Phosphorus dynamics associated with benthic microbial respiration in sediments from intertidal flats to deep sea

Abstract

ABSTRACT

Phosphorus dynamics associated with benthic microbial respiration in sediments from intertidal flats to deep sea

Jin-Sook Mok

Directed by Prof. Jung-Ho Hyun

Department of Marine Sciences and Convergent Technology

Graduated School of Hanyang University

Impacts of typhoon-induced heavy rainfalls and resultant freshwater runoff on the partitioning of organic carbon oxidation and nutrient dynamics in the intertidal sediments of the Han River estuary, Yellow Sea

Occurrence of typhoons accompanied by heavy precipitation has increased for the past 40 years in northeast Asia. To elucidate the impact of three consecutive typhoon-induced heavy rainfall events and resultant freshwater runoff on the partitioning of organic carbon (Corg) oxidation and nutrient dynamics, we investigated the geochemical constituents, the rate of anaerobic Corg oxidation, sulfate reduction (SR), iron reduction (FeR) and P speciation in the intertidal mud flat of the Han River estuary, Yellow Sea. Corg oxidation by SR and FeR and their metabolic products (∑CO2, NH4+, H2S, Fe2+) decreased significantly (P < 0.05) during and immediately after the heavy rainfall. Additional mesocosm experiments demonstrated that potential N2 production rates increased up to 2.4 times with increased nitrate concentrations during freshwater runoff. The results suggest that denitrification becomes a significant Corg oxidation pathway substituting for SR during high-nitrate freshwater runoff, which may remove substantial portion of the N introduced into the estuary. P speciation analysis further revealed that the concentrations of iron bound P decreased by 2.2 fold during the heavy rainfall compared to that measured before the rainfall. The results suggest that an excess supply of riverine Si keeps P from binding to Fe, thereby stimulating P release. Taking projections of enhanced rainfall events in the future into account, the results suggest that the intensified storm events and resultant riverine runoff induces a shift of Corg oxidation pathways in the sediments, which ultimately alters C-N-P-S-Fe dynamics and may deepen N-limiting conditions in coastal ecosystems of the Yellow Sea.

Phosphorus dynamics associated with organic carbon mineralization by sulfate- and iron reduction in sediment exposed to fish farming

To determine the phosphorus (P) dynamics in organic-rich sediments exposed to fish farming, we investigated P speciation and benthic P release together with partitioning of organic carbon (Corg) oxidation coupled to sulfate reduction and iron reduction. Biogenic-P and authigenic-P dominated in the P-enriched farm sediment, comprising 28% and 33% of total P, respectively, which well reflected the impact of high P containing fish feed. In the farm sediment where sulfate reduction dominated Corg oxidation, FeOOH reduction coupled to H2S oxidation stimulated release of the P bound to Fe-oxides, which ultimately enhanced P regeneration efficiency (122%) and/or bioavailability of P (58% of total P). Consequently, excessive benthic P release directly resulting from the supply of organic- and P-rich fish feed would be a significant internal P source for water column, which may induce an undesirable eutrophication and (harmful) algal bloom in shallow coastal ecosystems.

Phosphorus dynamics associated with organic matter mineralization in the surface sediments of the deep Ulleung Basin, East Sea

P speciation associated with the partitioning of organic carbon (Corg) oxidation in sediments was investigated to elucidate the P dynamics at two contrasting sediments in the continental shelf (EB1) and basin (EC1) in the Ulleung Basin (UB), East Sea. Sulfate reduction (SR) pre-dominated Corg oxidation at shelf site (EB1), comprising 77% of Corg oxidation, whereas combined Mn- and Fe-reduction accounted for > 80% of Corg oxidation at the Mn-oxide- and Fe-oxide-rich basin site (EC1). Under the SR-dominated condition (EB1), H2S oxidation coupled to reductive dissolution of FeOOH to form precipitation of FeS induced the accumulation of dissolved iron and phosphate in the pore water. On the other hand, phosphate in the Mn- and Fe-oxide-rich basin sediments (EC1) was depleted because the P released through organic matter decomposition or reductive dissolution of Fe-/Mn-oxide was effectively adsorbed to the metal-oxides in the surface sediments. Sequential extraction of P phases revealed that Fe bound P (52%-65% of total P) was the major phase in the surface sediments of both sites. Interestingly, the organic P (OP) fraction was 2.4-times higher at the basin site (12 μmol g-1) than at the shelf site (5 μmol g-1). Corg/Porg ratio presented as redox proxies in sediments was 644 and 191 for EB1 and EC1, respectively. The results indicate that OP was effectively preserved under sub-oxic conditions (EC1), whereas it was preferentially regenerated relative to Corg under anoxic conditions (EB1). P regeneration efficiency in the shelf (251%) and basin (30%) sediments suggest that significant amounts of P can release from the shelf sediments and there is potential of P release from the basin sediments, despite a removal mechanism by Fe- or Mn-oxides. Overall, the dynamics of P in the UB sediments were largely regulated by the partitioning of Corg oxidation pathways (i.e., sulfate reduction vs. metal reduction) and resultant interactions between Fe/Mn-S-P.