Organic matter quantity and quality across salinity gradients in conduit‐ vs. diffuse flow‐dominated subterranean estuaries

Abstract

Submarine groundwater discharge (SGD) is a source of water and bioreactive solutes to coastal zones but may be modified by organic matter (OM) remineralization dynamics within subterranean estuaries (STEs). We hypothesize that bioreactive solute fluxes should depend on water residence time in STEs, but links between OM transformations and residence time in STEs are poorly characterized. To test this hypothesis, we compare dissolved OM (DOM) quantity and quality in two hydrologically distinct STE systems: a reef lagoon on the east coast of the Yucatan Peninsula, Mexico, where semidiurnal mixing in submarine springs of a carbonate karst aquifer results in short residence times, and a barrier lagoon on the east coast of Florida, where slow flow through siliciclastic sediments results in long residence times. We measured dissolved organic carbon concentrations and characterized colored DOM (CDOM) with ultraviolet spectroscopy and fluorescence combined with Parallel Factor Analysis. Both sites exhibit similar shifts in OM quality with salinity and reflect a marine source of labile OM to the STEs. Nonconservative mixing and CDOM production occurs at all sites but the long water residence times in the siliciclastic STEs cause orders of magnitude greater production than the carbonate STE. Consistent CDOM production across sites with disparate characteristics indicates that STEs are common sources of CDOM to surface water. However, observed variation in the magnitudes of CDOM production indicates that estimating global, and even regional, solute fluxes associated with SGD will be complicated by hydrologic control on extents of OM remineralization.

Biogeochemical and Hydrological Drivers of Heterogeneous Nutrient Exports From Subterranean Estuaries

“…Flow rates also regulate the rate of delivery of terminal electron acceptors and organic matter, the primary electron donor, to the reaction zone. Therefore, flow rates should impact the extent and type of biogeochemical reactions in subterranean estuaries and thus SGD nutrient loads from any individual site (e.g., Pain et al, 2019b).…”

“…The subterranean estuary is characterized by steep salinity and redox gradients that drive a variety of reactions and nutrient transformations. The types and magnitudes of reactions depend on factors such as inflowing groundwater composition (Slomp and Van Cappellen, 2004), rock:water ratio and hydrogeological characteristics (Pain et al, 2019b), and characteristics of sediments and organic matter available to drive reactions (Moore, 1999). The net impact of these reactions may be site-specific and result in net increases or decreases in solutes of interest (Kroeger et al, 2007;Kroeger and Charette, 2008;Santos et al, 2009;Knee and Paytan, 2011;Erler et al, 2014).…”

Does Nitrate Enrichment Accelerate Organic Matter Turnover in Subterranean Estuaries?

“…Low C stocks and the refractory nature of both particulate and dissolved organic matter in the sub-surface landocean continuum limit heterotrophic NO 3 − reduction processes and can explain the large NO 3 − concentrations reaching STEs (Hartog et al, 2004;Rivett et al, 2008). On the other hand, fresh marine organic matter brought into the STE at the seepage face fuels a spatially heterogeneous cascade of biogeochemical processes that can determine final N loads to the coast (Ibánhez et al, 2013;Pain et al, 2019;Calvo-Martin et al, 2021). Due to the high mobility of N compounds in the environment and the role N plays in microbial and primary producer metabolism, anthropogenic N enrichment of STEs could also promote alterations on the cycling of other elements such as C. Hence, mobilization of organic C from high residence time reservoirs like peat bogs (Bragazza et al, 2006), the Artic tundra (Mack et al, 2004), or alpine steppes (Chen et al, 2020) is increasingly linked to the enhancement of microbial and enzymatic activity caused by anthropogenic N enrichment.…”

Tidal pumping and biogeochemical processes: Dissolution within the tidal capillary fringe of eogenetic coastal carbonates

“…The high primary porosity and permeability of the Yucatan Peninsula's eogenetic carbonate bedrock generally preclude perennial surface water except for where water tables exceed low points in surface topography (Gulley et al ., 2017). Consequently, caves replace rivers as the dominant sources of freshwater and nutrients to coastal carbonate environments and establish subterranean karst estuaries where mixing of fresh and saline water create biogeochemical hotspots (Gonneea et al, 2014; Menning et al, 2015; Gulley et al, 2016a; Young et al, 2018; Pain et al, 2019).…”