (SUB; depth ≈ 8-15 cm) to SML samples at a variety of locations and influences (fluvially-influenced, marine-influenced, marsh-influenced) in Delaware Bay over a seasonal cycle, this study aims to deconvolute the relationships between SML OM composition, SUB OM composition, microbial community composition and biological activity, and physical parameters including surface tension. Samples collected in the Delaware Bay (December 2018, March/June/August/October 2019) demonstrate SML enrichments in dissolved organic carbon (DOC) with enrichment factors up to 4.42 at the marine-influenced site and as low as 0.87 at the marsh-influenced site. Compositional differences in the dissolved OM pool between SML and SUB samples were assessed using Fourier-transform ion cyclotron resonance mass spectrometry (FTICR-MS). Preliminary results have identified formulas with H/C ratios above 1.7 and formulas with O/C ratios below 0.2 as unique to the SML, indicative of surfactant-like compounds. These results will be paired with further chemical analyses (excitation emission matrix spectroscopy, 1H NMR, particulate organic carbon) and analysis of microbial composition to refine a conceptual model of drivers of SML OM enrichment and composition. This work will thus further our understanding of SML formation and its potential impacts on air-sea exchange with implications for pollutant transport, carbon cycling, and climate change.