Speciation of dissolved inorganic iodine in a coastal fjord: a time-series study from Bedford Basin, Nova Scotia, Canada

Abstract

We report a long-term (4.5 year) time-series with weekly resolution of iodide and iodate measurements made at 4 depths within the Bedford Basin: a 70 m deep, seasonally stratified, coastal fjord located near Halifax, Nova Scotia, Canada. The subsurface data (60 m) reveal strong inverse correlations of both iodide and total dissolved iodine (TDI) with dissolved oxygen and indicate that there is in-situ reduction of iodate in subsurface waters (in the presence of oxygen) as well as an additional external source of iodide from the remineralization of sinking organic matter, a flux from sediments, or both. Surface water (<10 m) iodide concentrations increase gradually from spring (70 nmol L-1) through fall (120-150 nmol L-1) and are not well represented by the current empirical parameterizations used to predict surface water iodide levels globally. The vertical gradient of iodide between subsurface and surface waters increases over the summer as a result of subsurface processes and, together with diapycnal mixing, may contribute to the seasonal accumulation of iodide in surface water. Examination of a global surface water iodide data compilation reveals an inverse relationship with subsurface oxygen concentrations which suggests that subsurface remineralization and sediment-water fluxes coupled with vertical mixing may also contribute to surface water iodide variability on a global scale.

Iodide, iodate & dissolved organic iodine in the temperate coastal ocean

“…Importantly, observed and normalized dI T changes over time were similar (Figure 5B) indicating dilution of seawater by freshwater while in Plymouth Sound is not a significant driver of the concentration changes over the seasonal time series. This conclusion is also in agreement with Shi et al (2023) who did not find a relationship between salinity and dI T over a wider salinity range, S P = 26-31 during a 4.5-year time series in a fjord.…”

“…The relative contributions are of a similar magnitude to those observed in northeast Atlantic surface waters, 89% iodate and 12% iodide (He et al, 2013). The late autumn dI T maxima was caused by the gradual build-up of iodide through the warmer summer months, a process observed in the Mediterranean Sea and North Atlantic Fjords (Tian et al, 1996;Shi et al, 2023). The seasonal iodine speciation changes at the inner shelf, which were also observed at the outer shelf (Figure 2), are the first to be reported for UK coastal waters (Truesdale and Upstill-Goddard, 2003) though seasonal changes in iodine speciation have been reported in other locations (Jickells et al, 1988;Campos et al, 1996;Tian et al, 1996;Chance et al, 2010;Shi et al, 2023).…”

“…Relative to open ocean and outer shelf waters, the concentration of dissolved iodine in coastal inner shelf waters is lower (Truesdale, 1978;Truesdale, 1994;Truesdale and Upstill-Goddard, 2003;Satoh et al, 2019a;Satoh et al, 2023;Shi et al, 2023). The reason for this is not well known (Chance et al, 2014), though a chemical transformation to a particulate fraction may account for the apparent depreciated soluble iodine concentrations (Truesdale, 1994;Truesdale and Upstill-Goddard, 2003;Satoh et al, 2019a;Satoh et al, 2023).…”

“…The central tenet of iodine marine biogeochemistry is that transformations between soluble chemical species are biologically mediated (Tsunogai and Sase, 1969;Amachi et al, 2007), though chemical processes cannot be ruled out (Schnur et al, 2024). This tenet derives from observations of temporal changes in iodine speciation in ocean waters concluding that from spring through to autumn iodide accumulates and iodate is removed, and over winter the reverse occurs (Truesdale, 1978;Jickells et al, 1988;Campos et al, 1996;Tian et al, 1996;Truesdale and Jones, 2000;Truesdale and Upstill-Goddard, 2003;Waite et al, 2006;Chance et al, 2010;Satoh et al, 2019a;Shi et al, 2023). These observations support the observed global ocean iodide-nitrate anticorrelation and the supposition that iodate is either assimilated during primary production and iodide is released, or iodate is enzymatically reduced, possibly by nitrate reductase enzymes (Tsunogai and Sase, 1969;Wong and Zhang, 1992;Campos et al, 1999;Amachi et al, 2005;Amachi et al, 2007).…”

“…The reason for this is not well known (Chance et al, 2014), though a chemical transformation to a particulate fraction may account for the apparent depreciated soluble iodine concentrations (Truesdale, 1994;Truesdale and Upstill-Goddard, 2003;Satoh et al, 2019a;Satoh et al, 2023). Processes directly influencing the iodine cycle in wider coastal environments include interaction with dissolved organic material during its remineralization and sorption to particulate material (Truesdale, 1978;Tian et al, 1996;Satoh et al, 2019a;Shi et al, 2023), the mixing of deeper low oxygen high iodide waters to the surface (Shi et al, 2023), and significant changes in dissolved organic iodine (DOI) concentration (DDOI up to 100 nM; Satoh et al, 2019a). In coastal systems, the iodine cycle may also be influenced by higher life forms including macroalgae and invertebrates (e.g., jellyfish) that use organic and inorganic iodine species as antioxidants and signaling molecules (Silverstone et al, 1977;Küpper et al, 1998;Prieto et al, 2010;Cross et al, 2015;Gonzales et al, 2017;Tymon et al, 2017;Carrano et al, 2020;Carrano et al, 2021).…”