Influence of organic matter source and diagenetic state on photochemical release of dissolved organic matter and nutrients from resuspendable estuarine sediments

Presently there is a paucity of knowledge regarding the influence of sedimentary organic matter (SOM) source and diagenetic state on photolytic fluxes of dissolved organic matter and nutrients from sediments likely to be resuspended during high energy events such as storms. Here we present data on the photochemical release of dissolved organic carbon (DOC), total dissolved nitrogen (TDN), dissolved organic nitrogen (DON), NH4+, and PO43− from the fine fraction of sediments obtained along an estuarine gradient, coupled with analyses of biogeochemical and molecular indicators of SOM source. Photochemical release of DOC and PO43− was significantly higher than dark controls at all sites. DOC photorelease from fine (< ca. 10–20 μm) sediments was ten times higher than previous studies that used bulk sediment in suspensions. The magnitude, stoichiometry, and speciation of nutrients and dissolved organic matter released during sediment photolysis appear to be influenced by a combination of SOM source and diagenetic processing. Release of DOC and PO43− was higher at upstream sites, where sediments were dominated by relatively unaltered terrestrially-derived SOM. In contrast, TDN, DON, and NH4+ releases were higher at downstream sites, which exhibited more degraded organic matter of both terrestrial and aquatic origin. The ratio of inorganic nutrients released was low at all sites (NH4+: PO43− ~ 1–7) suggesting that SOM photolysis may be relatively more important as a source of inorganic P than N. Our results also demonstrate that SOM is broadly photolabile and that previous studies may underestimate photolytic release of DOC from the fine fraction of sediments.

Research highlights
► DOM and nutrient fluxes from fine sediments are 10× higher than from bulk sediments.
► Photorelease more important as a source of inorganic P than N.
► Even very degraded sediments released DOM and nutrients.
► Source and/or freshness influence magnitude of release and elemental stoichiometry.