Delineating Drivers of Dissolved Organic Matter Composition in a Major Freshwater Spring

摘要

Globally, karst ecosystems are experiencing anthropogenic impacts due to their high hydrologic connectivity, leading to issues including increasing color (browning) as noted in recent decades at the largest freshwater spring on Earth (Wakulla Springs, Florida, USA). Wakulla Springs is located within a karst landscape, characterized by numerous sinkholes connecting surface and groundwaters. Surface waters and associated dissolved organic matter (DOM) from both natural sources (e.g. Apalachicola National Forest) and anthropogenic sources (e.g. municipal wastewater facility, sprayfield farm and diffuse inputs from the city of Tallahassee) are transported through these features and discharged at Wakulla Springs' vent. In this study we assess dissolved organic matter (DOM) composition via absorbance and fluorescence spectroscopy and Fourier Transform ion cyclotron mass spectrometry (FT-ICR MS) to evaluate seasonal changes in DOM composition and dissolved organic carbon (DOC) concentration. Samples were collected between August 2015 and August 2016, bi-weekly, at three wells and two fluvial sites throughout Wakulla Springs State Park. These wells represent conduits feeding from different locations in the springshed, encompassing the major inputs and sources of DOM to the Spring vent and downstream Wakulla River. Sample sites separated into distinct groups based on DOC concentration and optical and FT-ICR MS parameters indicative of autochthonous (clear groundwater) versus allochthonous (terrestrial) DOM. Seasonal trends in DOM composition at Wakulla Springs vent are apparent and predominantly driven by high precipitation events and associated inputs of DOM from the Apalachicola National Forest with increased color, DOC, and terrestrially sourced molecular signatures. Principle component analysis highlights the ability of optical parameters to show the dominance of sample sites draining from the southwest (i.e. Apalachicola National Forest) as responsible for the color rich water at the vent, whereas water draining from the north was comparatively clear. Multivariate analyses demonstrates how FT-ICR MS can clearly distinguish the molecular signatures of clear groundwaters and afforested blackwater inputs, and emphasize the dominance of conduits draining from afforested sites as controlling the DOM signature and associated color at the vent. Despite the hydrologic complexity of the aquifer, FT-ICR MS allowed for clear discrimination between the different DOM sources and so these methodologies may be applied to aquifers around the world to trace inputs and evolved signatures. Increasing colored rich waters at the vent suggests that either input from conduits draining from the southwest have increased, or the relative dilution with clear groundwater has decreased in the conduit system prior to discharge from the vent. Sea-level rise impacts in the region have been suggested to result in more blackwaters delivered to the vent, and ongoing extraction of clear groundwater reduces the dilution capacity on color rich waters. Thus, anthropogenic impacts in the region need to be addressed if the trend of increased colored inputs at Wakulla Springs is to be reversed.