Spatiotemporal variations in carbon dynamics during a low flow period in a carbonate karst watershed: Santa Fe River, Florida, USA

Abstract

To understand role of biogeochemical reactions in controlling the amount and molecular form of dissolved carbon exported from carbonate terrains, spatiotemporal variations in dissolved organic carbon (DOC) and dissolved inorganic carbon (DIC) were observed over one year in the Santa Fe River system, a period of base flow or below. A water mixing model developed using concentrations of Na ? , Cl -, and SO 4 -2 identified three major water sources: soil water, groundwater and deep aquifer water. After accounting for mixing of these water sources, additional chemical signatures resulting from biogeochemical processes in the riparian zone were identified. Net mineralization of DOC occurred throughout the Santa Fe River watershed, particularly during the lowest flow conditions and in the upper watershed. However, natural dissolved organic matter was more labile during low flow and in the lower watershed, and predominantly derived from groundwater (rather than soil water) in all samples, likely via releases during carbonate dissolution. Carbonate dissolution commonly occurred in the upper watershed during low flow conditions, while carbonate minerals precipitated during baseflow, as well as in the lower watershed during very low flow conditions. Thus, riparian zone biogeochemical processes were strongly mediated by watershed hydrology, whose spatiotemporal variations resulted in greater inorganic and organic C export production in the lower watershed than the upper watershed, and during higher flow versus lower flow periods. During this lower flow period, the Santa Fe River watershed exported *1.0 and 10.3 ton km -2 year -1 DOC and DIC, respectively, representing higher C yields than many other types of watersheds.

River Corridor Sources Dominate CO2 Emissions From a Lowland River Network

“…Along these lower basin reaches (ICHE, SF2500, SF2800) the SF receives discrete lateral inputs from artesian Floridian aquifer springs. This water is clear, alkaline and thermally stable (Jin et al., 2015; Khadka et al., 2014), and dominates total discharge at low streamflow. At higher flow, discharge from the far more temporally dynamic upper basin dominates discharge, creating blackwater conditions throughout the network (Hensley et al., 2019) and even in the lower ICHE, a spring‐fed tributary to the SF.…”

Flow Extremes as Spatiotemporal Control Points on River Solute Fluxes and Metabolism

“…In the Upper Santa Fe River Basin, Plio‐Pleistocene sand deposits are underlain by the Hawthorne Formation, a layer of low‐permeability Miocene clay that is the primary confining unit of the Upper Floridan Aquifer (UFA; Katz et al, ). The attendant shallow water table creates numerous wetlands and a high density of streams draining wet flatwood forests (Myers & Ewel, ); water from the upper catchment is highly enriched in DOC, with concentrations often in excess of 50 mg‐C/L (Jin et al, ; Khadka et al, ).…”

“…In contrast, the upper river is highly dynamic, responding rapidly to rainfall inputs and routinely ceasing flow during droughts. The lower river is a dynamic blend (Jin et al, ) of these two contrasting source waters (Figure ).…”

“…In the Upper Santa Fe River Basin, Plio-Pleistocene sand deposits are underlain by the Hawthorne Formation, a layer of low-permeability Miocene clay that is the primary confining unit of the Upper Floridan Aquifer (UFA; Katz et al, 1997). The attendant shallow water table creates numerous wetlands and a high density of streams draining wet flatwood forests (Myers & Ewel, 1990); water from the upper catchment is highly enriched in DOC, with concentrations often in excess of 50 mg-C/L (Jin et al, 2015;Khadka et al, 2014). As the river crosses the Cody Escarpment, a relic of an interglacial shoreline where the Hawthorne is eroded away revealing underlying carbonate rocks, it is entirely captured by a large sinkhole at all but the highest Q. River water travels 6 km underground through a network of karst conduits (Martin & Dean, 2001), reemerging at the River Rise and creating a novel map artifact of a longitudinally discontinuous river ( Figure 1b).…”

Comparison of topsoil organic carbon and total nitrogen in different flood-risk riparian zones in a Chinese Karst area

“…Karst geomorphology is a unique and important geomorphological type in the world, which is usually characterized by extensive outcropping of soluble rocks and rapid and frequent hydrological and chemical processes. This karst area is a critical environment for regional water resources and the global carbon cycle (Rivera and Aide 1998;Farnleitner et al 2010;Jin et al 2015). The current assessments show that sedimentary carbonates of the earth contain more than 6.0 9 10 7 Pg (p = 10 15 ) of C, and the annual atmospheric CO 2 sink from carbonate weathering is approximately 0.5 Pg of C (Falkowski et al 2000;Liu et al 2011).…”