Groundwater as a source of dissolved organic matter to coastal waters: Insights from radon and CDOM observations in 12 shallow coastal systems

Abstract

The influence of groundwater and pore‐water exchange on dissolved organic matter (DOM) dynamics in coastal surface waters remains poorly understood. Here, we combine bottom up (i.e., groundwater‐derived flux estimates) and top down (i.e., water column response) evidence to assess whether groundwater exchange drives DOM dynamics in shallow coastal waters. We rely on automated chromophoric DOM (CDOM, a proxy for DOM) and radon (222Rn, groundwater proxy) measurements over tidal time scales in 12 shallow systems, including tidal freshwater wetlands, estuaries, mangroves, coral reefs, coastal lakes, a saltmarsh, and a residential canal estate. Groundwater‐derived dissolved organic carbon (DOC) fluxes ranged from 2 ± 2 mmol m−2 d−1 in a coral reef to 1941 ± 1325 mmol m−2 d−1 in a mangrove tidal creek. These groundwater fluxes replaced surface water DOC inventories on time scales ranging from ~ 0.5 d to several weeks. Systems with short replacement times displayed positive correlations between radon and CDOM in surface waters. Groundwater exchange diluted surface water DOC in four systems. Using multiple lines of evidence, we interpreted groundwater to be an important source of DOM to surface waters in 4 out of the 12 systems, including an offshore coral reef lagoon with low surface water DOC concentrations. Groundwater discharge was a negligible source of DOM in systems with high surface water DOC and CDOM concentrations such as tidal freshwater wetlands and coastal lakes. This investigation highlights the high variability in groundwater‐derived DOC fluxes and responses in the water column, and demonstrates that submarine groundwater discharge and advective pore‐water exchange should be considered in coastal carbon budgets.

Regional variation in δ13C of coral reef macroalgae

“…Dissolved CO 2(aq) can be high in inshore compared to offshore environments, where CO 2 is supplied by coastal systems and is associated with the decomposition of organic matter (Cai 2011). For example, dissolved CO 2(aq) is high in river flows and in groundwater from estuaries that have received organic material from catchments and have highly productive vegetative communities (Cartwright 2010;Maher et al 2013;Webb et al 2019). Our inshore study sites were in much closer proximity to these potential sources of CO 2 than that of Diaz-Pulido et al (2016) which likely explains the lower δ 13 C signature in macroalgae at our inshore compared to offshore sites.…”

Submarine Groundwater Discharge: Updates on Its Measurement Techniques, Geophysical Drivers, Magnitudes, and Effects

“…DOC and DIC are also formed within a STE by the bacterial degradation of particulate organic carbon (POC). Therefore, many STEs show higher DOC concentrations in groundwater than coastal seawater, indicating that SGD is a potential DOC source to the near-shore ocean (Webb et al, 2019). However, the contribution of SGD to the marine DOC budget remains unknown relative to other sources such as in-situ production of DOC in the euphotic zone.…”

Coloured dissolved organic matter in aquatic ecosystems of three representative regions of the Arctic according to the data obtained in year 2019

“…This happens due to the fact that at this time the lakes are switching to the groundwater feed. The concentration of dissolved organics can be positively affected by groundwater inflow in terrestrial lakes [8]. In addition, the growth of CDOM concentration at this period can be positively affected by decreasing in the activity of the aquatic biota vital processes.…”