Biogeochemical Processes Of Phosphorus In The Changjiang Estuary And Adjacent East China Sea Inner Shelf

Phosphorus (P) cycling in estuaries, especially in large-river delta-front estuaries (LDEs) and adjacent areas is an important part of oceanic P cycles. This work mainly focused on the Changjiang Estuary and adjacent East China Sea inner shelf and covered the results of P in water column, surface sediments, overlying water, pore water and core sediments of this region. Combined with other parameters, such as grain size and mineral compositions, sediment surface area (SSA), TOC,δ13C and δ18O, the primary goal of this work was to better constrain the biogeochemical processes in P cycling in the Changjiang LDE. Major results were summarized as follows.1. Surface water and suspended particulate matter (SPM) samples were collected along a salinity gradient in the Changjiang Estuary in June,2011 to examine the distribution, mixing behavior and transformation of dissolved and particulate phosphorus (P) species. Particulate P was fractionated into five size fractions using a water elutriation method, followed by a sequential extraction procedure (SEDEX). The variation of dissolved and particulate P species along the salinity gradient indicated significant transformation between different P phases, especially for labile P such as exchangeable P (Ex-P), organic P (Or-P) and Fe-bound P (Fe-P) in particulate P pool. Among all P forms, Detrital P (De-P) and Or-P in SPM were the most abundant, accounting for 43.9% and 26.1% of total particulate phosphorus (TPP). De-P increased with increasing particle grain size, while Or-P and other P forms decreased gradually with increasing grain size. The flux of bioavailable P (BAP, sum of Ex-P, Or-P and Fe-P) in clay fraction (<8 μm), defined as "truly" suspended particles, was estimated to be 0.21 ×104 tP yr-1 and accounted for 29% of bulk BAP flux. Particulate P to specific surface area ratios (P/SSA) were lower in fine suspended particles than in coarse particles, indicating lower preservation of P in these particles. When compared with the bed sediments of the Changjiang Estuary, the P/SSA loadings in fractionated suspended particles were maintained at the same level for De-P and Fe-P, slightly higher for authigenic P (Au-P) and refractory P (Re-P) and obviously higher for labile P such as Ex-P and Or-P, further indicating differential behavior of P forms during particle settlement in this dynamic estuary.2. P speciation in size-fractionated sediments was studied, and the phosphate adsorption characteristics associated with grain size were investigated in the laboratory. De-P was the most abundant form and had the highest proportion of total P (TP), followed by Or-P. BAP was much more concentrated in fine sediments than sandy sediments, which meant that fine sediments played a dominant role in P transfer from sediments to overlying water and/or ambient pore water. Native adsorbed phosphorus (NAP) was close to the Ex-P concentrations in sediments. The adsorption capacity was 116.0,84.8,73.6 and 50.0μg g"1 for fine silt, medium silt, coarse silt and sand, respectively. The zero equilibrium phosphate concentration (EPC0) values obtained from the adsorption isotherms of size-fractionated sediments from the same sampling site were very close in value (55.0,54.9,51.7 and 49.0 μg L-1 for fine silt, medium silt, coarse silt and sand, respectively), revealing a potential method for predicting the phosphate concentrations of overlying water and/or ambient pore water, which were also considered to be the equilibrium concentrations of phosphate through long-term adsorption and desorption processes by sediments. The adsorption capacity, adsorption rate and NAP were all largely controlled by sediment grain size. Based on our data, the maximum total phosphate adsorption amount (TQmax) of fine sediments (< 32μm) was almost six times as large as that of sediments larger than 32μm. Due to the much higher phosphate adsorption capacity of fine sediments, compared to the sandy sediments, the mud belt in the East China Sea (ECS) inner shelf should play a key role in controlling the phosphate concentration in the overlying water and/or ambient pore water and may act as an efficient "trapper" for the overloaded phosphate discharged by the Changjiang, greatly reducing the risk of eutrophication on the outer shelf of the ECS.3. The speciation, potential bioavailability, and preservation of phosphorus (P) in surface sediments of the Changjiang (Yangtze River) Estuary and adjacent ECS inner shelf were investigated through the analyses of P fractions and sediment bulk properties. TP in surface sediments ranged from 15.0 to 21.4μmol g-1 and was highest near the Changjiang river mouth. The average contribution of each form of P to TP was 