Trace element bias in the use of CO2 vents as analogues for low pHenvironments: Implications for contamination levels in acidifiedoceans

Research into the effects of ocean acidification on marine ecosystems has increasingly focused on naturalCO2 vents, although their intrinsic environmental complexity means observations from these areas maynot relate exclusively to pH gradients. In order to assess trace element levels and distribution in theLevante Bay (Vulcano Island, NE Sicily, Italy) and its suitability for studying biological effects of pHdecline, Ba, Fe and trace elements (As, Cd, Co, Cr, Cu, Hg, Mn, Mo, Ni, Pb, V and Zn) in sediment wereanalysed from 7 transects.Where present, Cymodocea nodosa leaves and epiphytes were also analysed. Atthe spatial scale of the bay, trace element concentrations in sediments and biota showed wide variability,possibly related to both input from fluid emissions and seawater physico-chemical variables (i.e. pH andEh), which may considerably affect the solubility and bioavailability of potentially harmful trace ele-ments. According to two pollution indices (MSPI: Marine Sediment Pollution Index and SQG-Q: SedimentQuality Guideline Quotient), the bay can be considered to be affected by low contamination withmoderate potential for adverse biological effects, especially in the area between about 150 and 350 mfrom the primary vent, where localized detrimental effects on biota may occur. Generally, biologicalsamples showed concentrations that were comparable with the lower values of seagrass ranges. Theoverall results of this study support the complex spatial dynamics of trace elements in the CO2 ventstudied, which are constrained by both direct input from the vent and/or biogeochemical processesaffecting element precipitation at the sedimenteseawater interface. Consequently, great caution shouldbe used when relating biological changes along pH gradients to the unifactorial effect of pH only, asinteractions with concurrent, multiple stressors, including trace element enrichments, may occur. Thisfinding has implications for the use of CO2 vents as analogues in ocean acidification research. Theyshould be considered more appropriately as analogues for low pH environments with non-negligibletrace element contamination which, in a scenario of continuous increase in anthropogenic pollution,may be very common.