Non-conservative Behavior of Dissolved Organic Matter and Trace Metals (Mn, Fe, Ba) Driven by Porewater Exchange in a Subtropical Mangrove-Estuary

Abstract

Estuaries play a key role in controlling the land-ocean fluxes of dissolved organic matter (DOM), nutrients and trace metals. Here, we study how mangrove-fringed areas affect the molecular DOM and trace metal composition in a subtropical estuary. We combined molecular analysis of solid-phase extractable (SPE) DOM using ultrahigh-resolution mass spectrometry with organic and inorganic bulk parameter analyses in surface and porewater along the estuarine gradient of a mangrove-fringed estuary in Australia (Coffs Creek). Statistical analysis and mixing models demonstrate that the fluvial and mangrove-porewater derived DOM and inorganic chemical species were altered and/or removed by the estuarine filter before reaching the coastal ocean. The mangrovefringed central estuary was a net source for dissolved Mn and Ba as well as total dissolved nitrogen (TDN) and dissolved organic carbon (DOC) to the tidal creek, likely due to the exchange of mangrove-porewater strongly enriched in these constituents. Dissolved Fe was removed from the water column, probably during the tidally driven circulation of creek water through the sulfidic mangrove sediments. In the mangroveporewater dominated tidal creek, sulfur-and nitrogen-containing as well as aromatic DOM compounds were relatively enriched, whereas phosphorous-containing DOM was relatively depleted compared to non-mangrove fringed areas. In areas with intense mixing of estuarine and marine water masses we observed a strong decrease of these DOM compounds relative to values expected from conservative mixing, suggesting their removal by photodegradation and co-precipitation with particles such as Mn(hydr)oxides and/or as organometallic complexes, leading to more aliphatic DOM signatures at the creek-mouth. Tidally driven porewater exchange and surface water runoff from the mangroves had a stronger effect on the biogeochemical cycling in the estuary than the fluvial input during a dry compared to a wet season. Our study confirms that mangroves can significantly contribute to biogeochemical budgets of (sub)tropical estuaries.

Trace metal stoichiometry of dissolved organic matter in the Amazon plume

“…There was some evidence for an outer-estuary increase in P SPE-amb , Zn SPE-amb , and Ni SPE-amb at salinities between 15 and 30. Concentrations of S SPE-amb and P SPE-amb offshore from the mangrove forests were similar to those observed in the riverine endmember, while A 254SPE-amb , Cu SPE-amb , Zn SPE-amb , Co SPE-amb , and Ni SPE-amb were slightly elevated close to the mangrove forests, possibly as a result of inputs from the organic carbon-rich mangrove sediment pore waters (46). The NBC showed elevated concentrations of S SPE-amb , P SPE-amb , and Zn SPE-amb but lower concentrations of Fe compared to high-salinity estuarine samples (Fig.…”

Seawater–Groundwater Interaction Governs Trace Metal Zonation in a Coastal Sandy Aquifer

“…According to the classical redox sequence between Fe and Mn, the reduced form of Fe is more easily oxidized than Mn, resulting in the preferential precipitation of Fe (hydr)oxides (Lee et al., 2007). Previous studies have suggested that the concentration of Ba in groundwater is controlled by adsorption and desorption (Gong et al., 2020; Hanor & Chan, 1977; Mori et al., 2019), while being affected by the Fe and Mn cycles (Coffey et al., 1997; Paytan & Kastner, 1996; Sanders et al., 2012). In comparison to the Fe cycle, the Mn cycle has a more pronounced impact on the concentration of Ba, as the affinity of Ba for Mn (hydr)oxides is significantly greater than that for Fe (hydr)oxides by several orders of magnitude (Balistrieri & Murray, 1986; Charette et al., 2005; Charette & Sholkovitz, 2006; Mishra & Tiwary, 1993).…”

The mangrove CO2 pump: Tidally driven pore‐water exchange

“…1). The mangroves of the Coffs Creek estuary are dominated by Avicennia marina and cover an area of 0.2 km 2 (Mori et al 2019). The catchment geology is composed of late carboniferous basaltic bedrock in the upper reaches and quaternary sediment deposits on the coastal floodplain.…”