55.6%(De-P),17.8%(Re-P),16.1%(Or-P),5.5%(Au-P),2.5%(Ex-P) and 2.5%(Fe-P), respectively. De-P showed relatively higher concentrations in the river mouth and the ECS shelf region, off the Changjiang Estuary. High concentrations of Or-P were found mainly in mud areas showing a similar distribution pattern with silt, SSA, and total organic carbon (TOC). Re-P was mainly distributed near the estuarine area and the Zhe-Min coast. BAP, accounted for 9.5 to 32.0% of TP (with a mean of 21.2%) and showed a similar distribution pattern to that of Or-P. De-P/SSA and TOC/SSA loadings both decreased with increasing of SSA, while Or-P/SSA loadings varied little with SSA, indicating that Or-P may have reached an adsorption-desorption equilibrium on mineral surfaces. TOC to total organic P (TOP; sum of Re-P and Or-P) ratios less than the Redfield ratio (84 in average) may have indicated efficient remineralization of organic matter in mobile muds of the Changjiang Estuary and adjacent ECS inner shelf. Furthermore, the relatively high TOC/Or-P ratios (72-422 with a mean of 188) likely suggest a higher degree of preferential regeneration of labile Or-P over TOC in sediments.4. In general, we measured Fe2+, Mn2+, and nutrients (PO43- and NH4+) in pore water, grain size composition, SSA,210Pb, TOC and stable carbon isotopic abundance (δ13C) in sediments of three short cores collected from mobile mud and non-mobile mud regions to characterize the basic properties of sediments and finally, the transformation, diagenetic processes, and preservation of P under different sedimentary environments were investigated from measurements of P speciation in these sediment cores. With input from the Changjiang River, TP and some P speciation, especially for De-P, were more abundant in the mud areas of the Changjiang Estuary and adjacent ECS inner shelf than the non-mud areas. Corresponding to the "mobile mud layer",210Pbexcess, median grain size (MGS), and various P species such as Ex-P, Or-P, Au-P, Re-P, and De-P in core 31 all showed a constant trend within a certain depth of top layer and an obvious "unconformity" with those in the sublayer, while 210Pbexcess and the inactive P forms (such as De-P, Au-P and Re-P) of core 12 were almost constant over the profile. The mobile-muds play a unique role on the release of BAP at the water-sediment interface. The Or-P and Fe-P were found to be transformed into Au-P in this region. Sediments of core 31 located in the mud area acted as the source of phosphate in pore water, while sediments of core 33 located in the non-mud area was a sink of phosphate.5. In this study, MGS, SSA, TOC contents, C/N molar ratios, stable carbon isotope, and P species in a sediment core, collected from the ECS inner-shelf were measured to explore the applicability of De-P as a potential indicator of past flooding events in the Changjiang River Basin (CRB). In particular, we examined the linkages between the evolution of floods with regional climate changes and anthropogenic activities in the CRB. Peaks of De-P concentrations in sediments corresponded well with the worst flooding events of the CRB over the past two centuries (e.g.,1850s, 1860s,1900s,1920s,1950s,1980s, and 2000s). Moreover, De-P also corresponded well with the extreme hypoxic events in 1981 and 1998 in the Changjiang Estuary as indicated by Mo/Al ratios, indicating potential linkages between De-P as a flooding proxy to flood-induced hypoxia events in this region. In addition, a robust relationship was found among De-P, the floods in 1950s,1980s,2000s of the CRB, the intensive El Nino-Southern Oscillation (ENSO), the abnormally weak East Asian Summer Monsoon (EASM) and the warm phase of Pacific Decadal Oscillation (PDO), suggesting that De-P also provided insights to linkages between regional climate change and flooding events in this region.6. Surface water samples were collected along a salinity gradient in the Changjiang Estuary in July,2013 to investigate the phosphate cycling through the analyse of oxygen isotope of phosphate. The Changjiang Estuary was best represented by a two end-member mixing model between the ECS water and Changjiang river water. Based on this, the two end-member mixing model and equilibrium mixing model were used to better constrain the cycling of phosphate in the Changjiang Estuary. This work indicated that phosphate was not heavily cycled in the freshwater end of the Changjiang Estuary, but largely controlled by physical mixing process. The cycling of phosphate in the Turbidity Maximum Zone was really weak and the phosphate with different δ18OP signatures from the deep layer water may have altered the original δ18OP value of this region. In the Plume Front, the measured δ18OP was nearly equated to the equilibrium value of δ18OP, indicating that phosphate was heavily cycled.