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

• Main Authors: Natalia Kapetanaki, Evangelia Krasakopoulou, Eleni Stathopoulou, Manos Dassenakis, Michael Scoullos
• Format: Article
• Language: English
• Published: MDPI AG 2020-06-01
• Series: Journal of Marine Science and Engineering
• Subjects: CO₂ addition; ocean acidification; pH decline; nitrification; microcosm experiment; anoxic sediment
• Online Access: https://www.mdpi.com/2077-1312/8/6/462

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 <i>p</i>CO₂ is already higher than global present atmospheric values, while near the bottom <i>p</i>CO₂ 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.