Severe Coastal Hypoxia Interchange with Ocean Acidification: An Experimental Perturbation Study on Carbon and Nutrient Biogeochemistry

Abstract

Normally atmospheric CO2 is the major driver of ocean acidification (OA); however, local discharge/degradation of organic matter (OM) and redox reactions can exacerbate OA in coastal areas. In this work we study the response of nutrient and carbon systems to pH decrease in relation to hydrographically induced intermittent characteristics and examine scenarios for future ocean acidification in a coastal system. Laboratory microcosm experiments were conducted using seawater and surface sediment collected from the deepest part of Elefsis Bay; the pH was constantly being monitored while CO2 gas addition was adjusted automatically. In Elefsis Bay surface pCO2 is already higher than global present atmospheric values, while near the bottom pCO2 reaches 1538 μatm and carbonate saturation states were calculated to be around 1.5. During the experiment, in more acidified conditions, limited alkalinity increase was observed and was correlated with the addition of bicarbonates and OM. Ammonium oxidation was decelerated and a nitrification mechanism was noticed, despite oxygen deficiency, paralleled by reduction of Mn-oxides. Phosphate was found significantly elevated for the first time in lower pH values, without reprecipitating after reoxygenation; this was linked with Fe(II) oxidation and Fe(III) reprecipitation without phosphate adsorption affecting both available dissolved phosphate and (dissolved inorganic nitrogen) DIN:DIP (dissolved inorganic phosphate)ratio.

Changes in Dissolved Inorganic Carbon Across Yangtze River Regulated by Dam and River‐Lake Exchange

“…The carbonate buffer system alleviated the decrease of pH by enhancing the outgassing of CO 2 , which further formed a positive feedback loop of water acidification in the reservoir area (Figure 9a). Previous studies indicated that more acidified conditions would promote carbonate dissolution and result in an increase in TA and HCO 3 − (Kapetanaki et al, 2018(Kapetanaki et al, , 2020. Notably, the SEM results show that pCO 2 variation has no significant impact on DIC concentration (p > 0.05), which can be attributed to the interannual variation of DIC in TGD driven by HCO 3 − generated by mineralization and water acidification, rather than the loss of DIC caused by CO 2 outgassing (Figures 9b and 9c).…”

Late Glacial Marine Transgression and Ecosystem Response in the Landlocked Elefsis Bay (Northern Saronikos Gulf, Greece)

“…The metropolitan urban center of Attiki, one of the most populated areas in the Mediterranean, is located in the northern borderland of the inner Saronikos Gulf that receives the treated sewage of the entire region, whereas in Elefsis area significant industrial and shipbuilding activities take place. Following the Water Framework Directive 2000/60 concerning the ecological status of European waters [45], extensive research and continuous monitoring of the coastal water quality in Elefsis Bay has been conducted during the last decades, based on biotic and abiotic quality criteria [46][47][48][49][50][51][52][53][54].…”