Human Influence on the Biogeochemical Reactivity of Subterranean Estuaries

Subterranean estuaries play a key role in the land-ocean interface by modulating groundwater-borne and recycled solutes discharged to the coast. Despite their importance for coastal ecosystems, the sensitivity of these systems to human activities is still unknown. To address this gap, dissolved organic matter (DOM) and dissolved inorganic nutrients of a pristine site were compared with those from two nearby, semiurban sites characterized by contrasting oxygen conditions. The local aquifers surrounding the semiurban subterranean estuaries contained more nitrogen, silicate, and DOM of higher molecular weight than the aquifers surrounding the pristine site. Despite the different chemical composition of the arriving fresh groundwater, N/P ratios and the quantity of humic-like DOM compounds at the pristine site were intermediate between those of the two semiurban sites. This pattern reflected the intermediate permeability and oxygenation of the pristine beach, highlighting the role of the sediment matrix in modulating the exported solutes. Enhanced oxygenation at one semiurban site resulted from a human-derived gravel layer that increased sediment permeability and reduced internal residence times. The anthropogenic alteration of the sediment permeability had a greater influence on the nutrients and DOM found in subterranean estuaries than did the chemical composition of the inland aquifers.