Date Published: August 11, 2017
Publisher: Public Library of Science
Author(s): Jennifer Gonzales, Teresa Tymon, Frithjof C. Küpper, Matthew S. Edwards, Carl J. Carrano, Alfonso Saiz-Lopez.
Kelps have a major role in marine and atmospheric iodine cycling in the coastal zone of temperate regions, with potential wide-ranging impacts on ozone destruction in the coastal marine boundary layer. However, little is known about the impact of kelp forests on iodine speciation in coastal sea water. To address this, we examined iodide and iodate concentrations in seawater in and around a giant kelp forest near San Diego, CA, USA, and a nearby site that was not influenced by kelp biology. Our data shows that while both iodide and iodate concentrations remained unchanged during the year at the nearby site, these concentrations changed significantly in and around the kelp forest, and were strongly related to changes in kelp canopy biomass. In particular, iodide reached its highest concentration and iodate reached its lowest concentration during the summer when the kelp canopies were near their maximum, while the opposite pattern was observed during the winter and spring when the kelp canopies were near their minimum. Further, comparisons of these changes with corresponding changes in seawater temperature and wind speed indicated that these relationships were relatively small compared to those with changes in kelp biomass. Together, our data show a strong relationship between kelp biomass and iodine metabolism.
Marine production of organic halogenated compounds has been proposed as an important link between ocean biology, atmospheric composition, and climate [1–5]. In particular, iodine atoms released from the photooxidation of organic and inorganic halogenated compounds enter catalytic ozone destruction cycles and can indirectly lead to a reduction in tropospheric ozone production by suppressing levels of nitrogen oxides . The climate relevance of the link between ocean biology and atmospheric halogens (including iodine) was demonstrated by estimating the change in the ozone radiative forcing due to halogen-driven ozone loss using a climate model [7,8]. These halogenated compounds enter the stratosphere through rapid convection from the marine boundary layer (MBL) , and once there may be more efficient than chlorine in destroying stratospheric ozone, a key greenhouse gas and air pollutant . Recent models have shown that iodine enters the stratosphere at larger concentrations than currently assumed by WMO . For example, computer simulations suggest tropospheric ozone may be reduced by 5–30% compared to simulations without halogenated compounds [5,12] and reactive iodine, together with bromine, may be responsible for up to 50% of ozone destruction in the MBL . Such ozone depletion may contribute approximately −0.10 W m−2 to the radiative flux at the tropical tropopause, which is of similar magnitude to the contribution of tropospheric ozone to the present-day radiative balance . Iodine compounds can also impact climate through modification of nitrogen oxide (NOx) and hydrogen oxide (HOx) cycles, and through the oxidation of dimethyl sulfide (DMS) , with resulting effects on the lifetimes of other climatically important trace gases [15,16]. Up to date, only two global models have evaluated the role of atmospheric iodine chemistry in the global concentration of atmospheric oxidants [17,18].
The marine biogeochemistry of iodine is likely important to global climate since the marine environment is likely the major source of emission of both molecular iodine and iodinated organic compounds into the atmosphere . There, these compounds can destroy tropospheric ozone, modify nitrogen oxide and hydrogen oxide cycles, and increase cloud formation . Together, these can increase rates of atmospheric warming, especially over coastal areas where iodine cycles are enhanced by biological activity of primary producers. Consequently, one of the original motivations for this work was to test the hypothesis that reduction of iodate, in at least some coastal ecosystems, occurs due to leakage of sulfur containing reducing agents (i.e. glutathione, cysteine, etc.) that accompany marine phytoplankton cell senescence. A natural laboratory for testing this hypothesis would make use of intense near shore harmful algal blooms (HABs). The Scripps Institution of Oceanography (SIO) Pier is a long-term HAB coastal monitoring site in the Southern California Bight that is well known to experience large cycles in phytoplankton abundance, such as the one that occurred during October 2011 . During these blooms, phytoplankton cell numbers can increase from just a few to more than 150,000,000 cells L-1. We proposed to monitor iodine speciation pre-bloom, during the bloom maximum, and post-bloom at this location with the expectation that detectable increases in iodide concentrations post-bloom would be consistent with the cell senescence model. However contrary to expectations, there were no phytoplankton blooms at Scripps Pier during any part of our 2014–2015 field campaign. Indeed we found that by examining the SIO-Scripps Pier HAB monitoring report monthly, total phytoplankton abundance was unusually low and unchanged during the year, possibly due to low nutrient availability that resulted from the very strong 2014–2015 El Niño Southern Oscillation. Consequently, the Scripps Pier site became not a testing site for the influence of phytoplankton activity on iodine speciation, but rather a near ideal “non-biological” control site where values were consistent with many other seawater iodine speciation studies  and apparently unaffected by changes in water temperature and/or wind speed.