POSTER Session 3

REGULATORS OF DISSOLVED ORGANIC MATTER PHOTODEGRADATION IN THE TURBID NEARSHORE OCEAN

Dissolved organic matter (DOM) is transported from land to ocean through the nearshore coastal zone, where DOM undergoes microbial and photochemical transformation. DOM transformation is strongly influenced by UV-visible solar exposure, which varies widely spatially and over timescales ranging from seasonal cycles to diurnal tides and episodic events. Here, we characterized the UV-visible solar exposure of coastal-saltmarsh-derived DOM in the northern Gulf of Mexico using field measurements of colored dissolved organic matter (CDOM, ag) absorption, diffuse attenuation (Kd), and other water inherent optical properties (IOPs: ap and bbp). To quantify the cumulative solar exposure of marsh-derived DOM in the dynamic nearshore ocean, we combined field measurements with hydrodynamic modeling of DOM export. DOM solar irradiation decreased during spring freshet, when the nearby Mississippi-Atchafalaya River System delivered large volumes of suspended sediment to the Gulf of Mexico limiting light availability for DOM photochemistry. To quantify the influence of solar radiation on DOM geochemistry, we also conducted laboratory simulated solar exposure experiments. We found that saltmarsh DOM experienced significant photobleaching (reduction in CDOM absorption), but little direct photomineralization of dissolved organic carbon (DOC). Rather than removing DOC, solar exposure degraded DOM and primed it for microbial uptake. DOM transformation in the northern Gulf of Mexico occurred as coupled photochemical and microbial processes primarily regulated by pulsed export of suspended sediment, which reduced available solar radiation.

*Joshua Harringmeyer, Boston University, USA, [email protected], https://orcid.org/0000-0001-8108-0644

Luca Cortese, Boston University, USA, [email protected]

Matthew Weiser, Boston University, USA, [email protected]

Andre Rovai, Louisiana State University, US Army Engineer Research and Development Center, USA, [email protected]

Sergio Fagherazzi, Boston University, USA, [email protected]

Robert Twilley, Louisiana State University, USA, [email protected]

Cédric Fichot, Boston University, USA, [email protected], https://orcid.org/0000-0002-1099-5764

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