Tidal Pumping Controls Dissolved Organic Matter Properties and Outwelling From Mangrove Groundwater to Coastal Water

Abstract

Dissolved organic matter (DOM) in highly productive intertidal mangroves is an important carbon and nutrient source to the coastal ocean. In highly dynamic mangrove intertidal systems, both tidal pumping or geochemical factors can control DOM characteristics. The influence of groundwater flow on DOM properties and fluxes remains poorly understood. Here, we compared the concentrations, sources, and spectroscopic characteristics of DOM across a recharge‐discharge gradient. We used a three‐dimensional array sampling strategy to reduce spatial biases, groundwater monitoring to quantify tidally driven exchange, and spectrofluoroscopy to quantify DOM fluorescent components. The results showed that tidal hydrology played the key role driving groundwater DOM properties in both horizontal (i.e., from the mangrove to the sea) and vertical directions. Due to the high spatial heterogeneity of dissolved inorganic (DIC) and organic (DOC) carbon concentrations in mangrove groundwater, carbon outwelling showed a high degree of variability at the intertidal scale (54.8–234.8 μmol m−2 d−1 for DIC and 2.5–12.4 μmol m−2 d−1 for DOC). The contribution of terrestrial DOM gradually decreased toward the ocean, but the microbial contributions for recently produced autochthonous DOM increased. Deep groundwater received less terrestrial DOM and experienced anaerobic microbial mineralization, resulting in lower organic carbon than shallow groundwater. Greater variations in spectroscopic characteristics were found in shallow groundwater than in deep groundwater. Overall, tidally driven surface water‐groundwater interactions control the variability in DOM properties and how DOM contributes to carbon budgets and sequestration in mangrove groundwater.

Outwelling of reduced porewater drives the biogeochemistry of dissolved organic matter and trace metals in a major mangrove‐fringed estuary in Amazonia

“…Higher values indicate more aromatic, red‐shifted FDOM. The HIX was highest in the porewater samples where components C1 and C2 were enriched (Supporting Information Table S7) since microbial alteration increases humic‐like DOM in deeper aquifers of mangrove ecosystems (Xiao et al, 2023). Thus, increased HIX levels during low tide confirm the outwelling of a terrestrial DOM from mangrove porewater to the estuary.…”

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

“…Different tidal stages are associated with varying degrees of seawater-groundwater mixing, seawater infiltration, dissolved oxygen level, and microbial activity (Liu, Jiao, & Liang, 2018;Robinson, Li, & Barry, 2007). Recent studies showed that tidal pumping controls the properties of dissolved organic matter and nutrient concentration in coastal aquifers (Liu et al, 2017;Xiao et al, 2023). These variations may lead to changes in the (bio)geochemical processes and rates related to trace metals in coastal aquifers.…”