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

Abstract

AbstractSubmarine 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.