Simulation of Coastal Processes affecting pH with Impacts on Carbon and Nutrient Biogeochemistry

Abstract

Naturally occurring microbial decomposition of organic matter (OM) in coastal marine environments cause increased acidity in deeper layers similar or even exceeding the future predictions for global ocean acidification (OA). Experimental studies in coastal areas characterized by increased inputs of OM and nutrients, coping with intermittent hypoxic/anoxic conditions, provide better understanding of the mechanisms affecting nutrients and carbon biogeochemistry under the emerging effects of coastal pH decrease. Laboratory CO 2 -manipulated microcosm experiments were conducted using seawater and surface sediment collected from the deepest part of Elefsis Bay (Saronikos Gulf, Eastern Mediterranean) focusing to study the co-evolution of processes affected by the decline of dissolved oxygen and pH induced by (a) OM remineralization and (b) the future anthropogenic increase of atmospheric CO 2 . Under more acidified conditions, a significant increase of total alkalinity was observed partially attributed to the sedimentary carbonate dissolution and the reactive nitrogen species shift towards ammonium. Νitrate and nitrite decline, in parallel with ammonium increase, demonstrated a deceleration of ammonium oxidation processes along with decrease in nitrate production. The decreased DIN:DIP ratio, the prevalence of organic nutrient species against the inorganic ones, the observations of constrained DON degradation and nitrate production decline and the higher DOC concentrations revealed the possible inhibition of OM decomposition under lower pH values. Finally, our results highlight the need for detailed studies of the carbonate system in coastal areas dominated by hypoxic/anoxic conditions, accompanied by other biogeochemical parameters and properly designed experiments to elucidate the processes sequence or alterations due to pH reduction.

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

“…The experimental setup follows a similar previous setup for the area [28]. Seawater from the deepest part (33 m) was collected unfiltered and untreated surface sediment (0-10 cm), which were then carefully transferred into four 25 L Nalgene Polycarbonate bottles (at a proportion 80% and 20%, respectively) in a thermostated room.…”

“…The nominal pH values selected for the two experiment conditions (treatments) were: (i) the pH value found in situ (7.75, NBS) as the control factor conditions/treatment (C), and (ii) the pH value This research aims in examining how further oxygen depletion and the establishment of severe hypoxic conditions/anoxic, due to enhanced OM degradation and concomitant nutrient accumulation, would act as a positive or negative feedback to bottom to surface acidification [28] and pointing out possible shifts in complex biogeochemical processes established in the area.…”

“…Seawater from the deepest part (33 m) was collected unfiltered and untreated surface sediment (0-10 cm), which were then carefully transferred into four 25 L Nalgene Polycarbonate bottles (at a proportion 80% and 20%, respectively) in a thermostated room. The seawater-sediment systems were kept in the dark, at in situ temperature (17.5 • C) and were left to equilibrate under controlled Ar supply for a week (instead of air supply used in the previous setup [28]), in order to maintain minimum DO conditions (Day-1 in all diagrams presented here suggests field values for maximum depth, 33 m). Subsequently, a 33 day-experimental period of CO 2 aeration followed, using a continuous flow system (IKS Aquastar, IKS Computer Systeme GmbH); Ar gas was supplied in parallel to avoid oxygen penetration.…”

“…The nominal pH values selected for the two experiment conditions (treatments) were: (i) the pH value found in situ (7.75, NBS) as the control factor conditions/treatment (C), and (ii) the pH value predicted for the year 2300 (6.80 NBS), which corresponds to the highest total cumulative future CO 2 emission scenario, for latitudes corresponding to the Mediterranean Sea [30] for the ocean acidification conditions/treatment (OA). The 2300 prediction for the pH value was chosen because in situ pH measurements below the pycnocline Elefsis' waters already exceeded the future projections for the year 2100 (data from present study; [22,27,28]). Each experimental condition was applied in two replicate tanks (Figure 2); seemingly, all results regarding seawater samples for both conditions refer to the mean value of the two replicates (including their standard deviation bars).…”

“…Individual studies within the warm period report pH fluctuations of 8.4-7.9 from surface to bottom, already during the 1970s, with decreased bottom pH values ranging between 7.5 and 7.9 [22,27]. Despite these findings, only one experimental study has been conducted to evaluate possible biogeochemical alterations due to pH decline during hypoxic conditions in the enclosed embayment of Elefsis Bay [28]. This research aims in examining how further oxygen depletion and the establishment of severe hypoxic conditions/anoxic, due to enhanced OM degradation and concomitant nutrient accumulation, would act as a positive or negative feedback to bottom to surface acidification [28] and pointing out possible shifts in complex biogeochemical processes established in the area.…”