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.
Highlights
Hypoxia constitutes the condition of low dissolved oxygen (DO); in most cases, hypoxia is associated with a semi-enclosed hydrogeomorphology that, combined with water-column stratification, restricts water exchange [1]
This study was closely associated with previous work concerning the area of Elefsis Bay [28] under hypoxic conditions coming after a previous preliminary experiment on the specific area [24]
The present work focused on the different alterations that severe hypoxia could implicate in the already high CO2 Elefsis bottom
Summary
Hypoxia constitutes the condition of low dissolved oxygen (DO); in most cases, hypoxia is associated with a semi-enclosed hydrogeomorphology that, combined with water-column stratification, restricts water exchange [1]. Various studies have related low oxygen with high CO2 partial pressure (pCO2) in both coastal and open ocean systems [6] often reaching an order of magnitude greater values than those predicted for ocean surface later this century (>1000 μatm; [7,8]). According to model simulations, a possible primary production increase due to eutrophication could poise ocean acidification effects on surface water carbonate chemistry in coastal environments [18]. Such coastal environments offer good case studies to better comprehend and possibly foresee potential interchanges between oxygen depletion, nutrient discharge and ocean acidification
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