Terrestrial Nutrients and Dissolved Organic Matter Input to the Coral Reef Ecosystem via Submarine Springs

Abstract

Submarine groundwater discharge (SGD) transports terrestrial nutrients and dissolved organic matter (DOM) to the ocean. An elevated concentration of nutrients and DOM can act as stressors enhancing coral disease and mortality, but only a few studies address the impacts of groundwater-borne nutrients and DOM on coral reef ecosystems. This study quantifies and characterizes nutrients, nitrate (NO3 –) stable isotopes, and DOM molecular composition of coastal groundwater discharging to the reef ecosystem via submarine springs in Lombok, Indonesia. NO3 – isotopic values point to both natural (soil) and anthropogenic (wastewater and fertilizer) origins of nutrients in the coastal aquifer. Submarine springs are fed by different groundwater sources and deliver land-based NO3 –, dissolved silica, phosphate, and labile DOM to the reef water column. Terrestrial nutrients and DOM undergo rapid turnover in the reef water column due to biogeochemical processes and biological uptake. Meanwhile, reef and offshore water likely act as sources of more stable, reworked DOM formulas and its mineralization product, ammonium. We observed that submarine springs consistently deliver similar nutrient loadings, creating a long-term environmental threat to coral reef sustainability. This study emphasizes the importance of understanding coastal biogeochemistry and hydrological processes in sensitive tropical ecosystems, particularly those adjacent to modified land-use watersheds.

Groundwater springs in the German Wadden Sea tidal flat: A fast-track terrestrial transfer route for nutrients and dissolved organic matter

“…The residence time in the coastal aquifer also plays an important role: In comparison, diffuse SGD appears to have greater potential of accumulating DOC along its flow paths than spring-type SGD, mainly because longer residence times favor reducing conditions with lower abundances of electron acceptors, as well as additional release of DOM due to particulate organic matter degradation and desorption from mineral phases (e.g., Seidel et al, 2014;Nelson et al, 2015;Linkhorst et al, 2017). However, submarine springs can be seasonally or locally enriched with high concentrations of DOC (Luzius et al, 2018;Pain et al, 2019;Adyasari et al, 2021) which may discharge rapidly into the adjacent water column. Therefore, while SGD clearly is an important player that needs to be considered for management of freshwater resources and protection of coastal areas (Burnett et al, 2006), its impact on the organic carbon budget still needs to be better constrained, especially in regard to coastal carbon sequestration (Santos et al, 2021).…”

Dissolved Organic Matter Sources in High Arsenic Groundwater From a Sand‐Gravel Confined Aquifer

“…From our previously conducted incubation experiments, it was also found that the consumption of more labile organic molecules produced more abundant microbially derived OM, and resulted in stronger Fe(III) oxide reduction and higher dissolved As concentrations (Qiao et al., 2020). Similarly, a greater extent of DOM degradation was also found to be responsible for higher dissolved Fe(II) and NH 4 + concentrations and more negative δ 13 C‐DIC values in the Pear River estuarine sediment porewater (Wang et al., 2021) and lower dissolved NO 3 − concentrations in coastal groundwater (Adyasari et al., 2021). Overall, the stronger DOM degradation state may cause more reducing conditions and drive the fundamental biogeochemical processes, especially Fe(III) oxide reduction and As release, in the aquifer systems (Figure 9).…”

“…The degradation index (I_deg) can be used to evaluate the average degradation state of DOM (Adyasari et al., 2021; Flerus et al., 2012; Koch et al., 2014). The I_deg ranges from 0 to 1 (Mentges et al., 2017), with higher I_deg values indicating stronger degradation states of DOM (Lechtenfeld et al., 2014).…”

“…(2012) established a degradation index (I_deg) based on the ratio of the intensity of five significantly positively correlated formulas to the sum intensity of the five positively correlated and five negatively correlated formulas with marine 14 C‐DOC aging, which can reflect the average degradation state of DOM (Adyasari et al., 2021; Koch et al., 2014). The index has also been used in other studies of marine DOM (Koch et al., 2014; Lechtenfeld et al., 2014; Mentges et al., 2017), marine sediment porewater DOM (Rossel et al., 2020), estuarine sediment porewater DOM (Wang et al., 2021), coastal groundwater DOM (Adyasari et al., 2021), and groundwater DOM (McDonough et al., 2022). Nevertheless, the degradation state of DOM in As‐prone groundwater is unknown, which significantly limits our understanding of DOM roles in As mobilization.…”

“…However, aromatic-rich DOM molecules are believed to be less labile (Stubbins et al, 2010), which can serve as electron shuttles and competitors (Qiao et al, 2021). Regarding both the biodegradable and recalcitrant DOM molecules, Flerus et al (2012) established a degradation index (I_deg) based on the ratio of the intensity of five significantly positively correlated formulas to the sum intensity of the five positively correlated and five negatively correlated formulas with marine 14 C-DOC aging, which can reflect the average degradation state of DOM (Adyasari et al, 2021;Koch et al, 2014). The index has also been used in other studies of marine DOM (Koch et al, 2014;Lechtenfeld et al, 2014;Mentges et al, 2017), marine sediment porewater DOM (Rossel et al, 2020), estuarine sediment porewater DOM (Wang et al, 2021), coastal groundwater DOM (Adyasari et al, 2021), andgroundwater DOM (McDonough et al, 2022).…”